llmnrd: Allow to bind to a specific network interface

Add a command line option -i/--interface which allows to bind the llmnrd
sockets to a specific interface. If used, requests are only answered on
the specified interface.

Example: llmnrd -i eth0

Closes #9
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>

7 files changed, 55 insertions, 14 deletions

diff --git a/iface.c b/iface.c
index 8cb88fc..333a142 100644
--- a/iface.c
+++ b/iface.c
@@ -42,6 +42,7 @@
 
 static bool iface_running = true;
 static bool iface_ipv6 = false;
+static unsigned int iface_ifindex = 0;
 static pthread_t iface_thread;
 static iface_event_handler_t iface_event_handler;
 
@@ -234,6 +235,13 @@ static void iface_nlmsg_change_addr(const struct nlmsghdr *nlh)
 	if ((ifa->ifa_flags & (IFA_F_TEMPORARY | IFA_F_TENTATIVE)) != 0)
 		return;
 
+	/*
+	 * If bound to a specific interface, don't report addresses of any other
+	 * interface.
+	 */
+	if (iface_ifindex > 0 && index != iface_ifindex)
+		return;
+
 	if_indextoname(index, ifname);
 
 	rta = (struct rtattr *)((const uint8_t *)nlh + NLMSG_SPACE(sizeof(*ifa)));
@@ -388,9 +396,11 @@ static void* iface_run_wrapper(void *data __unused)
 	return ERR_PTR(iface_run());
 }
 
-int iface_start_thread(bool ipv6)
+int iface_start_thread(bool ipv6, const char *iface)
 {
 	iface_ipv6 = ipv6;
+	if (iface)
+		iface_ifindex = if_nametoindex(iface);
 
 	if (pthread_create(&iface_thread, NULL, iface_run_wrapper, NULL) < 0) {
 		log_err("Failed to start interface monitoring thread\n");
diff --git a/iface.h b/iface.h
index 5032d87..11c49e7 100644
--- a/iface.h
+++ b/iface.h
@@ -31,7 +31,7 @@ typedef void (*iface_event_handler_t)(enum iface_event_type, unsigned char af,
 				      unsigned int ifindex);
 
 void iface_register_event_handler(iface_event_handler_t event_handler);
-int iface_start_thread(bool ipv6);
+int iface_start_thread(bool ipv6, const char *iface);
 void iface_stop(void);
 
 size_t iface_addr_lookup(unsigned int ifindex, unsigned char family,
diff --git a/llmnr.c b/llmnr.c
index a833ef1..206f165 100644
--- a/llmnr.c
+++ b/llmnr.c
@@ -61,19 +61,21 @@ static void llmnr_iface_event_handle(enum iface_event_type type, unsigned char a
 	}
 }
 
-int llmnr_init(const char *hostname, uint16_t port, bool ipv6)
+int llmnr_init(const char *hostname, uint16_t port, bool ipv6, const char *iface)
 {
 	llmnr_hostname[0] = strlen(hostname);
 	strncpy(&llmnr_hostname[1], hostname, LLMNR_LABEL_MAX_SIZE);
 	llmnr_hostname[LLMNR_LABEL_MAX_SIZE + 1] = '\0';
 	log_info("Starting llmnrd on port %u, hostname %s\n", port, hostname);
+	if (iface)
+		log_info("Binding to interface %s\n", iface);
 
-	llmnr_sock_ipv4 = socket_open_ipv4(port);
+	llmnr_sock_ipv4 = socket_open_ipv4(port, iface);
 	if (llmnr_sock_ipv4 < 0)
 		return -1;
 
 	if (ipv6) {
-		llmnr_sock_ipv6 = socket_open_ipv6(port);
+		llmnr_sock_ipv6 = socket_open_ipv6(port, iface);
 		if (llmnr_sock_ipv6 < 0)
 			return -1;
 	}
diff --git a/llmnr.h b/llmnr.h
index da86440..620e539 100644
--- a/llmnr.h
+++ b/llmnr.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2015 Tobias Klauser <tklauser@distanz.ch>
+ * Copyright (C) 2015-2016 Tobias Klauser <tklauser@distanz.ch>
  *
  * This file is part of llmnrd.
  *
@@ -22,7 +22,7 @@
 #include <stdbool.h>
 #include <stdint.h>
 
-int llmnr_init(const char *hostname, uint16_t port, bool ipv6);
+int llmnr_init(const char *hostname, uint16_t port, bool ipv6, const char *iface);
 int llmnr_run(void);
 void llmnr_stop(void);
 
diff --git a/llmnrd.c b/llmnrd.c
index 866e919..2bc4a20 100644
--- a/llmnrd.c
+++ b/llmnrd.c
@@ -39,9 +39,10 @@
 #include "llmnr.h"
 #include "llmnr-packet.h"
 
