Merge c15ab51249 into a526b9ddfc

Add a new flag to systemd-networkd-wait-online, --dns, to allow waiting
for DNS to be configured.

DNS is considered configured when at least one DNS server is accessible.
If a link has the property DefaultRoute=yes (either by explicit
configuration, or because there are no routing-only domains), or if the
search domain '.' is configured, wait for link-specific DNS to be
configured. Otherwise, global DNS servers may be considered.

man/systemd-networkd-wait-online.service.xml

@@ -144,6 +144,22 @@
         <xi:include href="version-info.xml" xpointer="v249"/></listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><option>--dns</option></term>
+
+        <listitem><para>Waiting for DNS servers to be accessible on each interface. If this
+        option is specified with <option>--any</option>, then
+        <command>systemd-networkd-wait-online</command> exits with success when at least one interface
+        becomes online and has an accessible DNS server.</para>
+        <para>If a link has the property <varname>DefaultRoute=yes</varname> (either because the
+        <varname>DNSDefaultRoute=</varname> network property is explicitly configured, or
+        because the link does not have any "routing-only" domains), or if the search domain "." is
+        configured, then wait for link-specific DNS servers to be accessible. Otherwise, allow global
+        DNS servers to satisfy the condition.</para>
+
+        <xi:include href="version-info.xml" xpointer="v257"/></listitem>
+      </varlistentry>
+
       <varlistentry>
         <term><option>--any</option></term>
 

src/libsystemd/sd-varlink/varlink-util.c

@@ -86,6 +86,19 @@ int varlink_callb_and_log(
         return varlink_call_and_log(v, method, parameters, ret_parameters);
 }
 
+int varlink_many_notify(Set *s, sd_json_variant *parameters) {
+        sd_varlink *link;
+        int r = 1;
+
+        if (set_isempty(s))
+                return 0;
+
+        SET_FOREACH(link, s)
+                RET_GATHER(r, sd_varlink_notify(link, parameters));
+
+        return r;
+}
+
 int varlink_many_notifyb(Set *s, ...) {
         int r;
 
@@ -102,12 +115,7 @@ int varlink_many_notifyb(Set *s, ...) {
         if (r < 0)
                 return r;
 
-        r = 1;
-        sd_varlink *link;
-        SET_FOREACH(link, s)
-                RET_GATHER(r, sd_varlink_notify(link, parameters));
-
-        return r;
+        return varlink_many_notify(s, parameters);
 }
 
 int varlink_many_reply(Set *s, sd_json_variant *parameters) {

src/libsystemd/sd-varlink/varlink-util.h

@@ -13,6 +13,7 @@ int varlink_callb_and_log(sd_varlink *v, const char *method, sd_json_variant **r
 #define varlink_callbo_and_log(v, method, ret_parameters, ...)          \
         varlink_callb_and_log((v), (method), (ret_parameters), SD_JSON_BUILD_OBJECT(__VA_ARGS__))
 
+int varlink_many_notify(Set *s, sd_json_variant *parameters);
 int varlink_many_notifyb(Set *s, ...);
 #define varlink_many_notifybo(s, ...)                                   \
         varlink_many_notifyb((s), SD_JSON_BUILD_OBJECT(__VA_ARGS__))

src/network/meson.build

@@ -114,6 +114,7 @@ sources = files(
 systemd_networkd_sources = files('networkd.c')
 
 systemd_networkd_wait_online_sources = files(
+        'wait-online/dns-configuration.c',
         'wait-online/link.c',
         'wait-online/manager.c',
         'wait-online/wait-online.c',

