[PATCH v14 04/10] 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 neigbour 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, if available.

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()
v12: - Some minor changes after feedback from Stefano
v13: - Removed call to nat_outbound() before MAC resolution, as
       we are now handling guest-side visible addresses only.
v14: - Removed obsolete memcpy() in ndp.c::ndp_na()
---
 arp.c   | 8 ++++++--
 inany.c | 1 +
 ndp.c   | 4 +++-
 3 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/arp.c b/arp.c
index ad088b1..b4a686f 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;
 	struct {
 		struct ethhdr eh;
 		struct arphdr ah;
@@ -102,8 +103,11 @@ 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_neigh_mac_get(c, &tgt, 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/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 588b48f..7e2ae2a 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;
 	struct ndp_na na = {
 		.ih = {
 			.icmp6_type		= NA,
@@ -213,7 +214,8 @@ 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);
+	inany_from_af(&tgt, AF_INET6, addr);
+	fwd_neigh_mac_get(c, &tgt, na.target_l2_addr.mac);
 
 	ndp_send(c, dst, &na, sizeof(na));
 }
-- 
2.50.1