resolved: add ability for external programs to hook into name resolution (for machined+networkd to synthesize records) (#39293)

Fixes: #8518

TODO

@@ -134,6 +134,14 @@ Deprecations and removals:
 
 Features:
 
+* networkd/machined: implement reverse name lookups in the resolved hook
+
+* networkd's resolved hook: optionally map all lease IP addresses handed out to
+  the same hostname which is configured on the .network file. Optionally, even
+  derive this single name from the network interface name (i.e. probably
+  altname or so). This way, when spawning a VM the host could pick the hostname
+  for it and the client gets no say.
+
 * systemd-repart: add --ghost, that creates file systems, updates the kernel's
   partition table but does *not* update partition table on disk. This way, we
   have disk backed file systems that go effectively disappear on reboot. This
@@ -230,12 +238,6 @@ Features:
   so that we add correct ordering between the two. Use MSG_PEEK + SCM_TIMESTAMP
   for this.
 
-* maybe replace nss-machines with logic in networkd that registers records with
-  systemd-resolved, based on DHCP leases, so that we gain compat with VMs.
-  Implementation idea: encode in an ifaltname the intended local name to expose this
-  under and then parse that out and map it to the combined A/AAAA of all handed
-  out leases.
-
 * bsod: add target "bsod.target" or so, which invokes systemd-bsod.target and
   waits and then reboots. Then use OnFailure=bsod.target from various jobs that
   should result in system reboots, such as TPM tamper detection cases.
@@ -2138,7 +2140,6 @@ Features:
     names, so that for the container case we can establish the same name
     (maybe "host") for referencing the server, everywhere.
   - allow clients to request DNSSEC for a single lookup even if DNSSEC is off (?)
-  - hook up resolved with machined-based address resolution
 
 * refcounting in sd-resolve is borked
 

man/org.freedesktop.resolve1.xml

@@ -497,6 +497,7 @@ node /org/freedesktop/resolve1 {
 #define SD_RESOLVED_FROM_ZONE          (UINT64_C(1) << 21)
 #define SD_RESOLVED_FROM_TRUST_ANCHOR  (UINT64_C(1) << 22)
 #define SD_RESOLVED_FROM_NETWORK       (UINT64_C(1) << 23)
+#define SD_RESOLVED_FROM_HOOK          (UINT64_C(1) << 27)
 </programlisting>
 
         <para>On input, the first five flags control the protocols to use for the look-up. They refer to

man/systemd.network.xml

@@ -4201,6 +4201,23 @@ ServerAddress=192.168.0.1/24</programlisting>
         </listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><varname>LocalLeaseDomain=</varname></term>
+        <listitem>
+          <para>Takes a DNS domain name as argument. If specified,
+          <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
+          will integrate with
+          <citerefentry><refentrytitle>systemd-resolved.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
+          and ensure that the hostnames associated with each handed out DHCP lease may be resolved to the IP
+          addresses of the lease. The hostnames are suffixed with the specified domain name.</para>
+
+          <para>Note that this purely about hostname resolution on the local system, i.e. from programs with
+          access to the same <filename>systemd-resolved</filename> instances via D-Bus IPC, Varlink IPC, or
+          the local DNS stub.</para>
+          <xi:include href="version-info.xml" xpointer="v259"/>
+        </listitem>
+      </varlistentry>
+
     </variablelist>
   </refsect1>
 

network/80-container-ve.network

@@ -30,3 +30,4 @@ IPv6SendRA=yes
 
 [DHCPServer]
 PersistLeases=runtime
+LocalLeaseDomain=_dhcp

network/80-container-vz.network

@@ -29,3 +29,4 @@ IPv6SendRA=yes
 
 [DHCPServer]
 PersistLeases=runtime
+LocalLeaseDomain=_dhcp

network/80-namespace-ns-tun.network

@@ -30,3 +30,4 @@ IPv6SendRA=yes
 
 [DHCPServer]
 PersistLeases=runtime
+LocalLeaseDomain=_dhcp

network/80-namespace-ns.network

@@ -30,3 +30,4 @@ IPv6SendRA=yes
 
 [DHCPServer]
 PersistLeases=runtime
+LocalLeaseDomain=_dhcp

network/80-vm-vt.network

@@ -29,3 +29,4 @@ IPv6SendRA=yes
 
 [DHCPServer]
 PersistLeases=runtime
+LocalLeaseDomain=_dhcp

network/80-wifi-ap.network.example

@@ -19,3 +19,6 @@ DHCPServer=yes
 IPMasquerade=both
 IPv6AcceptRA=no
 IPv6SendRA=yes
+
+[DHCPServer]
+LocalLeaseDomain=home.arpa

src/libsystemd-network/sd-dhcp-server.c

@@ -1711,3 +1711,38 @@ int sd_dhcp_server_set_lease_file(sd_dhcp_server *server, int dir_fd, const char
 
         return 0;
 }
+
+static int find_lease_address(Hashmap *h, const char *name, struct in_addr *ret) {
+        int r;
+
+        assert(name);
+
+        sd_dhcp_server_lease *lease;
+        HASHMAP_FOREACH(lease, h) {
+                if (!lease->hostname)
+                        continue;
+
+                r = dns_name_equal(lease->hostname, name);
+                if (r <= 0)
+                        continue;
+
+                if (ret)
+                        ret->s_addr = lease->address;
+                return 1;
+        }
+
+        return -ENOENT;
+}
+
+int sd_dhcp_server_get_lease_address_by_name(sd_dhcp_server *server, const char *name, struct in_addr *ret) {
+        int r;
+
+        assert_return(server, -EINVAL);
+        assert_return(dns_name_is_valid(name), -EINVAL);
+
+        r = find_lease_address(server->static_leases_by_address, name, ret);
+        if (r != -ENOENT)
+                return r;
+
+        return find_lease_address(server->bound_leases_by_address, name, ret);
+}

src/machine/machine-dbus.c

@@ -870,12 +870,11 @@ const BusObjectImplementation machine_object = {
         .node_enumerator = machine_node_enumerator,
 };
 
-int machine_send_signal(Machine *m, bool new_machine) {
-        _cleanup_free_ char *p = NULL;
-
+int machine_send_signal(Machine *m, const char *signal_name) {
         assert(m);
+        assert(signal_name);
 
-        p = machine_bus_path(m);
+        _cleanup_free_ char *p = machine_bus_path(m);
         if (!p)
                 return -ENOMEM;
 
@@ -883,7 +882,7 @@ int machine_send_signal(Machine *m, bool new_machine) {
                         m->manager->api_bus,
                         "/org/freedesktop/machine1",
                         "org.freedesktop.machine1.Manager",
-                        new_machine ? "MachineNew" : "MachineRemoved",
+                        signal_name,
                         "so", m->name, p);
 }
 

src/machine/machine-dbus.h

@@ -26,5 +26,5 @@ int bus_machine_method_copy(sd_bus_message *message, void *userdata, sd_bus_erro
 int bus_machine_method_open_root_directory(sd_bus_message *message, void *userdata, sd_bus_error *error);
 int bus_machine_method_get_uid_shift(sd_bus_message *message, void *userdata, sd_bus_error *error);
 
-int machine_send_signal(Machine *m, bool new_machine);
+int machine_send_signal(Machine *m, const char *signal_name);
 int machine_send_create_reply(Machine *m, sd_bus_error *error);

src/machine/machine.c

@@ -24,6 +24,7 @@
 #include "log.h"
 #include "machine.h"
 #include "machine-dbus.h"
+#include "machined-resolve-hook.h"
 #include "machined.h"
 #include "mkdir-label.h"
 #include "namespace-util.h"
@@ -672,7 +673,9 @@ int machine_start(Machine *m, sd_bus_message *properties, sd_bus_error *error) {
         /* Save new machine data */
         machine_save(m);
 
-        machine_send_signal(m, true);
+        machine_send_signal(m, "MachineNew");
+
+        (void) manager_notify_hook_filters(m->manager);
 
         return 0;
 }
@@ -730,8 +733,10 @@ int machine_finalize(Machine *m) {
         machine_add_to_gc_queue(m);
 
         if (m->started) {
-                machine_send_signal(m, false);
+                machine_send_signal(m, "MachineRemoved");
                 m->started = false;
+
+                (void) manager_notify_hook_filters(m->manager);
         }
 
         return 0;

src/machine/machined-resolve-hook.c

@@ -0,0 +1,200 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#include "sd-json.h"
+
+#include "dns-answer.h"
+#include "dns-domain.h"
+#include "dns-packet.h"
+#include "dns-question.h"
+#include "dns-rr.h"
+#include "hashmap.h"
+#include "json-util.h"
+#include "local-addresses.h"
+#include "log.h"
+#include "machine.h"
+#include "machined.h"
+#include "machined-resolve-hook.h"
+#include "resolve-hook-util.h"
+#include "set.h"
+#include "varlink-util.h"
+
+static int manager_make_machine_array(Manager *m, sd_json_variant **ret) {
+        int r;
+
+        assert(m);
+        assert(ret);
+
+        _cleanup_(sd_json_variant_unrefp) sd_json_variant *array = NULL;
+        Machine *machine;
+        HASHMAP_FOREACH(machine, m->machines) {
+                if (machine->class == MACHINE_HOST)
+                        continue;
+                if (!machine->started)
+                        continue;
+
+                r = sd_json_variant_append_arrayb(&array, SD_JSON_BUILD_STRING(machine->name));
+                if (r < 0)
+                        return r;
+        }
+
+        if (!array)
+                return sd_json_variant_new_array(ret, /* array= */ NULL, /* n= */ 0);
+
+        *ret = TAKE_PTR(array);
+        return 0;
+}
+
+int manager_notify_hook_filters(Manager *m) {
+        int r;
+
+        assert(m);
+
+        /* Called whenever a machine is added or dropped from the list */
+
+        if (set_isempty(m->query_filter_subscriptions))
+                return 0;
+
+        _cleanup_(sd_json_variant_unrefp) sd_json_variant *array = NULL;
+        r = manager_make_machine_array(m, &array);
+        if (r < 0)
+                return log_error_errno(r, "Failed to generate JSON array with machine names: %m");
+
+        r = varlink_many_notifybo(m->query_filter_subscriptions, SD_JSON_BUILD_PAIR_VARIANT("filterDomains", array));
+        if (r < 0)
+                return log_error_errno(r, "Failed to notify filter subscribers: %m");
+
+        return 0;
+}
+
+int vl_method_query_filter(
+                sd_varlink *link,
+                sd_json_variant *parameters,
+                sd_varlink_method_flags_t flags,
+                void *userdata) {
+
+        Manager *m = ASSERT_PTR(userdata);
+        int r;
+
+        assert(link);
+
+        _cleanup_(sd_json_variant_unrefp) sd_json_variant *array = NULL;
+        r = manager_make_machine_array(m, &array);
+        if (r < 0)
+                return r;
+
+        if (flags & SD_VARLINK_METHOD_MORE) {
+                /* If 'more' is set, this is a subscription request, keep track of the link */
+
+                r = sd_varlink_notifybo(link, SD_JSON_BUILD_PAIR_VARIANT("filterDomains", array));
+                if (r < 0)
+                        return log_error_errno(r, "Failed to notify filter subscribers: %m");
+
+                r = set_ensure_put(&m->query_filter_subscriptions, &varlink_hash_ops, link);
+                if (r < 0)
+                        return r;
+
+                sd_varlink_ref(link);
+        } else {
+                r = sd_varlink_replybo(link, SD_JSON_BUILD_PAIR_VARIANT("filterDomains", array));
+                if (r < 0)
+                        return log_error_errno(r, "Failed to notify filter subscribers: %m");
+        }
+
+        return 0;
+}
+
+int vl_method_resolve_record(
+                sd_varlink *link,
+                sd_json_variant *parameters,
+                sd_varlink_method_flags_t flags,
+                void *userdata) {
+
+        Manager *m = ASSERT_PTR(userdata);
+        int r;
+
+        assert(link);
+
+        _cleanup_(resolve_record_parameters_done) ResolveRecordParameters p = {};
+        r = sd_varlink_dispatch(link, parameters, resolve_record_parameters_dispatch_table, &p);
+        if (r != 0)
+                return r;
+
+        if (dns_question_isempty(p.question))
+                return sd_varlink_error_invalid_parameter_name(link, "question");
+
+        _cleanup_(dns_answer_unrefp) DnsAnswer *answer = NULL;
+
+        _cleanup_free_ struct local_address *addresses = NULL;
+        bool found = false, nxdomain = false;
+        int n_addresses = -1;
+
+        DnsResourceKey *key;
+        DNS_QUESTION_FOREACH(key, p.question) {
+                Machine *machine = hashmap_get(m->machines, dns_resource_key_name(key));
+                if (machine) {
+                        /* We found a perfect match, yay! */
+                        found = true;
+
+                        if (!dns_resource_key_is_address(key))
+                                continue;
+
+                        if (n_addresses < 0) {
+                                n_addresses = machine_get_addresses(machine, &addresses);
+                                if (n_addresses < 0)
+                                        return n_addresses;
+                        }
+
+                        int family = dns_type_to_af(key->type);
+                        FOREACH_ARRAY(address, addresses, n_addresses) {
+                                if (address->family != family)
+                                        continue;
+
+                                _cleanup_(dns_resource_record_unrefp) DnsResourceRecord *rr = NULL;
+                                r = dns_resource_record_new_address(&rr, address->family, &address->address, machine->name);
+                                if (r < 0)
+                                        return r;
+
+                                r = dns_answer_add_extend(
+                                                &answer,
+                                                rr,
+                                                machine->n_netif == 1 ? machine->netif[0] : -1,
+                                                DNS_ANSWER_AUTHENTICATED,
+                                                /* rrsig= */ NULL);
+                                if (r < 0)
+                                        return r;
+                        }
+                }
+
+                /* So this is not a direct match? Then check if we find a prefix match */
+                const char *q = dns_resource_key_name(key);
+                while (!nxdomain) {
+                        r = dns_name_parent(&q);
+                        if (r < 0)
+                                return r;
+                        if (r == 0)
+                                break;
+
+                        nxdomain = !!hashmap_get(m->machines, q);
+                }
+        }
+
+        if (!found) {
+                /* If we found a prefix match we own the subtree, and thus return NXDOMAIN because we know
+                 * that we only expose the machine A/AAAA records on the primary name, but nothing below. */
+                if (nxdomain)
+                        return sd_varlink_replybo(link, SD_JSON_BUILD_PAIR_INTEGER("rcode", DNS_RCODE_NXDOMAIN));
+
+                /* Otherwise we return an empty response, which means: continue with the usual lookup */
+                return sd_varlink_reply(link, /* parameters= */ NULL);
+        }
+
+        _cleanup_(sd_json_variant_unrefp) sd_json_variant *ja = NULL;
+        r = dns_answer_to_json(answer, &ja);
+        if (r < 0)
+                return r;
+
+        return sd_varlink_replybo(
+                        link,
+                        SD_JSON_BUILD_PAIR_INTEGER("rcode", DNS_RCODE_SUCCESS),
+                        SD_JSON_BUILD_PAIR_VARIANT("answer", ja));
+}

src/machine/machined-resolve-hook.h

@@ -0,0 +1,9 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+#pragma once
+
+#include "machine-forward.h"
+
+int vl_method_query_filter(sd_varlink *link, sd_json_variant *parameters, sd_varlink_method_flags_t flags, void *userdata);
+int vl_method_resolve_record(sd_varlink *link, sd_json_variant *parameters, sd_varlink_method_flags_t flags, void *userdata);
+
+int manager_notify_hook_filters(Manager *m);

src/machine/machined-varlink.c

@@ -17,14 +17,17 @@
 #include "machine.h"
 #include "machine-varlink.h"
 #include "machined.h"
+#include "machined-resolve-hook.h"
 #include "machined-varlink.h"
 #include "path-lookup.h"
+#include "set.h"
 #include "string-util.h"
 #include "strv.h"
 #include "user-util.h"
 #include "varlink-io.systemd.Machine.h"
 #include "varlink-io.systemd.MachineImage.h"
 #include "varlink-io.systemd.UserDatabase.h"
+#include "varlink-io.systemd.Resolve.Hook.h"
 #include "varlink-io.systemd.service.h"
 #include "varlink-util.h"
 
