fwd: Split notion of "our tap address" from gateway for IPv4

ip4.gw conflates 3 conceptually different things, which (for now) have the
same value:
  1. The router/gateway address as seen by the guest
  2. An address to NAT to the host with --no-map-gw isn't specified
  3. An address to use as source when nothing else makes sense

Case 3 occurs in two situations:

a) for our DHCP responses - since they come from passt internally there's
   no naturally meaningful address for them to come from
b) for forwarded connections coming from an address that isn't guest
   accessible (localhost or the guest's own address).

(b) occurs even with --no-map-gw, and the expected behaviour of forwarding
local connections requires it.

For IPv6 role (3) is now taken by ip6.our_tap_ll (which usually has the
same value as ip6.gw).  For future flexibility we may want to make this
"address of last resort" different from the gateway address, so split them
logically for IPv4 as well.

Specifically, add a new ip4.our_tap_addr field for the address with this
role, and initialise it to ip4.gw for now.  Unlike IPv6 where we can always
get a link-local address, we might not be able to get a (non 0.0.0.0)
address here (e.g. if the host is disconnected or only has a point to point
link with no gateway address).  In that case we have to disable forwarding
of inbound connections with guest-inaccessible source addresses.

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

3 files changed, 13 insertions, 7 deletions

diff --git a/dhcp.c b/dhcp.c
index acc5b03..353de32 100644
--- a/dhcp.c
+++ b/dhcp.c
@@ -345,9 +345,9 @@ int dhcp(const struct ctx *c, const struct pool *p)
 
 	m->yiaddr = c->ip4.addr;
 	mask.s_addr = htonl(0xffffffff << (32 - c->ip4.prefix_len));
-	memcpy(opts[1].s,  &mask,        sizeof(mask));
-	memcpy(opts[3].s,  &c->ip4.gw,   sizeof(c->ip4.gw));
-	memcpy(opts[54].s, &c->ip4.gw,   sizeof(c->ip4.gw));
+	memcpy(opts[1].s,  &mask,                sizeof(mask));
+	memcpy(opts[3].s,  &c->ip4.gw,           sizeof(c->ip4.gw));
+	memcpy(opts[54].s, &c->ip4.our_tap_addr, sizeof(c->ip4.our_tap_addr));
 
 	/* If the gateway is not on the assigned subnet, send an option 121
 	 * (Classless Static Routing) adding a dummy route to it.
@@ -377,7 +377,7 @@ int dhcp(const struct ctx *c, const struct pool *p)
 		opt_set_dns_search(c, sizeof(m->o));
 
 	dlen = offsetof(struct msg, o) + fill(m);
-	tap_udp4_send(c, c->ip4.gw, 67, c->ip4.addr, 68, m, dlen);
+	tap_udp4_send(c, c->ip4.our_tap_addr, 67, c->ip4.addr, 68, m, dlen);
 
 	return 1;
 }
diff --git a/fwd.c b/fwd.c
index d6f8a23..664b8ac 100644
--- a/fwd.c
+++ b/fwd.c
@@ -387,10 +387,14 @@ uint8_t fwd_nat_from_host(const struct ctx *c, uint8_t proto,
 	}
 
 	if (!fwd_guest_accessible(c, &ini->eaddr)) {
-		if (inany_v4(&ini->eaddr))
-			tgt->oaddr = inany_from_v4(c->ip4.gw);
-		else
+		if (inany_v4(&ini->eaddr)) {
+			if (IN4_IS_ADDR_UNSPECIFIED(&c->ip4.our_tap_addr))
+				/* No source address we can use */
+				return PIF_NONE;
+			tgt->oaddr = inany_from_v4(c->ip4.our_tap_addr);
+		} else {
 			tgt->oaddr.a6 = c->ip6.our_tap_ll;
+		}
 	} else {
 		tgt->oaddr = ini->eaddr;
 	}
diff --git a/passt.h b/passt.h
index 3b8a628..ecfed1e 100644
--- a/passt.h
+++ b/passt.h
@@ -97,6 +97,7 @@ enum passt_modes {
  * @gw:			Default IPv4 gateway
  * @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
  * @dns_host:		Use this DNS on the host for forwarding
  * @addr_out:		Optional source address for outbound traffic
  * @ifname_out:		Optional interface name to bind outbound sockets to
@@ -111,6 +112,7 @@ struct ip4_ctx {
 	struct in_addr gw;
 	struct in_addr dns[MAXNS + 1];
 	struct in_addr dns_match;
+	struct in_addr our_tap_addr;
 
 	/* PIF_HOST addresses */
 	struct in_addr dns_host;