src/network/wait-online/dns-configuration.c

@@ -0,0 +1,244 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#include "sd-json.h"
+
+#include "alloc-util.h"
+#include "dns-configuration.h"
+#include "iovec-util.h"
+#include "hash-funcs.h"
+#include "json-util.h"
+#include "set.h"
+#include "strv.h"
+
+DNSServer *dns_server_free(DNSServer *s) {
+        if (!s)
+                return NULL;
+
+        free(s->server_name);
+
+        return mfree(s);
+}
+
+DEFINE_PRIVATE_HASH_OPS_WITH_VALUE_DESTRUCTOR(
+        dns_server_hash_ops,
+        void,
+        trivial_hash_func,
+        trivial_compare_func,
+        DNSServer,
+        dns_server_free);
+
+static int dispatch_dns_server(const char *name, sd_json_variant *variant, sd_json_dispatch_flags_t flags, void *userdata) {
+        static const sd_json_dispatch_field dns_server_dispatch_table[] = {
+                { "address",    SD_JSON_VARIANT_ARRAY,    NULL,                     0,                                SD_JSON_MANDATORY },
+                { "family",     SD_JSON_VARIANT_UNSIGNED, sd_json_dispatch_uint,    offsetof(DNSServer, family),      SD_JSON_MANDATORY },
+                { "port",       SD_JSON_VARIANT_UNSIGNED, sd_json_dispatch_uint16,  offsetof(DNSServer, port),        0                 },
+                { "ifindex",    SD_JSON_VARIANT_UNSIGNED, json_dispatch_ifindex,    offsetof(DNSServer, ifindex),     SD_JSON_RELAX     },
+                { "name",       SD_JSON_VARIANT_STRING,   sd_json_dispatch_string,  offsetof(DNSServer, server_name), 0                 },
+                { "accessible", SD_JSON_VARIANT_BOOLEAN,  sd_json_dispatch_stdbool, offsetof(DNSServer, accessible),  SD_JSON_MANDATORY },
+                {},
+        };
+        DNSServer **ret = ASSERT_PTR(userdata);
+        _cleanup_(dns_server_freep) DNSServer *s = NULL;
+        _cleanup_(iovec_done) struct iovec iov = {};
+        int r;
+
+        s = new0(DNSServer, 1);
+        if (!s)
+                return log_oom();
+
+        r = sd_json_dispatch(variant, dns_server_dispatch_table, flags, s);
+        if (r < 0)
+                return r;
+
+        /* We ignored the address field above. Handle it now. */
+        r = json_dispatch_byte_array_iovec("address", sd_json_variant_by_key(variant, "address"), flags, &iov);
+        if (r < 0)
+                return r;
+
+        if (iov.iov_len != FAMILY_ADDRESS_SIZE_SAFE(s->family))
+                return json_log(variant, flags, SYNTHETIC_ERRNO(EINVAL), "JSON field 'address' is array of unexpected size.");
+
+        memcpy(&s->addr, iov.iov_base, iov.iov_len);
+
+        *ret = TAKE_PTR(s);
+
+        return 0;
+}
+
+static int dispatch_dns_server_array(const char *name, sd_json_variant *variant, sd_json_dispatch_flags_t flags, void *userdata) {
+        Set **ret = ASSERT_PTR(userdata);
+        Set *dns_servers;
+        sd_json_variant *v = NULL;
+        int r;
+
+        dns_servers = set_new(&dns_server_hash_ops);
+        if (!dns_servers)
+                return log_oom();
+
+        JSON_VARIANT_ARRAY_FOREACH(v, variant) {
+                _cleanup_(dns_server_freep) DNSServer *s = NULL;
+
+                s = new0(DNSServer, 1);
+                if (!s)
+                        return log_oom();
+
+                r = dispatch_dns_server(name, v, flags, &s);
+                if (r < 0)
+                        return json_log(v, flags, r, "JSON array element is not a valid DNSServer.");
+
+                r = set_put(dns_servers, TAKE_PTR(s));
+                if (r < 0)
+                        return log_oom();
+        }
+
+        set_free_and_replace(*ret, dns_servers);
+
+        return 0;
+}
+
+SearchDomain *search_domain_free(SearchDomain *d) {
+        if (!d)
+                return NULL;
+
+        free(d->name);
+
+        return mfree(d);
+}
+
+DEFINE_PRIVATE_HASH_OPS_WITH_VALUE_DESTRUCTOR(
+        search_domain_hash_ops,
+        void,
+        trivial_hash_func,
+        trivial_compare_func,
+        SearchDomain,
+        search_domain_free);
+
+static int dispatch_search_domain(const char *name, sd_json_variant *variant, sd_json_dispatch_flags_t flags, void *userdata) {
+        static const sd_json_dispatch_field search_domain_dispatch_table[] = {
+                { "name",      SD_JSON_VARIANT_STRING,   sd_json_dispatch_string,  offsetof(SearchDomain, name),       SD_JSON_MANDATORY },
+                { "routeOnly", SD_JSON_VARIANT_BOOLEAN,  sd_json_dispatch_stdbool, offsetof(SearchDomain, route_only), SD_JSON_MANDATORY },
+                { "ifindex",   SD_JSON_VARIANT_UNSIGNED, json_dispatch_ifindex,    offsetof(SearchDomain, ifindex),    SD_JSON_RELAX     },
+                {},
+        };
+        SearchDomain **ret = ASSERT_PTR(userdata);
+        _cleanup_(search_domain_freep) SearchDomain *d = NULL;
+        int r;
+
+        d = new0(SearchDomain, 1);
+        if (!d)
+                return log_oom();
+
+        r = sd_json_dispatch(variant, search_domain_dispatch_table, flags, d);
+        if (r < 0)
+                return r;
+
+        *ret = TAKE_PTR(d);
+
+        return 0;
+}
+
+static int dispatch_search_domain_array(const char *name, sd_json_variant *variant, sd_json_dispatch_flags_t flags, void *userdata) {
+        Set **ret = ASSERT_PTR(userdata);
+        Set *search_domains;
+        sd_json_variant *v = NULL;
+        int r;
+
+        search_domains = set_new(&search_domain_hash_ops);
+        if (!search_domains)
+                return log_oom();
+
+        JSON_VARIANT_ARRAY_FOREACH(v, variant) {
+                _cleanup_(search_domain_freep) SearchDomain *d = NULL;
+
+                d = new0(SearchDomain, 1);
+                if (!d)
+                        return log_oom();
+
+                r = dispatch_search_domain(name, v, flags, &d);
+                if (r < 0)
+                        return json_log(v, flags, r, "JSON array element is not a valid SearchDomain.");
+
+                r = set_put(search_domains, TAKE_PTR(d));
+                if (r < 0)
+                        return log_oom();
+        }
+
+        set_free_and_replace(*ret, search_domains);
+
+        return 0;
+}
+
+DNSConfiguration *dns_configuration_free(DNSConfiguration *c) {
+        if (!c)
+                return NULL;
+
+        dns_server_free(c->current_dns_server);
+        set_free(c->dns_servers);
+        set_free(c->search_domains);
+        free(c->ifname);
+
+        return mfree(c);
+}
+
+static int dispatch_dns_configuration(const char *name, sd_json_variant *variant, sd_json_dispatch_flags_t flags, void *userdata) {
+        static const sd_json_dispatch_field dns_configuration_dispatch_table[] = {
+                { "ifname",        SD_JSON_VARIANT_STRING,   sd_json_dispatch_string,      offsetof(DNSConfiguration, ifname),             0             },
+                { "ifindex",       SD_JSON_VARIANT_UNSIGNED, json_dispatch_ifindex,        offsetof(DNSConfiguration, ifindex),            SD_JSON_RELAX },
+                { "defaultRoute",  SD_JSON_VARIANT_BOOLEAN,  sd_json_dispatch_stdbool,     offsetof(DNSConfiguration, default_route),      0             },
+                { "currentServer", SD_JSON_VARIANT_OBJECT,   dispatch_dns_server,          offsetof(DNSConfiguration, current_dns_server), 0             },
+                { "servers",       SD_JSON_VARIANT_ARRAY,    dispatch_dns_server_array,    offsetof(DNSConfiguration, dns_servers),        0             },
+                { "searchDomains", SD_JSON_VARIANT_ARRAY,    dispatch_search_domain_array, offsetof(DNSConfiguration, search_domains),     0             },
+                {},
+
+        };
+        DNSConfiguration **ret = ASSERT_PTR(userdata);
+        _cleanup_(dns_configuration_freep) DNSConfiguration *c = NULL;
+        int r;
+
+        c = new0(DNSConfiguration, 1);
+        if (!c)
+                return log_oom();
+
+        r = sd_json_dispatch(variant, dns_configuration_dispatch_table, flags, c);
+        if (r < 0)
+                return r;
+
+        *ret = TAKE_PTR(c);
+
+        return 0;
+}
+
+int dns_configuration_from_json(sd_json_variant *variant, DNSConfiguration **ret) {
+        return dispatch_dns_configuration(NULL, variant, SD_JSON_LOG, ret);
+}
+
+bool dns_is_accessible(DNSConfiguration *c) {
+        DNSServer *s = NULL;
+
+        if (!c)
+                return false;
+
+        if (c->current_dns_server && c->current_dns_server->accessible)
+                return true;
+
+        SET_FOREACH(s, c->dns_servers)
+                if (s->accessible)
+                        return true;
+
+        return false;
+}
+
+bool dns_configuration_contains_search_domain(DNSConfiguration *c, const char *domain) {
+        SearchDomain *d = NULL;
+
+        assert(domain);
+
+        if (!c)
+                return false;
+
+        SET_FOREACH(d, c->search_domains)
+                if (streq(d->name, domain))
+                        return true;
+
+        return false;
+}

src/network/wait-online/dns-configuration.h

@@ -0,0 +1,49 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+#pragma once
+
+#include "sd-json.h"
+
+#include "in-addr-util.h"
+#include "macro-fundamental.h"
+#include "set.h"
+
+typedef struct DNSServer DNSServer;
+typedef struct SearchDomain SearchDomain;
+typedef struct DNSConfiguration DNSConfiguration;
+
+struct DNSServer {
+        union in_addr_union addr;
+        int family;
+        uint16_t port;
+        int ifindex;
+        char *server_name;
+        bool accessible;
+};
+
+DNSServer *dns_server_free(DNSServer *s);
+DEFINE_TRIVIAL_CLEANUP_FUNC(DNSServer*, dns_server_free);
+
+struct SearchDomain {
+        char *name;
+        bool route_only;
+        int ifindex;
+};
+
+SearchDomain *search_domain_free(SearchDomain *d);
+DEFINE_TRIVIAL_CLEANUP_FUNC(SearchDomain*, search_domain_free);
+
+struct DNSConfiguration {
+        char *ifname;
+        int ifindex;
+        bool default_route;
+        DNSServer *current_dns_server;
+        Set *dns_servers;
+        Set *search_domains;
+};
+
+int dns_configuration_from_json(sd_json_variant *variant, DNSConfiguration **ret);
+bool dns_is_accessible(DNSConfiguration *c);
+bool dns_configuration_contains_search_domain(DNSConfiguration *c, const char *domain);
+
+DNSConfiguration *dns_configuration_free(DNSConfiguration *c);
+DEFINE_TRIVIAL_CLEANUP_FUNC(DNSConfiguration*, dns_configuration_free);

src/network/wait-online/link.c

@@ -3,6 +3,7 @@
 #include "sd-network.h"
 
 #include "alloc-util.h"
+#include "dns-configuration.h"
 #include "format-ifname.h"
 #include "hashmap.h"
 #include "link.h"
@@ -62,6 +63,8 @@ Link *link_free(Link *l) {
                         hashmap_remove(l->manager->links_by_name, *n);
         }
 
+        dns_configuration_free(l->dns_configuration);
+
         free(l->state);
         free(l->ifname);
         strv_free(l->altnames);

src/network/wait-online/link.h

@@ -3,6 +3,7 @@
 
 #include "sd-netlink.h"
 
+#include "dns-configuration.h"
 #include "log-link.h"
 #include "network-util.h"
 
@@ -24,6 +25,7 @@ struct Link {
         LinkAddressState ipv4_address_state;
         LinkAddressState ipv6_address_state;
         char *state;
+        DNSConfiguration *dns_configuration;
 };
 
 int link_new(Manager *m, Link **ret, int ifindex, const char *ifname);

src/network/wait-online/manager.c

@@ -4,7 +4,13 @@
 #include <linux/if.h>
 #include <fnmatch.h>
 
+#include "sd-event.h"
+#include "sd-json.h"
+#include "sd-varlink.h"
+
 #include "alloc-util.h"
+#include "dns-configuration.h"
+#include "json-util.h"
 #include "link.h"
 #include "manager.h"
 #include "netlink-util.h"
@@ -133,6 +139,26 @@ static int manager_link_is_online(Manager *m, Link *l, const LinkOperationalStat
                                                     "No routable IPv6 address is configured.");
         }
 
