final words on udp/ICMP dest unreach issue [+PATCH]

From: clemens (
Date: Mon Jul 30 2001 - 07:13:59 EST


concerning the bug discussed in the "missing icmp errors for udp
packets"-thread on netdev a solution has been found.

here comes the bug (see net/ipv4/icmp.c):

int xrlim_allow(struct dst_entry *dst, int timeout)
         unsigned long now;

          now = jiffies;
          dst->rate_tokens += now - dst->rate_last;
          dst->rate_last = now;
#1: if (dst->rate_tokens > XRLIM_BURST_FACTOR*timeout)
#2: dst->rate_tokens = XRLIM_BURST_FACTOR*timeout;
#3: if (dst->rate_tokens >= timeout) {
                   dst->rate_tokens -= timeout;
                return 1;
         return 0;

for timeout=0 rate_tokens will be reset to 0 tokens (#2), since #1 always
(icmp ping does have timeout=0, for instance)
this doesn't cause the packet to be filtered, since in #2
holds, but will cause the following packet to be filtered, if sent
before (now - dst->rate_last) < timeout.
(note: timeout is not 0 in this inequation, since it's the
timeout of the icmp type of the following packet)

a patch is attached.

thanks to all contributors, especially pekka savola, for discovering
that pinging before udp scanning will cause the troubles, and alexey
for suppling an alternative patch (for those intrested:

alan, please take care of that.

greets, clemens

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