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From: Rahul <llvm.zentrik@gmail.com>
To: passt-user@passt.top
Subject: UDP drops when source port is in use in the init namespace
Date: Sun, 30 Aug 2026 11:12:44 +0100	[thread overview]
Message-ID: <CAOiLhOTSZuDVd79dHkdVgVurXXkr62wPTYKuD7F++47ssPeJhg@mail.gmail.com> (raw)

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Hi,

I'm seeing DNS requests being dropped when using pasta 20250217. As far as
I can tell, UDP
flows where the source port chosen in the namespace is already in use in the
init namespace get dropped.

All three fwd_nat_from_*() preserve the source port for UDP:

tgt->oport = 0;
if (proto == IPPROTO_UDP)
/* But for UDP preserve the source port */
tgt->oport = ini->eport;

That forces a bind() to it, and udp_flow_sock() treats bind failure as
fatal, so
the flow is cancelled and the datagram dropped with no error to the sender.

I don't think I ever hit this with slirp4netns, it looks like libslirp's
udp_attach() never
binds a port, so the kernel just gives it a random free port.

I'm not sure whether this is intended, but if it is, is there any way to
work
around it?

Thanks

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             reply	other threads:[~2026-08-30 10:12 UTC|newest]

Thread overview: 2+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2026-08-30 10:12 Rahul [this message]
2026-09-01 18:56 ` Stefano Brivio

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