+        if (m->requires_dns) {
+                if (!l->dns_configuration)
+                        return log_link_debug_errno(l, SYNTHETIC_ERRNO(EADDRNOTAVAIL),
+                                                    "No DNS configuration yet");
+
+                /* If a link is configured with DNSDefaultRoute=yes, or is configured with the
+                 * search domain '.', then require link-specific DNS servers to be available.
+                 * Otherwise, we check the global DNS configuration. */
+                if (l->dns_configuration->default_route ||
+                    dns_configuration_contains_search_domain(l->dns_configuration, ".")) {
+
+                        if (!dns_is_accessible(l->dns_configuration))
+                                return log_link_debug_errno(l, SYNTHETIC_ERRNO(EADDRNOTAVAIL),
+                                                            "No link-specific DNS server is accessible.");
+
+                } else if (!dns_is_accessible(m->dns_configuration))
+                        return log_link_debug_errno(l, SYNTHETIC_ERRNO(EADDRNOTAVAIL),
+                                                    "No DNS server is accessible.");
+        }
+
         log_link_debug(l, "link is configured by networkd and online.");
         return true;
 }
@@ -381,13 +407,102 @@ static int manager_network_monitor_listen(Manager *m) {
         return 0;
 }
 
+static int on_dns_configuration_event(
+                sd_varlink *link,
+                sd_json_variant *parameters,
+                const char *error_id,
+                sd_varlink_reply_flags_t flags,
+                void *userdata) {
+
+        Manager *m = ASSERT_PTR(userdata);
+        sd_json_variant *configurations = NULL, *v = NULL;
+        int r;
+
+        assert(link);
+
+        if (error_id) {
+                log_warning("DNS configuration event error, ignoring: %s", error_id);
+                return 0;
+        }
+
+        configurations = sd_json_variant_by_key(parameters, "configuration");
+        if (!sd_json_variant_is_array(configurations)) {
+                log_warning("DNS configuration JSON data does not have configuration key, ignoring.");
+                return 0;
+        }
+
+        JSON_VARIANT_ARRAY_FOREACH(v, configurations) {
+                _cleanup_(dns_configuration_freep) DNSConfiguration *c = NULL;
+
+                r = dns_configuration_from_json(v, &c);
+                if (r < 0) {
+                        log_warning_errno(r, "Failed to get DNS configuration JSON, ignoring: %m");
+                        continue;
+                }
+
+                if (c->ifindex > 0) {
+                        Link *l = hashmap_get(m->links_by_index, INT_TO_PTR(c->ifindex));
+                        if (l)
+                                free_and_replace_full(l->dns_configuration, c, dns_configuration_free);
+                } else
+                        /* Global DNS configuration */
+                        free_and_replace_full(m->dns_configuration, c, dns_configuration_free);
+        }
+
+        if (manager_configured(m))
+                sd_event_exit(m->event, 0);
+
+        return 0;
+}
+
+static int manager_dns_configuration_listen(Manager *m) {
+        _cleanup_(sd_varlink_unrefp) sd_varlink *vl = NULL;
+        int r;
+
+        assert(m);
+        assert(m->event);
+
+        if (!m->requires_dns)
+                return 0;
+
+        r = sd_varlink_connect_address(&vl, "/run/systemd/resolve/io.systemd.Resolve.Monitor");
+        if (r < 0)
+                return log_error_errno(r, "Failed to connect to io.systemd.Resolve.Monitor: %m");
+
+        r = sd_varlink_set_relative_timeout(vl, USEC_INFINITY);
+        if (r < 0)
+                return log_error_errno(r, "Failed to set varlink timeout: %m");
+
+        r = sd_varlink_attach_event(vl, m->event, SD_EVENT_PRIORITY_NORMAL);
+        if (r < 0)
+                return log_error_errno(r, "Failed to attach varlink connection to event loop: %m");
+
+        (void) sd_varlink_set_userdata(vl, m);
+
+        r = sd_varlink_bind_reply(vl, on_dns_configuration_event);
+        if (r < 0)
+                return log_error_errno(r, "Failed to bind varlink reply callback: %m");
+
+        r = sd_varlink_observebo(
+                        vl,
+                        "io.systemd.Resolve.Monitor.SubscribeDNSConfiguration",
+                        SD_JSON_BUILD_PAIR_BOOLEAN("allowInteractiveAuthentication", false));
+        if (r < 0)
+                return log_error_errno(r, "Failed to issue SubscribeDNSConfiguration: %m");
+
+        m->varlink_client = TAKE_PTR(vl);
+
+        return 0;
+}
+
 int manager_new(Manager **ret,
                 Hashmap *command_line_interfaces_by_name,
                 char **ignored_interfaces,
                 LinkOperationalStateRange required_operstate,
                 AddressFamily required_family,
                 bool any,
-                usec_t timeout) {
+                usec_t timeout,
+                bool requires_dns) {
 
         _cleanup_(manager_freep) Manager *m = NULL;
         int r;
@@ -404,6 +519,7 @@ int manager_new(Manager **ret,
                 .required_operstate = required_operstate,
                 .required_family = required_family,
                 .any = any,
+                .requires_dns = requires_dns,
         };
 
         r = sd_event_default(&m->event);
@@ -428,6 +544,10 @@ int manager_new(Manager **ret,
         if (r < 0)
                 return r;
 
+        r = manager_dns_configuration_listen(m);
+        if (r < 0)
+                return r;
+
         *ret = TAKE_PTR(m);
 
         return 0;
@@ -445,6 +565,9 @@ Manager* manager_free(Manager *m) {
         sd_event_source_unref(m->rtnl_event_source);
         sd_netlink_unref(m->rtnl);
         sd_event_unref(m->event);
+        sd_varlink_unref(m->varlink_client);
+
+        dns_configuration_free(m->dns_configuration);
 
         return mfree(m);
 }

src/network/wait-online/manager.h

@@ -4,7 +4,9 @@
 #include "sd-event.h"
 #include "sd-netlink.h"
 #include "sd-network.h"
+#include "sd-varlink.h"
 
+#include "dns-configuration.h"
 #include "hashmap.h"
 #include "network-util.h"
 #include "time-util.h"
@@ -23,6 +25,7 @@ struct Manager {
         LinkOperationalStateRange required_operstate;
         AddressFamily required_family;
         bool any;
+        bool requires_dns;
 
         sd_netlink *rtnl;
         sd_event_source *rtnl_event_source;
@@ -31,13 +34,16 @@ struct Manager {
         sd_event_source *network_monitor_event_source;
 
         sd_event *event;
+
+        sd_varlink *varlink_client;
+        DNSConfiguration *dns_configuration;
 };
 
 Manager* manager_free(Manager *m);
 int manager_new(Manager **ret, Hashmap *command_line_interfaces_by_name, char **ignored_interfaces,
                 LinkOperationalStateRange required_operstate,
                 AddressFamily required_family,
-                bool any, usec_t timeout);
+                bool any, usec_t timeout, bool requires_dns);
 
 DEFINE_TRIVIAL_CLEANUP_FUNC(Manager*, manager_free);
 

src/network/wait-online/wait-online.c

@@ -10,6 +10,7 @@
 #include "daemon-util.h"
 #include "main-func.h"
 #include "manager.h"
+#include "parse-argument.h"
 #include "pretty-print.h"
 #include "signal-util.h"
 #include "socket-util.h"
@@ -22,6 +23,7 @@ static char **arg_ignore = NULL;
 static LinkOperationalStateRange arg_required_operstate = LINK_OPERSTATE_RANGE_INVALID;
 static AddressFamily arg_required_family = ADDRESS_FAMILY_NO;
 static bool arg_any = false;
+static bool arg_requires_dns = false;
 
