[2/4] arp: respond with true mac address of LAN local remote hosts

[Jon Maloy <jmaloy@redhat.com>]

When we receive an ARP or NTP request over the tap interface for a
host on the local network, we respond with that host's real mac
address.

The local host, which is acting as a proxy for the default gateway,
is still exempted from this rule.

Signed-off-by: Jon Maloy <jmaloy@redhat.com>
---
 arp.c |  9 +++++++++
 fwd.c |  2 +-
 fwd.h |  3 ++-
 ndp.c | 11 +++++++++++
 4 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/arp.c b/arp.c
index fc482bb..fbd8219 100644
--- a/arp.c
+++ b/arp.c
@@ -29,6 +29,7 @@
 #include "dhcp.h"
 #include "passt.h"
 #include "tap.h"
+#include "netlink.h"
 
 /**
  * arp() - Check if this is a supported ARP message, reply as needed
@@ -39,6 +40,8 @@
  */
 int arp(const struct ctx *c, const struct pool *p)
 {
+	union inany_addr translated;
+	union inany_addr addr;
 	unsigned char swap[4];
 	struct ethhdr *eh;
 	struct arphdr *ah;
@@ -72,6 +75,12 @@ int arp(const struct ctx *c, const struct pool *p)
 	memcpy(am->tha,		am->sha,	sizeof(am->tha));
 	memcpy(am->sha,		c->our_tap_mac,	sizeof(am->sha));
 
+	/* If remote host on local network - respond with its mac address */
+	inany_from_af(&addr, AF_INET, am->tip);
+	nat_outbound(c, &addr, &translated);
+	if (!memcmp(&addr, &translated, sizeof(addr)))
+		nl_mac_get(nl_sock, &addr, am->sha);
+
 	memcpy(swap,		am->tip,	sizeof(am->tip));
 	memcpy(am->tip,		am->sip,	sizeof(am->tip));
 	memcpy(am->sip,		swap,		sizeof(am->sip));
diff --git a/fwd.c b/fwd.c
index 250cf56..02ebc9d 100644
--- a/fwd.c
+++ b/fwd.c
@@ -332,7 +332,7 @@ static bool fwd_guest_accessible(const struct ctx *c,
  * Only handles translations that depend *only* on the address.  Anything
  * related to specific ports or flows is handled elsewhere.
  */
-static void nat_outbound(const struct ctx *c, const union inany_addr *addr,
+void nat_outbound(const struct ctx *c, const union inany_addr *addr,
 			 union inany_addr *translated)
 {
 	if (inany_equals4(addr, &c->ip4.map_host_loopback))
diff --git a/fwd.h b/fwd.h
index 0458a3c..61632f2 100644
--- a/fwd.h
+++ b/fwd.h
@@ -56,5 +56,6 @@ uint8_t fwd_nat_from_splice(const struct ctx *c, uint8_t proto,
 			    const struct flowside *ini, struct flowside *tgt);
 uint8_t fwd_nat_from_host(const struct ctx *c, uint8_t proto,
 			  const struct flowside *ini, struct flowside *tgt);
-
+void nat_outbound(const struct ctx *c, const union inany_addr *addr,
+		  union inany_addr *translated);
 #endif /* FWD_H */
diff --git a/ndp.c b/ndp.c
index 3e15494..d702f37 100644
--- a/ndp.c
+++ b/ndp.c
@@ -32,6 +32,7 @@
 #include "passt.h"
 #include "tap.h"
 #include "log.h"
+#include "netlink.h"
 
 #define	RT_LIFETIME	65535
 
@@ -196,6 +197,9 @@ static void ndp_send(const struct ctx *c, const struct in6_addr *dst,
 static void ndp_na(const struct ctx *c, const struct in6_addr *dst,
 	    const struct in6_addr *addr)
 {
+	union inany_addr translated;
+	union inany_addr addr_any;
+
 	struct ndp_na na = {
 		.ih = {
 			.icmp6_type		= NA,
@@ -215,6 +219,13 @@ static void ndp_na(const struct ctx *c, const struct in6_addr *dst,
 
 	memcpy(na.target_l2_addr.mac, c->our_tap_mac, ETH_ALEN);
 
+	/* If remote host on local network - respond with its mac address */
+	inany_from_af(&addr_any, AF_INET6, addr);
+	nat_outbound(c, &addr_any, &translated);
+
+	if (!memcmp(&addr_any, &translated, sizeof(addr_any)))
+		nl_mac_get(nl_sock, &addr_any, na.target_l2_addr.mac);
+
 	ndp_send(c, dst, &na, sizeof(na));
 }
 
-- 
@@ -32,6 +32,7 @@
 #include "passt.h"
 #include "tap.h"
 #include "log.h"
+#include "netlink.h"
 
 #define	RT_LIFETIME	65535
 
@@ -196,6 +197,9 @@ static void ndp_send(const struct ctx *c, const struct in6_addr *dst,
 static void ndp_na(const struct ctx *c, const struct in6_addr *dst,
 	    const struct in6_addr *addr)
 {
+	union inany_addr translated;
+	union inany_addr addr_any;
+
 	struct ndp_na na = {
 		.ih = {
 			.icmp6_type		= NA,
@@ -215,6 +219,13 @@ static void ndp_na(const struct ctx *c, const struct in6_addr *dst,
 
 	memcpy(na.target_l2_addr.mac, c->our_tap_mac, ETH_ALEN);
 
+	/* If remote host on local network - respond with its mac address */
+	inany_from_af(&addr_any, AF_INET6, addr);
+	nat_outbound(c, &addr_any, &translated);
+
+	if (!memcmp(&addr_any, &translated, sizeof(addr_any)))
+		nl_mac_get(nl_sock, &addr_any, na.target_l2_addr.mac);
+
 	ndp_send(c, dst, &na, sizeof(na));
 }
 
-- 
2.48.1