netlink: add netlink monitoring for real-time iface/addr changes

Subscribe to RTMGRP_LINK, RTMGRP_IPV4_IFADDR, and RTMGRP_IPV6_IFADDR
events so interface and address changes are detected immediately instead
of waiting for the 10-second polling cycle.

The netlink socket fd is added to the main select() loop alongside the
per-interface mDNS sockets.  When an RTM_NEWLINK, RTM_DELLINK,
RTM_NEWADDR, or RTM_DELADDR event arrives, sys_init() is called to
rescan interfaces right away.  ENOBUFS (kernel event overflow) is
handled by treating it as an implicit change, ensuring no update is
ever silently missed.

The 10-second poll is retained as a safety net on all platforms.

Platform portability is handled entirely in netlink.h: Linux gets real
implementations in netlink.c, while other platforms get static inline
no-op stubs so mdnsd.c requires no preprocessor guards.

Closes #48

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

Author: troglobit

src/Makefile.am

@@ -8,7 +8,7 @@ if ENABLE_MQUERY
 bin_PROGRAMS            = mquery
 endif
 
-mdnsd_SOURCES           = mdnsd.c mdnsd.h addr.c conf.c queue.h mcsock.c mcsock.h
+mdnsd_SOURCES           = mdnsd.c mdnsd.h addr.c conf.c queue.h mcsock.c mcsock.h netlink.c netlink.h
 mdnsd_LDADD             = ../libmdnsd/libmdnsd.la $(LIBS) $(LIBOBJS)
 
 mquery_SOURCES          = mquery.c mcsock.c mcsock.h

src/mdnsd.c

@@ -44,8 +44,9 @@
 #include "config.h"
 #include "mcsock.h"
 #include "mdnsd.h"
+#include "netlink.h"
 
-#define SYS_INTERVAL 10		/* System interface poll interval */
+#define SYS_INTERVAL 10		/* System interface poll interval, safety net */
 
 static volatile sig_atomic_t running = 1;
 static volatile sig_atomic_t reload = 0;
@@ -270,6 +271,7 @@ int main(int argc, char *argv[])
 	fd_set fds;
 	int timeout = 0;
 	int c, rc;
+	int nl_sd = -1;
 
 	prognm = progname(argv[0]);
 	while ((c = getopt(argc, argv, "H:h"
@@ -342,11 +344,17 @@ int main(int argc, char *argv[])
 	sig_init();
 	sys_init();
 	pidfile(PACKAGE_NAME);
+	nl_sd = netlink_init();
 
 	while (running) {
 		int nfds = 0;
 
 		FD_ZERO(&fds);
+		if (nl_sd >= 0) {
+			FD_SET(nl_sd, &fds);
+			if (nl_sd > nfds)
+				nfds = nl_sd;
+		}
 		for (iface = iface_iterator(1); iface; iface = iface_iterator(0)) {
 			if (iface->sd < 0 || iface->unused)
 				continue;
@@ -377,6 +385,11 @@ int main(int argc, char *argv[])
 			continue;
 		}
 
+		if (nl_sd >= 0 && FD_ISSET(nl_sd, &fds)) {
+			if (netlink_read(nl_sd) > 0)
+				sys_init();
+		}
+
 		if (sys_timeout(&timeout))
 		    sys_init();
 
@@ -408,6 +421,7 @@ int main(int argc, char *argv[])
 	for (iface = iface_iterator(1); iface; iface = iface_iterator(0))
 		free_iface(iface);
 	iface_exit();
+	netlink_exit(nl_sd);
 
