[PATCH v6 3/8] 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.

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.
v4: - Moved NAT check into the function nat_outbound() to obtain more
      precise criteria for when NAT is used. We may in theory
      have NAT even if original and translated addresses are equal, and
      we want to catch this case.
    - I chose to keep the wrapper funtion inany_nat(), but moved it to
      fwd.h/fwd.c and renamed it to fwd_inany_nat().
v5: - Simplified criteria for when we do ARP/NDP lookup. Now, we
      just try with the potentially translated address after an
      attempted NAT check.
    - Using the new ARP/NDP cache table instead of using netlink
      directly.
v6: - Fixes after feedback from David:
      - Renamed nat_outbound() to fwd_nat_outbound()
      - Eliminated unnecessary temporary variable in arp.c::arp()
---
 arp.c   | 9 +++++++--
 fwd.c   | 8 ++++----
 fwd.h   | 2 ++
 inany.c | 1 +
 ndp.c   | 8 ++++++++
 5 files changed, 22 insertions(+), 6 deletions(-)

diff --git a/arp.c b/arp.c
index 44677ad..90ed5dc 100644
--- a/arp.c
+++ b/arp.c
@@ -69,6 +69,7 @@ static bool ignore_arp(const struct ctx *c,
  */
 int arp(const struct ctx *c, struct iov_tail *data)
 {
+	union inany_addr tgt, tgt_nat;
 	struct {
 		struct ethhdr eh;
 		struct arphdr ah;
@@ -102,8 +103,12 @@ int arp(const struct ctx *c, struct iov_tail *data)
 	resp.ah.ar_hln = ah->ar_hln;
 	resp.ah.ar_pln = ah->ar_pln;
 
-	/* ARP message */
-	memcpy(resp.am.sha,		c->our_tap_mac,	sizeof(resp.am.sha));
+	/* MAC address to return in ARP message */
+	inany_from_af(&tgt, AF_INET, am->tip);
+	fwd_nat_outbound(c, &tgt, &tgt_nat);
+	fwd_neigh_mac_get(c, &tgt_nat, resp.am.sha);
+
+	/* Rest of ARP message */
 	memcpy(resp.am.sip,		am->tip,	sizeof(resp.am.sip));
 	memcpy(resp.am.tha,		am->sha,	sizeof(resp.am.tha));
 	memcpy(resp.am.tip,		am->sip,	sizeof(resp.am.tip));
diff --git a/fwd.c b/fwd.c
index c58ba9f..127bd31 100644
--- a/fwd.c
+++ b/fwd.c
@@ -500,7 +500,7 @@ static bool fwd_guest_accessible(const struct ctx *c,
 }
 
 /**
- * nat_outbound() - Apply address translation for outbound (TAP to HOST)
+ * fwd_nat_outbound() - Apply address translation for outbound (TAP to HOST)
  * @c:		Execution context
  * @addr:	Input address (as seen on TAP interface)
  * @translated:	Output address (as seen on HOST interface)
@@ -508,8 +508,8 @@ 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,
-			 union inany_addr *translated)
+void fwd_nat_outbound(const struct ctx *c, const union inany_addr *addr,
+		      union inany_addr *translated)
 {
 	if (inany_equals4(addr, &c->ip4.map_host_loopback))
 		*translated = inany_loopback4;
@@ -543,7 +543,7 @@ uint8_t fwd_nat_from_tap(const struct ctx *c, uint8_t proto,
 		   inany_equals6(&ini->oaddr, &c->ip6.dns_match))
 		tgt->eaddr.a6 = c->ip6.dns_host;
 	else
-		nat_outbound(c, &ini->oaddr, &tgt->eaddr);
+		fwd_nat_outbound(c, &ini->oaddr, &tgt->eaddr);
 
 	tgt->eport = ini->oport;
 
diff --git a/fwd.h b/fwd.h
index 728601f..a4dd6bb 100644
--- a/fwd.h
+++ b/fwd.h
@@ -50,6 +50,8 @@ void fwd_scan_ports_init(struct ctx *c);
 
 bool nat_inbound(const struct ctx *c, const union inany_addr *addr,
 		 union inany_addr *translated);
+void fwd_nat_outbound(const struct ctx *c, const union inany_addr *addr,
+		      union inany_addr *translated);
 uint8_t fwd_nat_from_tap(const struct ctx *c, uint8_t proto,
 			 const struct flowside *ini, struct flowside *tgt);
 uint8_t fwd_nat_from_splice(const struct ctx *c, uint8_t proto,
diff --git a/inany.c b/inany.c
index 65a39f9..7680439 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);
diff --git a/ndp.c b/ndp.c
index eb090cd..944371c 100644
--- a/ndp.c
+++ b/ndp.c
@@ -196,6 +196,7 @@ 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, tgt_nat;
 	struct ndp_na na = {
 		.ih = {
 			.icmp6_type		= NA,
@@ -215,6 +216,13 @@ 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 MAC address if remote host's address or
+	 * NAT translated address can be found in NDP table.
+	 */
+	inany_from_af(&tgt, AF_INET6, addr);
+	fwd_nat_outbound(c, &tgt, &tgt_nat);
+	fwd_neigh_mac_get(c, &tgt_nat, na.target_l2_addr.mac);
+
 	ndp_send(c, dst, &na, sizeof(na));
 }
 
-- 
@@ -196,6 +196,7 @@ 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, tgt_nat;
 	struct ndp_na na = {
 		.ih = {
 			.icmp6_type		= NA,
@@ -215,6 +216,13 @@ 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 MAC address if remote host's address or
+	 * NAT translated address can be found in NDP table.
+	 */
+	inany_from_af(&tgt, AF_INET6, addr);
+	fwd_nat_outbound(c, &tgt, &tgt_nat);
+	fwd_neigh_mac_get(c, &tgt_nat, na.target_l2_addr.mac);
+
 	ndp_send(c, dst, &na, sizeof(na));
 }
 
-- 
2.50.1