resolved: let's preferably route reverse lookups for local subnets to matching interfaces

Let's preferably route traffic for reverse lookups to LLMNR/mDNS/DNS on
the matching interface if the IP address is in the local subnet. Also,
if looking up an IP address of our own host, let's avoid doing
LLMNR/mDNS at all.

This is useful if "~." is a routing domain to DNS, as it means, local
reverse lookups still go to LLMNR/mDNS, too.

Fixes: #16243 #10081

src/resolve/resolved-dns-scope.c

@@ -477,6 +477,65 @@ static DnsScopeMatch match_link_local_reverse_lookups(const char *domain) {
         return _DNS_SCOPE_MATCH_INVALID;
 }
 
+static DnsScopeMatch match_subnet_reverse_lookups(
+                DnsScope *s,
+                const char *domain,
+                bool exclude_own) {
+
+        union in_addr_union ia;
+        LinkAddress *a;
+        int f, r;
+
+        assert(s);
+        assert(domain);
+
+        /* Checks whether the specified domain is a reverse address domain (i.e. in the .in-addr.arpa or
+         * .ip6.arpa area), and if so, whether the address matches any of the local subnets of the link the
+         * scope is associated with. If so, our scope should consider itself relevant for any lookup in the
+         * domain, since it apparently refers to hosts on this link's subnet.
+         *
+         * If 'exclude_own' is true this will return DNS_SCOPE_NO for any IP addresses assigned locally. This
+         * is useful for LLMNR/mDNS as we never want to look up our own hostname on LLMNR/mDNS but always use
+         * the locally synthesized one. */
+
+        if (!s->link)
+                return _DNS_SCOPE_MATCH_INVALID; /* No link, hence no local addresses to check */
+
+        r = dns_name_address(domain, &f, &ia);
+        if (r < 0)
+                log_debug_errno(r, "Failed to determine whether '%s' is an address domain: %m", domain);
+        if (r <= 0)
+                return _DNS_SCOPE_MATCH_INVALID;
+
+        if (s->family != AF_UNSPEC && f != s->family)
+                return _DNS_SCOPE_MATCH_INVALID; /* Don't look for IPv4 addresses on LLMNR/mDNS over IPv6 and vice versa */
+
+        LIST_FOREACH(addresses, a, s->link->addresses) {
+
+                if (a->family != f)
+                        continue;
+
+                /* Equals our own address? nah, let's not use this scope. The local synthesizer will pick it up for us. */
+                if (exclude_own &&
+                    in_addr_equal(f, &a->in_addr, &ia) > 0)
+                        return DNS_SCOPE_NO;
+
+                if (a->prefixlen == UCHAR_MAX) /* don't know subnet mask */
+                        continue;
+
+                /* Check if the address is in the local subnet */
+                r = in_addr_prefix_covers(f, &a->in_addr, a->prefixlen, &ia);
+                if (r < 0)
+                        log_debug_errno(r, "Failed to determine whether link address covers lookup address '%s': %m", domain);
+                if (r > 0)
+                        /* Note that we only claim zero labels match. This is so that this is at the same
+                         * priority a DNS scope with "." as routing domain is. */
+                        return DNS_SCOPE_YES_BASE + 0;
+        }
+
+        return _DNS_SCOPE_MATCH_INVALID;
+}
+
 DnsScopeMatch dns_scope_good_domain(
                 DnsScope *s,
                 int ifindex,
@@ -533,6 +592,7 @@ DnsScopeMatch dns_scope_good_domain(
 
         case DNS_PROTOCOL_DNS: {
                 bool has_search_domains = false;
+                DnsScopeMatch m;
                 int n_best = -1;
 
                 /* Never route things to scopes that lack DNS servers */
@@ -579,6 +639,13 @@ DnsScopeMatch dns_scope_good_domain(
                     dns_name_endswith(domain, "local") > 0)
                         return DNS_SCOPE_NO;
 
+                /* If the IP address to look up matches the local subnet, then implicity synthesizes
+                 * DNS_SCOPE_YES_BASE + 0 on this interface, i.e. preferably resolve IP addresses via the DNS
+                 * server belonging to this interface. */
+                m = match_subnet_reverse_lookups(s, domain, false);
+                if (m >= 0)
+                        return m;
+
                 /* If there was no match at all, then see if this scope is suitable as default route. */
                 if (!dns_scope_is_default_route(s))
                         return DNS_SCOPE_NO;
@@ -593,6 +660,10 @@ DnsScopeMatch dns_scope_good_domain(
                 if (m >= 0)
                         return m;
 
+                m = match_subnet_reverse_lookups(s, domain, true);
+                if (m >= 0)
+                        return m;
+
                 if ((s->family == AF_INET && dns_name_endswith(domain, "in-addr.arpa") > 0) ||
                     (s->family == AF_INET6 && dns_name_endswith(domain, "ip6.arpa") > 0))
                         return DNS_SCOPE_MAYBE;
@@ -612,6 +683,10 @@ DnsScopeMatch dns_scope_good_domain(
                 if (m >= 0)
                         return m;
 
+                m = match_subnet_reverse_lookups(s, domain, true);
+                if (m >= 0)
+                        return m;
+
                 if ((s->family == AF_INET && dns_name_endswith(domain, "in-addr.arpa") > 0) ||
                     (s->family == AF_INET6 && dns_name_endswith(domain, "ip6.arpa") > 0))
                         return DNS_SCOPE_MAYBE;

src/resolve/resolved-link.c

@@ -802,6 +802,7 @@ int link_address_new(Link *l, LinkAddress **ret, int family, const union in_addr
                 .family = family,
                 .in_addr = *in_addr,
                 .link = l,
+                .prefixlen = UCHAR_MAX,
         };
 
         LIST_PREPEND(addresses, l->addresses, a);
@@ -1094,6 +1095,7 @@ fail:
 
 int link_address_update_rtnl(LinkAddress *a, sd_netlink_message *m) {
         int r;
+
         assert(a);
         assert(m);
 
@@ -1101,7 +1103,8 @@ int link_address_update_rtnl(LinkAddress *a, sd_netlink_message *m) {
         if (r < 0)
                 return r;
 
-        sd_rtnl_message_addr_get_scope(m, &a->scope);
+        (void) sd_rtnl_message_addr_get_prefixlen(m, &a->prefixlen);
+        (void) sd_rtnl_message_addr_get_scope(m, &a->scope);
 
         link_allocate_scopes(a->link);
         link_add_rrs(a->link, false);

src/resolve/resolved-link.h

@@ -23,6 +23,7 @@ struct LinkAddress {
 
         int family;
         union in_addr_union in_addr;
+        unsigned char prefixlen;
 
         unsigned char flags, scope;