 STATIC_DESTRUCTOR_REGISTER(arg_interfaces, hashmap_free_free_freep);
 STATIC_DESTRUCTOR_REGISTER(arg_ignore, strv_freep);
@@ -48,6 +50,7 @@ static int help(void) {
                "  -6 --ipv6                 Requires at least one IPv6 address\n"
                "     --any                  Wait until at least one of the interfaces is online\n"
                "     --timeout=SECS         Maximum time to wait for network connectivity\n"
+               "     --dns                  Requires at least one DNS server to be accessible\n"
                "\nSee the %s for details.\n",
                program_invocation_short_name,
                link);
@@ -106,6 +109,7 @@ static int parse_argv(int argc, char *argv[]) {
                 ARG_IGNORE,
                 ARG_ANY,
                 ARG_TIMEOUT,
+                ARG_DNS,
         };
 
         static const struct option options[] = {
@@ -119,6 +123,7 @@ static int parse_argv(int argc, char *argv[]) {
                 { "ipv6",              no_argument,       NULL, '6'         },
                 { "any",               no_argument,       NULL, ARG_ANY     },
                 { "timeout",           required_argument, NULL, ARG_TIMEOUT },
+                { "dns",               optional_argument, NULL, ARG_DNS     },
                 {}
         };
 
@@ -178,6 +183,12 @@ static int parse_argv(int argc, char *argv[]) {
                                 return r;
                         break;
 
+                case ARG_DNS:
+                        r = parse_boolean_argument("--dns", optarg, &arg_requires_dns);
+                        if (r < 0)
+                                return r;
+                        break;
+
                 case '?':
                         return -EINVAL;
 
@@ -204,7 +215,14 @@ static int run(int argc, char *argv[]) {
         if (arg_quiet)
                 log_set_max_level(LOG_ERR);
 
-        r = manager_new(&m, arg_interfaces, arg_ignore, arg_required_operstate, arg_required_family, arg_any, arg_timeout);
+        r = manager_new(&m,
+                        arg_interfaces,
+                        arg_ignore,
+                        arg_required_operstate,
+                        arg_required_family,
+                        arg_any,
+                        arg_timeout,
+                        arg_requires_dns);
         if (r < 0)
                 return log_error_errno(r, "Could not create manager: %m");
 

src/resolve/org.freedesktop.resolve1.policy

@@ -150,6 +150,17 @@
                 <annotate key="org.freedesktop.policykit.owner">unix-user:systemd-resolve</annotate>
         </action>
 
+        <action id="org.freedesktop.resolve1.subscribe-dns-configuration">
+                <description gettext-domain="systemd">Subscribe to DNS configuration</description>
+                <message gettext-domain="systemd">Authentication is required to subscribe to DNS configuration.</message>
+                <defaults>
+                        <allow_any>auth_admin</allow_any>
+                        <allow_inactive>auth_admin</allow_inactive>
+                        <allow_active>auth_admin_keep</allow_active>
+                </defaults>
+                <annotate key="org.freedesktop.policykit.owner">unix-user:systemd-resolve</annotate>
+        </action>
+
         <action id="org.freedesktop.resolve1.dump-cache">
                 <description gettext-domain="systemd">Dump cache</description>
                 <message gettext-domain="systemd">Authentication is required to dump cache.</message>

src/resolve/resolved-dns-search-domain.c

@@ -1,5 +1,7 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 
+#include "sd-json.h"
+
 #include "alloc-util.h"
 #include "dns-domain.h"
 #include "resolved-dns-search-domain.h"
@@ -197,3 +199,21 @@ int dns_search_domain_find(DnsSearchDomain *first, const char *name, DnsSearchDo
         *ret = NULL;
         return 0;
 }
+
+int dns_search_domain_dump_to_json(DnsSearchDomain *domain, sd_json_variant **ret) {
+        int ifindex = 0;
+
+        assert(domain);
+        assert(ret);
+
+        if (domain->type == DNS_SEARCH_DOMAIN_LINK) {
+                assert(domain->link);
+                ifindex = domain->link->ifindex;
+        }
+
+        return sd_json_buildo(
+                        ret,
+                        SD_JSON_BUILD_PAIR_STRING("name", domain->name),
+                        SD_JSON_BUILD_PAIR_BOOLEAN("routeOnly", domain->route_only),
+                        SD_JSON_BUILD_PAIR_CONDITION(ifindex > 0, "ifindex", SD_JSON_BUILD_UNSIGNED(ifindex)));
+}

src/resolve/resolved-dns-search-domain.h

@@ -1,6 +1,8 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
+#include "sd-json.h"
+
 #include "list.h"
 #include "macro.h"
 
@@ -54,3 +56,5 @@ static inline const char* DNS_SEARCH_DOMAIN_NAME(DnsSearchDomain *d) {
 }
 
 DEFINE_TRIVIAL_CLEANUP_FUNC(DnsSearchDomain*, dns_search_domain_unref);
+
+int dns_search_domain_dump_to_json(DnsSearchDomain *domain, sd_json_variant **ret);

src/resolve/resolved-dns-server.c

@@ -1,14 +1,20 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 
+#include <net/if_arp.h>
+
 #include "sd-messages.h"
 
 #include "alloc-util.h"
+#include "errno-util.h"
+#include "fd-util.h"
+#include "json-util.h"
 #include "resolved-bus.h"
 #include "resolved-dns-server.h"
 #include "resolved-dns-stub.h"
 #include "resolved-manager.h"
 #include "resolved-resolv-conf.h"
 #include "siphash24.h"
+#include "socket-util.h"
 #include "string-table.h"
 #include "string-util.h"
 
@@ -69,6 +75,7 @@ int dns_server_new(
                 .ifindex = ifindex,
                 .server_name = TAKE_PTR(name),
                 .config_source = config_source,
+                .accessible = -1,
         };
 
         dns_server_reset_features(s);
@@ -875,6 +882,7 @@ DnsServer *manager_set_dns_server(Manager *m, DnsServer *s) {
                 dns_cache_flush(&m->unicast_scope->cache);
 
         (void) manager_send_changed(m, "CurrentDNSServer");
+        (void) manager_send_dns_configuration_changed(m);
 
         return s;
 }
@@ -1128,3 +1136,58 @@ int dns_server_dump_state_to_json(DnsServer *server, sd_json_variant **ret) {
                         SD_JSON_BUILD_PAIR_BOOLEAN("PacketInvalid", server->packet_invalid),
                         SD_JSON_BUILD_PAIR_BOOLEAN("PacketDoOff", server->packet_do_off));
 }
+
+int dns_server_is_accessible(DnsServer *s) {
+        _cleanup_close_ int fd = -EBADF;
+        union sockaddr_union sa;
+        int ifindex, r;
+
+        assert(s);
+
+        if (s->accessible >= 0)
+                return s->accessible;
+
+        r = sockaddr_set_in_addr(&sa, s->family, &s->address, dns_server_port(s));
+        if (r < 0)
+                return r;
+
+        fd = socket(s->family, SOCK_DGRAM|SOCK_CLOEXEC, 0);
+        if (fd < 0)
+                return -errno;
+
+        ifindex = dns_server_ifindex(s);
+        if (ifindex > 0) {
+                r = socket_bind_to_ifindex(fd, ifindex);
+                if (r < 0)
+                        return r;
+        }
+
+        r = RET_NERRNO(connect(fd, &sa.sa, SOCKADDR_LEN(sa)));
+
+        return (s->accessible = r >= 0);
+}
+
+int dns_server_dump_configuration_to_json(DnsServer *server, sd_json_variant **ret) {
+        bool accessible = false;
+        int ifindex, r;
+
+        assert(server);
+        assert(ret);
+
+        ifindex = dns_server_ifindex(server);
+
+        r = dns_server_is_accessible(server);
+        if (r < 0)
+                log_debug_errno(r, "Failed to check if %s is accessible, assume not: %m", dns_server_string_full(server));
+        else
+                accessible = r;
+
+        return sd_json_buildo(
+                        ret,
+                        JSON_BUILD_PAIR_IN_ADDR("address", &server->address, server->family),
+                        SD_JSON_BUILD_PAIR_INTEGER("family", server->family),
+                        SD_JSON_BUILD_PAIR_UNSIGNED("port", dns_server_port(server)),
+                        SD_JSON_BUILD_PAIR_CONDITION(ifindex > 0, "ifindex", SD_JSON_BUILD_UNSIGNED(ifindex)),
+                        JSON_BUILD_PAIR_STRING_NON_EMPTY("name", server->server_name),
+                        SD_JSON_BUILD_PAIR_BOOLEAN("accessible", accessible));
+}