 	return 0;
 }

src/netlink.c

@@ -0,0 +1,126 @@
+/* Netlink interface monitor for Linux
+ *
+ * Copyright (c) 2026  Joachim Wiberg <troglobit@gmail.com>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *     * Redistributions of source code must retain the above copyright
+ *       notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above copyright
+ *       notice, this list of conditions and the following disclaimer in the
+ *       documentation and/or other materials provided with the distribution.
+ *     * Neither the name of the copyright holders nor the names of its
+ *       contributors may be used to endorse or promote products derived from
+ *       this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDERS OR CONTRIBUTORS BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#ifdef __linux__
+
+#include <errno.h>
+#include <string.h>
+#include <unistd.h>
+#include <sys/socket.h>
+#include <linux/netlink.h>
+#include <linux/rtnetlink.h>
+
+#include "mdnsd.h"
+#include "netlink.h"
+
+#define NL_BUFSZ 4096
+
+/*
+ * Open a netlink socket subscribed to interface link and address events.
+ * Returns the socket fd on success, -1 on failure.
+ */
+int netlink_init(void)
+{
+	struct sockaddr_nl sa = { 0 };
+	int sd;
+
+	sd = socket(AF_NETLINK, SOCK_RAW | SOCK_NONBLOCK | SOCK_CLOEXEC, NETLINK_ROUTE);
+	if (sd < 0) {
+		ERR("Failed creating netlink socket: %s", strerror(errno));
+		return -1;
+	}
+
+	sa.nl_family = AF_NETLINK;
+	sa.nl_groups = RTMGRP_LINK | RTMGRP_IPV4_IFADDR | RTMGRP_IPV6_IFADDR;
+	if (bind(sd, (struct sockaddr *)&sa, sizeof(sa)) < 0) {
+		ERR("Failed binding netlink socket: %s", strerror(errno));
+		close(sd);
+		return -1;
+	}
+
+	return sd;
+}
+
+/*
+ * Drain all pending netlink messages.  Returns 1 if any link or address
+ * change event was seen (caller should re-run interface discovery), 0 if
+ * nothing relevant arrived, or -1 on a fatal socket error.
+ *
+ * ENOBUFS means the kernel dropped events because we were too slow; treat
+ * that as an implicit change so the caller rescans to catch up.
+ */
+int netlink_read(int sd)
+{
+	char buf[NL_BUFSZ];
+	struct nlmsghdr *nh;
+	ssize_t len;
+	size_t l;
+	int changed = 0;
+
+	while (1) {
+		len = recv(sd, buf, sizeof(buf), 0);
+		if (len < 0) {
+			if (errno == EAGAIN || errno == EWOULDBLOCK)
+				break;
+			if (errno == EINTR)
+				continue;
+			if (errno == ENOBUFS) {
+				WARN("Netlink buffer overflow, interface events may have been lost");
+				changed = 1;
+				break;
+			}
+			ERR("Netlink recv error: %s", strerror(errno));
+			return -1;
+		}
+
+		l = (size_t)len;
+		for (nh = (struct nlmsghdr *)buf; NLMSG_OK(nh, l); nh = NLMSG_NEXT(nh, l)) {
+			switch (nh->nlmsg_type) {
+			case RTM_NEWLINK:
+			case RTM_DELLINK:
+			case RTM_NEWADDR:
+			case RTM_DELADDR:
+				changed = 1;
+				break;
+			default:
+				break;
+			}
+		}
+	}
+
+	return changed;
+}
+
+void netlink_exit(int sd)
+{
+	if (sd >= 0)
+		close(sd);
+}
+
+#endif /* __linux__ */

src/netlink.h

@@ -0,0 +1,46 @@
+/* Netlink interface monitor for Linux
+ *
+ * Copyright (c) 2026  Joachim Wiberg <troglobit@gmail.com>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *     * Redistributions of source code must retain the above copyright
+ *       notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above copyright
+ *       notice, this list of conditions and the following disclaimer in the
+ *       documentation and/or other materials provided with the distribution.
+ *     * Neither the name of the copyright holders nor the names of its
+ *       contributors may be used to endorse or promote products derived from
+ *       this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDERS OR CONTRIBUTORS BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#ifndef MDNSD_NETLINK_H_
+#define MDNSD_NETLINK_H_
+
+#ifdef __linux__
+
+int  netlink_init(void);
+int  netlink_read(int sd);
+void netlink_exit(int sd);
+
+#else /* non-Linux stubs */
+
+static inline int  netlink_init(void)        { return -1; }
+static inline int  netlink_read(int sd)      { (void)sd; return 0; }
+static inline void netlink_exit(int sd)      { (void)sd; }
+
+#endif /* __linux__ */
+#endif /* MDNSD_NETLINK_H_ */