@@ -847,6 +850,57 @@ static int manager_varlink_init_machine(Manager *m) {
         return 0;
 }
 
+static void on_resolve_hook_disconnect(sd_varlink_server *server, sd_varlink *link, void *userdata) {
+        Manager *m = ASSERT_PTR(userdata);
+
+        if (set_remove(m->query_filter_subscriptions, link))
+                sd_varlink_unref(link);
+}
+
+static int manager_varlink_init_resolve_hook(Manager *m) {
+        _cleanup_(sd_varlink_server_unrefp) sd_varlink_server *s = NULL;
+        int r;
+
+        assert(m);
+
+        if (m->varlink_resolve_hook_server)
+                return 0;
+        if (m->runtime_scope != RUNTIME_SCOPE_SYSTEM) /* no resolved in per-user mode! */
+                return 0;
+
+        r = varlink_server_new(&s, SD_VARLINK_SERVER_ACCOUNT_UID|SD_VARLINK_SERVER_INHERIT_USERDATA, m);
+        if (r < 0)
+                return log_error_errno(r, "Failed to allocate varlink server object: %m");
+
+        (void) sd_varlink_server_set_description(s, "varlink-resolve-hook");
+
+        r = sd_varlink_server_add_interface(s, &vl_interface_io_systemd_Resolve_Hook);
+        if (r < 0)
+                return log_error_errno(r, "Failed to add Resolve.Hook interface to varlink server: %m");
+
+        r = sd_varlink_server_bind_method_many(
+                        s,
+                        "io.systemd.Resolve.Hook.QueryFilter",   vl_method_query_filter,
+                        "io.systemd.Resolve.Hook.ResolveRecord", vl_method_resolve_record);
+        if (r < 0)
+                return log_error_errno(r, "Failed to register varlink methods: %m");
+
+        r = sd_varlink_server_bind_disconnect(s, on_resolve_hook_disconnect);
+        if (r < 0)
+                return log_error_errno(r, "Failed to bind on resolve hook disconnection events: %m");
+
+        r = sd_varlink_server_listen_address(s, "/run/systemd/resolve.hook/io.systemd.Machine", 0666 | SD_VARLINK_SERVER_MODE_MKDIR_0755);
+        if (r < 0)
+                return log_error_errno(r, "Failed to bind to varlink socket: %m");
+
+        r = sd_varlink_server_attach_event(s, m->event, SD_EVENT_PRIORITY_NORMAL);
+        if (r < 0)
+                return log_error_errno(r, "Failed to attach varlink connection to event loop: %m");
+
+        m->varlink_resolve_hook_server = TAKE_PTR(s);
+        return 0;
+}
+
 int manager_varlink_init(Manager *m) {
         int r;
 
@@ -858,6 +912,10 @@ int manager_varlink_init(Manager *m) {
         if (r < 0)
                 return r;
 
+        r = manager_varlink_init_resolve_hook(m);
+        if (r < 0)
+                return r;
+
         return 0;
 }
 
@@ -866,4 +924,5 @@ void manager_varlink_done(Manager *m) {
 
         m->varlink_userdb_server = sd_varlink_server_unref(m->varlink_userdb_server);
         m->varlink_machine_server = sd_varlink_server_unref(m->varlink_machine_server);
+        m->varlink_resolve_hook_server = sd_varlink_server_unref(m->varlink_resolve_hook_server);
 }

src/machine/machined.c

@@ -29,6 +29,7 @@
 #include "operation.h"
 #include "path-lookup.h"
 #include "service-util.h"
+#include "set.h"
 #include "signal-util.h"
 #include "socket-util.h"
 #include "special.h"
@@ -110,6 +111,8 @@ static Manager* manager_unref(Manager *m) {
 
         manager_varlink_done(m);
 
+        m->query_filter_subscriptions = set_free(m->query_filter_subscriptions);
+
         sd_bus_flush_close_unref(m->api_bus);
         sd_bus_flush_close_unref(m->system_bus);
         sd_event_unref(m->event);

src/machine/machined.h

@@ -32,6 +32,8 @@ typedef struct Manager {
 
         sd_varlink_server *varlink_userdb_server;
         sd_varlink_server *varlink_machine_server;
+        sd_varlink_server *varlink_resolve_hook_server;
+        Set *query_filter_subscriptions;
 
         RuntimeScope runtime_scope;
         char *state_dir;

src/machine/meson.build

@@ -16,6 +16,7 @@ systemd_machined_extract_sources = files(
         'machine.c',
         'machined-core.c',
         'machined-dbus.c',
+        'machined-resolve-hook.c',
         'machined-varlink.c',
         'operation.c',
 )

src/network/meson.build

@@ -74,6 +74,7 @@ systemd_networkd_extract_sources = files(
         'networkd-ntp.c',
         'networkd-queue.c',
         'networkd-radv.c',
+        'networkd-resolve-hook.c',
         'networkd-route.c',
         'networkd-route-metric.c',
         'networkd-route-nexthop.c',

src/network/networkd-dhcp-server.c

@@ -25,6 +25,7 @@
 #include "networkd-network.h"
 #include "networkd-ntp.h"
 #include "networkd-queue.h"
+#include "networkd-resolve-hook.h"
 #include "networkd-route-util.h"
 #include "path-util.h"
 #include "set.h"
@@ -747,6 +748,8 @@ static int dhcp4_server_configure(Link *link) {
         if (r < 0)
                 return log_link_error_errno(link, r, "Could not start DHCPv4 server instance: %m");
 
+        manager_notify_hook_filters(link->manager);
+
         return 0;
 }
 

src/network/networkd-link.c

@@ -23,6 +23,7 @@
 #include "arphrd-util.h"
 #include "bitfield.h"
 #include "device-util.h"
+#include "dns-domain.h"
 #include "errno-util.h"
 #include "ethtool-util.h"
 #include "event-util.h"
@@ -53,6 +54,7 @@
 #include "networkd-nexthop.h"
 #include "networkd-queue.h"
 #include "networkd-radv.h"
+#include "networkd-resolve-hook.h"
 #include "networkd-route.h"
 #include "networkd-route-util.h"
 #include "networkd-routing-policy-rule.h"
@@ -1070,6 +1072,8 @@ static Link *link_drop(Link *link) {
 
         assert(link->manager);
 
+        bool notify = link_has_local_lease_domain(link);
+
         link_set_state(link, LINK_STATE_LINGER);
 
         /* Drop all references from other links and manager. Note that async netlink calls may have
@@ -1098,6 +1102,10 @@ static Link *link_drop(Link *link) {
 
         /* The following must be called at last. */
         assert_se(hashmap_remove(link->manager->links_by_index, INT_TO_PTR(link->ifindex)) == link);
+
+        if (notify)
+                manager_notify_hook_filters(link->manager);
+
         return link_unref(link);
 }
 
@@ -1351,6 +1359,8 @@ static void link_enter_unmanaged(Link *link) {
         if (link->state == LINK_STATE_UNMANAGED)
                 return;
 
+        bool notify = link_has_local_lease_domain(link);
+
         log_link_full(link, link->state == LINK_STATE_INITIALIZED ? LOG_DEBUG : LOG_INFO,
                       "Unmanaging interface.");
 
@@ -1367,6 +1377,9 @@ static void link_enter_unmanaged(Link *link) {
 
         link->network = network_unref(link->network);
         link_set_state(link, LINK_STATE_UNMANAGED);
+
+        if (notify)
+                manager_notify_hook_filters(link->manager);
 }
 
 static int link_managed_by_us(Link *link) {
@@ -3061,3 +3074,12 @@ static const char * const kernel_operstate_table[] = {
 };
 
 DEFINE_STRING_TABLE_LOOKUP_TO_STRING(kernel_operstate, int);
+
+bool link_has_local_lease_domain(Link *link) {
+        assert(link);
+
+        return link->dhcp_server &&
+                link->network &&
+                link->network->dhcp_server_local_lease_domain &&
+                !dns_name_is_root(link->network->dhcp_server_local_lease_domain);
+}

src/network/networkd-link.h

@@ -257,3 +257,5 @@ const char* kernel_operstate_to_string(int t) _const_;
 
 void link_required_operstate_for_online(Link *link, LinkOperationalStateRange *ret);
 AddressFamily link_required_family_for_online(Link *link);
+
+bool link_has_local_lease_domain(Link *link);

src/network/networkd-manager-varlink.c

@@ -279,9 +279,9 @@ static int vl_method_set_persistent_storage(sd_varlink *vlink, sd_json_variant *
         return sd_varlink_reply(vlink, NULL);
 }
 
-int manager_connect_varlink(Manager *m, int fd) {
+int manager_varlink_init(Manager *m, int fd) {
         _cleanup_(sd_varlink_server_unrefp) sd_varlink_server *s = NULL;
-        _unused_ _cleanup_close_ int fd_close = fd;
+        _unused_ _cleanup_close_ int fd_close = fd; /* take possession */
         int r;
 
         assert(m);
@@ -335,9 +335,3 @@ int manager_connect_varlink(Manager *m, int fd) {
         m->varlink_server = TAKE_PTR(s);
         return 0;
 }
-
-void manager_varlink_done(Manager *m) {
-        assert(m);
-
-        m->varlink_server = sd_varlink_server_unref(m->varlink_server);
-}

src/network/networkd-manager-varlink.h

@@ -3,5 +3,4 @@
 
 #include "networkd-forward.h"
 
-int manager_connect_varlink(Manager *m, int fd);
-void manager_varlink_done(Manager *m);
+int manager_varlink_init(Manager *m, int fd);

src/network/networkd-manager.c

@@ -8,6 +8,7 @@
 #include "sd-event.h"
 #include "sd-netlink.h"
 #include "sd-resolve.h"
+#include "sd-varlink.h"
 
 #include "alloc-util.h"
 #include "bus-error.h"
@@ -37,6 +38,7 @@
 #include "networkd-neighbor.h"
 #include "networkd-nexthop.h"
 #include "networkd-queue.h"
+#include "networkd-resolve-hook.h"
 #include "networkd-route.h"
 #include "networkd-routing-policy-rule.h"
 #include "networkd-serialize.h"
@@ -205,13 +207,14 @@ static int manager_connect_udev(Manager *m) {
         return 0;
 }
 
-static int manager_listen_fds(Manager *m, int *ret_rtnl_fd, int *ret_varlink_fd) {
+static int manager_listen_fds(Manager *m, int *ret_rtnl_fd, int *ret_varlink_fd, int *ret_resolve_hook_fd) {
         _cleanup_strv_free_ char **names = NULL;
-        int n, rtnl_fd = -EBADF, varlink_fd = -EBADF;
+        int n, rtnl_fd = -EBADF, varlink_fd = -EBADF, resolve_hook_fd = -EBADF;
 
         assert(m);
         assert(ret_rtnl_fd);
         assert(ret_varlink_fd);
+        assert(ret_resolve_hook_fd);
 
         n = sd_listen_fds_with_names(/* unset_environment = */ true, &names);
         if (n < 0)
@@ -235,6 +238,11 @@ static int manager_listen_fds(Manager *m, int *ret_rtnl_fd, int *ret_varlink_fd)
                         continue;
                 }
 
+                if (streq(names[i], "resolve-hook")) {
+                        resolve_hook_fd = fd;
+                        continue;
+                }
+
                 if (manager_set_serialization_fd(m, fd, names[i]) >= 0)
                         continue;
 
@@ -250,6 +258,7 @@ static int manager_listen_fds(Manager *m, int *ret_rtnl_fd, int *ret_varlink_fd)
 
         *ret_rtnl_fd = rtnl_fd;
         *ret_varlink_fd = varlink_fd;
+        *ret_resolve_hook_fd = resolve_hook_fd;
 
         return 0;
 }
@@ -543,7 +552,7 @@ static int manager_set_keep_configuration(Manager *m) {
 }
 
 int manager_setup(Manager *m) {
-        _cleanup_close_ int rtnl_fd = -EBADF, varlink_fd = -EBADF;
+        _cleanup_close_ int rtnl_fd = -EBADF, varlink_fd = -EBADF, resolve_hook_fd = -EBADF;
         int r;
 
         assert(m);
@@ -567,7 +576,7 @@ int manager_setup(Manager *m) {
         if (r < 0)
                 return r;
 
-        r = manager_listen_fds(m, &rtnl_fd, &varlink_fd);
+        r = manager_listen_fds(m, &rtnl_fd, &varlink_fd, &resolve_hook_fd);
         if (r < 0)
                 return r;
 
