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SubjectRe: udp traceroute dropping packets
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Russell Miller <rmiller@duskglow.com> writes:

> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
>
> I came across this archived linux-kernel message:
>
> http://www.uwsg.iu.edu/hypermail/linux/net/0007.2/0111.html
>
> I am having the exact same problem as is outlined there, from a post three
> years ago. Here's a summary:
>
> "I think I've found a bug in UDP/ICMP code in the kernel using
> traceroute.
>
> To reproduce: Launch traceroute -n to some Linux system nearby
> really quickly 3 times in the row; localhost won't work, it has to go
> through network. Quick response is crucial. I used systems w/ in
> the same physical network and a few routers between (still < 5 ms
> response).
>
> The effect: On third traceroute (or perhaps second/first, if you're quick
> enough), ICMP port unreachable will not be sent to the UDP datagram. "
>
> I reproduced this on a redhat 8.0 machine running kernel 2.4.23.
> Changing to the -I option of traceroute (to use ICMP) works
> flawlessly. I'll be glad to provide more information if you need
> it. Please CC, as I'm not subscribed.

You're probably hitting the ICMP rate limit.

Play with /proc/sys/net/ipv4/icmp_rate*.

- increase /proc/sys/net/ipv4/icmp_ratelimit

or

- clear bit 3 in /proc/sys/net/ipv4/icmp_ratemask

Phil.
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