Re: [PATCH] Send an initial ARP request to resolve the guest IP address

On Sun, Sep 07, 2025 at 01:01:08PM +0200, Volker Diels-Grabsch wrote:

When restarting passt while QEMU keeps running with a configured "reconnect-ms" setting, the port forwardings will stop working until the guest sends some outgoing network traffic.

Reason: Although QEMU reconnects successfully to the unix domain socket of the new passt process, that one no longer knows the guest's MAC address and uses instead a broadcast MAC address. However, this is ignored by the guest, at least if the guest runs Linux.

Huh... I thought Linux would respond to that. I wonder if there's some sysctl config or version difference going on here.

Only after the guest sends some network package on its own initiative, passt will

[Aside: Although "packet" and "package" usually mean the same thing in English, it's always "network packets" not "network packages" and always "software packages" not "software packets"]

know the MAC address and will be able to establish forwarded connections.

This change fixes this issue by sending an ARP request to resolve the guest's MAC address via its IP address, which we do know, right after the unix domain socket (re)connection.

Right. The guest doesn't necessarily use the IP we give it, but it usually will, so this will work most of the time.

The only case where the IP is "wrong" would be if the configuration changed, and/or on the very first start right after qemu started. But in those cases, we just wouldn't get an ARP response, and can't do anything until we receive the guest's DHCP request - just as before. In other words, in the worst case an ARP request would be harmless.

Right. This seems like a good idea to me. It won't help in all cases, but it will help in some, and I can't see any harm it could do. Some comments on the implementation details below.

Signed-off-by: Volker Diels-Grabsch --- arp.c | 34 ++++++++++++++++++++++++++++++++++ arp.h | 1 + tap.c | 15 +++++++++++++-- 3 files changed, 48 insertions(+), 2 deletions(-)

diff --git a/arp.c b/arp.c index 44677ad..561581a 100644 --- a/arp.c +++ b/arp.c @@ -112,3 +112,37 @@ int arp(const struct ctx *c, struct iov_tail *data)

return 1; } + +/** + * send_initial_arp_req() - Send initial ARP request to retrieve guest MAC address

Wrap to 80 columns, please. Also, since this is exposed from this module the name should start with 'arp_'.

+ * @c: Execution context + */ +void send_initial_arp_req(const struct ctx *c) +{ + struct { + struct ethhdr eh; + struct arphdr ah; + struct arpmsg am; + } __attribute__((__packed__)) req; + + /* Ethernet header */ + req.eh.h_proto = htons(ETH_P_ARP); + memcpy(req.eh.h_dest, c->guest_mac, sizeof(req.eh.h_dest));

At this point we expect guest_mac to be the broadcast address, but it seems like explicitly using broadcast would be a little more robust if that changes at some point.

+ memcpy(req.eh.h_source, c->our_tap_mac, sizeof(req.eh.h_source)); + + /* ARP header */ + req.ah.ar_op = htons(ARPOP_REQUEST); + req.ah.ar_hrd = htons(ARPHRD_ETHER); + req.ah.ar_pro = htons(ETH_P_IP); + req.ah.ar_hln = ETH_ALEN; + req.ah.ar_pln = 4; + + /* ARP message */ + memcpy(req.am.sha, c->our_tap_mac, sizeof(req.am.sha)); + memcpy(req.am.sip, &c->ip4.our_tap_addr, sizeof(req.am.sip)); + memcpy(req.am.tha, c->guest_mac, sizeof(req.am.tha)); + memcpy(req.am.tip, &c->ip4.addr, sizeof(req.am.tip)); + + info("sending initial ARP request to retrieve guest MAC address after reconnect"); + tap_send_single(c, &req, sizeof(req)); +} diff --git a/arp.h b/arp.h index 86bcbf8..5490144 100644 --- a/arp.h +++ b/arp.h @@ -21,5 +21,6 @@ struct arpmsg { } __attribute__((__packed__));

int arp(const struct ctx *c, struct iov_tail *data); +void send_initial_arp_req(const struct ctx *c);

#endif /* ARP_H */ diff --git a/tap.c b/tap.c index 7ba6399..47dedd5 100644 --- a/tap.c +++ b/tap.c @@ -1355,6 +1355,16 @@ static void tap_start_connection(const struct ctx *c) ev.events = EPOLLIN | EPOLLRDHUP; ev.data.u64 = ref.u64; epoll_ctl(c->epollfd, EPOLL_CTL_ADD, c->fd_tap, &ev); + + switch (c->mode) { + case MODE_PASST: + send_initial_arp_req(c); + break; + case MODE_PASTA: + break; + case MODE_VU: + break; + }

I don't think we want to make this conditional on MODE_PASST. I think MODE_VU could suffer from the same problem. So could MODE_PASTA, although it would take a more unusual setup to trigger it. In any case, sending it unconditionally should be at worst harmless and is simpler.

}

/** @@ -1504,8 +1514,9 @@ void tap_backend_init(struct ctx *c) tap_sock_unix_init(c);

/* In passt mode, we don't know the guest's MAC address until it - * sends us packets. Use the broadcast address so that our - * first packets will reach it. + * sends us packets (e.g. responds to our initial ARP request). + * Until then, use the broadcast address so that our first + * packets will reach it. */ memset(&c->guest_mac, 0xff, sizeof(c->guest_mac)); break; -- 2.47.2

-- 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