conf, tcp, udp: Allow specification of interface to bind to

Since kernel version 5.7, commit c427bfec18f2 ("net: core: enable
SO_BINDTODEVICE for non-root users"), we can bind sockets to
interfaces, if they haven't been bound yet (as in bind()).

Introduce an optional interface specification for forwarded ports,
prefixed by %, that can be passed together with an address.

Reported use case: running local services that use ports we want
to have externally forwarded:
  https://github.com/containers/podman/issues/14425

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

9 files changed, 88 insertions, 47 deletions

diff --git a/conf.c b/conf.c
index 779371f..56f33f0 100644
--- a/conf.c
+++ b/conf.c
@@ -180,8 +180,8 @@ static int conf_ports(const struct ctx *c, char optname, const char *optarg,
 		      struct port_fwd *fwd)
 {
 	char addr_buf[sizeof(struct in6_addr)] = { 0 }, *addr = addr_buf;
+	char buf[BUFSIZ], *spec, *ifname = NULL, *p;
 	uint8_t exclude[PORT_BITMAP_SIZE] = { 0 };
-	char buf[BUFSIZ], *spec, *p;
 	sa_family_t af = AF_UNSPEC;
 	bool exclude_only = true;
 
@@ -209,9 +209,9 @@ static int conf_ports(const struct ctx *c, char optname, const char *optarg,
 
 		for (i = 0; i < PORT_EPHEMERAL_MIN; i++) {
 			if (optname == 't')
-				tcp_sock_init(c, 0, AF_UNSPEC, NULL, i);
+				tcp_sock_init(c, 0, AF_UNSPEC, NULL, NULL, i);
 			else if (optname == 'u')
-				udp_sock_init(c, 0, AF_UNSPEC, NULL, i);
+				udp_sock_init(c, 0, AF_UNSPEC, NULL, NULL, i);
 		}
 
 		return 0;
@@ -231,6 +231,14 @@ static int conf_ports(const struct ctx *c, char optname, const char *optarg,
 		if (optname != 't' && optname != 'u')
 			goto bad;
 
+		if ((ifname = strchr(buf, '%'))) {
+			if (spec - ifname >= IFNAMSIZ - 1)
+				goto bad;
+
+			*ifname = 0;
+			ifname++;
+		}
+
 		if (inet_pton(AF_INET, buf, addr))
 			af = AF_INET;
 		else if (inet_pton(AF_INET6, buf, addr))
@@ -278,9 +286,9 @@ static int conf_ports(const struct ctx *c, char optname, const char *optarg,
 			bitmap_set(fwd->map, i);
 
 			if (optname == 't')
-				tcp_sock_init(c, 0, af, addr, i);
+				tcp_sock_init(c, 0, af, addr, ifname, i);
 			else if (optname == 'u')
-				udp_sock_init(c, 0, af, addr, i);
+				udp_sock_init(c, 0, af, addr, ifname, i);
 		}
 
 		return 0;
@@ -324,9 +332,9 @@ static int conf_ports(const struct ctx *c, char optname, const char *optarg,
 			fwd->delta[i] = mapped_range.first - orig_range.first;
 
 			if (optname == 't')
-				tcp_sock_init(c, 0, af, addr, i);
+				tcp_sock_init(c, 0, af, addr, ifname, i);
 			else if (optname == 'u')
-				udp_sock_init(c, 0, af, addr, i);
+				udp_sock_init(c, 0, af, addr, ifname, i);
 		}
 	} while ((p = next_chunk(p, ',')));
 
@@ -720,8 +728,9 @@ static void usage(const char *name)
 	info(   "      'all': forward all unbound, non-ephemeral ports");
 	info(   "      a comma-separated list, optionally ranged with '-'");
 	info(   "        and optional target ports after ':', with optional");
-	info(   "        address specification suffixed by '/'. Ranges can be");
-	info(   "        reduced by excluding ports or ranges prefixed by '~'");
+	info(   "        address specification suffixed by '/' and optional");
+	info(   "        interface prefixed by '%%'. Ranges can be reduced by");
+	info(   "        excluding ports or ranges prefixed by '~'");
 	info(   "        Examples:");
 	info(   "        -t 22		Forward local port 22 to 22 on guest");
 	info(   "        -t 22:23	Forward local port 22 to 23 on guest");
@@ -740,6 +749,7 @@ static void usage(const char *name)
 	exit(EXIT_FAILURE);
 