@@ -586,7 +595,11 @@ int manager_setup(Manager *m) {
         if (m->test_mode)
                 return 0;
 
-        r = manager_connect_varlink(m, TAKE_FD(varlink_fd));
+        r = manager_varlink_init(m, TAKE_FD(varlink_fd));
+        if (r < 0)
+                return r;
+
+        r = manager_varlink_init_resolve_hook(m, TAKE_FD(resolve_hook_fd));
         if (r < 0)
                 return r;
 
@@ -737,7 +750,9 @@ Manager* manager_free(Manager *m) {
 
         sd_device_monitor_unref(m->device_monitor);
 
-        manager_varlink_done(m);
+        m->varlink_server = sd_varlink_server_unref(m->varlink_server);
+        m->varlink_resolve_hook_server = sd_varlink_server_unref(m->varlink_resolve_hook_server);
+        m->query_filter_subscriptions = set_free(m->query_filter_subscriptions);
         hashmap_free(m->polkit_registry);
         sd_bus_flush_close_unref(m->bus);
 

src/network/networkd-manager.h

@@ -22,6 +22,8 @@ typedef struct Manager {
         sd_resolve *resolve;
         sd_bus *bus;
         sd_varlink_server *varlink_server;
+        sd_varlink_server *varlink_resolve_hook_server;
+        Set *query_filter_subscriptions;
         sd_device_monitor *device_monitor;
         Hashmap *polkit_registry;
         int ethtool_fd;

src/network/networkd-network-gperf.gperf

@@ -393,6 +393,7 @@ DHCPServer.BootServerName,                       config_parse_dns_name,
 DHCPServer.BootFilename,                         config_parse_string,                            CONFIG_PARSE_STRING_SAFE_AND_ASCII,     offsetof(Network, dhcp_server_boot_filename)
 DHCPServer.RapidCommit,                          config_parse_bool,                              0,                                      offsetof(Network, dhcp_server_rapid_commit)
 DHCPServer.PersistLeases,                        config_parse_dhcp_server_persist_leases,        0,                                      offsetof(Network, dhcp_server_persist_leases)
+DHCPServer.LocalLeaseDomain,                     config_parse_dns_name,                          0,                                      offsetof(Network, dhcp_server_local_lease_domain)
 DHCPServerStaticLease.Address,                   config_parse_dhcp_static_lease_address,         0,                                      0
 DHCPServerStaticLease.MACAddress,                config_parse_dhcp_static_lease_hwaddr,          0,                                      0
 DHCPServerStaticLease.Hostname,                  config_parse_dhcp_static_lease_hostname,        0,                                      0

src/network/networkd-network.c

@@ -761,6 +761,7 @@ static Network *network_free(Network *network) {
                 free(network->dhcp_server_emit[t].addresses);
         ordered_hashmap_free(network->dhcp_server_send_options);
         ordered_hashmap_free(network->dhcp_server_send_vendor_options);
+        free(network->dhcp_server_local_lease_domain);
 
         /* DHCP client */
         free(network->dhcp_vendor_class_identifier);

src/network/networkd-network.h

@@ -233,6 +233,7 @@ typedef struct Network {
         usec_t dhcp_server_ipv6_only_preferred_usec;
         bool dhcp_server_rapid_commit;
         DHCPServerPersistLeases dhcp_server_persist_leases;
+        char *dhcp_server_local_lease_domain;
 
         /* link-local addressing support */
         AddressFamily link_local;

src/network/networkd-resolve-hook.c

@@ -0,0 +1,261 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#include "sd-dhcp-server.h"
+#include "sd-json.h"
+#include "sd-varlink.h"
+
+#include "alloc-util.h"
+#include "dns-answer.h"
+#include "dns-domain.h"
+#include "dns-packet.h"
+#include "dns-question.h"
+#include "dns-rr.h"
+#include "env-util.h"
+#include "fd-util.h"
+#include "networkd-link.h"
+#include "networkd-manager.h"
+#include "networkd-resolve-hook.h"
+#include "resolve-hook-util.h"
+#include "set.h"
+#include "varlink-io.systemd.Resolve.Hook.h"
+#include "varlink-util.h"
+
+static int manager_make_domain_array(Manager *m, sd_json_variant **ret) {
+        int r;
+
+        assert(m);
+        assert(ret);
+
+        _cleanup_(set_freep) Set *domains = NULL;
+        Link *link;
+        HASHMAP_FOREACH(link, m->links_by_index) {
+                if (!link_has_local_lease_domain(link))
+                        continue;
+
+                r = set_put_strdup_full(&domains, &dns_name_hash_ops_free, link->network->dhcp_server_local_lease_domain);
+                if (r < 0 && r != -EEXIST)
+                        return r;
+        }
+
+        _cleanup_(sd_json_variant_unrefp) sd_json_variant *array = NULL;
+        char *s;
+        SET_FOREACH(s, domains) {
+                r = sd_json_variant_append_arrayb(&array, SD_JSON_BUILD_STRING(s));
+                if (r < 0)
+                        return r;
+        }
+
+        if (!array)
+                return sd_json_variant_new_array(ret, /* array= */ NULL, /* n= */ 0);
+
+        *ret = TAKE_PTR(array);
+        return 0;
+}
+
+int manager_notify_hook_filters(Manager *m) {
+        int r;
+
+        assert(m);
+
+        /* Called whenever a machine is added or dropped from the list */
+
+        if (set_isempty(m->query_filter_subscriptions))
+                return 0;
+
+        _cleanup_(sd_json_variant_unrefp) sd_json_variant *array = NULL;
+        r = manager_make_domain_array(m, &array);
+        if (r < 0)
+                return log_error_errno(r, "Failed to generate JSON array with machine names: %m");
+
+        r = varlink_many_notifybo(m->query_filter_subscriptions, SD_JSON_BUILD_PAIR_VARIANT("filterDomains", array));
+        if (r < 0)
+                return log_error_errno(r, "Failed to notify filter subscribers: %m");
+
+        return 0;
+}
+
+static int vl_method_query_filter(
+                sd_varlink *link,
+                sd_json_variant *parameters,
+                sd_varlink_method_flags_t flags,
+                void *userdata) {
+
+        Manager *m = ASSERT_PTR(userdata);
+        int r;
+
+        assert(link);
+
+        _cleanup_(sd_json_variant_unrefp) sd_json_variant *array = NULL;
+        r = manager_make_domain_array(m, &array);
+        if (r < 0)
+                return r;
+
+        if (flags & SD_VARLINK_METHOD_MORE) {
+                /* If 'more' is set, this is a subscription request, keep track of the link */
+
+                r = sd_varlink_notifybo(link, SD_JSON_BUILD_PAIR_VARIANT("filterDomains", array));
+                if (r < 0)
+                        return log_error_errno(r, "Failed to notify filter subscribers: %m");
+
+                r = set_ensure_put(&m->query_filter_subscriptions, &varlink_hash_ops, link);
+                if (r < 0)
+                        return r;
+
+                sd_varlink_ref(link);
+        } else {
+                r = sd_varlink_replybo(link, SD_JSON_BUILD_PAIR_VARIANT("filterDomains", array));
+                if (r < 0)
+                        return log_error_errno(r, "Failed to notify filter subscribers: %m");
+        }
+
+        return 0;
+}
+
+static int vl_method_resolve_record(
+                sd_varlink *link,
+                sd_json_variant *parameters,
+                sd_varlink_method_flags_t flags,
+                void *userdata) {
+
+        Manager *m = ASSERT_PTR(userdata);
+        int r;
+
+        assert(link);
+
+        _cleanup_(resolve_record_parameters_done) ResolveRecordParameters p = {};
+        r = sd_varlink_dispatch(link, parameters, resolve_record_parameters_dispatch_table, &p);
+        if (r != 0)
+                return r;
+
+        if (dns_question_isempty(p.question))
+                return sd_varlink_error_invalid_parameter_name(link, "question");
+
+        _cleanup_(dns_answer_unrefp) DnsAnswer *answer = NULL;
+        bool found_address = false, found_domain = false;
+        DnsResourceKey *key;
+        DNS_QUESTION_FOREACH(key, p.question) {
+                const char *name = dns_resource_key_name(key);
+
+                Link *l;
+                HASHMAP_FOREACH(l, m->links_by_index) {
+
+                        if (!link_has_local_lease_domain(l))
+                                continue;
+
+                        /* Try to strip the local lease domain suffix from name, so that we have the short hostname left. */
+                        _cleanup_free_ char *prefix = NULL;
+                        r = dns_name_change_suffix(name, l->network->dhcp_server_local_lease_domain, /* new_suffix= */ NULL, &prefix);
+                        if (r <= 0) /* no match? */
+                                continue;
+
+                        found_domain = true;
+
+                        struct in_addr address;
+                        r = sd_dhcp_server_get_lease_address_by_name(l->dhcp_server, prefix, &address);
+                        if (r <= 0)
+                                continue;
+
+                        /* The domain exists, so we can give a positive reply. But only for A lookups we have addresses to return. */
+                        if (key->type != DNS_TYPE_A)
+                                continue;
+
+                        found_address = true;
+
+                        _cleanup_(dns_resource_record_unrefp) DnsResourceRecord *rr = NULL;
+                        r = dns_resource_record_new_address(&rr, AF_INET, (union in_addr_union*) &address, name);
+                        if (r < 0)
+                                return r;
+
+                        r = dns_answer_add_extend(
+                                        &answer,
+                                        rr,
+                                        l->ifindex,
+                                        DNS_ANSWER_AUTHENTICATED,
+                                        /* rrsig= */ NULL);
+                        if (r < 0)
+                                return r;
+                }
+        }
+
+        if (!found_address) {
+                /* If this was a lookup in one of our domains, return NXDOMAIN, we are authoritative on that */
+                if (found_domain)
+                        return sd_varlink_replybo(link, SD_JSON_BUILD_PAIR_INTEGER("rcode", DNS_RCODE_NXDOMAIN));
+
+                /* Otherwise we return an empty response, which means: continue with the usual lookup */
+                return sd_varlink_reply(link, /* parameters= */ NULL);
+        }
+
+        _cleanup_(sd_json_variant_unrefp) sd_json_variant *ja = NULL;
+        r = dns_answer_to_json(answer, &ja);
+        if (r < 0)
+                return r;
+
+        return sd_varlink_replybo(
+                        link,
+                        SD_JSON_BUILD_PAIR_INTEGER("rcode", DNS_RCODE_SUCCESS),
+                        SD_JSON_BUILD_PAIR_VARIANT("answer", ja));
+}
+
+static void on_resolve_hook_disconnect(sd_varlink_server *server, sd_varlink *link, void *userdata) {
+        Manager *m = ASSERT_PTR(userdata);
+
+        if (set_remove(m->query_filter_subscriptions, link))
+                sd_varlink_unref(link);
+}
+
+int manager_varlink_init_resolve_hook(Manager *m, int fd) {
+        _cleanup_(sd_varlink_server_unrefp) sd_varlink_server *s = NULL;
+        _unused_ _cleanup_close_ int fd_close = fd; /* take possession */
+        int r;
+
+        assert(m);
+
+        if (m->varlink_resolve_hook_server)
+                return 0;
+
+        r = getenv_bool("SYSTEMD_NETWORK_RESOLVE_HOOK");
+        if (r < 0 && r != -ENXIO)
+                log_warning_errno(r, "Failed to parse $SYSTEMD_NETWORK_RESOLVE_HOOK, ignoring: %m");
+        if (r == 0) {
+                log_notice("Resolve hook disabled via $SYSTEMD_NETWORK_RESOLVE_HOOK.");
+                return 0;
+        }
+
+        r = varlink_server_new(&s, SD_VARLINK_SERVER_ACCOUNT_UID|SD_VARLINK_SERVER_INHERIT_USERDATA, m);
+        if (r < 0)
+                return log_error_errno(r, "Failed to allocate varlink server object: %m");
+
+        (void) sd_varlink_server_set_description(s, "varlink-resolve-hook");
+
+        r = sd_varlink_server_add_interface(s, &vl_interface_io_systemd_Resolve_Hook);
+        if (r < 0)
+                return log_error_errno(r, "Failed to add Resolve.Hook interface to varlink server: %m");
+
+        r = sd_varlink_server_bind_method_many(
+                        s,
+                        "io.systemd.Resolve.Hook.QueryFilter",   vl_method_query_filter,
+                        "io.systemd.Resolve.Hook.ResolveRecord", vl_method_resolve_record);
+        if (r < 0)
+                return log_error_errno(r, "Failed to register varlink methods: %m");
+
+        r = sd_varlink_server_bind_disconnect(s, on_resolve_hook_disconnect);
+        if (r < 0)
+                return log_error_errno(r, "Failed to bind on resolve hook disconnection events: %m");
+
+        if (fd < 0)
+                r = sd_varlink_server_listen_address(s, "/run/systemd/resolve.hook/io.systemd.Network", 0666 | SD_VARLINK_SERVER_MODE_MKDIR_0755);
+        else
+                r = sd_varlink_server_listen_fd(s, fd);
+        if (r < 0)
+                return log_error_errno(r, "Failed to bind to systemd-resolved hook Varlink socket: %m");
+
+        TAKE_FD(fd_close);
+
+        r = sd_varlink_server_attach_event(s, m->event, SD_EVENT_PRIORITY_NORMAL);
+        if (r < 0)
+                return log_error_errno(r, "Failed to attach varlink connection to event loop: %m");
+
+        m->varlink_resolve_hook_server = TAKE_PTR(s);
+        return 0;
+}

src/network/networkd-resolve-hook.h

@@ -0,0 +1,7 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+#pragma once
+
+#include "networkd-forward.h"
+
+int manager_notify_hook_filters(Manager *m);
+int manager_varlink_init_resolve_hook(Manager *m, int fd);

src/resolve/meson.build

@@ -28,6 +28,7 @@ systemd_resolved_extract_sources = files(
         'resolved-dnssd-bus.c',
         'resolved-dnssd.c',
         'resolved-etc-hosts.c',
+        'resolved-hook.c',
         'resolved-link-bus.c',
         'resolved-link.c',
         'resolved-llmnr.c',

src/resolve/resolvectl.c

@@ -253,13 +253,14 @@ static void print_source(uint64_t flags, usec_t rtt) {
                ansi_normal());
 
         if ((flags & (SD_RESOLVED_FROM_MASK|SD_RESOLVED_SYNTHETIC)) != 0)
-                printf("%s-- Data from:%s%s%s%s%s%s\n",
+                printf("%s-- Data from:%s%s%s%s%s%s%s\n",
                        ansi_grey(),
                        FLAGS_SET(flags, SD_RESOLVED_SYNTHETIC) ? " synthetic" : "",
                        FLAGS_SET(flags, SD_RESOLVED_FROM_CACHE) ? " cache" : "",
                        FLAGS_SET(flags, SD_RESOLVED_FROM_ZONE) ? " zone" : "",
                        FLAGS_SET(flags, SD_RESOLVED_FROM_TRUST_ANCHOR) ? " trust-anchor" : "",
                        FLAGS_SET(flags, SD_RESOLVED_FROM_NETWORK) ? " network" : "",
+                       FLAGS_SET(flags, SD_RESOLVED_FROM_HOOK) ? " hook" : "",
                        ansi_normal());
 }
 

src/resolve/resolved-bus.c

@@ -1017,7 +1017,7 @@ static void resolve_service_all_complete(DnsQuery *query) {
 
         assert(q);
 
