ci: switch back to meson-0.56.2

It seems there is another meson (0.57.0) regression preventing clang from
building systemd with --optimization=3 -Db_lto=true

By analogy with https://github.com/google/oss-fuzz/pull/5199 let's just switch
to 0.56.2 for the time being

.github/workflows/build_test.sh

@@ -95,7 +95,7 @@ apt-get -y install "${PACKAGES[@]}"
 # support all the features we need (like --optimization=). Since the build-dep
 # command above installs the distro versions, let's install the pip ones just
 # locally and add the local bin directory to the $PATH.
-pip3 install --user -U meson ninja
+pip3 install --user -U meson==0.56.2 ninja
 export PATH="$HOME/.local/bin:$PATH"
 
 $CC --version

man/resolvectl.xml

@@ -262,6 +262,65 @@
         returned.</para></listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><option>--validate=</option><replaceable>BOOL</replaceable></term>
+
+        <listitem><para>Takes a boolean parameter; used in conjunction with <command>query</command>. If true
+        (the default), DNSSEC validation is applied as usual — under the condition that it is enabled for the
+        network and for <filename>systemd-resolved.service</filename> as a whole. If false, DNSSEC validation
+        is disabled for the specific query, regardless of whether it is enabled for the network or in the
+        service. Note that setting this option to true does not force DNSSEC validation on systems/networks
+        where DNSSEC is turned off. This option is only suitable to turn off such validation where otherwise
+        enabled, not enable validation where otherwise disabled.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><option>--synthesize=</option><replaceable>BOOL</replaceable></term>
+
+        <listitem><para>Takes a boolean parameter; used in conjunction with <command>query</command>. If true
+        (the default), select domains are resolved on the local system, among them
+        <literal>localhost</literal> and <literal>_gateway</literal> or entries from
+        <filename>/etc/hosts</filename>. If false these domains are not resolved locally, and either fail (in
+        case of <literal>localhost</literal> or <literal>_gateway</literal> and suchlike) or go to the
+        network via regular DNS/mDNS/LLMNR lookups (in case of <filename>/etc/hosts</filename>
+        entries).</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><option>--cache=</option><replaceable>BOOL</replaceable></term>
+
+        <listitem><para>Takes a boolean parameter; used in conjunction with <command>query</command>. If true
+        (the default), lookups use the local DNS resource record cache. If false, lookups are routed to the
+        network instead, regardless if already available in the local cache.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><option>--zone=</option><replaceable>BOOL</replaceable></term>
+
+        <listitem><para>Takes a boolean parameter; used in conjunction with <command>query</command>. If true
+        (the default), lookups are answered from locally registered LLMNR or mDNS resource records, if
+        defined. If false, locally registered LLMNR/mDNS records are not considered for the lookup
+        request.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><option>--trust-anchor=</option><replaceable>BOOL</replaceable></term>
+
+        <listitem><para>Takes a boolean parameter; used in conjunction with <command>query</command>. If true
+        (the default), lookups for DS and DNSKEY are answered from the local DNSSEC trust anchors if
+        possible. If false, the local trust store is not considered for the lookup request.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><option>--network=</option><replaceable>BOOL</replaceable></term>
+
+        <listitem><para>Takes a boolean parameter; used in conjunction with <command>query</command>. If true
+        (the default), lookups are answered via DNS, LLMNR or mDNS network requests if they cannot be
+        synthesized locally, or be answered from the local cache, zone or trust anchors (see above). If false,
+        the request is not answered from the network and will thus fail if none of the indicated sources can
+        answer them.</para></listitem>
+      </varlistentry>
+
       <varlistentry>
         <term><option>--search=</option><replaceable>BOOL</replaceable></term>
 

src/libsystemd/sd-bus/bus-common-errors.c

@@ -76,6 +76,7 @@ BUS_ERROR_MAP_ELF_REGISTER const sd_bus_error_map bus_common_errors[] = {
         SD_BUS_ERROR_MAP(BUS_ERROR_LINK_BUSY,                    EBUSY),
         SD_BUS_ERROR_MAP(BUS_ERROR_NETWORK_DOWN,                 ENETDOWN),
         SD_BUS_ERROR_MAP(BUS_ERROR_NO_SOURCE,                    ESRCH),
+        SD_BUS_ERROR_MAP(BUS_ERROR_STUB_LOOP,                    ELOOP),
         SD_BUS_ERROR_MAP(BUS_ERROR_NO_SUCH_DNSSD_SERVICE,        ENOENT),
         SD_BUS_ERROR_MAP(BUS_ERROR_DNSSD_SERVICE_EXISTS,         EEXIST),
 