src/resolve/resolved-dns-server.h

@@ -106,6 +106,9 @@ struct DnsServer {
 
         /* Servers registered via D-Bus are not removed on reload */
         ResolveConfigSource config_source;
+
+        /* Tri-state to indicate if the DNS server is accessible. */
+        int accessible;
 };
 
 int dns_server_new(
@@ -187,3 +190,9 @@ static inline bool dns_server_is_fallback(DnsServer *s) {
 }
 
 int dns_server_dump_state_to_json(DnsServer *server, sd_json_variant **ret);
+int dns_server_dump_configuration_to_json(DnsServer *server, sd_json_variant **ret);
+
+int dns_server_is_accessible(DnsServer *s);
+static inline void dns_server_reset_accessible(DnsServer *s) {
+        s->accessible = -1;
+}

src/resolve/resolved-link-bus.c

@@ -24,6 +24,7 @@ static BUS_DEFINE_PROPERTY_GET(property_get_dnssec_supported, "b", Link, link_dn
 static BUS_DEFINE_PROPERTY_GET2(property_get_dnssec_mode, "s", Link, link_get_dnssec_mode, dnssec_mode_to_string);
 static BUS_DEFINE_PROPERTY_GET2(property_get_llmnr_support, "s", Link, link_get_llmnr_support, resolve_support_to_string);
 static BUS_DEFINE_PROPERTY_GET2(property_get_mdns_support, "s", Link, link_get_mdns_support, resolve_support_to_string);
+static BUS_DEFINE_PROPERTY_GET(property_get_default_route, "b", Link, link_get_default_route);
 
 static int property_get_dns_over_tls_mode(
                 sd_bus *bus,
@@ -160,30 +161,6 @@ static int property_get_domains(
         return sd_bus_message_close_container(reply);
 }
 
-static int property_get_default_route(
-                sd_bus *bus,
-                const char *path,
-                const char *interface,
-                const char *property,
-                sd_bus_message *reply,
-                void *userdata,
-                sd_bus_error *error) {
-
-        Link *l = ASSERT_PTR(userdata);
-
-        assert(reply);
-
-        /* Return what is configured, if there's something configured */
-        if (l->default_route >= 0)
-                return sd_bus_message_append(reply, "b", l->default_route);
-
-        /* Otherwise report what is in effect */
-        if (l->unicast_scope)
-                return sd_bus_message_append(reply, "b", dns_scope_is_default_route(l->unicast_scope));
-
-        return sd_bus_message_append(reply, "b", false);
-}
-
 static int property_get_scopes_mask(
                 sd_bus *bus,
                 const char *path,
@@ -293,6 +270,7 @@ static int bus_link_method_set_dns_servers_internal(sd_bus_message *message, voi
                 (void) link_save_user(l);
                 (void) manager_write_resolv_conf(l->manager);
                 (void) manager_send_changed(l->manager, "DNS");
+                (void) manager_send_dns_configuration_changed(l->manager);
 
                 if (j)
                         log_link_info(l, "Bus client set DNS server list to: %s", j);
@@ -768,6 +746,7 @@ int bus_link_method_revert(sd_bus_message *message, void *userdata, sd_bus_error
         (void) link_save_user(l);
         (void) manager_write_resolv_conf(l->manager);
         (void) manager_send_changed(l->manager, "DNS");
+        (void) manager_send_dns_configuration_changed(l->manager);
 
         return sd_bus_reply_method_return(message, NULL);
 }

src/resolve/resolved-link.c

@@ -14,6 +14,7 @@
 #include "mkdir.h"
 #include "netif-util.h"
 #include "parse-util.h"
+#include "resolved-dns-scope.h"
 #include "resolved-link.h"
 #include "resolved-llmnr.h"
 #include "resolved-mdns.h"
@@ -299,6 +300,12 @@ static int link_update_dns_servers(Link *l) {
         }
 
         dns_server_unlink_marked(l->dns_servers);
+
+        LIST_FOREACH(servers, s, l->dns_servers)
+                /* If the link state changed, we should re-check if DNS servers
+                 * are accessible. */
+                dns_server_reset_accessible(s);
+
         return 0;
 
 clear:
@@ -831,6 +838,20 @@ ResolveSupport link_get_mdns_support(Link *link) {
         return MIN(link->mdns_support, link->manager->mdns_support);
 }
 
+bool link_get_default_route(Link *l) {
+        assert(l);
+
+        /* Return what is configured, if there's something configured */
+        if (l->default_route >= 0)
+                return l->default_route;
+
+        /* Otherwise report what is in effect */
+        if (l->unicast_scope)
+                return dns_scope_is_default_route(l->unicast_scope);
+
+        return false;
+}
+
 int link_address_new(Link *l,
                 LinkAddress **ret,
                 int family,

src/resolve/resolved-link.h

@@ -109,6 +109,8 @@ DnsOverTlsMode link_get_dns_over_tls_mode(Link *l);
 ResolveSupport link_get_llmnr_support(Link *link);
 ResolveSupport link_get_mdns_support(Link *link);
 
+bool link_get_default_route(Link *l);
+
 int link_save_user(Link *l);
 int link_load_user(Link *l);
 void link_remove_user(Link *l);

src/resolve/resolved-manager.c

@@ -22,6 +22,7 @@
 #include "idn-util.h"
 #include "io-util.h"
 #include "iovec-util.h"
+#include "json-util.h"
 #include "memstream-util.h"
 #include "missing_network.h"
 #include "missing_socket.h"
@@ -286,6 +287,7 @@ static int on_network_event(sd_event_source *s, int fd, uint32_t revents, void *
 
         (void) manager_write_resolv_conf(m);
         (void) manager_send_changed(m, "DNS");
+        (void) manager_send_dns_configuration_changed(m);
 
         return 0;
 }
@@ -1184,7 +1186,7 @@ int manager_monitor_send(Manager *m, DnsQuery *q) {
 
         assert(m);
 
-        if (set_isempty(m->varlink_subscription))
+        if (set_isempty(m->varlink_query_results_subscription))
                 return 0;
 
         /* Merge all questions into one */
@@ -1234,7 +1236,7 @@ int manager_monitor_send(Manager *m, DnsQuery *q) {
         }
 