-        if (q->block_all_complete > 0) {
+        if (q->hook_query || q->block_all_complete > 0) {
                 TAKE_PTR(q);
                 return;
         }
@@ -1028,6 +1028,12 @@ static void resolve_service_all_complete(DnsQuery *query) {
 
                 LIST_FOREACH(auxiliary_queries, aux, q->auxiliary_queries) {
 
+                        if (aux->hook_query) {
+                                /* If an auxiliary query's hook is still pending, let's wait */
+                                TAKE_PTR(q);
+                                return;
+                        }
+
                         switch (aux->state) {
 
                         case DNS_TRANSACTION_PENDING:

src/resolve/resolved-dns-query.c

@@ -19,6 +19,7 @@
 #include "resolved-dns-synthesize.h"
 #include "resolved-dns-transaction.h"
 #include "resolved-etc-hosts.h"
+#include "resolved-hook.h"
 #include "resolved-manager.h"
 #include "resolved-timeouts.h"
 #include "set.h"
@@ -524,6 +525,8 @@ DnsQuery *dns_query_free(DnsQuery *q) {
 
         dns_service_browser_unref(q->service_browser_request);
 
+        hook_query_free(q->hook_query);
+
         if (q->manager) {
                 LIST_REMOVE(queries, q->manager->dns_queries, q);
                 q->manager->n_dns_queries--;
@@ -907,24 +910,17 @@ static int dns_query_try_etc_hosts(DnsQuery *q) {
         return 1;
 }
 
-int dns_query_go(DnsQuery *q) {
-        DnsScopeMatch found = DNS_SCOPE_NO;
-        DnsScope *first = NULL;
+static int dns_query_go_scopes(DnsQuery *q) {
         int r;
 
         assert(q);
+        assert(!q->hook_query);
+        assert(q->state == DNS_TRANSACTION_NULL);
 
-        if (q->state != DNS_TRANSACTION_NULL)
-                return 0;
-
-        r = dns_query_try_etc_hosts(q);
-        if (r < 0)
-                return r;
-        if (r > 0) {
-                dns_query_complete(q, DNS_TRANSACTION_SUCCESS);
-                return 1;
-        }
+        /* Start the lookup via the scopes */
 
+        DnsScopeMatch found = DNS_SCOPE_NO;
+        DnsScope *first = NULL;
         LIST_FOREACH(scopes, s, q->manager->dns_scopes) {
                 DnsScopeMatch match;
 
@@ -999,6 +995,72 @@ fail:
         return r;
 }
 
+static void on_hook_complete(HookQuery *hq, int rcode, DnsAnswer *answer, void *userdata) {
+        DnsQuery *q = ASSERT_PTR(userdata);
+        int r;
+
+        assert(hq);
+        assert(q->hook_query == hq);
+        assert(q->state == DNS_TRANSACTION_NULL);
+
+        q->hook_query = hook_query_free(q->hook_query);
+        TAKE_PTR(hq);
+
+        if (rcode < 0) {
+                log_debug("Hook yielded no results, proceeding.");
+                r = dns_query_go_scopes(q);
+                if (r < 0) {
+                        dns_query_reset_answer(q);
+                        q->answer_errno = r;
+                        dns_query_complete(q, DNS_TRANSACTION_ERRNO);
+                }
+
+                return;
+        }
+
+        dns_query_reset_answer(q);
+
+        q->answer = dns_answer_ref(answer);
+        q->answer_rcode = rcode;
+        q->answer_protocol = dns_synthesize_protocol(q->flags);
+        q->answer_family = dns_synthesize_family(q->flags);
+        q->answer_query_flags = SD_RESOLVED_FROM_HOOK;
+        dns_query_complete(q, rcode == DNS_RCODE_SUCCESS ? DNS_TRANSACTION_SUCCESS : DNS_TRANSACTION_RCODE_FAILURE);
+}
+
+int dns_query_go(DnsQuery *q) {
+        int r;
+
+        assert(q);
+
+        /* Already ongoing? Then suppress */
+        if (q->hook_query ||
+            q->state != DNS_TRANSACTION_NULL)
+                return 0;
+
+        r = dns_query_try_etc_hosts(q);
+        if (r < 0)
+                return r;
+        if (r > 0) {
+                dns_query_complete(q, DNS_TRANSACTION_SUCCESS);
+                return 1;
+        }
+
+        r = manager_hook_query(
+                        q->manager,
+                        q->question_bypass ? q->question_bypass->question : q->question_idna,
+                        q->question_bypass ? q->question_bypass->question : q->question_utf8,
+                        on_hook_complete,
+                        q,
+                        &q->hook_query);
+        if (r < 0)
+                return r;
+        if (r > 0) /* hook calls are pending */
+                return 0;
+
+        return dns_query_go_scopes(q);
+}
+
 static void dns_query_accept(DnsQuery *q, DnsQueryCandidate *c) {
         DnsTransactionState state = DNS_TRANSACTION_NO_SERVERS;
         bool has_authenticated = false, has_non_authenticated = false, has_confidential = false, has_non_confidential = false;
@@ -1137,6 +1199,9 @@ void dns_query_ready(DnsQuery *q) {
          * after calling this function, unless the block_ready
          * counter was explicitly bumped before doing so. */
 
+        if (q->hook_query)
+                return;
+
         if (q->block_ready > 0)
                 return;
 

src/resolve/resolved-dns-query.h

@@ -110,6 +110,9 @@ typedef struct DnsQuery {
         DnssdDiscoveredService *dnsservice_request;
         DnsServiceBrowser *service_browser_request;
 
+        /* Pending query to any installed hooks */
+        HookQuery *hook_query;
+
         /* Completion callback */
         void (*complete)(DnsQuery* q);
 

src/resolve/resolved-forward.h

@@ -34,6 +34,7 @@ typedef struct DnsSvcParam DnsSvcParam;
 typedef struct DnsTransaction DnsTransaction;
 typedef struct DnsTxtItem DnsTxtItem;
 typedef struct DnsZoneItem DnsZoneItem;
+typedef struct HookQuery HookQuery;
 typedef struct Link Link;
 typedef struct LinkAddress LinkAddress;
 typedef struct Manager Manager;