 pasta_opts:
+
 	info(   "  -t, --tcp-ports SPEC	TCP port forwarding to namespace");
 	info(   "    can be specified multiple times"); 
 	info(   "    SPEC can be:");
@@ -747,7 +757,8 @@ pasta_opts:
 	info(   "      'auto': forward all ports currently bound in namespace");
 	info(   "      a comma-separated list, optionally ranged with '-'");
 	info(   "        and optional target ports after ':', with optional");
-	info(   "        address specification suffixed by '/'. Examples:");
+	info(   "        address specification suffixed by '/' and optional");
+	info(   "        interface prefixed by '%%'. Examples:");
 	info(   "        -t 22	Forward local port 22 to port 22 in netns");
 	info(   "        -t 22:23	Forward local port 22 to port 23");
 	info(   "        -t 22,25	Forward ports 22, 25 to ports 22, 25");
diff --git a/icmp.c b/icmp.c
index f02f89f..6bd87fd 100644
--- a/icmp.c
+++ b/icmp.c
@@ -169,7 +169,7 @@ int icmp_tap_handler(const struct ctx *c, int af, const void *addr,
 		iref.icmp.id = id = ntohs(ih->un.echo.id);
 
 		if ((s = icmp_id_map[V4][id].sock) <= 0) {
-			s = sock_l4(c, AF_INET, IPPROTO_ICMP, NULL, id,
+			s = sock_l4(c, AF_INET, IPPROTO_ICMP, NULL, NULL, id,
 				    iref.u32);
 			if (s < 0)
 				goto fail_sock;
@@ -207,7 +207,7 @@ int icmp_tap_handler(const struct ctx *c, int af, const void *addr,
 
 		iref.icmp.id = id = ntohs(ih->icmp6_identifier);
 		if ((s = icmp_id_map[V6][id].sock) <= 0) {
-			s = sock_l4(c, AF_INET6, IPPROTO_ICMPV6, NULL, id,
+			s = sock_l4(c, AF_INET6, IPPROTO_ICMPV6, NULL, NULL, id,
 				    iref.u32);
 			if (s < 0)
 				goto fail_sock;
diff --git a/passt.1 b/passt.1
index 555a50c..7d113f2 100644
--- a/passt.1
+++ b/passt.1
@@ -325,7 +325,8 @@ For low (< 1024) ports, see \fBNOTES\fR.
 .BR ports
 A comma-separated list of ports, optionally ranged with \fI-\fR, and,
 optionally, with target ports after \fI:\fR, if they differ. Specific addresses
-can be bound as well, separated by \fI/\fR. Within given ranges, selected ports
+can be bound as well, separated by \fI/\fR, and also, since Linux 5.7, limited
+to specific interfaces, prefixed by \fI%\fR. Within given ranges, selected ports
 and ranges can be excluded by an additional specification prefixed by \fI~\fR.
 Specifying excluded ranges only implies that all other ports are forwarded.
 Examples:
@@ -349,6 +350,9 @@ Forward local ports 22 to 80 to corresponding ports on the guest plus 10
 -t 192.0.2.1/22
 Forward local port 22, bound to 192.0.2.1, to port 22 on the guest
 .TP
+-t 192.0.2.1%eth0/22
+Forward local port 22, bound to 192.0.2.1 and interface eth0, to port 22
+.TP
 -t 2000-5000,~3000-3010
 Forward local ports 2000 to 5000, but not 3000 to 3010
 .TP
@@ -399,7 +403,8 @@ periodically derived (every second) from listening sockets reported by
 .BR ports
 A comma-separated list of ports, optionally ranged with \fI-\fR, and,
 optionally, with target ports after \fI:\fR, if they differ. Specific addresses
-can be bound as well, separated by \fI/\fR. Within given ranges, selected ports
+can be bound as well, separated by \fI/\fR, and also, since Linux 5.7, limited
+to specific interfaces, prefixed by \fI%\fR. Within given ranges, selected ports
 and ranges can be excluded by an additional specification prefixed by \fI~\fR.
 Specifying excluded ranges only implies that all other ports are forwarded.
 Examples:
@@ -424,6 +429,9 @@ namespace
 -t 192.0.2.1/22
 Forward local port 22, bound to 192.0.2.1, to port 22 in the target namespace
 .TP
+-t 192.0.2.1%eth0/22
+Forward local port 22, bound to 192.0.2.1 and interface eth0, to port 22
+.TP
 -t 2000-5000,~3000-3010
 Forward local ports 2000 to 5000, but not 3000 to 3010
 .TP
diff --git a/tcp.c b/tcp.c
index 830dc88..7e82589 100644
--- a/tcp.c
+++ b/tcp.c
@@ -3078,10 +3078,11 @@ void tcp_sock_handler(struct ctx *c, union epoll_ref ref, uint32_t events,
  * @ns:		In pasta mode, if set, bind with loopback address in namespace
  * @af:		Address family to select a specific IP version, or AF_UNSPEC
  * @addr:	Pointer to address for binding, NULL if not configured
+ * @ifname:	Name of interface to bind to, NULL if not configured
  * @port:	Port, host order
  */
 void tcp_sock_init(const struct ctx *c, int ns, sa_family_t af,
-		   const void *addr, in_port_t port)
+		   const void *addr, const char *ifname, in_port_t port)
 {
 	union tcp_epoll_ref tref = { .tcp.listen = 1 };
 	const void *bind_addr;
@@ -3103,8 +3104,8 @@ void tcp_sock_init(const struct ctx *c, int ns, sa_family_t af,
 		tref.tcp.splice = 0;
 
