resolved: rework what ResolveHostname() with family == AF_UNSPEC means

Previously, if a hostanem is resolved with AF_UNSPEC specified, this would be used as indication to resolve both an
AF_INET and an AF_INET6 address. With this change this logic is altered: an AF_INET address is only resolved if there's
actually a routable IPv4 address on the specific interface, and similar an AF_INET6 address is only resolved if there's
a routable IPv6 address. With this in place, it's ensured that the returned data is actually connectable by
applications. This logic mimics glibc's resolver behaviour.

Note that if the client asks explicitly for AF_INET or AF_INET6 it will get what it asked for.

This also simplifies the logic how it is determined whether a specific lookup shall take place on a scope.
Specifically, the checks with dns_scope_good_key() are now moved out of the transaction code and into the query code,
so that we don't even create a transaction object on a specific scope if we cannot execute the resolution on it anyway.

1 files changed, 45 insertions, 2 deletions

diff --git a/src/resolve/resolved-dns-query.c b/src/resolve/resolved-dns-query.c
index a00851658e..06d30d7863 100644
--- a/src/resolve/resolved-dns-query.c
+++ b/src/resolve/resolved-dns-query.c
@@ -21,6 +21,7 @@
 
 #include "alloc-util.h"
 #include "dns-domain.h"
+#include "dns-type.h"
 #include "hostname-util.h"
 #include "local-addresses.h"
 #include "resolved-dns-query.h"
@@ -161,6 +162,7 @@ static int dns_query_candidate_go(DnsQueryCandidate *c) {
         DnsTransaction *t;
         Iterator i;
         int r;
+        unsigned n = 0;
 
         assert(c);
 
@@ -172,8 +174,14 @@ static int dns_query_candidate_go(DnsQueryCandidate *c) {
                 r = dns_transaction_go(t);
                 if (r < 0)
                         return r;
+
+                n++;
         }
 
+        /* If there was nothing to start, then let's proceed immediately */
+        if (n == 0)
+                dns_query_candidate_notify(c);
+
         return 0;
 }
 
@@ -222,6 +230,31 @@ static DnsTransactionState dns_query_candidate_state(DnsQueryCandidate *c) {
         return state;
 }
 
+static bool dns_query_candidate_is_routable(DnsQueryCandidate *c, uint16_t type) {
+        int family;
+
+        assert(c);
+
+        /* Checks whether the specified RR type matches an address family that is routable on the link(s) the scope of
+         * this candidate belongs to. Specifically, whether there's a routable IPv4 address on it if we query an A RR,
+         * or a routable IPv6 address if we query an AAAA RR. */
+
+        if (!c->query->suppress_unroutable_family)
+                return true;
+
+        if (c->scope->protocol != DNS_PROTOCOL_DNS)
+                return true;
+
+        family = dns_type_to_af(type);
+        if (family < 0)
+                return true;
+
+        if (c->scope->link)
+                return link_relevant(c->scope->link, family, false);
+        else
+                return manager_routable(c->scope->manager, family);
+}
+
 static int dns_query_candidate_setup_transactions(DnsQueryCandidate *c) {
         DnsQuestion *question;
         DnsResourceKey *key;
@@ -236,14 +269,24 @@ static int dns_query_candidate_setup_transactions(DnsQueryCandidate *c) {
         /* Create one transaction per question key */
         DNS_QUESTION_FOREACH(key, question) {
                 _cleanup_(dns_resource_key_unrefp) DnsResourceKey *new_key = NULL;
+                DnsResourceKey *qkey;
+
+                if (!dns_query_candidate_is_routable(c, key->type))
+                        continue;
 
                 if (c->search_domain) {
                         r = dns_resource_key_new_append_suffix(&new_key, key, c->search_domain->name);
                         if (r < 0)
                                 goto fail;
-                }
 
-                r = dns_query_candidate_add_transaction(c, new_key ?: key);
+                        qkey = new_key;
+                } else
+                        qkey = key;
+
+                if (!dns_scope_good_key(c->scope, qkey))
+                        continue;
+
+                r = dns_query_candidate_add_transaction(c, qkey);
                 if (r < 0)
                         goto fail;