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, 12 insertions, 9 deletions

diff --git a/src/resolve/resolved-link.c b/src/resolve/resolved-link.c
index 37dd4a6e78..7412e64622 100644
--- a/src/resolve/resolved-link.c
+++ b/src/resolve/resolved-link.c
@@ -515,14 +515,17 @@ int link_update_monitor(Link *l) {
         return 0;
 }
 
-bool link_relevant(Link *l, int family, bool multicast) {
+bool link_relevant(Link *l, int family, bool local_multicast) {
         _cleanup_free_ char *state = NULL;
         LinkAddress *a;
 
         assert(l);
 
-        /* A link is relevant for multicast traffic if it isn't a loopback or pointopoint device, has a link beat, can
-         * do multicast and has at least one relevant IP address */
+        /* A link is relevant for local multicast traffic if it isn't a loopback or pointopoint device, has a link
+         * beat, can do multicast and has at least one link-local (or better) IP address.
+         *
+         * A link is relevant for non-multicast traffic if it isn't a loopback device, has a link beat, and has at
+         * least one routable address.*/
 
         if (l->flags & (IFF_LOOPBACK|IFF_DORMANT))
                 return false;
@@ -530,7 +533,7 @@ bool link_relevant(Link *l, int family, bool multicast) {
         if ((l->flags & (IFF_UP|IFF_LOWER_UP)) != (IFF_UP|IFF_LOWER_UP))
                 return false;
 
-        if (multicast) {
+        if (local_multicast) {
                 if (l->flags & IFF_POINTOPOINT)
                         return false;
 
@@ -548,7 +551,7 @@ bool link_relevant(Link *l, int family, bool multicast) {
                 return false;
 
         LIST_FOREACH(addresses, a, l->addresses)
-                if ((family == AF_UNSPEC || a->family == family) && link_address_relevant(a))
+                if ((family == AF_UNSPEC || a->family == family) && link_address_relevant(a, local_multicast))
                         return true;
 
         return false;
@@ -692,7 +695,7 @@ void link_address_add_rrs(LinkAddress *a, bool force_remove) {
         if (a->family == AF_INET) {
 
                 if (!force_remove &&
-                    link_address_relevant(a) &&
+                    link_address_relevant(a, true) &&
                     a->link->llmnr_ipv4_scope &&
                     a->link->llmnr_support == RESOLVE_SUPPORT_YES &&
                     a->link->manager->llmnr_support == RESOLVE_SUPPORT_YES) {
@@ -749,7 +752,7 @@ void link_address_add_rrs(LinkAddress *a, bool force_remove) {
         if (a->family == AF_INET6) {
 
                 if (!force_remove &&
-                    link_address_relevant(a) &&
+                    link_address_relevant(a, true) &&
                     a->link->llmnr_ipv6_scope &&
                     a->link->llmnr_support == RESOLVE_SUPPORT_YES &&
                     a->link->manager->llmnr_support == RESOLVE_SUPPORT_YES) {
@@ -826,13 +829,13 @@ int link_address_update_rtnl(LinkAddress *a, sd_netlink_message *m) {
         return 0;
 }
 
-bool link_address_relevant(LinkAddress *a) {
+bool link_address_relevant(LinkAddress *a, bool local_multicast) {
         assert(a);
 
         if (a->flags & (IFA_F_DEPRECATED|IFA_F_TENTATIVE))
                 return false;
 
-        if (IN_SET(a->scope, RT_SCOPE_HOST, RT_SCOPE_NOWHERE))
+        if (a->scope >= (local_multicast ? RT_SCOPE_HOST : RT_SCOPE_LINK))
                 return false;
 
         return true;