resolved: when there's already somebody listening on the LLMNR ports, simple disable LLMNR and warn, but continue

This allows us to run resolved inside an nspawn container that shares
the network namespace with the host, if there's already an instance
running.

src/resolve/resolved-manager.c

@@ -392,6 +392,63 @@ static int manager_watch_hostname(Manager *m) {
         return 0;
 }
 
+static void manager_llmnr_stop(Manager *m) {
+        assert(m);
+
+        m->llmnr_ipv4_udp_event_source = sd_event_source_unref(m->llmnr_ipv4_udp_event_source);
+        m->llmnr_ipv4_udp_fd = safe_close(m->llmnr_ipv4_udp_fd);
+
+        m->llmnr_ipv6_udp_event_source = sd_event_source_unref(m->llmnr_ipv6_udp_event_source);
+        m->llmnr_ipv6_udp_fd = safe_close(m->llmnr_ipv6_udp_fd);
+
+        m->llmnr_ipv4_tcp_event_source = sd_event_source_unref(m->llmnr_ipv4_tcp_event_source);
+        m->llmnr_ipv4_tcp_fd = safe_close(m->llmnr_ipv4_tcp_fd);
+
+        m->llmnr_ipv6_tcp_event_source = sd_event_source_unref(m->llmnr_ipv6_tcp_event_source);
+        m->llmnr_ipv6_tcp_fd = safe_close(m->llmnr_ipv6_tcp_fd);
+}
+
+static int manager_llmnr_start(Manager *m) {
+        int r;
+
+        assert(m);
+
+        if (m->llmnr_support == SUPPORT_NO)
+                return 0;
+
+        r = manager_llmnr_ipv4_udp_fd(m);
+        if (r == -EADDRINUSE)
+                goto eaddrinuse;
+        if (r < 0)
+                return r;
+
+        r = manager_llmnr_ipv6_udp_fd(m);
+        if (r == -EADDRINUSE)
+                goto eaddrinuse;
+        if (r < 0)
+                return r;
+
+        r = manager_llmnr_ipv4_tcp_fd(m);
+        if (r == -EADDRINUSE)
+                goto eaddrinuse;
+        if (r < 0)
+                return r;
+
+        r = manager_llmnr_ipv6_tcp_fd(m);
+        if (r == -EADDRINUSE)
+                goto eaddrinuse;
+        if (r < 0)
+                return r;
+
+        return 0;
+
+eaddrinuse:
+        log_warning("There appears to be another LLMNR respondering running. Turning off LLMNR support.");
+        m->llmnr_support = SUPPORT_NO;
+        manager_llmnr_stop(m);
+        return 0;
+}
+
 int manager_new(Manager **ret) {
         _cleanup_(manager_freep) Manager *m = NULL;
         int r;
@@ -443,25 +500,24 @@ int manager_new(Manager **ret) {
         if (r < 0)
                 return r;
 
-        r = manager_llmnr_ipv4_udp_fd(m);
-        if (r < 0)
-                return r;
-        r = manager_llmnr_ipv6_udp_fd(m);
-        if (r < 0)
-                return r;
-        r = manager_llmnr_ipv4_tcp_fd(m);
-        if (r < 0)
-                return r;
-        r = manager_llmnr_ipv6_tcp_fd(m);
-        if (r < 0)
-                return r;
-
         *ret = m;
         m = NULL;
 
         return 0;
 }
 
+int manager_start(Manager *m) {
+        int r;
+
+        assert(m);
+
+        r = manager_llmnr_start(m);
+        if (r < 0)
+                return r;
+
+        return 0;
+}
+
 Manager *manager_free(Manager *m) {
         Link *l;
 
@@ -492,15 +548,7 @@ Manager *manager_free(Manager *m) {
         safe_close(m->dns_ipv4_fd);
         safe_close(m->dns_ipv6_fd);
 
-        sd_event_source_unref(m->llmnr_ipv4_udp_event_source);
-        sd_event_source_unref(m->llmnr_ipv6_udp_event_source);
-        safe_close(m->llmnr_ipv4_udp_fd);
-        safe_close(m->llmnr_ipv6_udp_fd);
-
-        sd_event_source_unref(m->llmnr_ipv4_tcp_event_source);
-        sd_event_source_unref(m->llmnr_ipv6_tcp_event_source);
-        safe_close(m->llmnr_ipv4_tcp_fd);
-        safe_close(m->llmnr_ipv6_tcp_fd);
+        manager_llmnr_stop(m);
 
         sd_event_source_unref(m->bus_retry_event_source);
         sd_bus_unref(m->bus);

src/resolve/resolved-manager.h

@@ -114,6 +114,7 @@ struct Manager {
 int manager_new(Manager **ret);
 Manager* manager_free(Manager *m);
 
+int manager_start(Manager *m);
 int manager_read_resolv_conf(Manager *m);
 int manager_write_resolv_conf(Manager *m);
 

src/resolve/resolved.c

@@ -75,6 +75,12 @@ int main(int argc, char *argv[]) {
         if (r < 0)
                 log_warning("Failed to parse configuration file: %s", strerror(-r));
 
+        r = manager_start(m);
+        if (r < 0) {
+                log_error("Failed to start manager: %s", strerror(-r));
+                goto finish;
+        }
+
         /* Write finish default resolv.conf to avoid a dangling
          * symlink */
         r = manager_write_resolv_conf(m);