Re: [PATCH v11 4/9] arp/ndp: respond with true MAC address of LAN local remote hosts

Two nits only: On Sat, 27 Sep 2025 15:25:17 -0400 Jon Maloy wrote:

When we receive an ARP request or NDP neigbor solicitation over

neighbour (it doesn't matter so much but I actually grep through commit messages from time to time...)

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 Reviewed-by: David Gibson

--- 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 57e7b59..4f6b7df 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 7f38b40..fef9d3d 100644 --- a/fwd.c +++ b/fwd.c @@ -496,7 +496,7 @@ 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) @@ -504,8 +504,8 @@ 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; @@ -539,7 +539,7 @@ 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 5464349..241f527 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 d7f64a3..610a06c 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

Maybe s/true/recorded/? Outside of the context of this series, otherwise, it's pretty hard to figure out what you mean with "true" here.

+ * 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)); }

-- Stefano