From: Bret Comnes <bret@comnes.org>
Date: Thu, 26 Mar 2026 05:59:09 +0000 (-0700)
Subject: Revert "resolve: refuse traffic from the local host only for queries"
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=658e5ac06f80ee2078b034f7cc483204d7f91c5e;p=thirdparty%2Fsystemd.git

Revert "resolve: refuse traffic from the local host only for queries"

This reverts commit 526f1594daec073269c3e70ee7914f6dd8740d5c.

This revert is necessary because the change breaks mDNS hostname stability
whenever a DNS-SD service calls UnregisterService. When a service
unregisters (e.g. on process restart), manager_refresh_rrs() clears and
re-adds all RRs in PROBING state, which sends a multicast announcement
(QR=1). The kernel reflects this back to resolved's own socket. Because
the local-address check was moved inside the query-only branch by the
reverted commit, the reply path in on_mdns_packet() is now unguarded.
The looped-back announcement matches the pending probe transaction and
completes it with DNS_TRANSACTION_SUCCESS. Since the zone item is still
in PROBING state (not ESTABLISHED), dns_zone_item_notify() sets
we_lost=true and calls dns_zone_item_conflict(), which invokes
manager_next_hostname() and renames the hostname (e.g. foo.local →
foo4.local). This happens reliably on every restart of any service using
RegisterService/UnregisterService (homebridge, avahi-compat wrappers,
etc.).

The top-level local-address check in on_mdns_packet() suppresses all
looped-back multicast traffic before the reply/query split. Restoring it
there is consistent with the overall design: dns_scope_check_conflicts()
already has its own manager_packet_from_local_address() guard and is
unaffected.

A more targeted long-term fix (e.g. guarding dns_transaction_process_reply()
for mDNS, or avoiding unnecessary re-probing of already-established records
in manager_refresh_rrs()) can be pursued separately.
---

diff --git a/src/resolve/resolved-mdns.c b/src/resolve/resolved-mdns.c
index 5026b10ff4c..fb20ba9cd02 100644
--- a/src/resolve/resolved-mdns.c
+++ b/src/resolve/resolved-mdns.c
@@ -413,6 +413,14 @@ static int on_mdns_packet(sd_event_source *s, int fd, uint32_t revents, void *us
         if (r <= 0)
                 return r;
 
+        /* Refuse traffic from the local host, to avoid query loops. However, allow legacy mDNS
+         * unicast queries through anyway (we never send those ourselves, hence no risk).
+         * i.e. check for the source port nr. */
+        if (p->sender_port == MDNS_PORT && manager_packet_from_local_address(m, p)) {
+                log_debug("Got mDNS UDP packet from local host, ignoring.");
+                return 0;
+        }
+
         scope = manager_find_scope(m, p);
         if (!scope) {
                 log_debug("Got mDNS UDP packet on unknown scope. Ignoring.");
@@ -529,14 +537,6 @@ static int on_mdns_packet(sd_event_source *s, int fd, uint32_t revents, void *us
                 if (unsolicited_packet)
                         mdns_notify_browsers_unsolicited_updates(m, p->answer, p->family);
         } else if (dns_packet_validate_query(p) > 0)  {
-                /* Refuse traffic from the local host, to avoid query loops. However, allow legacy mDNS
-                 * unicast queries through anyway (we never send those ourselves, hence no risk).
-                 * i.e. check for the source port nr. */
-                if (p->sender_port == MDNS_PORT && manager_packet_from_local_address(m, p)) {
-                        log_debug("Got mDNS UDP packet from local host, ignoring.");
-                        return 0;
-                }
-
                 log_debug("Got mDNS query packet for id %u", DNS_PACKET_ID(p));
 
                 r = mdns_scope_process_query(scope, p);