src/libsystemd/sd-bus/bus-common-errors.h

@@ -74,6 +74,7 @@
 #define BUS_ERROR_LINK_BUSY                    "org.freedesktop.resolve1.LinkBusy"
 #define BUS_ERROR_NETWORK_DOWN                 "org.freedesktop.resolve1.NetworkDown"
 #define BUS_ERROR_NO_SOURCE                    "org.freedesktop.resolve1.NoSource"
+#define BUS_ERROR_STUB_LOOP                    "org.freedesktop.resolve1.StubLoop"
 #define BUS_ERROR_NO_SUCH_DNSSD_SERVICE        "org.freedesktop.resolve1.NoSuchDnssdService"
 #define BUS_ERROR_DNSSD_SERVICE_EXISTS         "org.freedesktop.resolve1.DnssdServiceExists"
 #define _BUS_ERROR_DNS                         "org.freedesktop.resolve1.DnsError."

src/nss-resolve/nss-resolve.c

@@ -8,6 +8,7 @@
 #include <sys/types.h>
 #include <unistd.h>
 
+#include "env-util.h"
 #include "errno-util.h"
 #include "in-addr-util.h"
 #include "macro.h"
@@ -184,6 +185,21 @@ static const JsonDispatch address_parameters_dispatch_table[] = {
         {}
 };
 
+static uint64_t query_flags(void) {
+        uint64_t f = 0;
+        int r;
+
+        /* Allow callers to turn off validation, when we resolve via nss-resolve */
+
+        r = getenv_bool_secure("SYSTEMD_NSS_RESOLVE_VALIDATE");
+        if (r < 0 && r != -ENXIO)
+                log_debug_errno(r, "Failed to parse $SYSTEMD_NSS_RESOLVE_VALIDATE value, ignoring.");
+        else if (r == 0)
+                f |= SD_RESOLVED_NO_VALIDATE;
+
+        return f;
+}
+
 enum nss_status _nss_resolve_gethostbyname4_r(
                 const char *name,
                 struct gaih_addrtuple **pat,
@@ -215,7 +231,8 @@ enum nss_status _nss_resolve_gethostbyname4_r(
                 goto fail;
 
         r = json_build(&cparams, JSON_BUILD_OBJECT(
-                                       JSON_BUILD_PAIR("name", JSON_BUILD_STRING(name))));
+                                       JSON_BUILD_PAIR("name", JSON_BUILD_STRING(name)),
+                                       JSON_BUILD_PAIR("flags", JSON_BUILD_UNSIGNED(query_flags()))));
         if (r < 0)
                 goto fail;
 
@@ -367,7 +384,8 @@ enum nss_status _nss_resolve_gethostbyname3_r(
                 goto fail;
 
         r = json_build(&cparams, JSON_BUILD_OBJECT(JSON_BUILD_PAIR("name", JSON_BUILD_STRING(name)),
-                                                   JSON_BUILD_PAIR("family", JSON_BUILD_INTEGER(af))));
+                                                   JSON_BUILD_PAIR("family", JSON_BUILD_INTEGER(af)),
+                                                   JSON_BUILD_PAIR("flags", JSON_BUILD_UNSIGNED(query_flags()))));
         if (r < 0)
                 goto fail;
 
