[PATCH v3 2/8] arp/ndp: respond with true MAC address of LAN local remote hosts

[Jon Maloy <jmaloy@redhat.com>]

When we receive an ARP request or NDP neigbor solicitation over
the tap interface for a host on the local network segment attached
to the template interface, 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>

---
v3: - Added helper function to find out if a remote ip address is subject
      to NAT. This filters out local host addresses which should be
      presented with the passt/pasta local MAC address 9a:55:9a:55:9a:55 even
      though it is on the local segment.
    - Adapted to the change in nl_mac_get() function, so that we now consider
      only the template interface when checking the ARP/NDP table.
---
 arp.c   |  9 +++++++++
 fwd.c   |  2 +-
 fwd.h   |  3 ++-
 inany.c | 15 +++++++++++++++
 inany.h |  1 +
 ndp.c   |  9 +++++++++
 6 files changed, 37 insertions(+), 2 deletions(-)

diff --git a/arp.c b/arp.c
index fc482bb..1952a63 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
@@ -40,6 +41,7 @@
 int arp(const struct ctx *c, const struct pool *p)
 {
 	unsigned char swap[4];
+	union inany_addr tgt;
 	struct ethhdr *eh;
 	struct arphdr *ah;
 	struct arpmsg *am;
@@ -72,6 +74,13 @@ 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));
 
+	/* Respond with true MAC address if remote host is on
+	 * the template interface's network segment
+	 */
+	inany_from_af(&tgt, AF_INET, am->tip);
+	if (!inany_nat(c, &tgt))
+		nl_mac_get(nl_sock, &tgt, c->ifi4, 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/inany.c b/inany.c
index f5483bf..0ecf10a 100644
--- a/inany.c
+++ b/inany.c
@@ -16,6 +16,7 @@
 #include "ip.h"
 #include "siphash.h"
 #include "inany.h"
+#include "fwd.h"
 
 const union inany_addr inany_loopback4 = INANY_INIT4(IN4ADDR_LOOPBACK_INIT);
 const union inany_addr inany_any4 = INANY_INIT4(IN4ADDR_ANY_INIT);
@@ -56,3 +57,17 @@ int inany_pton(const char *src, union inany_addr *dst)
 
 	return 0;
 }
+
+/** inany_nat - Find if a remote IPv[46] address is subject to NAT
+ * @c:		Execution context
+ * @addr:	IPv[46] address
+ *
+ * Return: true if translated, false otherwise
+ */
+bool inany_nat(const struct ctx *c, const union inany_addr *addr)
+{
+	union inany_addr addr_nat;
+
+	nat_outbound(c, addr, &addr_nat);
+	return !inany_equals(addr, &addr_nat);
+}
diff --git a/inany.h b/inany.h
index 7ca5cbd..b2a66b0 100644
--- a/inany.h
+++ b/inany.h
@@ -278,5 +278,6 @@ static inline void inany_siphash_feed(struct siphash_state *state,
 
 const char *inany_ntop(const union inany_addr *src, char *dst, socklen_t size);
 int inany_pton(const char *src, union inany_addr *dst);
+bool inany_nat(const struct ctx *c, const union inany_addr *addr);
 
 #endif /* INANY_H */
diff --git a/ndp.c b/ndp.c
index 3e15494..2e7f0bc 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 +197,7 @@ 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 tgt;
 	struct ndp_na na = {
 		.ih = {
 			.icmp6_type		= NA,
@@ -215,6 +217,13 @@ 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);
 
+	/* Respond with true link-layer address if remote host is on
+	 * the template interface's network segment
+	 */
+	inany_from_af(&tgt, AF_INET6, addr);
+	if (!inany_nat(c, &tgt))
+		nl_mac_get(nl_sock, &tgt, c->ifi6, 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 +197,7 @@ 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 tgt;
 	struct ndp_na na = {
 		.ih = {
 			.icmp6_type		= NA,
@@ -215,6 +217,13 @@ 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);
 
+	/* Respond with true link-layer address if remote host is on
+	 * the template interface's network segment
+	 */
+	inany_from_af(&tgt, AF_INET6, addr);
+	if (!inany_nat(c, &tgt))
+		nl_mac_get(nl_sock, &tgt, c->ifi6, na.target_l2_addr.mac);
+
 	ndp_send(c, dst, &na, sizeof(na));
 }
 
-- 
2.48.1