 		if (!ns) {
-			s = sock_l4(c, AF_INET, IPPROTO_TCP, bind_addr, port,
-				    tref.u32);
+			s = sock_l4(c, AF_INET, IPPROTO_TCP, bind_addr, ifname,
+				    port, tref.u32);
 			if (s >= 0)
 				tcp_sock_set_bufsize(c, s);
 			else
@@ -3118,8 +3119,8 @@ void tcp_sock_init(const struct ctx *c, int ns, sa_family_t af,
 			bind_addr = &(uint32_t){ htonl(INADDR_LOOPBACK) };
 
 			tref.tcp.splice = 1;
-			s = sock_l4(c, AF_INET, IPPROTO_TCP, bind_addr, port,
-				    tref.u32);
+			s = sock_l4(c, AF_INET, IPPROTO_TCP, bind_addr, ifname,
+				    port, tref.u32);
 			if (s >= 0)
 				tcp_sock_set_bufsize(c, s);
 			else
@@ -3144,8 +3145,8 @@ void tcp_sock_init(const struct ctx *c, int ns, sa_family_t af,
 
 		tref.tcp.splice = 0;
 		if (!ns) {
-			s = sock_l4(c, AF_INET6, IPPROTO_TCP, bind_addr, port,
-				    tref.u32);
+			s = sock_l4(c, AF_INET6, IPPROTO_TCP, bind_addr, ifname,
+				    port, tref.u32);
 			if (s >= 0)
 				tcp_sock_set_bufsize(c, s);
 			else
@@ -3159,8 +3160,8 @@ void tcp_sock_init(const struct ctx *c, int ns, sa_family_t af,
 			bind_addr = &in6addr_loopback;
 
 			tref.tcp.splice = 1;
-			s = sock_l4(c, AF_INET6, IPPROTO_TCP, bind_addr, port,
-				    tref.u32);
+			s = sock_l4(c, AF_INET6, IPPROTO_TCP, bind_addr, ifname,
+				    port, tref.u32);
 			if (s >= 0)
 				tcp_sock_set_bufsize(c, s);
 			else
@@ -3193,7 +3194,7 @@ static int tcp_sock_init_ns(void *arg)
 		if (!bitmap_isset(c->tcp.fwd_out.map, port))
 			continue;
 
-		tcp_sock_init(c, 1, AF_UNSPEC, NULL, port);
+		tcp_sock_init(c, 1, AF_UNSPEC, NULL, NULL, port);
 	}
 
 	return 0;
@@ -3410,7 +3411,8 @@ static int tcp_port_rebind(void *arg)
 
 			if ((a->c->ifi4 && tcp_sock_ns[port][V4] == -1) ||
 			    (a->c->ifi6 && tcp_sock_ns[port][V6] == -1))
-				tcp_sock_init(a->c, 1, AF_UNSPEC, NULL, port);
+				tcp_sock_init(a->c, 1, AF_UNSPEC, NULL, NULL,
+					      port);
 		}
 	} else {
 		for (port = 0; port < NUM_PORTS; port++) {
@@ -3443,7 +3445,8 @@ static int tcp_port_rebind(void *arg)
 
 			if ((a->c->ifi4 && tcp_sock_init_ext[port][V4] == -1) ||
 			    (a->c->ifi6 && tcp_sock_init_ext[port][V6] == -1))
-				tcp_sock_init(a->c, 0, AF_UNSPEC, NULL, port);
+				tcp_sock_init(a->c, 0, AF_UNSPEC, NULL, NULL,
+					      port);
 		}
 	}
 
