On Tue, Sep 09, 2025 at 09:59:14PM -0400, Jon Maloy wrote: > 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 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 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)); > } > > -- > 2.50.1 > -- David Gibson (he or they) | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you, not the other way | around. http://www.ozlabs.org/~dgibson