conf, fwd: Split notion of gateway/router from guest-visible host address

The @gw fields in the ip4_ctx and ip6_ctx give the (host's) default
gateway.  We use this for two quite distinct things: advertising the
gateway that the guest should use (via DHCP, NDP and/or --config-net)
and for a limited form of NAT.  So that the guest can access services
on the host, we map the gateway address within the guest to the
loopback address on the host.

Using the gateway address for this isn't necessarily the best choice
for this purpose, certainly not for all circumstances.  So, start off
by splitting the notion of these into two different values: @guest_gw
which is the gateway address the guest should use and @nat_host_loopback,
which is the guest visible address to remap to the host's loopback.

Usually nat_host_loopback will have the same value as guest_gw.  However
when --no-map-gw is specified we leave them unspecified instead.  This
means when we use nat_host_loopback, we don't need to separately check
c->no_map_gw to see if it's relevant.

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

1 files changed, 10 insertions, 6 deletions

diff --git a/passt.h b/passt.h
index c6c67ff..7cdba85 100644
--- a/passt.h
+++ b/passt.h
@@ -101,7 +101,9 @@ enum passt_modes {
  * @addr:		IPv4 address assigned to guest
  * @addr_seen:		Latest IPv4 address seen as source from tap
  * @prefixlen:		IPv4 prefix length (netmask)
- * @gw:			Default IPv4 gateway
+ * @guest_gw:		IPv4 gateway as seen by the guest
+ * @map_host_loopback:	Outbound connections to this address are NATted to the
+ *                      host's 127.0.0.1
  * @dns:		DNS addresses for DHCP, zero-terminated
  * @dns_match:		Forward DNS query if sent to this address
  * @our_tap_addr:	IPv4 address for passt's use on tap
@@ -116,7 +118,8 @@ struct ip4_ctx {
 	struct in_addr addr;
 	struct in_addr addr_seen;
 	int prefix_len;
-	struct in_addr gw;
+	struct in_addr guest_gw;
+	struct in_addr map_host_loopback;
 	struct in_addr dns[MAXNS + 1];
 	struct in_addr dns_match;
 	struct in_addr our_tap_addr;
@@ -136,7 +139,9 @@ struct ip4_ctx {
  * @addr:		IPv6 address assigned to guest
  * @addr_seen:		Latest IPv6 global/site address seen as source from tap
  * @addr_ll_seen:	Latest IPv6 link-local address seen as source from tap
- * @gw:			Default IPv6 gateway
+ * @guest_gw:		IPv6 gateway as seen by the guest
+ * @map_host_loopback:	Outbound connections to this address are NATted to the
+ *                      host's [::1]
  * @dns:		DNS addresses for DHCPv6 and NDP, zero-terminated
  * @dns_match:		Forward DNS query if sent to this address
  * @our_tap_ll:		Link-local IPv6 address for passt's use on tap
@@ -151,7 +156,8 @@ struct ip6_ctx {
 	struct in6_addr addr;
 	struct in6_addr addr_seen;
 	struct in6_addr addr_ll_seen;
-	struct in6_addr gw;
+	struct in6_addr guest_gw;
+	struct in6_addr map_host_loopback;
 	struct in6_addr dns[MAXNS + 1];
 	struct in6_addr dns_match;
 	struct in6_addr our_tap_ll;
@@ -213,7 +219,6 @@ struct ip6_ctx {
  * @no_dhcpv6:		Disable DHCPv6 server
  * @no_ndp:		Disable NDP handler altogether
  * @no_ra:		Disable router advertisements
- * @no_map_gw:		Don't map connections, untracked UDP to gateway to host
  * @low_wmem:		Low probed net.core.wmem_max
  * @low_rmem:		Low probed net.core.rmem_max
  */
@@ -273,7 +278,6 @@ struct ctx {
 	int no_dhcpv6;
 	int no_ndp;
 	int no_ra;
-	int no_map_gw;
 
 	int low_wmem;
 	int low_rmem;