Allow different external interfaces for IPv4 and IPv6 connectivity

It's quite plausible for a host to have both IPv4 and IPv6 connectivity,
but only via different interfaces.  For example, this will happen in the
case that IPv6 connectivity is via a tunnel (e.g. 6in4 or 6rd).  It would
also happen in the case that IPv4 access is via a tunnel on an otherwise
IPv6 only local network, which is a setup that might become more common in
the post IPv4 address exhaustion world.

In turns out there's no real need for passt/pasta to get its IPv4 and IPv6
connectivity via the same interface, so we can handle this situation fairly
easily.  Change the core to allow eparate external interfaces for IPv4 and
IPv6.  We don't actually set these separately for now.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

conf.c

@@ -630,17 +630,17 @@ static void conf_ip(struct ctx *c)
 		v4 = v6		= IP_VERSION_PROBE;
 	}
 
-	if (!c->ifi)
-		c->ifi = nl_get_ext_if(&v4, &v6);
+	if (!c->ifi4 && !c->ifi6)
+		c->ifi4 = c->ifi6 = nl_get_ext_if(&v4, &v6);
 
 	if (v4 != IP_VERSION_DISABLED) {
 		if (!c->gw4)
-			nl_route(0, c->ifi, AF_INET, &c->gw4);
+			nl_route(0, c->ifi4, AF_INET, &c->gw4);
 
 		if (!c->addr4) {
 			int mask_len = 0;
 
-			nl_addr(0, c->ifi, AF_INET, &c->addr4, &mask_len, NULL);
+			nl_addr(0, c->ifi4, AF_INET, &c->addr4, &mask_len, NULL);
 			c->mask4 = htonl(0xffffffff << (32 - mask_len));
 		}
 
@@ -658,7 +658,7 @@ static void conf_ip(struct ctx *c)
 		memcpy(&c->addr4_seen, &c->addr4, sizeof(c->addr4_seen));
 
 		if (!memcmp(c->mac, MAC_ZERO, ETH_ALEN))
-			nl_link(0, c->ifi, c->mac, 0, 0);
+			nl_link(0, c->ifi4, c->mac, 0, 0);
 	}
 
 	if (c->mode == MODE_PASST)
@@ -668,9 +668,9 @@ static void conf_ip(struct ctx *c)
 		int prefix_len = 0;
 
 		if (IN6_IS_ADDR_UNSPECIFIED(&c->gw6))
-			nl_route(0, c->ifi, AF_INET6, &c->gw6);
+			nl_route(0, c->ifi6, AF_INET6, &c->gw6);
 
-		nl_addr(0, c->ifi, AF_INET6,
+		nl_addr(0, c->ifi6, AF_INET6,
 			IN6_IS_ADDR_UNSPECIFIED(&c->addr6) ? &c->addr6 : NULL,
 			&prefix_len, &c->addr6_ll);
 
@@ -883,12 +883,12 @@ static void conf_print(const struct ctx *c)
 	char buf4[INET_ADDRSTRLEN], ifn[IFNAMSIZ];
 	int i;
 
-	if (c->mode == MODE_PASTA) {
-		info("Outbound interface: %s, namespace interface: %s",
-		     if_indextoname(c->ifi, ifn), c->pasta_ifn);
-	} else {
-		info("Outbound interface: %s", if_indextoname(c->ifi, ifn));
-	}
+	if (c->ifi4)
+		info("Outbound interface (IPv4): %s", if_indextoname(c->ifi4, ifn));
+	if (c->ifi6)
+		info("Outbound interface (IPv6): %s", if_indextoname(c->ifi6, ifn));
+	if (c->mode == MODE_PASTA)
+		info("Namespace interface: %s", c->pasta_ifn);
 
 	if (c->v4) {
 		info("ARP:");
@@ -1395,12 +1395,12 @@ void conf(struct ctx *c, int argc, char **argv)
 			usage(argv[0]);
 			break;
 		case 'i':
-			if (c->ifi) {
+			if (c->ifi4 || c->ifi6) {
 				err("Redundant interface: %s", optarg);
 				usage(argv[0]);
 			}
 