-static const char *short_opts = "H:p:6dhV";
+static const char *short_opts = "H:i:p:6dhV";
 static const struct option long_opts[] = {
 	{ "hostname",	required_argument,	NULL, 'H' },
+	{ "interface",  required_argument,	NULL, 'i' },
 	{ "port",	required_argument,	NULL, 'p' },
 	{ "ipv6",	no_argument,		NULL, '6' },
 	{ "daemonize",	no_argument,		NULL, 'd' },
@@ -55,6 +56,7 @@ static void __noreturn usage_and_exit(int status)
 	fprintf(stdout, "Usage: llmnrd [OPTIONS]\n"
 			"Options:\n"
 			"  -H, --hostname NAME  set hostname to respond with (default: system hostname)\n"
+			"  -i, --interface DEV  bind socket to a specific interface, e.g. eth0\n"
 			"  -p, --port NUM       set port number to listen on (default: %d)\n"
 			"  -6, --ipv6           enable LLMNR name resolution over IPv6\n"
 			"  -d, --daemonize      run as daemon in the background\n"
@@ -113,6 +115,7 @@ int main(int argc, char **argv)
 	long num_arg;
 	bool daemonize = false, ipv6 = false;
 	char *hostname = NULL;
+	char *iface = NULL;
 	uint16_t port = LLMNR_UDP_PORT;
 
 	while ((c = getopt_long(argc, argv, short_opts, long_opts, NULL)) != -1) {
@@ -123,6 +126,9 @@ int main(int argc, char **argv)
 		case 'H':
 			hostname = xstrdup(optarg);
 			break;
+		case 'i':
+			iface = xstrdup(optarg);
+			break;
 		case 'p':
 			num_arg = strtol(optarg, NULL, 0);
 			if (num_arg < 0 || num_arg > UINT16_MAX) {
@@ -164,10 +170,10 @@ int main(int argc, char **argv)
 		}
 	}
 
-	if (llmnr_init(hostname, port, ipv6) < 0)
+	if (llmnr_init(hostname, port, ipv6, iface) < 0)
 		goto out;
 
-	if (iface_start_thread(ipv6) < 0)
+	if (iface_start_thread(ipv6, iface) < 0)
 		goto out;
 
 	ret = llmnr_run();
diff --git a/socket.c b/socket.c
index 21e715a..1103779 100644
--- a/socket.c
+++ b/socket.c
@@ -36,7 +36,24 @@
 static const int YES = 1;
 static const int TTL = 255;
 
-int socket_open_ipv4(uint16_t port)
+static int socket_bind_to_device(int sock, const char *iface) {
+#ifdef SO_BINDTODEVICE
+	/* bind socket to specific interface */
+	if (iface) {
+		struct ifreq ifr;
+
+		memset(&ifr, 0, sizeof(ifr));
+		strncpy(ifr.ifr_name, iface, sizeof(ifr.ifr_name) - 1);
+		if (setsockopt(sock, SOL_SOCKET, SO_BINDTODEVICE, &ifr, sizeof(ifr)) < 0) {
+			log_err("Failed to bind socket to device %s: %s\n", iface, strerror(errno));
+			return -1;
+		}
+	}
+#endif
+	return 0;
+}
+
+int socket_open_ipv4(uint16_t port, const char *iface)
 {
 	int sock;
 	struct sockaddr_in sa;
@@ -64,6 +81,9 @@ int socket_open_ipv4(uint16_t port)
 		goto err;
 	}
 
+	if (socket_bind_to_device(sock, iface) < 0)
+		goto err;
+
 	/* bind the socket */
 	memset(&sa, 0, sizeof(sa));
 	sa.sin_family = AF_INET;
@@ -81,7 +101,7 @@ err:
 	return -1;
 }
 
-int socket_open_ipv6(uint16_t port)
+int socket_open_ipv6(uint16_t port, const char *iface)
 {
 	int sock, opt_pktinfo;
 	struct sockaddr_in6 sa;
@@ -120,6 +140,9 @@ int socket_open_ipv6(uint16_t port)
 		goto err;
 	}
 
+	if (socket_bind_to_device(sock, iface) < 0)
+		goto err;
+
 	/* bind the socket */
 	memset(&sa, 0, sizeof(sa));
 	sa.sin6_family = AF_INET6;
diff --git a/socket.h b/socket.h
index 93c41a0..49d999b 100644
--- a/socket.h
+++ b/socket.h
@@ -22,8 +22,8 @@
 #include <stdbool.h>
 #include <stdint.h>
 
-int socket_open_ipv4(uint16_t port);
-int socket_open_ipv6(uint16_t port);
+int socket_open_ipv4(uint16_t port, const char *iface);
+int socket_open_ipv6(uint16_t port, const char *iface);
 int socket_open_rtnl(bool ipv6);
 
 int socket_mcast_group_ipv4(int sock, unsigned int ifindex, bool join);