@@ -571,7 +589,8 @@ enum nss_status _nss_resolve_gethostbyaddr2_r(
                 goto fail;
 
         r = json_build(&cparams, JSON_BUILD_OBJECT(JSON_BUILD_PAIR("address", JSON_BUILD_BYTE_ARRAY(addr, len)),
-                                                   JSON_BUILD_PAIR("family", JSON_BUILD_INTEGER(af))));
+                                                   JSON_BUILD_PAIR("family", JSON_BUILD_INTEGER(af)),
+                                                   JSON_BUILD_PAIR("flags", JSON_BUILD_UNSIGNED(query_flags()))));
         if (r < 0)
                 goto fail;
 

src/resolve/resolvectl.c

@@ -2620,8 +2620,14 @@ static int native_help(void) {
                "     --service-address=BOOL    Resolve address for services (default: yes)\n"
                "     --service-txt=BOOL        Resolve TXT records for services (default: yes)\n"
                "     --cname=BOOL              Follow CNAME redirects (default: yes)\n"
-               "     --search=BOOL             Use search domains for single-label names\n"
-               "                                                              (default: yes)\n"
+               "     --validate=BOOL           Allow DNSSEC validation (default: yes)\n"
+               "     --synthesize=BOOL         Allow synthetic response (default: yes)\n"
+               "     --cache=BOOL              Allow response from cache (default: yes)\n"
+               "     --zone=BOOL               Allow response from locally registered mDNS/LLMNR\n"
+               "                               records (default: yes)\n"
+               "     --trust-anchor=BOOL       Allow response from local trust anchor (default: yes)\n"
+               "     --network=BOOL            Allow response from network (default: yes)\n"
+               "     --search=BOOL             Use search domains for single-label names (default: yes)\n"
                "     --raw[=payload|packet]    Dump the answer as binary data\n"
                "     --legend=BOOL             Print headers and additional info (default: yes)\n"
                "\nSee the %s for details.\n",
@@ -2961,6 +2967,12 @@ static int native_parse_argv(int argc, char *argv[]) {
                 ARG_VERSION = 0x100,
                 ARG_LEGEND,
                 ARG_CNAME,
+                ARG_VALIDATE,
+                ARG_SYNTHESIZE,
+                ARG_CACHE,
+                ARG_ZONE,
+                ARG_TRUST_ANCHOR,
+                ARG_NETWORK,
                 ARG_SERVICE_ADDRESS,
                 ARG_SERVICE_TXT,
                 ARG_RAW,
@@ -2977,6 +2989,12 @@ static int native_parse_argv(int argc, char *argv[]) {
                 { "interface",             required_argument, NULL, 'i'                       },
                 { "protocol",              required_argument, NULL, 'p'                       },
                 { "cname",                 required_argument, NULL, ARG_CNAME                 },
+                { "validate",              required_argument, NULL, ARG_VALIDATE              },
+                { "synthesize",            required_argument, NULL, ARG_SYNTHESIZE            },
+                { "cache",                 required_argument, NULL, ARG_CACHE                 },
+                { "zone",                  required_argument, NULL, ARG_ZONE                  },
+                { "trust-anchor",          required_argument, NULL, ARG_TRUST_ANCHOR          },
+                { "network",               required_argument, NULL, ARG_NETWORK               },
                 { "service-address",       required_argument, NULL, ARG_SERVICE_ADDRESS       },
                 { "service-txt",           required_argument, NULL, ARG_SERVICE_TXT           },
                 { "raw",                   optional_argument, NULL, ARG_RAW                   },
@@ -3100,6 +3118,48 @@ static int native_parse_argv(int argc, char *argv[]) {
                         SET_FLAG(arg_flags, SD_RESOLVED_NO_CNAME, r == 0);
                         break;
 
+                case ARG_VALIDATE:
+                        r = parse_boolean(optarg);
+                        if (r < 0)
+                                return log_error_errno(r, "Failed to parse --validate= argument.");
+                        SET_FLAG(arg_flags, SD_RESOLVED_NO_VALIDATE, r == 0);
+                        break;
+
+                case ARG_SYNTHESIZE:
+                        r = parse_boolean(optarg);
+                        if (r < 0)
+                                return log_error_errno(r, "Failed to parse --synthesize= argument.");
+                        SET_FLAG(arg_flags, SD_RESOLVED_NO_SYNTHESIZE, r == 0);
+                        break;
+
+                case ARG_CACHE:
+                        r = parse_boolean(optarg);
+                        if (r < 0)
+                                return log_error_errno(r, "Failed to parse --cache= argument.");
+                        SET_FLAG(arg_flags, SD_RESOLVED_NO_CACHE, r == 0);
+                        break;
+
+                case ARG_ZONE:
+                        r = parse_boolean(optarg);
+                        if (r < 0)
+                                return log_error_errno(r, "Failed to parse --zone= argument.");
+                        SET_FLAG(arg_flags, SD_RESOLVED_NO_ZONE, r == 0);
+                        break;
+
+                case ARG_TRUST_ANCHOR:
+                        r = parse_boolean(optarg);
+                        if (r < 0)
+                                return log_error_errno(r, "Failed to parse --trust-anchor= argument.");
+                        SET_FLAG(arg_flags, SD_RESOLVED_NO_TRUST_ANCHOR, r == 0);
+                        break;
+
+                case ARG_NETWORK:
+                        r = parse_boolean(optarg);
+                        if (r < 0)
+                                return log_error_errno(r, "Failed to parse --network= argument.");
+                        SET_FLAG(arg_flags, SD_RESOLVED_NO_NETWORK, r == 0);
+                        break;
+
                 case ARG_SERVICE_ADDRESS:
                         r = parse_boolean(optarg);
                         if (r < 0)