-			if (!(c->ifi = if_nametoindex(optarg))) {
+			if (!(c->ifi4 = c->ifi6 = if_nametoindex(optarg))) {
 				err("Invalid interface name %s: %s", optarg,
 				    strerror(errno));
 				usage(argv[0]);
@@ -1559,8 +1559,12 @@ void conf(struct ctx *c, int argc, char **argv)
 
 	get_dns(c);
 
-	if (!*c->pasta_ifn)
-		if_indextoname(c->ifi, c->pasta_ifn);
+	if (!*c->pasta_ifn) {
+		if (c->ifi4)
+			if_indextoname(c->ifi4, c->pasta_ifn);
+		else
+			if_indextoname(c->ifi6, c->pasta_ifn);
+	}
 
 	c->tcp.ns_detect_ports   = c->udp.ns_detect_ports   = 0;
 	c->tcp.init_detect_ports = c->udp.init_detect_ports = 0;

passt.h

@@ -122,6 +122,7 @@ enum passt_modes {
  * @mac:		Host MAC address
  * @mac_guest:		MAC address of guest or namespace, seen or configured
  * @v4:			Enable IPv4 transport
+ * @ifi4:		Index of routable interface for IPv4
  * @addr4:		IPv4 address for external, routable interface
  * @addr4_seen:		Latest IPv4 address seen as source from tap
  * @mask4:		IPv4 netmask, network order
@@ -130,6 +131,7 @@ enum passt_modes {
  * @dns4_fwd:		Address forwarded (UDP) to first IPv4 DNS, network order
  * @dns_search:		DNS search list
  * @v6:			Enable IPv6 transport
+ * @ifi6:		Index of routable interface for IPv6
  * @addr6:		IPv6 address for external, routable interface
  * @addr6_ll:		Link-local IPv6 address on external, routable interface
  * @addr6_seen:		Latest IPv6 global/site address seen as source from tap
@@ -137,7 +139,6 @@ enum passt_modes {
  * @gw6:		Default IPv6 gateway
  * @dns6:		IPv6 DNS addresses, zero-terminated
  * @dns6_fwd:		Address forwarded (UDP) to first IPv6 DNS, network order
- * @ifi:		Index of routable interface
  * @pasta_ifn:		Name of namespace interface for pasta
  * @pasta_ifn:		Index of namespace interface for pasta
  * @pasta_conf_ns:	Configure namespace interface after creating it
@@ -193,6 +194,7 @@ struct ctx {
 	unsigned char mac_guest[ETH_ALEN];
 
 	int v4;
+	unsigned int ifi4;
 	uint32_t addr4;
 	uint32_t addr4_seen;
 	uint32_t mask4;
@@ -203,6 +205,7 @@ struct ctx {
 	struct fqdn dns_search[MAXDNSRCH];
 
 	int v6;
+	unsigned int ifi6;
 	struct in6_addr addr6;
 	struct in6_addr addr6_ll;
 	struct in6_addr addr6_seen;
@@ -211,7 +214,6 @@ struct ctx {
 	struct in6_addr dns6[MAXNS + 1];
 	struct in6_addr dns6_fwd;
 
-	unsigned int ifi;
 	char pasta_ifn[IF_NAMESIZE];
 	unsigned int pasta_ifi;
 	int pasta_conf_ns;

tcp.c

@@ -2207,7 +2207,7 @@ static void tcp_conn_from_tap(struct ctx *c, int af, const void *addr,
 		struct sockaddr_in6 addr6_ll = {
 			.sin6_family = AF_INET6,
 			.sin6_addr = c->addr6_ll,
-			.sin6_scope_id = c->ifi,
+			.sin6_scope_id = c->ifi6,
 		};
 		if (bind(s, (struct sockaddr *)&addr6_ll, sizeof(addr6_ll))) {
 			close(s);

util.c

@@ -280,7 +280,7 @@ int sock_l4(const struct ctx *c, int af, uint8_t proto,
 
 			if (!memcmp(bind_addr, &c->addr6_ll,
 			    sizeof(c->addr6_ll)))
-				addr6.sin6_scope_id = c->ifi;
+				addr6.sin6_scope_id = c->ifi6;
 		} else {
 			addr6.sin6_addr = in6addr_any;
 		}