src/resolve/resolved-hook.c

@@ -0,0 +1,881 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#include "sd-event.h"
+#include "sd-varlink.h"
+
+#include "dirent-util.h"
+#include "dns-domain.h"
+#include "env-util.h"
+#include "errno-util.h"
+#include "fd-util.h"
+#include "hash-funcs.h"
+#include "iovec-util.h"
+#include "json-util.h"
+#include "ratelimit.h"
+#include "resolved-hook.h"
+#include "resolved-manager.h"
+#include "set.h"
+#include "stat-util.h"
+#include "varlink-util.h"
+
+/* Controls how many idle connections to keep around at max. This is purely an optimization: an established
+ * socket that has gone through connect()/accept() already is just quicker to use. Since we might get a flood
+ * of resolution requests we keep multiple connections open thus, but not too many. */
+#define HOOK_IDLE_CONNECTIONS_MAX 4U
+
+/* Encapsulates a specific hook, i.e. bound socket in in the /run/systemd/resolve.hook/ directory */
+typedef struct Hook {
+        unsigned n_ref;
+
+        Manager *manager;
+        char *socket_path;
+
+        sd_varlink *filter_link;
+        Set *idle_links; /* we retry to recycle varlink connections */
+
+        /* This hook only shall be applied to names matching the following filter parameters */
+        Set *filter_domains;          /* if NULL → no filtering; if empty → do not accept anything */
+        unsigned filter_labels_min;   /* minimum number of labels */
+        unsigned filter_labels_max;   /* maximum number of labels (this is useful to hook only into single-label lookups á la LLMNR) */
+
+        /* timestamp we last saw this in CLOCK_MONOTONIC, for GC handling */
+        uint64_t seen_usec;
+
+        /* When a hook never responds correctly, we'll eventually give up trying */
+        RateLimit reconnect_ratelimit;
+} Hook;
+
+static Hook* hook_free(Hook *h) {
+        if (!h)
+                return NULL;
+
+        mfree(h->socket_path);
+        sd_varlink_unref(h->filter_link);
+        set_free(h->idle_links);
+
+        set_free(h->filter_domains);
+
+        return mfree(h);
+}
+
+DEFINE_PRIVATE_TRIVIAL_REF_UNREF_FUNC(Hook, hook, hook_free);
+DEFINE_TRIVIAL_CLEANUP_FUNC(Hook*, hook_unref);
+
+static Hook *hook_unlink(Hook *h) {
+        if (!h)
+                return NULL;
+
+        if (!h->manager)
+                return NULL;
+
+        if (h->socket_path)
+                hashmap_remove(h->manager->hooks, h->socket_path);
+        h->manager = NULL;
+
+        return hook_unref(h);
+}
+
+static int dispatch_filter_domains(const char *name, sd_json_variant *variant, sd_json_dispatch_flags_t flags, void *userdata) {
+        Hook *h = ASSERT_PTR(userdata);
+        int r;
+
+        if (!sd_json_variant_is_array(variant))
+                return json_log(variant, flags, SYNTHETIC_ERRNO(EINVAL), "JSON field '%s' is not an array.", strna(name));
+
+        /* Let's explicitly allocate the set here, since we want that a NULL set means: let everything
+         * through; but an empty set shall mean: let nothing through */
+        r = set_ensure_allocated(&h->filter_domains, &dns_name_hash_ops_free);
+        if (r < 0)
+                return json_log_oom(variant, flags);
+
+        sd_json_variant *i;
+        JSON_VARIANT_ARRAY_FOREACH(i, variant) {
+                if (!sd_json_variant_is_string(i))
+                        return json_log(variant, flags, SYNTHETIC_ERRNO(EINVAL),
+                                        "Element of JSON field '%s' is not a string.", strna(name));
+
+                r = set_put_strdup_full(&h->filter_domains, &dns_name_hash_ops_free, sd_json_variant_string(i));
+                if (r < 0 && r != -EEXIST)
+                        return json_log_oom(variant, flags);
+        }
+
+        return 0;
+}
+
+static void hook_reset_filter(Hook *h) {
+        assert(h);
+
+        h->filter_domains = set_free(h->filter_domains);
+        h->filter_labels_min = UINT_MAX;
+        h->filter_labels_max = UINT_MAX;
+}
+
+static int hook_acquire_filter(Hook *h);
+
+static int on_filter_reply(
+                sd_varlink *link,
+                sd_json_variant *parameters,
+                const char *error_id,
+                sd_varlink_reply_flags_t flags,
+                void *userdata) {
+
+        Hook *h = ASSERT_PTR(userdata);
+        int r;
+
+        if (error_id) {
+                if (streq(error_id, SD_VARLINK_ERROR_DISCONNECTED)) {
+                        /* When we are are disconnected, that's fine, maybe the other side wants to clean up
+                         * open connections every now and then, or is being restarted and thus a moment
+                         * offline. Try to reconnect immediately to recover. However, a service that
+                         * continously fails should not be able to get us into a busy loop, hence we apply a
+                         * ratelimit, and when it is hit we stop reconnecting. */
+                        if (ratelimit_below(&h->reconnect_ratelimit)) {
+                                log_debug("Connection terminated while querying filter of hook '%s', trying to reconnect.", h->socket_path);
+
+                                h->filter_link = sd_varlink_unref(h->filter_link);
+
+                                r = hook_acquire_filter(h);
+                                if (r < 0)
+                                        goto terminate;
+                        } else
+                                log_warning("Connection terminated while querying filter of hook '%s', and reconnection attempts failed too quickly, giving up.", h->socket_path);
+
+                        goto terminate;
+                }
+
+                if (streq(error_id, SD_VARLINK_ERROR_METHOD_NOT_FOUND)) {
+                        log_debug("Hook '%s' does not implement querying filter.", h->socket_path);
+                        goto terminate;
+                }
+
+                log_warning("Received error while requesting query filter: %s", error_id);
+                goto terminate;
+        }
+
+        if (!FLAGS_SET(flags, SD_VARLINK_REPLY_CONTINUES)) {
+                log_debug("Final message received while querying filter, terminating connection.");
+                goto terminate;
+        }
+
+        hook_reset_filter(h);
+
+        static const struct sd_json_dispatch_field dispatch_table[] = {
+                { "filterDomains",   SD_JSON_VARIANT_ARRAY,   dispatch_filter_domains, 0,                                 SD_JSON_NULLABLE },
+                { "filterLabelsMin", SD_JSON_VARIANT_INTEGER, sd_json_dispatch_uint,   offsetof(Hook, filter_labels_min), 0                },
+                { "filterLabelsMax", SD_JSON_VARIANT_INTEGER, sd_json_dispatch_uint,   offsetof(Hook, filter_labels_max), 0                },
+                {},
+        };
+
+        r = sd_json_dispatch(parameters, dispatch_table, SD_JSON_LOG|SD_JSON_ALLOW_EXTENSIONS, h);
+        if (r < 0)
+                goto terminate;
+
+        return 1;
+
+terminate:
+        h->filter_link = sd_varlink_unref(h->filter_link);
+        hook_reset_filter(h);
+        return 1;
+}
+
+static int hook_varlink_connect(Hook *h, int64_t priority, sd_varlink **ret) {
+        int r;
+
+        assert(h);
+        assert(ret);
+
+        _cleanup_(sd_varlink_unrefp) sd_varlink *v = NULL;
+        r = sd_varlink_connect_address(&v, h->socket_path);
+        if (ERRNO_IS_NEG_DISCONNECT(r) || r == -ENOENT) {
+                log_debug_errno(r, "Socket '%s' is not connectible, probably stale, ignoring: %m", h->socket_path);
+                *ret = NULL;
+                return 0; /* dead socket */
+        }
+        if (r < 0)
+                return log_error_errno(r, "Failed to connect to '%s': %m", h->socket_path);
+
+        _cleanup_free_ char *bn = NULL;
+        r = path_extract_filename(h->socket_path, &bn);
+        if (r < 0)
+                return log_error_errno(r, "Failed to extract filename from path '%s': %m", h->socket_path);
+
+        _cleanup_free_ char *j = strjoin("hook-", bn);
+        if (!j)
+                return log_oom();
+
+        (void) sd_varlink_set_description(v, j);
+
+        r = sd_varlink_attach_event(v, h->manager->event, priority);
+        if (r < 0)
+                return log_error_errno(r, "Failed to attach Varlink connection to event loop: %m");
+
+        *ret = TAKE_PTR(v);
+        return 1; /* worked */
+}
+
+static int hook_acquire_filter(Hook *h) {
+        int r;
+
+        assert(h);
+        assert(h->manager);
+
+        if (h->filter_link)
+                return 0;
+
+        _cleanup_(sd_varlink_unrefp) sd_varlink *v = NULL;
+        r = hook_varlink_connect(h, SD_EVENT_PRIORITY_NORMAL-10, &v); /* Give the querying of the filter a bit of priority */
+        if (r <= 0)
+                return r;
+
+        /* Turn off timeout, after all we want to continously monitor filter changes */
+        r = sd_varlink_set_relative_timeout(v, UINT64_MAX);
+        if (r < 0)
+                return log_error_errno(r, "Failed to disable timeout on Varlink connection %m");
+
+        sd_varlink_set_userdata(v, h);
+        r = sd_varlink_bind_reply(v, on_filter_reply);
+        if (r < 0)
+                return log_error_errno(r, "Failed to set filter reply callback on Varlink connection: %m");
+
+        r = sd_varlink_observe(
+                        v,
+                        "io.systemd.Resolve.Hook.QueryFilter",
+                        /* parameters= */ NULL);
+        if (r < 0)
+                return log_error_errno(r, "Failed to issue QueryFilter() varlink call: %m");
+
+        h->filter_link = TAKE_PTR(v);
+        return 0;
+}
+
+static int hook_test_filter(Hook *h, DnsQuestion *question) {
+        int r;
+
+        assert(h);
+        assert(question);
+
+        const char *name = dns_question_first_name(question);
+        if (!name)
+                return -EINVAL;
+
+        if (h->filter_labels_max != UINT_MAX || h->filter_labels_min != UINT_MAX) {
+                int n = dns_name_count_labels(name);
+                if (n < 0)
+                        return n;
+
+                if (h->filter_labels_max != UINT_MAX && (unsigned) n > h->filter_labels_max)
+                        return false;
+                if (h->filter_labels_min != UINT_MAX && (unsigned) n < h->filter_labels_min)
+                        return false;
+        }
+
+        if (h->filter_domains)
+                for (const char *p = name;;) {
+                        if (set_contains(h->filter_domains, p))
+                                break;
+
+                        r = dns_name_parent(&p);
+                        if (r < 0)
+                                return r;
+                        if (r == 0)
+                                return false;
+                }
+
+        return true;
+}
+
+static int hook_compare(const Hook *a, const Hook *b) {
+        assert(a);
+
+        /* Hooks take preference based on the name of their socket */
+        return path_compare(a->socket_path, b->socket_path);
+}
+
+static void hook_recycle_varlink(Hook *h, sd_varlink *vl) {
+        int r;
+
+        assert(h);
+        assert(vl);
+
+        /* Disable any potential callbacks while we are recycling the thing */
+        sd_varlink_set_userdata(vl, NULL);
+        sd_varlink_bind_reply(vl, NULL);
+
+        if (set_size(h->idle_links) > HOOK_IDLE_CONNECTIONS_MAX)
+                return;
+
+        /* If we are done with a lookup don't close the connection right-away, but keep it open so that we
+         * can possibly reuse it later, and can save a bit of time on future lookups. We only keep a few
+         * around however. */
+
+        r = set_ensure_put(&h->idle_links, &varlink_hash_ops, vl);
+        if (r < 0)
+                log_debug_errno(r, "Failed to add varlink connection to idle set, ignoring: %m");
+        else
+                sd_varlink_ref(vl);
+}
+
+static void manager_gc_hooks(Manager *m, usec_t seen_usec) {
+        assert(m);
+
+        Hook *h;
+        HASHMAP_FOREACH(h, m->hooks) {
+                /* Keep hooks around that have been seen in this iteration */
+                if (h->seen_usec == seen_usec)
+                        continue;
+
+                hook_unlink(h);
+        }
+}
+
+DEFINE_HASH_OPS_WITH_VALUE_DESTRUCTOR(
+                hook_hash_ops,
+                char, string_hash_func, string_compare_func,
+                Hook, hook_unlink);
+
+static int manager_hook_add(Manager *m, const char *p, usec_t seen_usec) {
+        int r;
+
+        assert(m);
+        assert(p);
+
+        Hook *found = hashmap_get(m->hooks, p);
+        if (found) {
+                found->seen_usec = seen_usec;
+                return 0;
+        }
+
+        _cleanup_free_ char *s = strdup(p);
+        if (!s)
+                return log_oom();
+
+        _cleanup_(hook_unrefp) Hook *h = new(Hook, 1);
+        if (!h)
+                return log_oom();
+
+        *h = (Hook) {
+                .n_ref = 1,
+                .socket_path = TAKE_PTR(s),
+                .filter_labels_min = UINT_MAX,
+                .filter_labels_max = UINT_MAX,
+                .reconnect_ratelimit = { 1 * USEC_PER_SEC, 5 },
+                .seen_usec = seen_usec,
+        };
+
+        if (hashmap_ensure_put(&m->hooks, &hook_hash_ops, h->socket_path, h) < 0)
+                return log_oom();
+
+        hook_ref(h);
+        h->manager = m;
+
+        r = hook_acquire_filter(h);
+        if (r < 0) {
+                hook_unlink(h);
+                return r;
+        }
+
+        return 0;
+}
+
+static int manager_hook_discover(Manager *m) {
+        /* You might wonder, why is this /run/systemd/resolve.hook/ and not /run/systemd/resolve/hook/?
+         * That's because of permissions: resolved runs as "systemd-resolve" user and owns
+         * /run/systemd/resolve/, but the hook directory is where other privileged code shall bind a socket
+         * in (and where root ownership hence makes sense). Hence we do not nest the directories, but put
+         * them side by side, so that they can have different ownership. */
+        static const char dp[] = "/run/systemd/resolve.hook";
+        _cleanup_closedir_ DIR *d = NULL;
+        int r;
+
+        assert(m);
+
+        usec_t seen_usec = now(CLOCK_MONOTONIC);
+
+        struct stat st;
+        if (stat(dp, &st) < 0) {
+                if (errno == ENOENT)
+                        r = 0;
+                else
+                        r = log_warning_errno(errno, "Failed to stat %s/: %m", dp);
+
+                goto finish;
+        }
+
+        if (stat_inode_unmodified(&st, &m->hook_stat))
+                return 0;
+
+        d = opendir(dp);
+        if (!d) {
+                if (errno == ENOENT)
+                        r = 0;
+                else
+                        r = log_warning_errno(errno, "Failed to enumerate %s/ contents: %m", dp);
+
+                goto finish;
+        }
+
+        for (;;) {
+                errno = 0;
+                struct dirent *de = readdir_no_dot(d);
+                if (!de) {
+                        if (errno == 0) /* EOD */
+                                break;
+
+                        r = log_error_errno(errno, "Failed to enumerate %s/: %m", dp);
+                        goto finish;
+                }
+
+                if (!IN_SET(de->d_type, DT_SOCK, DT_UNKNOWN))
+                        continue;
+
+                _cleanup_free_ char *p = path_join(dp, de->d_name);
+                if (!p) {
+                        r = log_oom();
+                        goto finish;
+                }
+
+                (void) manager_hook_add(m, p, seen_usec);
+        }
+
+        m->hook_stat = st;
+        r = 0;
+
+finish:
+        manager_gc_hooks(m, seen_usec);
+        return r;
+}
+
+typedef struct HookQuery HookQuery;
+typedef struct HookQueryCandidate HookQueryCandidate;
+
+/* Encapsulates a query currently being processed by various hooks */
+struct HookQuery {
+        /* Question */
+        DnsQuestion *question_idna;
+        DnsQuestion *question_utf8;
+
+        /* Selected answer */
+        DnsAnswer *answer;
+        int answer_rcode;
+        Hook *answer_hook;
+
+        /* Candidates for a reply, i.e, one entry for each hook */
+        LIST_HEAD(HookQueryCandidate, candidates);
+
+        /* Completion callback to invoke */
+        void (*complete)(HookQuery *q, int answer_rcode, DnsAnswer *answer, void *userdata);
+        void *userdata;
+};
+
+/* Encapsulates the state of a hook query to one specific hook */
+struct HookQueryCandidate {
+        HookQuery *query;
+        Hook *hook;
+        sd_varlink *link;
+        LIST_FIELDS(HookQueryCandidate, candidates);
+};
+
+static HookQueryCandidate* hook_query_candidate_free(HookQueryCandidate *c) {
+        if (!c)
+                return NULL;
+
+        c->link = sd_varlink_unref(c->link);
+
+        if (c->query)
+                LIST_REMOVE(candidates, c->query->candidates, c);
+
+        hook_unref(c->hook);
+        return mfree(c);
+}
+
+DEFINE_TRIVIAL_CLEANUP_FUNC(HookQueryCandidate*, hook_query_candidate_free);
+
+HookQuery* hook_query_free(HookQuery *hq) {
+        if (!hq)
+                return NULL;
+
+        /* Free candidates as long as there are candidates */
+        while (hq->candidates)
+                hook_query_candidate_free(hq->candidates);
+
+        dns_question_unref(hq->question_utf8);
+        dns_question_unref(hq->question_idna);
+        dns_answer_unref(hq->answer);
+        hook_unref(hq->answer_hook);
+
+        return mfree(hq);
+}
+
+DEFINE_TRIVIAL_CLEANUP_FUNC(HookQuery*, hook_query_free);
+
+static void hook_query_ready(HookQuery *hq) {
+        assert(hq);
+
+        bool done = true;
+        LIST_FOREACH(candidates, c, hq->candidates)
+                if (c->link) { /* ongoing connection? */
+                        done = false;
+                        break;
+                }
+
+        if (!done)
+                return;
+
+        /* The complete() callback quite likely will destroy 'hq', which might be what keeps the answer
+         * object alive. Let's take an explicit ref here hence, so that it definitely remains alive for the
+         * whole callback lifetime */
+        _cleanup_(dns_answer_unrefp) DnsAnswer *answer = dns_answer_ref(hq->answer);
+        hq->complete(hq, hq->answer_rcode, answer, hq->userdata);
+}
+
+static int dispatch_rcode(const char *name, sd_json_variant *variant, sd_json_dispatch_flags_t flags, void *userdata) {
+        int *u = ASSERT_PTR(userdata), r;
+
+        assert(variant);
+
+        int rcode;
+        r = sd_json_dispatch_int(name, variant, flags, &rcode);
+        if (r < 0)
+                return r;
+
+        if (rcode < 0 || rcode >= _DNS_RCODE_MAX)
+                return json_log(variant, flags, SYNTHETIC_ERRNO(ERANGE), "JSON field '%s' contains an invalid DNS rcode.", strna(name));
+
+        *u = rcode;
+        return 0;
+}
+
+static int dispatch_answer(const char *name, sd_json_variant *variant, sd_json_dispatch_flags_t flags, void *userdata) {
+        DnsAnswer **a = ASSERT_PTR(userdata);
+        int r;
+
+        assert(variant);
+
+        if (sd_json_variant_is_null(variant)) {
+                *a = dns_answer_unref(*a);
+                return 0;
+        }
+
+        if (!sd_json_variant_is_array(variant))
+                return json_log(variant, flags, SYNTHETIC_ERRNO(EINVAL), "JSON field '%s' is not an array.", strna(name));
+
+        _cleanup_(dns_answer_unrefp) DnsAnswer *l = NULL;
+        sd_json_variant *e;
+        JSON_VARIANT_ARRAY_FOREACH(e, variant) {
+                if (!sd_json_variant_is_object(e))
+                        return json_log(e, flags, SYNTHETIC_ERRNO(EINVAL), "JSON array element is not an object");
+
+                _cleanup_(iovec_done) struct iovec iovec = {};
+                static const sd_json_dispatch_field dispatch_table[] = {
+                        { "raw", SD_JSON_VARIANT_STRING, json_dispatch_unbase64_iovec, 0, SD_JSON_MANDATORY },
+                        { "rr",  SD_JSON_VARIANT_OBJECT, NULL,                         0, 0                 },
+                        {}
+                };
+
+                r = sd_json_dispatch(e, dispatch_table, flags|SD_JSON_ALLOW_EXTENSIONS, &iovec);
+                if (r < 0)
+                        return r;
+
+                _cleanup_(dns_resource_record_unrefp) DnsResourceRecord *rr = NULL;
+                r = dns_resource_record_new_from_raw(&rr, iovec.iov_base, iovec.iov_len);
+                if (r < 0)
+                        return json_log(variant, flags, SYNTHETIC_ERRNO(EINVAL),
+                                        "JSON field '%s' contains an invalid resource record.", strna(name));
+
+                if (dns_answer_add_extend(&l, rr, /* ifindex= */ 0, /* flags= */ 0, /* rrsig= */ NULL) < 0)
+                        return json_log_oom(e, flags);
+        }
+
+        dns_answer_unref(*a);
+        *a = TAKE_PTR(l);
+
+        return 0;
+}
+
+typedef struct QueryReplyParameters {
+        int rcode;
+        DnsAnswer *answer;
+} QueryReplyParameters;
+
+static void query_reply_parameters_done(QueryReplyParameters *p) {
+        assert(p);
+
+        p->answer = dns_answer_unref(p->answer);
+}
+
+static int on_query_reply(
+                sd_varlink *link,
+                sd_json_variant *parameters,
+                const char *error_id,
+                sd_varlink_reply_flags_t flags,
+                void *userdata) {
+
+        HookQueryCandidate *qc = ASSERT_PTR(userdata);
+        HookQuery *q = ASSERT_PTR(qc->query); /* save early in case we destroy 'qc' half-way through this function */
+        int r;
+
+        assert(link);
+
+        _cleanup_(query_reply_parameters_done) QueryReplyParameters p = {
+                .rcode = -1,
+        };
+
+        if (error_id) {
+                log_notice("Query on hook '%s' failed with error '%s', ignoring.", qc->hook->socket_path, error_id);
+                r = -EBADR;
+                goto destroy;
+        }
+
+        static const sd_json_dispatch_field dispatch_table[] = {
+                { "rcode",  _SD_JSON_VARIANT_TYPE_INVALID, dispatch_rcode,  offsetof(QueryReplyParameters, rcode),  0 },
+                { "answer", SD_JSON_VARIANT_ARRAY,         dispatch_answer, offsetof(QueryReplyParameters, answer), 0 },
+                {},
+        };
+
+        r = sd_json_dispatch(parameters, dispatch_table, SD_JSON_LOG|SD_JSON_ALLOW_EXTENSIONS, &p);
+        if (r < 0)
+                goto destroy;
+
+        if (p.rcode < 0) {
+                /* If no rcode is specified, then this means "continue with regular DNS based resolving" to us */
+                log_debug("Query on hook '%s' returned empty reply, skipping.", qc->hook->socket_path);
+                r = 0;
+                goto destroy;
+        }
+
+        bool win = false;
+        if (p.rcode == DNS_RCODE_SUCCESS)
+                /* if this is a successful lookup, let it win if the so far best lookup was a failure or
+                 * empty, or ordered later than us */
+                win = q->answer_rcode != DNS_RCODE_SUCCESS ||
+                        dns_answer_isempty(q->answer) ||
+                        (!dns_answer_isempty(p.answer) &&
+                         hook_compare(qc->hook, q->answer_hook) < 0);
+        else
+                /* if this is a failure lookup, let it win if we so far haven't seen any reply at all, or the
+                 * winner so far us ordered later than us. */
+                win = q->answer_rcode < 0 ||
+                        hook_compare(qc->hook, q->answer_hook) < 0;
+
+        if (win) {
+                /* This reply wins over whatever was stored before. Let's track that */
+                dns_answer_unref(q->answer);
+                q->answer = TAKE_PTR(p.answer);
+                q->answer_rcode = p.rcode;
+                hook_unref(q->answer_hook);
+                q->answer_hook = hook_ref(qc->hook);
+        }
+
+        hook_recycle_varlink(qc->hook, qc->link);
+        qc->link = sd_varlink_unref(qc->link);
+
+        /* Check if we are ready now, and have processed all hooks on this query (this might destroy our
+         * candidate and our hook query!) */
+        hook_query_ready(q);
+        return 0;
+
+destroy:
+        qc = hook_query_candidate_free(qc);
+        hook_query_ready(q);
+        return r;
+}
+
+static int dns_questions_to_json(DnsQuestion *a, DnsQuestion *b, sd_json_variant **ret) {
+        int r;
+
+        assert(ret);
+
+        /* Takes both questions and turns them into a JSON array of objects with the key. Note this takes two
+         * questions, one in IDNA and one in UTF-8 encoding, and merges them, removing duplicates. */
+
+        _cleanup_(sd_json_variant_unrefp) sd_json_variant *l = NULL;
+
+        DnsResourceKey *key;
+        DNS_QUESTION_FOREACH(key, a) {
+                _cleanup_(sd_json_variant_unrefp) sd_json_variant *v = NULL;
+                r = dns_resource_key_to_json(key, &v);
+                if (r < 0)
+                        return r;
+
+                r = sd_json_variant_append_arraybo(&l, SD_JSON_BUILD_PAIR_VARIANT("key", v));
+                if (r < 0)
+                        return r;
+        }
+
+        if (a != b) {
+                DNS_QUESTION_FOREACH(key, b) {
+                        if (dns_question_contains_key(a, key))
+                                continue;
+
+                        _cleanup_(sd_json_variant_unrefp) sd_json_variant *v = NULL;
+                        r = dns_resource_key_to_json(key, &v);
+                        if (r < 0)
+                                return r;
+
+                        r = sd_json_variant_append_arraybo(&l, SD_JSON_BUILD_PAIR_VARIANT("key", v));
+                        if (r < 0)
+                                return r;
+                }
+        }
+
+        *ret = TAKE_PTR(l);
+        return 0;
+}
+
+static int hook_query_add_candidate(HookQuery *hq, Hook *h) {
+        int r;
+
+        assert(hq);
+        assert(h);
+
+        _cleanup_(sd_varlink_unrefp) sd_varlink *vl = NULL;
+        for (;;) {
+                /* Before we create a new connection, let's see if there are still idle connections we can
+                 * use. */
+                vl = set_steal_first(h->idle_links);
+                if (!vl) {
+                        /* Nope, there's nothing, let's create a new connection */
+                        r = hook_varlink_connect(h, SD_EVENT_PRIORITY_NORMAL, &vl);
+                        if (r <= 0)
+                                return r;
+
+                        break;
+                }
+
+                r = sd_varlink_is_connected(vl);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to check if varlink connection is connected: %m");
+                if (r > 0)
+                        break;
+
+                vl = sd_varlink_unref(vl);
+        }
+
+        /* Set a short timeout for hooks. Hooks should not be able to cause the DNS part of the lookup to fail. */
+        r = sd_varlink_set_relative_timeout(vl, SD_RESOLVED_QUERY_TIMEOUT_USEC/4);
+        if (r < 0)
+                return log_error_errno(r, "Failed to set Varlink connection timeout: %m");
+
+        r = sd_varlink_bind_reply(vl, on_query_reply);
+        if (r < 0)
+                return log_error_errno(r, "Failed to bind reply callback to Varlink connection: %m");
+
+        _cleanup_(sd_json_variant_unrefp) sd_json_variant *jq = NULL;
+        r = dns_questions_to_json(hq->question_idna, hq->question_utf8, &jq);
+        if (r < 0)
+                return log_error_errno(r, "Failed to convert question to JSON: %m");
+
+        r = sd_varlink_invokebo(
+                        vl,
+                        "io.systemd.Resolve.Hook.ResolveRecord",
+                        SD_JSON_BUILD_PAIR_VARIANT("question", jq));
+        if (r < 0)
+                return log_error_errno(r, "Failed to enqueue question onto Varlink connection: %m");
+
+        _cleanup_(hook_query_candidate_freep) HookQueryCandidate *qc = new(HookQueryCandidate, 1);
+        if (!qc)
+                return log_oom();
+
+        qc->query = hq;
+        qc->hook = hook_ref(h);
+        qc->link = TAKE_PTR(vl);
+        LIST_PREPEND(candidates, hq->candidates, qc);
+
+        sd_varlink_set_userdata(qc->link, qc);
+
+        TAKE_PTR(qc);
+
+        return 0;
+}
+
+static bool use_hooks(void) {
+        static int cache = -1;
+        int r;
+
+        if (cache >= 0)
+                return cache;
+
+        r = secure_getenv_bool("SYSTEMD_RESOLVED_HOOK");
+        if (r < 0) {
+                if (r != -ENXIO)
+                        log_debug_errno(r, "Failed to parse $SYSTEMD_RESOLVED_HOOK, ignoring: %m");
+
+                return (cache = true);
+        }
+
+        return (cache = r);
+}
+
+int manager_hook_query(
+                Manager *m,
+                DnsQuestion *question_idna,
+                DnsQuestion *question_utf8,
+                HookCompleteCallback complete_cb,
+                void *userdata,
+                HookQuery **ret) {
+
+        int r;
+
+        assert(m);
+        assert(ret);
+
+        if (!use_hooks()) {
+                *ret = NULL;
+                return 0; /* no relevant hooks, continue immediately */
+        }
+
+        /* Let's bring our list of hooks up-to-date */
+        (void) manager_hook_discover(m);
+
+        _cleanup_(hook_query_freep) HookQuery *hq = NULL;
+
+        Hook *h;
+        HASHMAP_FOREACH(h, m->hooks) {
+                r = hook_test_filter(h, question_idna);
+                if (r < 0) {
+                        log_warning_errno(
+                                        r, "Failed to test if hook '%s' matches IDNA  question (%s), assuming not.",
+                                        h->socket_path, dns_question_first_name(question_idna));
+                        continue;
+                }
+                if (r == 0) {
+                        r = hook_test_filter(h, question_utf8);
+                        if (r < 0) {
+                                log_warning_errno(
+                                                r, "Failed to test if hook '%s' matches UTF-8 question (%s), assuming not.",
+                                                h->socket_path, dns_question_first_name(question_utf8));
+                                continue;
+                        }
+                        if (r == 0) {
+                                log_debug("Hook %s does not match question, skipping.", h->socket_path);
+                                continue;
+                        }
+                }
+
+                if (!hq) {
+                        hq = new(HookQuery, 1);
+                        if (!hq)
+                                return log_oom();
+
+                        *hq = (HookQuery) {
+                                .question_idna = dns_question_ref(question_idna),
+                                .question_utf8 = dns_question_ref(question_utf8),
+                                .answer_rcode = -1,
+                                .complete = complete_cb,
+                                .userdata = userdata,
+                        };
+                }
+
+                r = hook_query_add_candidate(hq, h);
+                if (r < 0)
+                        return r;
+        }
+
+        if (!hq || !hq->candidates) {
+                *ret = NULL;
+                return 0; /* no relevant hooks, continue immediately */
+        }
+
+        *ret = TAKE_PTR(hq);
+        return 1; /* please wait for the hooks to reply */
+}