src/resolve/resolved-bus.c

@@ -104,6 +104,9 @@ static int reply_query_state(DnsQuery *q) {
         case DNS_TRANSACTION_NO_SOURCE:
                 return sd_bus_reply_method_errorf(q->bus_request, BUS_ERROR_NO_SOURCE, "All suitable resolution sources turned off");
 
+        case DNS_TRANSACTION_STUB_LOOP:
+                return sd_bus_reply_method_errorf(q->bus_request, BUS_ERROR_STUB_LOOP, "Configured DNS server loops back to us");
+
         case DNS_TRANSACTION_RCODE_FAILURE: {
                 _cleanup_(sd_bus_error_free) sd_bus_error error = SD_BUS_ERROR_NULL;
 

src/resolve/resolved-dns-answer.c

@@ -740,16 +740,21 @@ int dns_answer_reserve(DnsAnswer **a, size_t n_free) {
                 if ((*a)->n_ref > 1)
                         return -EBUSY;
 
-                ns = (*a)->n_rrs + n_free;
-                if (ns > UINT16_MAX) /* Maximum number of RRs we can stick into a DNS packet section */
+                ns = (*a)->n_rrs;
+                assert(ns <= UINT16_MAX); /* Maximum number of RRs we can stick into a DNS packet section */
+
+                if (n_free > UINT16_MAX - ns) /* overflow check */
                         ns = UINT16_MAX;
+                else
+                        ns += n_free;
 
                 if ((*a)->n_allocated >= ns)
                         return 0;
 
-                /* Allocate more than we need */
-                ns *= 2;
-                if (ns > UINT16_MAX)
+                /* Allocate more than we need, but not more than UINT16_MAX */
+                if (ns <= UINT16_MAX/2)
+                        ns *= 2;
+                else
                         ns = UINT16_MAX;
 
                 /* This must be done before realloc() below. Otherwise, the original DnsAnswer object
@@ -780,33 +785,50 @@ int dns_answer_reserve(DnsAnswer **a, size_t n_free) {
 }
 
 int dns_answer_reserve_or_clone(DnsAnswer **a, size_t n_free) {
-        _cleanup_(dns_answer_unrefp) DnsAnswer *n = NULL;
         int r;
 
         assert(a);
 
-        /* Tries to extend the DnsAnswer object. And if that's not
-         * possible, since we are not the sole owner, then allocate a
-         * new, appropriately sized one. Either way, after this call
-         * the object will only have a single reference, and has room
-         * for at least the specified number of RRs. */
+        /* Tries to extend the DnsAnswer object. And if that's not possible, since we are not the sole owner,
+         * then allocate a new, appropriately sized one. Either way, after this call the object will only
+         * have a single reference, and has room for at least the specified number of RRs. */
 
-        r = dns_answer_reserve(a, n_free);
-        if (r != -EBUSY)
-                return r;
+        if (*a && (*a)->n_ref > 1) {
+                _cleanup_(dns_answer_unrefp) DnsAnswer *n = NULL;
+                size_t ns;
 
-        assert(*a);
+                ns = (*a)->n_rrs;
+                assert(ns <= UINT16_MAX); /* Maximum number of RRs we can stick into a DNS packet section */
 
