conf: Allow address remapped to host to be configured

Because the host and guest share the same IP address with passt/pasta, it's
not possible for the guest to directly address the host.  Therefore we
allow packets from the guest going to a special "NAT to host" address to be
redirected to the host, appearing there as though they have both source and
destination address of loopback.

Currently that special address is always the address of the default
gateway (or none).  That can be a problem if we want that gateway to be
addressable by the guest.  Therefore, allow the special "NAT to host"
address to be overridden on the command line with a new --map-host-loopback
option.

In order to exercise and test it, update the passt_in_ns and perf
tests to use this option and give different mapping addresses for the
two layers of the environment.

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

1 files changed, 50 insertions, 2 deletions

diff --git a/conf.c b/conf.c
index d4a3e9c..d605c21 100644
--- a/conf.c
+++ b/conf.c
@@ -817,6 +817,9 @@ static void usage(const char *name, FILE *f, int status)
 		fprintf(f, "  --no-dhcp-search	No list in DHCP/DHCPv6/NDP\n");
 
 	fprintf(f,
+		"  --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"
 		"  --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"
@@ -959,6 +962,11 @@ static void conf_print(const struct ctx *c)
 	info("    host: %s", eth_ntop(c->our_tap_mac, bufmac, sizeof(bufmac)));
 
 	if (c->ifi4) {
+		if (!IN4_IS_ADDR_UNSPECIFIED(&c->ip4.map_host_loopback))
+			info("    NAT to host 127.0.0.1: %s",
+			     inet_ntop(AF_INET, &c->ip4.map_host_loopback,
+				       buf4, sizeof(buf4)));
+
 		if (!c->no_dhcp) {
 			uint32_t mask;
 
@@ -989,6 +997,11 @@ static void conf_print(const struct ctx *c)
 	}
 
 	if (c->ifi6) {
+		if (!IN6_IS_ADDR_UNSPECIFIED(&c->ip6.map_host_loopback))
+			info("    NAT to host ::1: %s",
+			     inet_ntop(AF_INET6, &c->ip6.map_host_loopback,
+				       buf6, sizeof(buf6)));
+
 		if (!c->no_ndp && !c->no_dhcpv6)
 			info("NDP/DHCPv6:");
 		else if (!c->no_ndp)
@@ -1123,6 +1136,35 @@ 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
+ */
+static void conf_nat(struct ctx *c, const char *arg, 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;
+	}
+
+	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))
+		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))
+		return;
+
+	die("Invalid address to remap to host: %s", optarg);
+}
+
+/**
  * conf_open_files() - Open files as requested by configuration
  * @c:		Execution context
  */
@@ -1231,6 +1273,7 @@ void conf(struct ctx *c, int argc, char **argv)
 		{"no-copy-routes", no_argument,		NULL,		18 },
 		{"no-copy-addrs", no_argument,		NULL,		19 },
 		{"netns-only",	no_argument,		NULL,		20 },
+		{"map-host-loopback", required_argument, NULL,		21 },
 		{ 0 },
 	};
 	const char *logname = (c->mode == MODE_PASTA) ? "pasta" : "passt";
@@ -1400,6 +1443,9 @@ void conf(struct ctx *c, int argc, char **argv)
 			netns_only = 1;
 			*userns = 0;
 			break;
+		case 21:
+			conf_nat(c, optarg, &no_map_gw);
+			break;
 		case 'd':
 			c->debug = 1;
 			c->quiet = 0;
@@ -1639,10 +1685,12 @@ void conf(struct ctx *c, int argc, char **argv)
 	    (*c->ip6.ifname_out && !c->ifi6))
 		die("External interface not usable");
 
-	if (c->ifi4 && !no_map_gw)
+	if (c->ifi4 && !no_map_gw &&
+	    IN4_IS_ADDR_UNSPECIFIED(&c->ip4.map_host_loopback))
 		c->ip4.map_host_loopback = c->ip4.guest_gw;
 
-	if (c->ifi6 && !no_map_gw)
+	if (c->ifi6 && !no_map_gw &&
+	    IN6_IS_ADDR_UNSPECIFIED(&c->ip6.map_host_loopback))
 		c->ip6.map_host_loopback = c->ip6.guest_gw;
 
 	if (c->ifi4 && IN4_IS_ADDR_UNSPECIFIED(&c->ip4.guest_gw))