src/resolve/resolved-hook.h

@@ -0,0 +1,10 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+#pragma once
+
+#include "resolved-forward.h"
+
+typedef void (HookCompleteCallback)(HookQuery *q, int rcode, DnsAnswer *answer, void *userdata);
+
+int manager_hook_query(Manager *m, DnsQuestion *question_idna, DnsQuestion *question_utf8, HookCompleteCallback complete_cb, void *userdata, HookQuery **ret);
+
+HookQuery* hook_query_free(HookQuery *hq);

src/resolve/resolved-manager.c

@@ -918,6 +918,8 @@ Manager* manager_free(Manager *m) {
                 dns_service_browser_free(sb);
         hashmap_free(m->dns_service_browsers);
 
+        hashmap_free(m->hooks);
+
         return mfree(m);
 }
 

src/resolve/resolved-manager.h

@@ -159,6 +159,9 @@ typedef struct Manager {
 
         /* Map varlink links to DnsServiceBrowser instances. */
         Hashmap *dns_service_browsers;
+
+        Hashmap *hooks;
+        struct stat hook_stat;
 } Manager;
 
 /* Manager */

src/resolve/resolved-varlink.c

@@ -700,7 +700,7 @@ static void resolve_service_all_complete(DnsQuery *query) {
 
         assert(q);
 
-        if (q->block_all_complete > 0) {
+        if (q->hook_query || q->block_all_complete > 0) {
                 TAKE_PTR(q);
                 return;
         }
@@ -710,6 +710,13 @@ static void resolve_service_all_complete(DnsQuery *query) {
                 bool have_success = false;
 
                 LIST_FOREACH(auxiliary_queries, aux, q->auxiliary_queries) {
+
+                        if (aux->hook_query) {
+                                /* If an auxiliary query's hook is still pending, let's wait */
+                                TAKE_PTR(q);
+                                return;
+                        }
+
                         switch (aux->state) {
 
                         case DNS_TRANSACTION_PENDING:

src/resolve/test-dns-query.c

@@ -1,5 +1,7 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 
+#include <stdlib.h>
+
 #include "sd-event.h"
 
 #include "dns-answer.h"
@@ -890,4 +892,10 @@ TEST(dns_query_go) {
         exercise_dns_query_go(&cfg, NULL);
 }
 
-DEFINE_TEST_MAIN(LOG_DEBUG);
+static int intro(void) {
+        /* Disable hooks in order to make test cases hermetic */
+        ASSERT_OK_ERRNO(setenv("SYSTEMD_RESOLVED_HOOK", "0", /* overwrite= */ false));
+        return EXIT_SUCCESS;
+}
+
+DEFINE_TEST_MAIN_WITH_INTRO(LOG_DEBUG, intro);

src/shared/dns-answer.c

@@ -2,6 +2,8 @@
 
 #include <stdio.h>
 
+#include "sd-json.h"
+
 #include "alloc-util.h"
 #include "dns-answer.h"
 #include "dns-domain.h"
@@ -866,3 +868,36 @@ uint32_t dns_answer_min_ttl(DnsAnswer *a) {
 
         return ttl;
 }
+
+int dns_answer_to_json(DnsAnswer *answer, sd_json_variant **ret) {
+        int r;
+
+        assert(ret);
+
+        _cleanup_(sd_json_variant_unrefp) sd_json_variant *ja = NULL;
+        DnsResourceRecord *rr;
+        DNS_ANSWER_FOREACH(rr, answer) {
+                _cleanup_(sd_json_variant_unrefp) sd_json_variant *v = NULL;
+
+                r = dns_resource_record_to_json(rr, &v);
+                if (r < 0)
+                        return r;
+
+                r = dns_resource_record_to_wire_format(rr, /* canonical= */ false);
+                if (r < 0)
+                        return r;
+
+                r = sd_json_variant_append_arraybo(
+                                &ja,
+                                SD_JSON_BUILD_PAIR_VARIANT("rr", v),
+                                SD_JSON_BUILD_PAIR_BASE64("raw", rr->wire_format, rr->wire_format_size));
+                if (r < 0)
+                        return r;
+        }
+
+        if (!ja)
+                return sd_json_variant_new_array(ret, /* array=*/ NULL, /* n= */ 0);
+
+        *ret = TAKE_PTR(ja);
+        return 0;
+}

src/shared/dns-answer.h

@@ -104,6 +104,8 @@ void dns_answer_randomize(DnsAnswer *a);
 
 uint32_t dns_answer_min_ttl(DnsAnswer *a);
 