-        n = dns_answer_new(((*a)->n_rrs + n_free) * 2);
-        if (!n)
-                return -ENOMEM;
+                if (n_free > UINT16_MAX - ns) /* overflow check */
+                        ns = UINT16_MAX;
+                else if (n_free > 0) { /* Increase size and double the result, just in case — except if the
+                                        * increase is specified as 0, in which case we just allocate the
+                                        * exact amount as before, under the assumption this is just a request
+                                        * to copy the answer. */
+                        ns += n_free;
 
-        r = dns_answer_add_raw_all(n, *a);
-        if (r < 0)
-                return r;
+                        if (ns <= UINT16_MAX/2) /* overflow check */
+                                ns *= 2;
+                        else
+                                ns = UINT16_MAX;
+                }
 
-        dns_answer_unref(*a);
-        *a = TAKE_PTR(n);
+                n = dns_answer_new(ns);
+                if (!n)
+                        return -ENOMEM;
+
+                r = dns_answer_add_raw_all(n, *a);
+                if (r < 0)
+                        return r;
+
+                dns_answer_unref(*a);
+                assert_se(*a = TAKE_PTR(n));
+        } else if (n_free > 0) {
+                r = dns_answer_reserve(a, n_free);
+                if (r < 0)
+                        return r;
+        }
 
         return 0;
 }

src/resolve/resolved-dns-packet.c

@@ -2253,6 +2253,18 @@ static int dns_packet_extract_answer(DnsPacket *p, DnsAnswer **ret_answer) {
                 bool cache_flush = false;
                 size_t start;
 
+                if (p->rindex == p->size) {
+                        /* If we reached the end of the packet already, but there are still more RRs
+                         * declared, then that's a corrupt packet. Let's accept the packet anyway, since it's
+                         * apparently a common bug in routers. Let's however suppress OPT support in this
+                         * case, so that we force the rest of the logic into lowest DNS baseline support. Or
+                         * to say this differently: if the DNS server doesn't even get the RR counts right,
+                         * it's highly unlikely it gets EDNS right. */
+                        log_debug("More resource records declared in packet than included, suppressing OPT.");
+                        bad_opt = true;
+                        break;
+                }
+
                 r = dns_packet_read_rr(p, &rr, &cache_flush, &start);
                 if (r < 0)
                         return r;
@@ -2352,8 +2364,10 @@ static int dns_packet_extract_answer(DnsPacket *p, DnsAnswer **ret_answer) {
                 previous = dns_resource_record_ref(rr);
         }
 
-        if (bad_opt)
+        if (bad_opt) {
                 p->opt = dns_resource_record_unref(p->opt);
+                p->opt_start = p->opt_size = SIZE_MAX;
+        }
 
         *ret_answer = TAKE_PTR(answer);
 
@@ -2380,6 +2394,12 @@ int dns_packet_extract(DnsPacket *p) {
         if (r < 0)
                 return r;
 
+        if (p->rindex < p->size)  {
+                log_debug("Trailing garbage in packet, suppressing OPT.");
+                p->opt = dns_resource_record_unref(p->opt);
+                p->opt_start = p->opt_size = SIZE_MAX;
+        }
+
         p->question = TAKE_PTR(question);
         p->answer = TAKE_PTR(answer);
 
@@ -2535,6 +2555,10 @@ static int dns_packet_compare_func(const DnsPacket *x, const DnsPacket *y) {
 
 DEFINE_HASH_OPS(dns_packet_hash_ops, DnsPacket, dns_packet_hash_func, dns_packet_compare_func);
 
+bool dns_packet_equal(const DnsPacket *a, const DnsPacket *b) {
+        return dns_packet_compare_func(a, b) == 0;
+}
+
 static const char* const dns_rcode_table[_DNS_RCODE_MAX_DEFINED] = {
         [DNS_RCODE_SUCCESS] = "SUCCESS",
         [DNS_RCODE_FORMERR] = "FORMERR",

src/resolve/resolved-dns-packet.h

@@ -227,6 +227,8 @@ void dns_packet_rewind(DnsPacket *p, size_t idx);
 int dns_packet_skip_question(DnsPacket *p);
 int dns_packet_extract(DnsPacket *p);
 