         r = varlink_many_notifybo(
-                        m->varlink_subscription,
+                        m->varlink_query_results_subscription,
                         SD_JSON_BUILD_PAIR("state", SD_JSON_BUILD_STRING(dns_transaction_state_to_string(q->state))),
                         SD_JSON_BUILD_PAIR_CONDITION(q->state == DNS_TRANSACTION_DNSSEC_FAILED,
                                                      "result", SD_JSON_BUILD_STRING(dnssec_result_to_string(q->answer_dnssec_result))),
@@ -1932,3 +1934,124 @@ void dns_manager_reset_statistics(Manager *m) {
         m->n_failure_responses_served_stale_total = 0;
         zero(m->n_dnssec_verdict);
 }
+
+static int dns_configuration_json_append(
+                const char *ifname,
+                int ifindex,
+                int default_route,
+                DnsServer *current_dns_server,
+                DnsServer *dns_servers,
+                DnsSearchDomain *search_domains,
+                sd_json_variant **configuration) {
+
+        _cleanup_(sd_json_variant_unrefp) sd_json_variant *dns_servers_json = NULL,
+                                                          *search_domains_json = NULL,
+                                                          *current_dns_server_json = NULL;
+        int r;
+
+        assert(configuration);
+
+        r = sd_json_variant_new_array(&dns_servers_json, NULL, 0);
+        if (r < 0)
+                return r;
+
+        r = sd_json_variant_new_array(&search_domains_json, NULL, 0);
+        if (r < 0)
+                return r;
+
+        if (current_dns_server) {
+                r = dns_server_dump_configuration_to_json(current_dns_server, &current_dns_server_json);
+                if (r < 0)
+                        return r;
+        }
+
+        LIST_FOREACH(servers, s, dns_servers) {
+                _cleanup_(sd_json_variant_unrefp) sd_json_variant *v = NULL;
+
+                r = dns_server_dump_configuration_to_json(s, &v);
+                if (r < 0)
+                        return r;
+
+                r = sd_json_variant_append_array(&dns_servers_json, v);
+                if (r < 0)
+                        return r;
+        }
+
+        LIST_FOREACH(domains, d, search_domains) {
+                _cleanup_(sd_json_variant_unrefp) sd_json_variant *v = NULL;
+
+                r = dns_search_domain_dump_to_json(d, &v);
+                if (r < 0)
+                        return r;
+
+                r = sd_json_variant_append_array(&search_domains_json, v);
+                if (r < 0)
+                        return r;
+        }
+
+        return sd_json_variant_append_arraybo(
+                        configuration,
+                        JSON_BUILD_PAIR_STRING_NON_EMPTY("ifname", ifname),
+                        SD_JSON_BUILD_PAIR_CONDITION(ifindex > 0, "ifindex", SD_JSON_BUILD_UNSIGNED(ifindex)),
+                        SD_JSON_BUILD_PAIR_CONDITION(ifindex > 0, "defaultRoute", SD_JSON_BUILD_BOOLEAN(default_route > 0)),
+                        JSON_BUILD_PAIR_VARIANT_NON_NULL("currentServer", current_dns_server_json),
+                        SD_JSON_BUILD_PAIR_VARIANT("servers", dns_servers_json),
+                        SD_JSON_BUILD_PAIR_VARIANT("searchDomains", search_domains_json));
+}
+
+int manager_dump_dns_configuration_json(Manager *m, sd_json_variant **ret) {
+        _cleanup_(sd_json_variant_unrefp) sd_json_variant *configuration = NULL;
+        Link *l;
+        int r;
+
+        assert(m);
+        assert(ret);
+
+        /* Global DNS configuration */
+        r = dns_configuration_json_append(
+                        /* ifname = */ NULL,
+                        /* ifindex = */ 0,
+                        /* default_route = */ 0,
+                        manager_get_dns_server(m),
+                        m->dns_servers,
+                        m->search_domains,
+                        &configuration);
+        if (r < 0)
+                return r;
+
+        /* Append configuration for each link */
+        HASHMAP_FOREACH(l, m->links) {
+                r = dns_configuration_json_append(
+                                l->ifname,
+                                l->ifindex,
+                                link_get_default_route(l),
+                                link_get_dns_server(l),
+                                l->dns_servers,
+                                l->search_domains,
+                                &configuration);
+                if (r < 0)
+                        return r;
+        }
+
+        return sd_json_buildo(ret, SD_JSON_BUILD_PAIR_VARIANT("configuration", configuration));
+}
+
+int manager_send_dns_configuration_changed(Manager *m) {
+        _cleanup_(sd_json_variant_unrefp) sd_json_variant *configuration = NULL;
+        int r;
+
+        assert(m);
+
+        if (set_isempty(m->varlink_dns_configuration_subscription))
+                return 0;
+
+        r = manager_dump_dns_configuration_json(m, &configuration);
+        if (r < 0)
+                return log_warning_errno(r, "Failed to dump DNS configuration json: %m");
+
+        r = varlink_many_notify(m->varlink_dns_configuration_subscription, configuration);
+        if (r < 0)
+                return log_warning_errno(r, "Failed to send DNS configuration event: %m");
+
+        return 0;
+}

src/resolve/resolved-manager.h

@@ -152,7 +152,8 @@ struct Manager {
         sd_varlink_server *varlink_server;
         sd_varlink_server *varlink_monitor_server;
 
-        Set *varlink_subscription;
+        Set *varlink_query_results_subscription;
+        Set *varlink_dns_configuration_subscription;
 
         sd_event_source *clock_change_event_source;
 
@@ -225,3 +226,6 @@ int socket_disable_pmtud(int fd, int af);
 int dns_manager_dump_statistics_json(Manager *m, sd_json_variant **ret);
 
 void dns_manager_reset_statistics(Manager *m);
+
+int manager_dump_dns_configuration_json(Manager *m, sd_json_variant **ret);
+int manager_send_dns_configuration_changed(Manager *m);

src/resolve/resolved-varlink.c

@@ -125,14 +125,21 @@ static void vl_on_disconnect(sd_varlink_server *s, sd_varlink *link, void *userd
 
 static void vl_on_notification_disconnect(sd_varlink_server *s, sd_varlink *link, void *userdata) {
         Manager *m = ASSERT_PTR(userdata);
+        sd_varlink *removed_link = NULL;
 
         assert(s);
         assert(link);
 
-        sd_varlink *removed_link = set_remove(m->varlink_subscription, link);
+        removed_link = set_remove(m->varlink_query_results_subscription, link);
         if (removed_link) {
                 sd_varlink_unref(removed_link);
-                log_debug("%u monitor clients remain active", set_size(m->varlink_subscription));
+                log_debug("%u query result monitor clients remain active", set_size(m->varlink_query_results_subscription));
+        }
+
+        removed_link = set_remove(m->varlink_dns_configuration_subscription, link);
+        if (removed_link) {
+                sd_varlink_unref(removed_link);
+                log_debug("%u DNS monitor clients remain active", set_size(m->varlink_dns_configuration_subscription));
         }
 }
 
@@ -1227,12 +1234,12 @@ static int vl_method_subscribe_query_results(sd_varlink *link, sd_json_variant *
         if (r < 0)
                 return log_error_errno(r, "Failed to report monitor to be established: %m");
 
-        r = set_ensure_put(&m->varlink_subscription, NULL, link);
+        r = set_ensure_put(&m->varlink_query_results_subscription, NULL, link);
         if (r < 0)
                 return log_error_errno(r, "Failed to add subscription to set: %m");
         sd_varlink_ref(link);
 
-        log_debug("%u clients now attached for varlink notifications", set_size(m->varlink_subscription));
+        log_debug("%u clients now attached for query result varlink notifications", set_size(m->varlink_query_results_subscription));
 
         return 1;
 }
@@ -1352,6 +1359,38 @@ static int vl_method_reset_statistics(sd_varlink *link, sd_json_variant *paramet
         return sd_varlink_replyb(link, SD_JSON_BUILD_EMPTY_OBJECT);
 }
 
+static int vl_method_subscribe_dns_configuration(sd_varlink *link, sd_json_variant *parameters, sd_varlink_method_flags_t flags, void *userdata) {
+        _cleanup_(sd_json_variant_unrefp) sd_json_variant *configuration = NULL;
+        Manager *m = ASSERT_PTR(sd_varlink_get_userdata(ASSERT_PTR(link)));
+        int r;
+
+        /* if the client didn't set the more flag, it is using us incorrectly */
+        if (!FLAGS_SET(flags, SD_VARLINK_METHOD_MORE))
+                return sd_varlink_error(link, SD_VARLINK_ERROR_EXPECTED_MORE, NULL);
+
+        r = verify_polkit(link, parameters, "org.freedesktop.resolve1.subscribe-dns-configuration");
+        if (r <= 0)
+                return r;
+
+        r = manager_dump_dns_configuration_json(m, &configuration);
+        if (r < 0)
+                return log_error_errno(r, "Failed to dump current DNS configuration: %m");
+
+        r = sd_varlink_notify(link, configuration);
+        if (r < 0)
+                return log_error_errno(r, "Failed to send current DNS configuration: %m");
+
+        r = set_ensure_put(&m->varlink_dns_configuration_subscription, NULL, link);
+        if (r < 0)
+                return log_error_errno(r, "Failed to add subscription to set: %m");
+        sd_varlink_ref(link);
+
+        log_debug("%u clients now attached for link configuration varlink notifications",
+                  set_size(m->varlink_dns_configuration_subscription));
+
+        return 1;
+}
+
 static int varlink_monitor_server_init(Manager *m) {
         _cleanup_(sd_varlink_server_unrefp) sd_varlink_server *server = NULL;
         int r;
@@ -1377,7 +1416,8 @@ static int varlink_monitor_server_init(Manager *m) {
                         "io.systemd.Resolve.Monitor.DumpCache", vl_method_dump_cache,
                         "io.systemd.Resolve.Monitor.DumpServerState", vl_method_dump_server_state,
                         "io.systemd.Resolve.Monitor.DumpStatistics", vl_method_dump_statistics,
-                        "io.systemd.Resolve.Monitor.ResetStatistics", vl_method_reset_statistics);
+                        "io.systemd.Resolve.Monitor.ResetStatistics", vl_method_reset_statistics,
+                        "io.systemd.Resolve.Monitor.SubscribeDNSConfiguration", vl_method_subscribe_dns_configuration);
         if (r < 0)
                 return log_error_errno(r, "Failed to register varlink methods: %m");
 

