Re: [PATCH 2/2] udp: support traceroute for IPv4

[David Gibson <david@gibson.dropbear.id.au>]

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On Mon, Mar 17, 2025 at 01:40:51PM -0400, Jon Maloy wrote:
> 
> 
> On 2025-03-16 22:58, David Gibson wrote:
> > On Sat, Mar 15, 2025 at 11:32:45AM -0400, Jon Maloy wrote:
> > > Now that ICMP pass-through from socket-to-tap is in place, it is
> > > easy to support UDP based traceroute functionality in direction
> > > tap-to-socket.
> 
> [...]
> 
> > > +		    uint8_t ttl, const struct pool *p, int idx,
> > > +		    const struct timespec *now)
> > >   {
> > >   	const struct flowside *toside;
> > >   	struct mmsghdr mm[UIO_MAXIOV];
> > > @@ -933,6 +935,12 @@ int udp_tap_handler(const struct ctx *c, uint8_t pif,
> > >   		mm[i].msg_hdr.msg_controllen = 0;
> > >   		mm[i].msg_hdr.msg_flags = 0;
> > > +		if (ttl <= 30) {
> > > +			if (setsockopt(s, IPPROTO_IP, IP_TTL,
> > > +				       &ttl, sizeof(ttl)) < 0)
> > 
> > AFAIK this will set the TTL on every subsequent packet, not just the
> > next one, so this isn't quite right.  To use this I guess we'd have to
> > store the correct TTL in the flow, and do the sockopt if it's
> > different from the guest side one.
> 
> I forgot one thing at our discussion about this at our meeting.
> Traceroute works by sending three packets with ttl=1, then
> three with ttl=2, then three with ttl=3 and so forth.
> For each packet sent it steps the destination port number,
> starting at 33334, and in the most extreme case (assuming it
> stops at ttl=32, which linux traceroute does in practice)
> stopping at port number 33524.
> The whole port range [33434:33535] is unassigned by IANA, and
> no sane developer would use  it for his application, for obvious reasons.
> 
> This means that every single traceroute/udp message will become
> a separate flow, and there is no need to complicate things by
> storing the ttl in the flow or use it as criteria. It also
> means that my current approach works fine. I don't even need
> to restore the ttl in the outgoing socket, since that, just
> like the associated flow, is a single-shot affair.

Urgh.  I mean, yes it works for (the current implementation of)
traceroute, but doing it like this really isn't correct for anything
else that manipulates TTL.  I mean, yes, traceroute is *probably* the
only thing to do that, but I think we should endeavour to be more
generally correct if it's not too complicated.  And in this case, I
don't think it's too complicated.

> > Unless there's a way to set TTL
> > via cmsg.  Maybe there is, but I haven't spotted it in a quick glance.
> 
> I seems I can add it as ancillary data if I add a control buffer to struct
> mmsghdr.msghdr. However, it is hard to see any benefit of this, given the
> above. It will cost more performance on the critical code path
> than the simple test I am doing now.

Thinking about this more, I don't actually think setting the TTL
per-packet with a cmsg is particularly useful anyway.  For traceroute
to a small extent - and I suspect in the unlikely case of anything
else manipulating TTL - it seems like the guest is likely to use the
same TTL for multiple packets in a row.  That means just calling the
setsockopt() when it changes from the current value should work well.

-- 
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

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