+bool dns_packet_equal(const DnsPacket *a, const DnsPacket *b);
+
 /* https://www.iana.org/assignments/dns-parameters/dns-parameters.xhtml#dns-parameters-6 */
 enum {
         DNS_RCODE_SUCCESS = 0,

src/resolve/resolved-dns-scope.c

@@ -1126,7 +1126,7 @@ void dns_scope_check_conflicts(DnsScope *scope, DnsPacket *p) {
                         return;
         }
 
-        if (manager_our_packet(scope->manager, p))
+        if (manager_packet_from_local_address(scope->manager, p))
                 return;
 
         r = dns_packet_extract(p);

src/resolve/resolved-dns-stub.c

@@ -85,6 +85,15 @@ DnsStubListenerExtra *dns_stub_listener_extra_free(DnsStubListenerExtra *p) {
         return mfree(p);
 }
 
+uint16_t dns_stub_listener_extra_port(DnsStubListenerExtra *p) {
+        assert(p);
+
+        if (p->port > 0)
+                return p->port;
+
+        return 53;
+}
+
 static int dns_stub_collect_answer_by_question(
                 DnsAnswer **reply,
                 DnsAnswer *answer,
@@ -639,6 +648,7 @@ static void dns_stub_query_complete(DnsQuery *q) {
         case DNS_TRANSACTION_RR_TYPE_UNSUPPORTED:
         case DNS_TRANSACTION_NETWORK_DOWN:
         case DNS_TRANSACTION_NO_SOURCE:
+        case DNS_TRANSACTION_STUB_LOOP:
                 (void) dns_stub_send_reply(q, DNS_RCODE_SERVFAIL);
                 break;
 
@@ -688,6 +698,11 @@ static void dns_stub_process_query(Manager *m, DnsStubListenerExtra *l, DnsStrea
                 return;
         }
 
+        if (manager_packet_from_our_transaction(m, p)) {
+                log_debug("Got our own packet looped back, ignoring.");
+                return;
+        }
+
         r = dns_packet_extract(p);
         if (r < 0) {
                 log_debug_errno(r, "Failed to extract resources from incoming packet, ignoring packet: %m");
@@ -951,13 +966,13 @@ static int manager_dns_stub_fd_extra(Manager *m, DnsStubListenerExtra *l, int ty
         if (l->family == AF_INET)
                 sa = (union sockaddr_union) {
                         .in.sin_family = l->family,
-                        .in.sin_port = htobe16(l->port != 0 ? l->port : 53U),
+                        .in.sin_port = htobe16(dns_stub_listener_extra_port(l)),
                         .in.sin_addr = l->address.in,
                 };
         else
                 sa = (union sockaddr_union) {
                         .in6.sin6_family = l->family,
-                        .in6.sin6_port = htobe16(l->port != 0 ? l->port : 53U),
+                        .in6.sin6_port = htobe16(dns_stub_listener_extra_port(l)),
                         .in6.sin6_addr = l->address.in6,
                 };
 

src/resolve/resolved-dns-stub.h

@@ -33,6 +33,7 @@ extern const struct hash_ops dns_stub_listener_extra_hash_ops;
 
 int dns_stub_listener_extra_new(Manager *m, DnsStubListenerExtra **ret);
 DnsStubListenerExtra *dns_stub_listener_extra_free(DnsStubListenerExtra *p);
+uint16_t dns_stub_listener_extra_port(DnsStubListenerExtra *p);
 
 void manager_dns_stub_stop(Manager *m);
 int manager_dns_stub_start(Manager *m);

src/resolve/resolved-dns-transaction.c

@@ -317,8 +317,9 @@ static void dns_transaction_tentative(DnsTransaction *t, DnsPacket *p) {
 
         assert(t);
         assert(p);
+        assert(t->scope->protocol == DNS_PROTOCOL_LLMNR);
 
-        if (manager_our_packet(t->scope->manager, p) != 0)
+        if (manager_packet_from_local_address(t->scope->manager, p) != 0)
                 return;
 
         (void) in_addr_to_string(p->family, &p->sender, &pretty);
@@ -639,6 +640,9 @@ static int dns_transaction_emit_tcp(DnsTransaction *t) {
                 if (r < 0)
                         return r;
 