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

@@ -112,6 +112,52 @@ static SD_VARLINK_DEFINE_METHOD(
                 ResetStatistics,
                 VARLINK_DEFINE_POLKIT_INPUT);
 
+static SD_VARLINK_DEFINE_STRUCT_TYPE(
+                DNSServer,
+                SD_VARLINK_FIELD_COMMENT("IPv4 or IPv6 address of the server."),
+                SD_VARLINK_DEFINE_FIELD(address, SD_VARLINK_INT, SD_VARLINK_ARRAY),
+                SD_VARLINK_FIELD_COMMENT("Address family of the server, one of AF_INET or AF_INET6."),
+                SD_VARLINK_DEFINE_FIELD(family, SD_VARLINK_INT, 0),
+                SD_VARLINK_FIELD_COMMENT("Port number of the server."),
+                SD_VARLINK_DEFINE_FIELD(port, SD_VARLINK_INT, 0),
+                SD_VARLINK_FIELD_COMMENT("Interface index for which this server is configured."),
+                SD_VARLINK_DEFINE_FIELD(ifindex, SD_VARLINK_INT, SD_VARLINK_NULLABLE),
+                SD_VARLINK_FIELD_COMMENT("Server Name Indication (SNI) of the server."),
+                SD_VARLINK_DEFINE_FIELD(name, SD_VARLINK_STRING, SD_VARLINK_NULLABLE),
+                SD_VARLINK_FIELD_COMMENT("Indicates if the DNS server is accessible or not."),
+                SD_VARLINK_DEFINE_FIELD(accessible, SD_VARLINK_BOOL, 0));
+
+static SD_VARLINK_DEFINE_STRUCT_TYPE(
+                SearchDomain,
+                SD_VARLINK_FIELD_COMMENT("Domain name."),
+                SD_VARLINK_DEFINE_FIELD(name, SD_VARLINK_STRING, 0),
+                SD_VARLINK_FIELD_COMMENT("Indicates whether or not this is a routing-only domain."),
+                SD_VARLINK_DEFINE_FIELD(routeOnly, SD_VARLINK_BOOL, 0),
+                SD_VARLINK_FIELD_COMMENT("Interface index for which this search domain is configured."),
+                SD_VARLINK_DEFINE_FIELD(ifindex, SD_VARLINK_INT, SD_VARLINK_NULLABLE));
+
+static SD_VARLINK_DEFINE_STRUCT_TYPE(
+                DNSConfiguration,
+                SD_VARLINK_FIELD_COMMENT("Interface name, if any, associated with this configuration. Empty for global configuration."),
+                SD_VARLINK_DEFINE_FIELD(ifname, SD_VARLINK_STRING, SD_VARLINK_NULLABLE),
+                SD_VARLINK_FIELD_COMMENT("Interface index, if any, associated with this configuration. Empty for global configuration."),
+                SD_VARLINK_DEFINE_FIELD(ifindex, SD_VARLINK_INT, SD_VARLINK_NULLABLE),
+                SD_VARLINK_FIELD_COMMENT("Indicates whether or not this link's DNS servers will be used for resolving domain names that do not match any link's configured domains."),
+                SD_VARLINK_DEFINE_FIELD(defaultRoute, SD_VARLINK_BOOL, SD_VARLINK_NULLABLE),
+                SD_VARLINK_FIELD_COMMENT("DNS server currently selected to use for lookups."),
+                SD_VARLINK_DEFINE_FIELD_BY_TYPE(currentServer, DNSServer, SD_VARLINK_NULLABLE),
+                SD_VARLINK_FIELD_COMMENT("Array of configured DNS servers."),
+                SD_VARLINK_DEFINE_FIELD_BY_TYPE(servers, DNSServer, SD_VARLINK_ARRAY|SD_VARLINK_NULLABLE),
+                SD_VARLINK_FIELD_COMMENT("Array of configured search domains."),
+                SD_VARLINK_DEFINE_FIELD_BY_TYPE(searchDomains, SearchDomain, SD_VARLINK_ARRAY|SD_VARLINK_NULLABLE));
+
+static SD_VARLINK_DEFINE_METHOD_FULL(
+                SubscribeDNSConfiguration,
+                SD_VARLINK_REQUIRES_MORE,
+                SD_VARLINK_DEFINE_INPUT(allowInteractiveAuthentication, SD_VARLINK_BOOL, SD_VARLINK_NULLABLE),
+                SD_VARLINK_FIELD_COMMENT("The current global and per-interface DNS configurations"),
+                SD_VARLINK_DEFINE_OUTPUT_BY_TYPE(configuration, DNSConfiguration, SD_VARLINK_ARRAY));
+
 SD_VARLINK_DEFINE_INTERFACE(
                 io_systemd_Resolve_Monitor,
                 "io.systemd.Resolve.Monitor",
@@ -129,4 +175,12 @@ SD_VARLINK_DEFINE_INTERFACE(
                 &vl_type_TransactionStatistics,
                 &vl_type_CacheStatistics,
                 &vl_type_DnssecStatistics,
-                &vl_type_ServerState);
+                &vl_type_ServerState,
+                SD_VARLINK_SYMBOL_COMMENT("Encapsulates a DNS server address specification."),
+                &vl_type_DNSServer,
+                SD_VARLINK_SYMBOL_COMMENT("Encapsulates a search domain specification."),
+                &vl_type_SearchDomain,
+                SD_VARLINK_SYMBOL_COMMENT("Encapsulates a global or per-link DNS configuration, including configured DNS servers, search domains, and more."),
+                &vl_type_DNSConfiguration,
+                SD_VARLINK_SYMBOL_COMMENT("Sends the complete global and per-link DNS configurations when any changes are made to them. The current configurations are given immediately when this method is invoked."),
+                &vl_method_SubscribeDNSConfiguration);

test/networkd-test.py

@@ -1086,6 +1086,139 @@ DNS=127.0.0.1
             self.show_journal('systemd-timedated.service')
             self.fail(f'Timezone: {tz}, expected: Pacific/Honolulu')
 