diff --git a/tcp.h b/tcp.h
index 2548d4d..7ba7ab7 100644
--- a/tcp.h
+++ b/tcp.h
@@ -21,7 +21,7 @@ void tcp_sock_handler(struct ctx *c, union epoll_ref ref, uint32_t events,
 int tcp_tap_handler(struct ctx *c, int af, const void *addr,
 		    const struct pool *p, const struct timespec *now);
 void tcp_sock_init(const struct ctx *c, int ns, sa_family_t af,
-		   const void *addr, in_port_t port);
+		   const void *addr, const char *ifname, in_port_t port);
 int tcp_init(struct ctx *c);
 void tcp_timer(struct ctx *c, const struct timespec *ts);
 void tcp_defer_handler(struct ctx *c);
diff --git a/udp.c b/udp.c
index 5422fdd..cac9c65 100644
--- a/udp.c
+++ b/udp.c
@@ -1005,7 +1005,7 @@ int udp_tap_handler(struct ctx *c, int af, const void *addr,
 			union udp_epoll_ref uref = { .udp.bound = 1,
 						     .udp.port = src };
 
-			s = sock_l4(c, AF_INET, IPPROTO_UDP, NULL, src,
+			s = sock_l4(c, AF_INET, IPPROTO_UDP, NULL, NULL, src,
 				    uref.u32);
 			if (s < 0)
 				return p->count;
@@ -1057,8 +1057,8 @@ int udp_tap_handler(struct ctx *c, int af, const void *addr,
 						     .udp.v6 = 1,
 						     .udp.port = src };
 
-			s = sock_l4(c, AF_INET6, IPPROTO_UDP, bind_addr, src,
-				    uref.u32);
+			s = sock_l4(c, AF_INET6, IPPROTO_UDP, bind_addr, NULL,
+				    src, uref.u32);
 			if (s < 0)
 				return p->count;
 
@@ -1111,10 +1111,11 @@ int udp_tap_handler(struct ctx *c, int af, const void *addr,
  * @ns:		In pasta mode, if set, bind with loopback address in namespace
  * @af:		Address family to select a specific IP version, or AF_UNSPEC
  * @addr:	Pointer to address for binding, NULL if not configured
+ * @ifname:	Name of interface to bind to, NULL if not configured
  * @port:	Port, host order
  */
 void udp_sock_init(const struct ctx *c, int ns, sa_family_t af,
-		   const void *addr, in_port_t port)
+		   const void *addr, const char *ifname, in_port_t port)
 {
 	union udp_epoll_ref uref = { .udp.bound = 1 };
 	const void *bind_addr;
@@ -1138,8 +1139,8 @@ void udp_sock_init(const struct ctx *c, int ns, sa_family_t af,
 
 		if (!ns) {
 			uref.udp.splice = 0;
-			s = sock_l4(c, AF_INET, IPPROTO_UDP, bind_addr, port,
-				    uref.u32);
+			s = sock_l4(c, AF_INET, IPPROTO_UDP, bind_addr, ifname,
+				    port, uref.u32);
 
 			udp_tap_map[V4][uref.udp.port].sock = s;
 		}
@@ -1148,16 +1149,16 @@ void udp_sock_init(const struct ctx *c, int ns, sa_family_t af,
 			bind_addr = &(uint32_t){ htonl(INADDR_LOOPBACK) };
 			uref.udp.splice = UDP_TO_NS;
 
-			sock_l4(c, AF_INET, IPPROTO_UDP, bind_addr, port,
-				uref.u32);
+			sock_l4(c, AF_INET, IPPROTO_UDP, bind_addr, ifname,
+				port, uref.u32);
 		}
 
 		if (ns) {
 			bind_addr = &(uint32_t){ htonl(INADDR_LOOPBACK) };
 			uref.udp.splice = UDP_TO_INIT;
 
-			sock_l4(c, AF_INET, IPPROTO_UDP, bind_addr, port,
-				uref.u32);
+			sock_l4(c, AF_INET, IPPROTO_UDP, bind_addr, ifname,
+				port, uref.u32);
 		}
 	}
 
@@ -1171,8 +1172,8 @@ void udp_sock_init(const struct ctx *c, int ns, sa_family_t af,
 
 		if (!ns) {
 			uref.udp.splice = 0;
-			s = sock_l4(c, AF_INET6, IPPROTO_UDP, bind_addr, port,
-				    uref.u32);
+			s = sock_l4(c, AF_INET6, IPPROTO_UDP, bind_addr, ifname,
+				    port, uref.u32);
 
 			udp_tap_map[V6][uref.udp.port].sock = s;
 		}
@@ -1181,16 +1182,16 @@ void udp_sock_init(const struct ctx *c, int ns, sa_family_t af,
 			bind_addr = &in6addr_loopback;
 			uref.udp.splice = UDP_TO_NS;
 
