fwd, conf: Allow NAT of the guest's assigned address

The guest is usually assigned one of the host's IP addresses.  That means
it can't access the host itself via its usual address.  The
--map-host-loopback option (enabled by default with the gateway address)
allows the guest to contact the host.  However, connections forwarded this
way appear on the host to have originated from the loopback interface,
which isn't always desirable.

Add a new --map-guest-addr option, which acts similarly but forwarded
connections will go to the host's external address, instead of loopback.

If '-a' is used, so the guest's address is not the same as the host's, this
will instead forward to whatever host-visible site is shadowed by the
guest's assigned address.

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

1 files changed, 29 insertions, 17 deletions

diff --git a/conf.c b/conf.c
index d605c21..6794508 100644
--- a/conf.c
+++ b/conf.c
@@ -820,6 +820,9 @@ static void usage(const char *name, FILE *f, int status)
 		"  --map-host-loopback ADDR	Translate ADDR to refer to host\n"
 	        "    can be specified zero to two times (for IPv4 and IPv6)\n"
 		"    default: gateway address\n"
+		"  --map-guest-addr ADDR	Translate ADDR to guest's address\n"
+	        "    can be specified zero to two times (for IPv4 and IPv6)\n"
+		"    default: none\n"
 		"  --dns-forward ADDR	Forward DNS queries sent to ADDR\n"
 		"    can be specified zero to two times (for IPv4 and IPv6)\n"
 		"    default: don't forward DNS queries\n"
@@ -1136,29 +1139,32 @@ static void conf_ugid(char *runas, uid_t *uid, gid_t *gid)
 }
 
 /**
- * conf_nat() - Parse --map-host-loopback option
- * @c:		Execution context
- * @arg:	String argument to --map-host-loopback
- * @no_map_gw:	--no-map-gw flag, updated for "none" argument
+ * conf_nat() - Parse --map-host-loopback or --map-guest-addr option
+ * @arg:	String argument to option
+ * @addr4:	IPv4 to update with parsed address
+ * @addr6:	IPv6 to update with parsed address
+ * @no_map_gw:	--no-map-gw flag, or NULL, updated for "none" argument
  */
-static void conf_nat(struct ctx *c, const char *arg, int *no_map_gw)
+static void conf_nat(const char *arg, struct in_addr *addr4,
+		     struct in6_addr *addr6, int *no_map_gw)
 {
 	if (strcmp(arg, "none") == 0) {
-		c->ip4.map_host_loopback = in4addr_any;
-		c->ip6.map_host_loopback = in6addr_any;
-		*no_map_gw = 1;
+		*addr4 = in4addr_any;
+		*addr6 = in6addr_any;
+		if (no_map_gw)
+			*no_map_gw = 1;
 	}
 
-	if (inet_pton(AF_INET6, arg, &c->ip6.map_host_loopback) &&
-	    !IN6_IS_ADDR_UNSPECIFIED(&c->ip6.map_host_loopback)	&&
-	    !IN6_IS_ADDR_LOOPBACK(&c->ip6.map_host_loopback)	&&
-	    !IN6_IS_ADDR_MULTICAST(&c->ip6.map_host_loopback))
+	if (inet_pton(AF_INET6, arg, addr6)	&&
+	    !IN6_IS_ADDR_UNSPECIFIED(addr6)	&&
+	    !IN6_IS_ADDR_LOOPBACK(addr6)	&&
+	    !IN6_IS_ADDR_MULTICAST(addr6))
 		return;
 
-	if (inet_pton(AF_INET, arg, &c->ip4.map_host_loopback)	&&
-	    !IN4_IS_ADDR_UNSPECIFIED(&c->ip4.map_host_loopback)	&&
-	    !IN4_IS_ADDR_LOOPBACK(&c->ip4.map_host_loopback)	&&
-	    !IN4_IS_ADDR_MULTICAST(&c->ip4.map_host_loopback))
+	if (inet_pton(AF_INET, arg, addr4)	&&
+	    !IN4_IS_ADDR_UNSPECIFIED(addr4)	&&
+	    !IN4_IS_ADDR_LOOPBACK(addr4)	&&
+	    !IN4_IS_ADDR_MULTICAST(addr4))
 		return;
 
 	die("Invalid address to remap to host: %s", optarg);
@@ -1274,6 +1280,7 @@ void conf(struct ctx *c, int argc, char **argv)
 		{"no-copy-addrs", no_argument,		NULL,		19 },
 		{"netns-only",	no_argument,		NULL,		20 },
 		{"map-host-loopback", required_argument, NULL,		21 },
+		{"map-guest-addr", required_argument,	NULL,		22 },
 		{ 0 },
 	};
 	const char *logname = (c->mode == MODE_PASTA) ? "pasta" : "passt";
@@ -1444,7 +1451,12 @@ void conf(struct ctx *c, int argc, char **argv)
 			*userns = 0;
 			break;
 		case 21:
-			conf_nat(c, optarg, &no_map_gw);
+			conf_nat(optarg, &c->ip4.map_host_loopback,
+				 &c->ip6.map_host_loopback, &no_map_gw);
+			break;
+		case 22:
+			conf_nat(optarg, &c->ip4.map_guest_addr,
+				 &c->ip6.map_guest_addr, NULL);
 			break;
 		case 'd':
 			c->debug = 1;