+    def do_test_wait_online_dns(
+        self,
+        iface,
+        global_dns='',
+        fallback_dns='',
+        expect_timeout=False
+    ):
+        if expect_timeout:
+            timeout = 5
+        else:
+            timeout = 10
+
+        # Unless given, clear global DNS configuration
+        conf = '/run/systemd/resolved.conf.d/global-dns.conf'
+        os.makedirs(os.path.dirname(conf), exist_ok=True)
+        with open(conf, 'w') as f:
+            f.write((
+                '[Resolve]\n'
+                f'DNS={global_dns}\n'
+                f'FallbackDNS={fallback_dns}\n'
+            ))
+        self.addCleanup(os.remove, conf)
+        subprocess.check_call(
+            ['systemctl', 'reload', 'systemd-resolved']
+        )
+
+        self.start_unit('systemd-networkd')
+
+        env = os.environ.copy()
+        env['SYSTEMD_LOG_LEVEL'] = 'debug'
+        env['SYSTEMD_LOG_TARGET'] = 'console'
+        r = subprocess.run(
+            [NETWORKD_WAIT_ONLINE, '--dns', '--interface', iface, f'--timeout={timeout}'],
+            stdout=subprocess.PIPE,
+            stderr=subprocess.PIPE,
+            env=env,
+        )
+
+        if expect_timeout:
+            self.assertNotEqual(
+                r.returncode,
+                0,
+                f'Command succeeded unexpectedly:\n{r.stderr.decode()}'
+            )
+            self.assertRegex(
+                r.stderr.decode(),
+                rf'{iface}: No link-specific DNS server is accessible',
+                f'Missing expected log message:\n{r.stderr.decode()}'
+            )
+        else:
+            self.assertEqual(
+                r.returncode,
+                0,
+                f'Command timed out:\n{r.stderr.decode()}'
+            )
+
+    def test_wait_online_dns(self):
+        ''' test systemd-networkd-wait-online with --dns '''
+        self.write_network(
+            self.config,
+            (
+                '[Match]\n'
+                f'Name={self.iface}\n'
+                '[Network]\n'
+                'DHCP=ipv4\n'
+            )
+        )
+        self.create_iface()
+        self.do_test_wait_online_dns(self.iface)
+
+    def test_wait_online_dns_global(self):
+        '''
+        test systemd-networkd-wait-online with --dns, expect pass due to global DNS
+        '''
+
+        # Explicitly set DNSDefaultRoute=no, and allow global DNS to be used.
+        self.write_network(
+            self.config,
+            (
+                '[Match]\n'
+                f'Name={self.iface}\n'
+                '[Network]\n'
+                'DHCP=ipv4\n'
+                'DNSDefaultRoute=no\n'
+                '[DHCPv4]\n'
+                'UseDNS=no\n'
+            )
+        )
+        self.create_iface()
+        self.do_test_wait_online_dns(self.iface, global_dns='192.168.5.1')
+
+    def test_wait_online_dns_expect_timeout(self):
+        ''' test systemd-networkd-wait-online with --dns, and expect timeout '''
+
+        # Explicitly set DNSDefaultRoute=yes, and require link-specific DNS to be used.
+        self.write_network(
+            self.config,
+            (
+                '[Match]\n'
+                f'Name={self.iface}\n'
+                '[Network]\n'
+                'DHCP=ipv4\n'
+                'DNSDefaultRoute=yes\n'
+                '[DHCPv4]\n'
+                'UseDNS=no\n'
+            )
+        )
+        self.create_iface()
+        self.do_test_wait_online_dns(self.iface, expect_timeout=True)
+
+    def test_wait_online_dns_expect_timeout_global(self):
+        '''
+        test systemd-networkd-wait-online with --dns, and expect timeout
+        despite global DNS
+        '''
+
+        # Configure Domains=~., and expect timeout despite global DNS servers
+        # being available.
+        self.write_network(
+            self.config,
+            (
+                '[Match]\n'
+                f'Name={self.iface}\n'
+                '[Network]\n'
+                'DHCP=ipv4\n'
+                'Domains=~.\n'
+                '[DHCPv4]\n'
+                'UseDNS=no\n'
+            )
+        )
+        self.create_iface()
+        self.do_test_wait_online_dns(self.iface, expect_timeout=True, global_dns='192.168.5.1')
+
 
 class MatchClientTest(unittest.TestCase, NetworkdTestingUtilities):
     """Test [Match] sections in .network files.

test/units/TEST-75-RESOLVED.sh

@@ -934,6 +934,15 @@ testcase_11_nft() {
 
 # Test resolvectl show-server-state
 testcase_12_resolvectl2() {
+    # Cleanup
+    # shellcheck disable=SC2317
+    cleanup() {
+        rm -f /run/systemd/resolved.conf.d/reload.conf
+        systemctl reload systemd-resolved.service
+    }
+
+    trap cleanup RETURN
+
     run resolvectl show-server-state
     grep -qF "10.0.0.1" "$RUN_OUT"
     grep -qF "Interface" "$RUN_OUT"
@@ -996,6 +1005,78 @@ testcase_12_resolvectl2() {
     restart_resolved
 }
 
+# Test io.systemd.Resolve.Monitor.SubscribeDNSConfiguration
+testcase_13_varlink_subscribe_dns_configuration() {
+    # Cleanup
+    # shellcheck disable=SC2317
+    cleanup() {
+        echo "===== io.systemd.Resolve.Monitor.SubscribeDNSConfiguration output: ====="
+        cat "$tmpfile"
+        echo "=========="
+        rm -f /run/systemd/resolved.conf.d/global-dns.conf
+        restart_resolved
+    }
+
+    trap cleanup RETURN
+
+    local unit
+    local tmpfile
+
+    unit="subscribe-dns-configuration-$(systemd-id128 new -u).service"
+    tmpfile=$(mktemp)
+
+    # Clear global and per-interface DNS before monitoring the configuration change.
+    mkdir -p /run/systemd/resolved.conf.d/
+    {
+        echo "[Resolve]"
+        echo "DNS="
+    } > /run/systemd/resolved.conf.d/global-dns.conf
+    systemctl reload systemd-resolved.service
+    resolvectl dns dns0 ""
+
+    # Start the call to io.systemd.Resolve.Monitor.SubscribeDNSConfiguration
+    systemd-run -u "$unit" -p "Type=exec" -p "StandardOutput=truncate:$tmpfile" \
+        varlinkctl call --more --timeout=5 --graceful=io.systemd.TimedOut /run/systemd/resolve/io.systemd.Resolve.Monitor io.systemd.Resolve.Monitor.SubscribeDNSConfiguration '{}'
+
+    # Update the global configuration.
+    mkdir -p /run/systemd/resolved.conf.d/
+    {
+        echo "[Resolve]"
+        echo "DNS=8.8.8.8"
+        echo "Domains=lan"
+    } > /run/systemd/resolved.conf.d/global-dns.conf
+    systemctl reload systemd-resolved.service
+
+    # Update a link configuration.
+    resolvectl dns dns0 8.8.4.4 1.1.1.1
+
+    # Wait for the monitor to exit gracefully.
+    while systemctl --quiet is-active "$unit"; do
+        sleep 0.5
+    done
+
+    # Hack to remove the "Method call returned expected error" line from the output.
+    sed -i '/^Method call.*returned expected error/d' "$tmpfile"
+
+    # Check that an initial reply was given with the settings applied BEFORE the monitor started.
+    grep -qF \
+        '{"global":{"servers":[],"domains":[]}}' \
+        <(jq -cr --seq  '.configuration[] | select(.ifname == null) | {"global": {servers: .servers, domains: .searchDomains}}' "$tmpfile")
+    grep -qF \
+        '{"dns0":{"servers":[],"domains":[]}}' \
+        <(jq -cr --seq  '.configuration[] | select(.ifname == "dns0") | {"dns0": {servers: .servers, domains: .searchDomains}}' "$tmpfile")
+
+    # Check that the global configuration change was reflected.
+    grep -qF \
+        '{"global":{"servers":[[8,8,8,8]],"domains":["lan"]}}' \
+        <(jq -cr --seq  '.configuration[] | select(.ifname == null) | {"global":{servers: [.servers[] | .address], domains: [.searchDomains[] | .name]}}' "$tmpfile")
+
+    # Check that the link configuration change was reflected.
+    grep -qF \
+        '{"dns0":{"servers":[[8,8,4,4],[1,1,1,1]],"domains":[]}}' \
+        <(jq -cr --seq  '.configuration[] | select(.ifname == "dns0") | {"dns0":{servers: [.servers[] | .address], domains: [.searchDomains[] | .name]}}' "$tmpfile")
+}
+
 # PRE-SETUP
 systemctl unmask systemd-resolved.service
 systemctl enable --now systemd-resolved.service

units/systemd-networkd-wait-online.service.in

@@ -14,7 +14,7 @@ ConditionCapability=CAP_NET_ADMIN
 DefaultDependencies=no
 Conflicts=shutdown.target
 BindsTo=systemd-networkd.service
-After=systemd-networkd.service
+After=systemd-networkd.service systemd-resolved.service
 Before=network-online.target shutdown.target
 
 [Service]