-			sock_l4(c, AF_INET6, IPPROTO_UDP, bind_addr, port,
-				uref.u32);
+			sock_l4(c, AF_INET6, IPPROTO_UDP, bind_addr, ifname,
+				port, uref.u32);
 		}
 
 		if (ns) {
 			bind_addr = &in6addr_loopback;
 			uref.udp.splice = UDP_TO_INIT;
 
-			sock_l4(c, AF_INET6, IPPROTO_UDP, bind_addr, port,
-				uref.u32);
+			sock_l4(c, AF_INET6, IPPROTO_UDP, bind_addr, ifname,
+				port, uref.u32);
 		}
 	}
 }
@@ -1213,7 +1214,7 @@ int udp_sock_init_ns(void *arg)
 		if (!bitmap_isset(c->udp.fwd_out.f.map, dst))
 			continue;
 
-		udp_sock_init(c, 1, AF_UNSPEC, NULL, dst);
+		udp_sock_init(c, 1, AF_UNSPEC, NULL, NULL, dst);
 	}
 
 	return 0;
diff --git a/udp.h b/udp.h
index d14df0a..b4ee8b7 100644
--- a/udp.h
+++ b/udp.h
@@ -13,7 +13,7 @@ void udp_sock_handler(const struct ctx *c, union epoll_ref ref, uint32_t events,
 int udp_tap_handler(struct ctx *c, int af, const void *addr,
 		    const struct pool *p, const struct timespec *now);
 void udp_sock_init(const struct ctx *c, int ns, sa_family_t af,
-		   const void *addr, in_port_t port);
+		   const void *addr, const char *ifname, in_port_t port);
 int udp_init(struct ctx *c);
 void udp_timer(struct ctx *c, const struct timespec *ts);
 void udp_update_l2_buf(const unsigned char *eth_d, const unsigned char *eth_s,
diff --git a/util.c b/util.c
index 5b1e08a..76057be 100644
--- a/util.c
+++ b/util.c
@@ -90,13 +90,15 @@ found:
  * @af:		Address family, AF_INET or AF_INET6
  * @proto:	Protocol number
  * @bind_addr:	Address for binding, NULL for any
+ * @ifname:	Interface for binding, NULL for any
  * @port:	Port, host order
  * @data:	epoll reference portion for protocol handlers
  *
  * Return: newly created socket, -1 on error
  */
 int sock_l4(const struct ctx *c, int af, uint8_t proto,
-	    const void *bind_addr, uint16_t port, uint32_t data)
+	    const void *bind_addr, const char *ifname, uint16_t port,
+	    uint32_t data)
 {
 	union epoll_ref ref = { .r.proto = proto, .r.p.data = data };
 	struct sockaddr_in addr4 = {
@@ -163,6 +165,21 @@ int sock_l4(const struct ctx *c, int af, uint8_t proto,
 	if (setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, &y, sizeof(y)))
 		debug("Failed to set SO_REUSEADDR on socket %i", fd);
 
+	if (ifname) {
+		/* Supported since kernel version 5.7, commit c427bfec18f2
+		 * ("net: core: enable SO_BINDTODEVICE for non-root users"). If
+		 * it's unsupported, don't bind the socket at all, because the
+		 * user might rely on this to filter incoming connections.
+		 */
+		if (setsockopt(fd, SOL_SOCKET, SO_BINDTODEVICE,
+			       ifname, strlen(ifname))) {
+			warn("Can't bind socket for %s port %u to %s, closing",
+			     ip_proto_str[proto], port, ifname);
+			close(fd);
+			return -1;
+		}
+	}
+
 	if (bind(fd, sa, sl) < 0) {
 		/* We'll fail to bind to low ports if we don't have enough
 		 * capabilities, and we'll fail to bind on already bound ports,
diff --git a/util.h b/util.h
index f9a8ec6..7dc3d18 100644
--- a/util.h
+++ b/util.h
@@ -194,7 +194,8 @@ __attribute__ ((weak)) int ffsl(long int i) { return __builtin_ffsl(i); }
 char *ipv6_l4hdr(const struct pool *p, int index, size_t offset, uint8_t *proto,
 		 size_t *dlen);
 int sock_l4(const struct ctx *c, int af, uint8_t proto,
-	    const void *bind_addr, uint16_t port, uint32_t data);
+	    const void *bind_addr, const char *ifname, uint16_t port,
+	    uint32_t data);
 void sock_probe_mem(struct ctx *c);
 int timespec_diff_ms(const struct timespec *a, const struct timespec *b);
 void bitmap_set(uint8_t *map, int bit);