+                if (manager_server_is_stub(t->scope->manager, t->server))
+                        return -ELOOP;
+
                 if (!t->bypass) {
                         if (!dns_server_dnssec_supported(t->server) && dns_type_is_dnssec(dns_transaction_key(t)->type))
                                 return -EOPNOTSUPP;
@@ -1303,6 +1307,9 @@ static int dns_transaction_emit_udp(DnsTransaction *t) {
                 if (r < 0)
                         return r;
 
+                if (manager_server_is_stub(t->scope->manager, t->server))
+                        return -ELOOP;
+
                 if (t->current_feature_level < DNS_SERVER_FEATURE_LEVEL_UDP || DNS_SERVER_FEATURE_LEVEL_IS_TLS(t->current_feature_level))
                         return -EAGAIN; /* Sorry, can't do UDP, try TCP! */
 
@@ -1845,7 +1852,23 @@ int dns_transaction_go(DnsTransaction *t) {
                 if (IN_SET(r, -EMSGSIZE, -EAGAIN))
                         r = dns_transaction_emit_tcp(t);
         }
+        if (r == -ELOOP) {
+                if (t->scope->protocol != DNS_PROTOCOL_DNS)
+                        return r;
 
+                /* One of our own stub listeners */
+                log_debug_errno(r, "Detected that specified DNS server is our own extra listener, switching DNS servers.");
+
+                dns_scope_next_dns_server(t->scope);
+
+                if (dns_scope_get_dns_server(t->scope) == t->server) {
+                        log_debug_errno(r, "Still pointing to extra listener after switching DNS servers, refusing operation.");
+                        dns_transaction_complete(t, DNS_TRANSACTION_STUB_LOOP);
+                        return 0;
+                }
+
+                return dns_transaction_go(t);
+        }
         if (r == -ESRCH) {
                 /* No servers to send this to? */
                 dns_transaction_complete(t, DNS_TRANSACTION_NO_SERVERS);
@@ -3386,6 +3409,7 @@ static const char* const dns_transaction_state_table[_DNS_TRANSACTION_STATE_MAX]
         [DNS_TRANSACTION_NETWORK_DOWN] = "network-down",
         [DNS_TRANSACTION_NOT_FOUND] = "not-found",
         [DNS_TRANSACTION_NO_SOURCE] = "no-source",
+        [DNS_TRANSACTION_STUB_LOOP] = "stub-loop",
 };
 DEFINE_STRING_TABLE_LOOKUP(dns_transaction_state, DnsTransactionState);
 

src/resolve/resolved-dns-transaction.h

@@ -33,6 +33,7 @@ enum DnsTransactionState {
         DNS_TRANSACTION_NETWORK_DOWN,
         DNS_TRANSACTION_NOT_FOUND, /* like NXDOMAIN, but when LLMNR/TCP connections fail */
         DNS_TRANSACTION_NO_SOURCE, /* All suitable DnsTransactionSource turned off */
+        DNS_TRANSACTION_STUB_LOOP,
         _DNS_TRANSACTION_STATE_MAX,
         _DNS_TRANSACTION_STATE_INVALID = -EINVAL,
 };

src/resolve/resolved-llmnr.c

@@ -83,7 +83,7 @@ static int on_llmnr_packet(sd_event_source *s, int fd, uint32_t revents, void *u
         if (r <= 0)
                 return r;
 
-        if (manager_our_packet(m, p))
+        if (manager_packet_from_local_address(m, p))
                 return 0;
 
         scope = manager_find_scope(m, p);

src/resolve/resolved-manager.c

@@ -1255,13 +1255,31 @@ LinkAddress* manager_find_link_address(Manager *m, int family, const union in_ad
         return NULL;
 }
 
-bool manager_our_packet(Manager *m, DnsPacket *p) {
+bool manager_packet_from_local_address(Manager *m, DnsPacket *p) {
         assert(m);
         assert(p);
 
+        /* Let's see if this packet comes from an IP address we have on any local interface */
+
         return !!manager_find_link_address(m, p->family, &p->sender);
 }
 
