Continuous icmp host-unreachables

Brian Candler (B.Candler@pobox.com)
Fri, 9 Oct 1998 14:54:12 +0100


My machine, running 2.0.35, seems to have got into a state where it is
sending an icmp host unreachable every 10 seconds to a particular host, and
has been doing so for several hours.

Although I know this particular remote host has tried to connect in
previously, it is currently not sending any traffic (otherwise tcpdump would
show it)

# tcpdump -i eth0 -n -e icmp
tcpdump: listening on eth0
14:40:44.895803 0:a0:c9:12:52:c6 0:c0:95:f0:1c:de 0800 220: xxx.xxx.xxx.162 > yyy.yyy.yyy.86: icmp: host xxx.xxx.xxx.163 unreachable [tos 0xc0]
14:40:54.905803 0:a0:c9:12:52:c6 0:c0:95:f0:1c:de 0800 220: xxx.xxx.xxx.162 > yyy.yyy.yyy.86: icmp: host xxx.xxx.xxx.163 unreachable [tos 0xc0]
14:41:04.975803 0:a0:c9:12:52:c6 0:c0:95:f0:1c:de 0800 220: xxx.xxx.xxx.162 > yyy.yyy.yyy.86: icmp: host xxx.xxx.xxx.163 unreachable [tos 0xc0]
14:41:14.915803 0:a0:c9:12:52:c6 0:c0:95:f0:1c:de 0800 220: xxx.xxx.xxx.162 > yyy.yyy.yyy.86: icmp: host xxx.xxx.xxx.163 unreachable [tos 0xc0]
14:41:24.935803 0:a0:c9:12:52:c6 0:c0:95:f0:1c:de 0800 220: xxx.xxx.xxx.162 > yyy.yyy.yyy.86: icmp: host xxx.xxx.xxx.163 unreachable [tos 0xc0]
14:41:34.955803 0:a0:c9:12:52:c6 0:c0:95:f0:1c:de 0800 220: xxx.xxx.xxx.162 > yyy.yyy.yyy.86: icmp: host xxx.xxx.xxx.163 unreachable [tos 0xc0]

.163 is a host connected to eth1 on this box which is currently down, hence
no ARP responses. It is listed as "incomplete" in the arp table:

# arp -n
Address HWtype HWaddress Flags Mask Iface
...
xxx.xxx.xxx.163 (incomplete) eth1
...
xxx.xxx.xxx.163 ether 00:A0:C9:12:52:C6 CMP eth0
...

and seems to remain there permanently.

The only dodgy thing going on is that I _also_ have a proxyARP entry for this
IP address on a different interface (also shown above).

Could it be that the ARP entry is timing out, generating a host unreachable,
trying to remove itself from the ARP cache but failing because of the
existing proxy entry, and is thus doomed to repeat forever?

Brian.
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