+int dns_answer_to_json(DnsAnswer *answer, sd_json_variant **ret);
+
 DEFINE_TRIVIAL_CLEANUP_FUNC(DnsAnswer*, dns_answer_unref);
 
 typedef struct DnsAnswerIterator {

src/shared/dns-question.c

@@ -7,6 +7,7 @@
 #include "dns-question.h"
 #include "dns-rr.h"
 #include "dns-type.h"
+#include "json-util.h"
 #include "socket-util.h"
 #include "string-util.h"
 
@@ -599,3 +600,37 @@ int dns_question_merge(DnsQuestion *a, DnsQuestion *b, DnsQuestion **ret) {
         *ret = TAKE_PTR(k);
         return 0;
 }
+
+int dns_json_dispatch_question(const char *name, sd_json_variant *variant, sd_json_dispatch_flags_t flags, void *userdata) {
+        DnsQuestion **q = ASSERT_PTR(userdata);
+        int r;
+
+        if (!sd_json_variant_is_array(variant))
+                return json_log(variant, flags, SYNTHETIC_ERRNO(EINVAL), "JSON field '%s' is not an array.", strna(name));
+
+        _cleanup_(dns_question_unrefp) DnsQuestion *nq = NULL;
+        nq = dns_question_new(sd_json_variant_elements(variant));
+        if (!nq)
+                return json_log_oom(variant, flags);
+
+        sd_json_variant *i;
+        JSON_VARIANT_ARRAY_FOREACH(i, variant) {
+                _cleanup_(dns_resource_key_unrefp) DnsResourceKey *key = NULL;
+
+                static const sd_json_dispatch_field dispatch_table[] = {
+                        { "key", SD_JSON_VARIANT_OBJECT, dns_json_dispatch_resource_key, 0, SD_JSON_MANDATORY },
+                        {}
+                };
+
+                r = sd_json_dispatch(i, dispatch_table, flags, &key);
+                if (r < 0)
+                        return r;
+
+                if (dns_question_add(nq, key, /* flags= */ 0) < 0)
+                        return json_log_oom(variant, flags);
+        }
+
+        dns_question_unref(*q);
+        *q = TAKE_PTR(nq);
+        return 0;
+}

src/shared/dns-question.h

@@ -1,6 +1,8 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
+#include "sd-json.h"
+
 #include "shared-forward.h"
 
 /* A simple array of resource keys */
@@ -58,6 +60,8 @@ static inline bool dns_question_isempty(DnsQuestion *q) {
 
 int dns_question_merge(DnsQuestion *a, DnsQuestion *b, DnsQuestion **ret);
 
+int dns_json_dispatch_question(const char *name, sd_json_variant *variant, sd_json_dispatch_flags_t flags, void *userdata);
+
 DEFINE_TRIVIAL_CLEANUP_FUNC(DnsQuestion*, dns_question_unref);
 
 #define _DNS_QUESTION_FOREACH(u, k, q)                                     \

src/shared/dns-rr.c

@@ -2558,3 +2558,16 @@ static const char* const sshfp_key_type_table[_SSHFP_KEY_TYPE_MAX_DEFINED] = {
         [SSHFP_KEY_TYPE_SHA256]   = "SHA-256",   /* RFC 4255 */
 };
 DEFINE_STRING_TABLE_LOOKUP_WITH_FALLBACK(sshfp_key_type, int, 255);
+
+int dns_json_dispatch_resource_key(const char *name, sd_json_variant *variant, sd_json_dispatch_flags_t flags, void *userdata) {
+        DnsResourceKey **k = ASSERT_PTR(userdata);
+        int r;
+
+        _cleanup_(dns_resource_key_unrefp) DnsResourceKey *nk = NULL;
+        r = dns_resource_key_from_json(variant, &nk);
+        if (r < 0)
+                return json_log(variant, flags, SYNTHETIC_ERRNO(EINVAL), "JSON field '%s' is not a valid resource record key.", strna(name));
+
+        DNS_RESOURCE_KEY_REPLACE(*k, TAKE_PTR(nk));
+        return 0;
+}

src/shared/dns-rr.h

@@ -444,3 +444,5 @@ int sshfp_algorithm_from_string(const char *s) _pure_;
 
 int sshfp_key_type_to_string_alloc(int i, char **ret);
 int sshfp_key_type_from_string(const char *s) _pure_;
+
+int dns_json_dispatch_resource_key(const char *name, sd_json_variant *variant, sd_json_dispatch_flags_t flags, void *userdata);

src/shared/meson.build

@@ -169,6 +169,7 @@ shared_sources = files(
         'recovery-key.c',
         'reread-partition-table.c',
         'resize-fs.c',
+        'resolve-hook-util.c',
         'resolve-util.c',
         'rm-rf.c',
         'seccomp-util.c',
@@ -217,6 +218,7 @@ shared_sources = files(
         'varlink-io.systemd.PCRLock.c',
         'varlink-io.systemd.Repart.c',
         'varlink-io.systemd.Resolve.c',
+        'varlink-io.systemd.Resolve.Hook.c',
         'varlink-io.systemd.Resolve.Monitor.c',
         'varlink-io.systemd.Udev.c',
         'varlink-io.systemd.Unit.c',

src/shared/resolve-hook-util.c

@@ -0,0 +1,15 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#include "resolve-hook-util.h"
+#include "dns-question.h"
+
+void resolve_record_parameters_done(ResolveRecordParameters *p) {
+        assert(p);
+
+        dns_question_unref(p->question);
+}
+
+const sd_json_dispatch_field resolve_record_parameters_dispatch_table[] = {
+        { "question", SD_JSON_VARIANT_ARRAY, dns_json_dispatch_question, offsetof(ResolveRecordParameters, question), SD_JSON_MANDATORY },
+        {}
+};

src/shared/resolve-hook-util.h

@@ -0,0 +1,14 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+#pragma once
+
+#include "sd-json.h"
+
+#include "shared-forward.h"
+
+typedef struct ResolveRecordParameters {
+        DnsQuestion *question;
+} ResolveRecordParameters;
+
+void resolve_record_parameters_done(ResolveRecordParameters *p);
+
+extern const sd_json_dispatch_field resolve_record_parameters_dispatch_table[];

src/shared/resolved-def.h

@@ -80,10 +80,13 @@
 #define SD_RESOLVED_QUERY_CONTINUOUS            \
                                     (UINT64_C(1) << 26)
 
+/* Output: Result was answered by hook */
+#define SD_RESOLVED_FROM_HOOK       (UINT64_C(1) << 27)
+
 #define SD_RESOLVED_LLMNR           (SD_RESOLVED_LLMNR_IPV4|SD_RESOLVED_LLMNR_IPV6)
 #define SD_RESOLVED_MDNS            (SD_RESOLVED_MDNS_IPV4|SD_RESOLVED_MDNS_IPV6)
 #define SD_RESOLVED_PROTOCOLS_ALL   (SD_RESOLVED_MDNS|SD_RESOLVED_LLMNR|SD_RESOLVED_DNS)
 
-#define SD_RESOLVED_FROM_MASK       (SD_RESOLVED_FROM_CACHE|SD_RESOLVED_FROM_ZONE|SD_RESOLVED_FROM_TRUST_ANCHOR|SD_RESOLVED_FROM_NETWORK)
+#define SD_RESOLVED_FROM_MASK       (SD_RESOLVED_FROM_CACHE|SD_RESOLVED_FROM_ZONE|SD_RESOLVED_FROM_TRUST_ANCHOR|SD_RESOLVED_FROM_NETWORK|SD_RESOLVED_FROM_HOOK)
 
 #define SD_RESOLVED_QUERY_TIMEOUT_USEC (120 * USEC_PER_SEC)

src/shared/varlink-io.systemd.Resolve.Hook.c

@@ -0,0 +1,81 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#include "varlink-io.systemd.Resolve.Hook.h"
+
+/* We want to reuse the ResourceKey structure from the io.systemd.Resolve interface, hence import it here */
+#include "varlink-io.systemd.Resolve.h"
+
+static SD_VARLINK_DEFINE_STRUCT_TYPE(
+                Answer,
+                SD_VARLINK_FIELD_COMMENT("A resource record that shall be looked up. Note that this field is (currently) mostly "
+                                         "decoration, useful for debugging, and may be omitted. The data actually used is encoded in the "
+                                         "'raw' field."),
+                SD_VARLINK_DEFINE_FIELD_BY_TYPE(rr, ResourceRecord, SD_VARLINK_NULLABLE),
+                SD_VARLINK_FIELD_COMMENT("A resource record encoded in DNS wire format, in turn encoded in Base64. This is the actual data "
+                                         "returned to the application, and should carry the same information as the 'rr' field, just in a "
+                                         "different encoding."),
+                SD_VARLINK_DEFINE_FIELD(raw, SD_VARLINK_STRING, 0));
+
+static SD_VARLINK_DEFINE_STRUCT_TYPE(
+                Question,
+                SD_VARLINK_FIELD_COMMENT("A resource record key that shall be looked up."),
+                SD_VARLINK_DEFINE_FIELD_BY_TYPE(key, ResourceKey, 0));
+
+static SD_VARLINK_DEFINE_METHOD_FULL(
+                QueryFilter,
+                SD_VARLINK_SUPPORTS_MORE,
+                SD_VARLINK_FIELD_COMMENT("A list of domains this hook is interested in. Lookups for domains not listed here will not be "
+                                         "passed to the Hook via ResolveRecord(). If this field is not set, requests for all domains "
+                                         "will be passed to the hook. Note that this applies recursively, i.e. a domain of a lookup is "
+                                         "considered matching the listed domains both if it exactly matches it, and in case only a suffix "
+                                         "of it matches it. If this is set to an empty array the hook is disabled."),
+                SD_VARLINK_DEFINE_OUTPUT(filterDomains, SD_VARLINK_STRING, SD_VARLINK_NULLABLE|SD_VARLINK_ARRAY),
+                SD_VARLINK_FIELD_COMMENT("Require the specified number of labels or more in a domain for the hook to be considered."),
+                SD_VARLINK_DEFINE_OUTPUT(filterLabelsMin, SD_VARLINK_INT, SD_VARLINK_NULLABLE),
+                SD_VARLINK_FIELD_COMMENT("Require the specified number of labels or less in a domain for the hook to be considered."),
+                SD_VARLINK_DEFINE_OUTPUT(filterLabelsMax, SD_VARLINK_INT, SD_VARLINK_NULLABLE));
+
+static SD_VARLINK_DEFINE_METHOD(
+                ResolveRecord,
+                SD_VARLINK_FIELD_COMMENT("The question being looked up, i.e. a combination of resource record keys. Note that unlike DNS "
+                                         "queries on the wire these lookups can carry multiple key requests, albeit closely related ones. "
+                                         "Specifically, lookups for A+AAAA for the the same hostname are submitted as one question, as "
+                                         "are lookups for TXT+SRV when doing DNS-SD resolution. Moreover, when looking up resources with "
+                                         "non-ASCII characters, they are placed together in a single question, once with labels encoded in "
+                                         "UTF-8, and once in IDNA. Hook implementations must be able to deal with these and other similar "
+                                         "combinations of resource key requests, and reply with all matching answers at once, or fail them "
+                                         "as one. Partial success/failure combinations are not supported."),
+                SD_VARLINK_DEFINE_INPUT_BY_TYPE(question, Question, SD_VARLINK_ARRAY),
+                SD_VARLINK_FIELD_COMMENT("A DNS response code. If a hook sets this return parameter further processing of the lookup via "
+                                         "regular proocols such as DNS, LLMNR, mDNS is skipped, and the return code returned immediately. "
+                                         "In other words, if a hook intends to let the request pass to normal resolution, it should not "
+                                         "set this return parameter."),
+                SD_VARLINK_DEFINE_OUTPUT(rcode, SD_VARLINK_INT, SD_VARLINK_NULLABLE),
+                SD_VARLINK_FIELD_COMMENT("An answer for a lookup, i.e. a combination of resource records, matching the request. This "
+                                         "should only be set when the 'rcode' parameter is returned as 0 (SUCCESS)."),
+                SD_VARLINK_DEFINE_OUTPUT_BY_TYPE(answer, Answer, SD_VARLINK_ARRAY|SD_VARLINK_NULLABLE));
+
+SD_VARLINK_DEFINE_INTERFACE(
+                io_systemd_Resolve_Hook,
+                "io.systemd.Resolve.Hook",
+                SD_VARLINK_INTERFACE_COMMENT("Generic interface for implementing a domain name resolution hook."),
+                SD_VARLINK_SYMBOL_COMMENT("Encapsulates a positive lookup answer"),
+                &vl_type_Answer,
+                SD_VARLINK_SYMBOL_COMMENT("Encapsulates a lookup question"),
+                &vl_type_Question,
+                SD_VARLINK_SYMBOL_COMMENT("Encapsulates the class/type/name part of a DNS resource record."),
+                &vl_type_ResourceKey,
+                SD_VARLINK_SYMBOL_COMMENT("Encapsulates a DNS resource record."),
+                &vl_type_ResourceRecord,
+                SD_VARLINK_SYMBOL_COMMENT("Returns filter parameters for this hook. A hook service can implement this to reduce lookup "
+                                          "requests, by enabling itself only for certain domains or certain numbers of labels in the name. "
+                                          "It's recommended to implement this to reduce the number of redundant calls to each hook. Note "
+                                          "that this is advisory only, and implementing services must be able to gracefully handle lookup "
+                                          "requests that do not match this filter. This call is usually made with the 'more' flag set, in "
+                                          "which case the connection is left open after the first reply, and the implementing hook "
+                                          "services can send updates to the filter at any time. Whenever a further reply is sent the "
+                                          "filter configured therein fully replaces any previously communicated filter."),
+                &vl_method_QueryFilter,
+                SD_VARLINK_SYMBOL_COMMENT("Sent whenever a resolution request is made. This typically takes the filter paramaters returned "
+                                          "by QueryFilter() into account, but this is not guaranteed."),
+                &vl_method_ResolveRecord);

src/shared/varlink-io.systemd.Resolve.Hook.h

@@ -0,0 +1,6 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+#pragma once
+
+#include "sd-varlink-idl.h"
+
+extern const sd_varlink_interface vl_interface_io_systemd_Resolve_Hook;

src/systemd/sd-dhcp-server.h

@@ -98,6 +98,8 @@ int sd_dhcp_server_is_in_relay_mode(sd_dhcp_server *server);
 int sd_dhcp_server_set_relay_target(sd_dhcp_server *server, const struct in_addr* address);
 int sd_dhcp_server_set_relay_agent_information(sd_dhcp_server *server, const char* circuit_id, const char* remote_id);
 