+bool manager_packet_from_our_transaction(Manager *m, DnsPacket *p) {
+        DnsTransaction *t;
+
+        assert(m);
+        assert(p);
+
+        /* Let's see if we have a transaction with a query message with the exact same binary contents as the
+         * one we just got. If so, it's almost definitely a packet loop of some kind. */
+
+        t = hashmap_get(m->dns_transactions, UINT_TO_PTR(DNS_PACKET_ID(p)));
+        if (!t)
+                return false;
+
+        return t->sent && dns_packet_equal(t->sent, p);
+}
+
 DnsScope* manager_find_scope(Manager *m, DnsPacket *p) {
         Link *l;
 
@@ -1600,3 +1618,27 @@ bool manager_next_dnssd_names(Manager *m) {
 
         return tried;
 }
+
+bool manager_server_is_stub(Manager *m, DnsServer *s) {
+        DnsStubListenerExtra *l;
+
+        assert(m);
+        assert(s);
+
+        /* Safety check: we generally already skip the main stub when parsing configuration. But let's be
+         * extra careful, and check here again */
+        if (s->family == AF_INET &&
+            s->address.in.s_addr == htobe32(INADDR_DNS_STUB) &&
+            dns_server_port(s) == 53)
+                return true;
+
+        /* Main reason to call this is to check server data against the extra listeners, and filter things
+         * out. */
+        ORDERED_SET_FOREACH(l, m->dns_extra_stub_listeners)
+                if (s->family == l->family &&
+                    in_addr_equal(s->family, &s->address, &l->address) &&
+                    dns_server_port(s) == dns_stub_listener_extra_port(l))
+                        return true;
+
+        return false;
+}

src/resolve/resolved-manager.h

@@ -165,7 +165,9 @@ LinkAddress* manager_find_link_address(Manager *m, int family, const union in_ad
 void manager_refresh_rrs(Manager *m);
 int manager_next_hostname(Manager *m);
 
-bool manager_our_packet(Manager *m, DnsPacket *p);
+bool manager_packet_from_local_address(Manager *m, DnsPacket *p);
+bool manager_packet_from_our_transaction(Manager *m, DnsPacket *p);
+
 DnsScope* manager_find_scope(Manager *m, DnsPacket *p);
 
 void manager_verify_all(Manager *m);
@@ -195,3 +197,5 @@ void manager_reset_server_features(Manager *m);
 void manager_cleanup_saved_user(Manager *m);
 
 bool manager_next_dnssd_names(Manager *m);
+
+bool manager_server_is_stub(Manager *m, DnsServer *s);

src/resolve/resolved-mdns.c

@@ -254,7 +254,7 @@ static int on_mdns_packet(sd_event_source *s, int fd, uint32_t revents, void *us
         if (r <= 0)
                 return r;
 
-        if (manager_our_packet(m, p))
+        if (manager_packet_from_local_address(m, p))
                 return 0;
 
         scope = manager_find_scope(m, p);

src/resolve/resolved-varlink.c

@@ -60,6 +60,9 @@ static int reply_query_state(DnsQuery *q) {
         case DNS_TRANSACTION_NO_SOURCE:
                 return varlink_error(q->varlink_request, "io.systemd.Resolve.NoSource", NULL);
 
+        case DNS_TRANSACTION_STUB_LOOP:
+                return varlink_error(q->varlink_request, "io.systemd.Resolve.StubLoop", NULL);
+
         case DNS_TRANSACTION_NOT_FOUND:
                 /* We return this as NXDOMAIN. This is only generated when a host doesn't implement LLMNR/TCP, and we
                  * thus quickly know that we cannot resolve an in-addr.arpa or ip6.arpa address. */

units/systemd-timesyncd.service.in

@@ -22,6 +22,10 @@ Wants=time-set.target
 AmbientCapabilities=CAP_SYS_TIME
 BusName=org.freedesktop.timesync1
 CapabilityBoundingSet=CAP_SYS_TIME
+# Turn off DNSSEC validation for hostname look-ups, since those need the
+# correct time to work, but we likely won't acquire that without NTP. Let's
+# break this chicken-and-egg cycle here.
+Environment=SYSTEMD_NSS_RESOLVE_VALIDATE=0
 ExecStart=!!@rootlibexecdir@/systemd-timesyncd
 LockPersonality=yes
 MemoryDenyWriteExecute=yes