+int sd_dhcp_server_get_lease_address_by_name(sd_dhcp_server *server, const char *name, struct in_addr *ret);
+
 _SD_DEFINE_POINTER_CLEANUP_FUNC(sd_dhcp_server, sd_dhcp_server_unref);
 
 _SD_END_DECLARATIONS;

src/test/test-varlink-idl.c

@@ -37,6 +37,7 @@
 #include "varlink-io.systemd.PCRLock.h"
 #include "varlink-io.systemd.Repart.h"
 #include "varlink-io.systemd.Resolve.h"
+#include "varlink-io.systemd.Resolve.Hook.h"
 #include "varlink-io.systemd.Resolve.Monitor.h"
 #include "varlink-io.systemd.Udev.h"
 #include "varlink-io.systemd.Unit.h"
@@ -192,6 +193,7 @@ TEST(parse_format) {
                 &vl_interface_io_systemd_PCRLock,
                 &vl_interface_io_systemd_Repart,
                 &vl_interface_io_systemd_Resolve,
+                &vl_interface_io_systemd_Resolve_Hook,
                 &vl_interface_io_systemd_Resolve_Monitor,
                 &vl_interface_io_systemd_Udev,
                 &vl_interface_io_systemd_Unit,

test/networkd-test.py

@@ -65,6 +65,7 @@ def setUpModule():
     for u in [
         'systemd-networkd.socket',
         'systemd-networkd-varlink.socket',
+        'systemd-networkd-resolve-hook.socket',
         'systemd-networkd.service',
         'systemd-resolved-varlink.socket',
         'systemd-resolved-monitor.socket',
@@ -90,13 +91,15 @@ def setUpModule():
         subprocess.call(['useradd', '--system', '--no-create-home', 'systemd-network'])
 
     for d in ['/etc/systemd/network', '/run/systemd/network',
-              '/run/systemd/netif', '/run/systemd/resolve']:
-        if os.path.isdir(d):
-            subprocess.check_call(["mount", "-t", "tmpfs", "none", d])
+              '/run/systemd/netif', '/run/systemd/resolve', '/run/systemd/resolve.hook']:
+        subprocess.check_call(["mount", "-m", "-t", "tmpfs", "none", d])
         tmpmounts.append(d)
     if os.path.isdir('/run/systemd/resolve'):
         os.chmod('/run/systemd/resolve', 0o755)
         shutil.chown('/run/systemd/resolve', 'systemd-resolve', 'systemd-resolve')
+    if os.path.isdir('/run/systemd/resolve.hook'):
+        os.chmod('/run/systemd/resolve.hook', 0o755)
+        shutil.chown('/run/systemd/resolve.hook', 'systemd-network', 'systemd-network')
     if os.path.isdir('/run/systemd/netif'):
         os.chmod('/run/systemd/netif', 0o755)
         shutil.chown('/run/systemd/netif', 'systemd-network', 'systemd-network')
@@ -279,6 +282,8 @@ Gateway=192.168.250.1
 
     def tearDown(self):
         subprocess.check_call(['systemctl', 'stop', 'systemd-networkd.socket'])
+        subprocess.check_call(['systemctl', 'stop', 'systemd-networkd-varlink.socket'])
+        subprocess.check_call(['systemctl', 'stop', 'systemd-networkd-resolve-hook.socket'])
         subprocess.check_call(['systemctl', 'stop', 'systemd-networkd.service'])
         subprocess.check_call(['ip', 'link', 'del', 'mybridge'])
         subprocess.check_call(['ip', 'link', 'del', 'port1'])
@@ -374,6 +379,8 @@ class ClientTestBase(NetworkdTestingUtilities):
     def tearDown(self):
         self.shutdown_iface()
         subprocess.call(['systemctl', 'stop', 'systemd-networkd.socket'])
+        subprocess.call(['systemctl', 'stop', 'systemd-networkd-varlink.socket'])
+        subprocess.call(['systemctl', 'stop', 'systemd-networkd-resolve-hook.socket'])
         subprocess.call(['systemctl', 'stop', 'systemd-networkd.service'])
         subprocess.call(['ip', 'link', 'del', 'dummy0'],
                         stderr=subprocess.DEVNULL)
@@ -931,9 +938,11 @@ class NetworkdClientTest(ClientTestBase, unittest.TestCase):
 set -eu
 mkdir -p /run/systemd/network
 mkdir -p /run/systemd/netif
+mkdir -p /run/systemd/resolve.hook
 mkdir -p /var/lib/systemd/network
 mount -t tmpfs none /run/systemd/network
 mount -t tmpfs none /run/systemd/netif
+mount -t tmpfs none /run/systemd/resolve.hook
 mount -t tmpfs none /var/lib/systemd/network
 [ ! -e /run/dbus ] || mount -t tmpfs none /run/dbus
 # create router/client veth pair
@@ -967,6 +976,9 @@ EOF
 # Hence, 'networkctl persistent-storage yes' cannot be used.
 export SYSTEMD_NETWORK_PERSISTENT_STORAGE_READY=1
 
+# Don't try to register resolved hook for our testcase
+export SYSTEMD_NETWORK_RESOLVE_HOOK=0
+
 # Generate debugging logs.
 export SYSTEMD_LOG_LEVEL=debug
 
@@ -983,6 +995,7 @@ exec $(systemctl cat systemd-networkd.service | sed -n '/^ExecStart=/ {{ s/^.*=/
                                '-p', 'InaccessibleDirectories=-/etc/systemd/network',
                                '-p', 'InaccessibleDirectories=-/run/systemd/network',
                                '-p', 'InaccessibleDirectories=-/run/systemd/netif',
+                               '-p', 'InaccessibleDirectories=-/run/systemd/resolve.hook',
                                '-p', 'InaccessibleDirectories=-/var/lib/systemd/network',
                                '--service-type=notify', script])
 

test/test-network/conf/25-dhcp-client-resolve-hook.network

@@ -0,0 +1,10 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+[Match]
+Name=veth99
+
+[Network]
+DHCP=yes
+
+[DHCPv4]
+SendHostname=yes
+Hostname=flummy

test/test-network/conf/25-dhcp-server-resolve-hook.network

@@ -0,0 +1,13 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+[Match]
+Name=veth-peer
+
+[Network]
+IPv6AcceptRA=false
+DHCPServer=yes
+
+[DHCPServer]
+ServerAddress=192.168.5.1/24
+PoolOffset=20
+PoolSize=10
+LocalLeaseDomain=_networkdtest

test/test-network/systemd-networkd-tests.py

@@ -426,6 +426,7 @@ def save_active_units():
     for u in [
             'systemd-networkd.socket',
             'systemd-networkd-varlink.socket',
+            'systemd-networkd-resolve-hook.socket',
             'systemd-networkd.service',
             'systemd-resolved-monitor.socket',
             'systemd-resolved-varlink.socket',
@@ -449,6 +450,10 @@ def restore_active_units():
         call('systemctl stop systemd-networkd-varlink.socket')
         has_network_socket = True
 
+    if 'systemd-networkd-resolve-hook.socket' in active_units:
+        call('systemctl stop systemd-networkd-resolve-hook.socket')
+        has_network_socket = True
+
     if 'systemd-resolved-monitor.socket' in active_units:
         call('systemctl stop systemd-resolved-monitor.socket')
         has_resolve_socket = True
@@ -525,6 +530,7 @@ def setup_system_units():
                 'systemd-networkd.service',
                 'systemd-networkd.socket',
                 'systemd-networkd-varlink.socket',
+                'systemd-networkd-resolve-hook.socket',
                 'systemd-networkd-persistent-storage.service',
                 'systemd-resolved.service',
                 'systemd-timesyncd.service',
@@ -572,6 +578,13 @@ def setup_system_units():
             'StartLimitIntervalSec=0',
         ]
     )
+    create_unit_dropin(
+        'systemd-networkd-resolve-hook.socket',
+        [
+            '[Unit]',
+            'StartLimitIntervalSec=0',
+        ]
+    )
     create_unit_dropin(
         'systemd-networkd-persistent-storage.service',
         [
@@ -604,6 +617,7 @@ def clear_system_units():
     rm_unit('systemd-networkd.service')
     rm_unit('systemd-networkd.socket')
     rm_unit('systemd-networkd-varlink.socket')
+    rm_unit('systemd-networkd-resolve-hook.socket')
     rm_unit('systemd-networkd-persistent-storage.service')
     rm_unit('systemd-resolved.service')
     rm_unit('systemd-timesyncd.service')
@@ -990,10 +1004,12 @@ def stop_networkd(show_logs=True, check_failed=True):
     if check_failed:
         check_output('systemctl stop systemd-networkd.socket')
         check_output('systemctl stop systemd-networkd-varlink.socket')
+        check_output('systemctl stop systemd-networkd-resolve-hook.socket')
         check_output('systemctl stop systemd-networkd.service')
     else:
         call('systemctl stop systemd-networkd.socket')
         call('systemctl stop systemd-networkd-varlink.socket')
+        call('systemctl stop systemd-networkd-resolve-hook.socket')
         call('systemctl stop systemd-networkd.service')
 
     if show_logs:
@@ -7366,6 +7382,16 @@ class NetworkdDHCPServerTests(unittest.TestCase, Utilities):
         data = json.loads(output)
         self.assertEqual(data['DHCPv4Client']['Lease']['Hostname'], 'fqdn.example.com')
 
+    def test_dhcp_server_resolve_hook(self):
+        copy_network_unit('25-veth.netdev', '25-dhcp-client-resolve-hook.network', '25-dhcp-server-resolve-hook.network')
+        start_networkd()
+        self.wait_online('veth99:routable', 'veth-peer:routable')
+
+        output = check_output('resolvectl query flummy._networkdtest')
+        print(output)
+        self.assertIn('192.168.5.2', output)
+
+
 class NetworkdDHCPServerRelayAgentTests(unittest.TestCase, Utilities):
 
     def setUp(self):

test/units/TEST-13-NSPAWN.nss-mymachines.sh

@@ -136,4 +136,12 @@ done
 (! getent group -s mymachines foo 11)
 (! getent passwd -s mymachines foo 11)
 
+# Now check the machined's hook for resolved too
+run_and_grep "10\.1\.0\.2" resolvectl query nss-mymachines-singleip
+
+run_and_grep "fd00:dead:beef:cafe::2" resolvectl query nss-mymachines-manyips
+for i in {100..120}; do
+    run_and_grep "10\.2\.0\.$i" resolvectl query nss-mymachines-manyips
+done
+
 machinectl stop nss-mymachines-{noip,singleip,manyips}

test/units/TEST-88-UPGRADE.sh

@@ -83,6 +83,9 @@ timer2=$(systemctl show -P NextElapseUSecRealtime upgrade_timer_test.timer)
 
 dnf downgrade -y --allowerasing --disablerepo '*' "$pkgdir"/distro/*.rpm
 
+# FIXME: See https://github.com/systemd/systemd/pull/39293
+systemctl stop systemd-networkd-resolve-hook.socket || true
+
 # Some distros don't ship networkd, so the test will always fail
 if command -v networkctl >/dev/null; then
     networkd=1

units/meson.build

@@ -497,6 +497,10 @@ units = [
           'file' : 'systemd-networkd-varlink.socket',
           'conditions' : ['ENABLE_NETWORKD'],
         },
+        {
+          'file' : 'systemd-networkd-resolve-hook.socket',
+          'conditions' : ['ENABLE_NETWORKD'],
+        },
         {
           'file' : 'systemd-networkd-wait-online.service.in',
           'conditions' : ['ENABLE_NETWORKD'],

units/systemd-networkd-resolve-hook.socket

@@ -0,0 +1,26 @@
+#  SPDX-License-Identifier: LGPL-2.1-or-later
+#
+#  This file is part of systemd.
+#
+#  systemd is free software; you can redistribute it and/or modify it
+#  under the terms of the GNU Lesser General Public License as published by
+#  the Free Software Foundation; either version 2.1 of the License, or
+#  (at your option) any later version.
+
+[Unit]
+Description=Network Management Resolve Hook Socket
+Documentation=man:systemd-networkd.service(8)
+ConditionCapability=CAP_NET_ADMIN
+DefaultDependencies=no
+Before=sockets.target shutdown.target
+Conflicts=shutdown.target
+
+[Socket]
+ListenStream=/run/systemd/resolve.hook/io.systemd.Network
+FileDescriptorName=resolve-hook
+SocketMode=0666
+Service=systemd-networkd.service
+RemoveOnStop=yes
+
+[Install]
+WantedBy=sockets.target

units/systemd-networkd.service.in

@@ -46,7 +46,7 @@ RestrictRealtime=yes
 RestrictSUIDSGID=yes
 RuntimeDirectory=systemd/netif
 RuntimeDirectoryPreserve=yes
-Sockets=systemd-networkd.socket systemd-networkd-varlink.socket
+Sockets=systemd-networkd.socket systemd-networkd-varlink.socket systemd-networkd-resolve-hook.socket
 SystemCallArchitectures=native
 SystemCallErrorNumber=EPERM
 SystemCallFilter=@system-service bpf
@@ -56,7 +56,7 @@ User=systemd-network
 
 [Install]
 WantedBy=multi-user.target
-Also=systemd-networkd.socket systemd-networkd-varlink.socket
+Also=systemd-networkd.socket systemd-networkd-varlink.socket systemd-networkd-resolve-hook.socket
 Alias=dbus-org.freedesktop.network1.service
 
 # The output from this generator is used by udevd and networkd. Enable it by

units/systemd-networkd.socket

@@ -18,6 +18,7 @@ Conflicts=shutdown.target
 [Socket]
 ReceiveBuffer=128M
 ListenNetlink=route 1361
+FileDescriptorName=rtnl
 PassPacketInfo=yes
 
 [Install]