Extra bytes at start when tcpdump'ing ippp device.

David Woodhouse (David.Woodhouse@axiom.demon.co.uk)
Fri, 10 Jul 1998 10:26:18 +0100

I've just set up our internet gateway with diald and ISDN. However, diald isn't
behaving correctly because it can't understand the outgoing packets. tcpdump
(from Red Hat 5.1) doesn't show them correctly either - they all seem to have a
prefix of ten bytes of crap. What causes this? Is it just that libpcap and
diald need to be taught about IPPP so that they can strip the headers properly,
or is the kernel doing something wrong?

The kernel is a stock 2.1.108, and the card is a Teles 16.3c PnP.

Here's a sample: pinging the other end of the PPP link.

11:39:56.937230 truncated-ip - 5026 bytes missing!0.84.34.37 > 0.0.64.1: (frag 32769:5104@63512+) [ttl 0]
1400 1400 8001 ff03 0021 4500 0054 2225
0000 4001 0944 9e98 1032 9e98 01de 0800
81d7 0a56 1f00 7c4c a335 344d 0e00 0809
0a0b 0c0d 0e0f 1011 1213 1415 1617 1819
1a1b 1c1d 1e1f 2021 2223 2425 2627 2829
2a2b 2c2d 2e2f 3031 3233 3435 3637
11:39:56.967230 demon-du.demon.co.uk > axiom.demon.co.uk: icmp: echo reply
4500 0054 c42a 0000 ff01 a83d 9e98 01de
9e98 1032 0000 89d7 0a56 1f00 7c4c a335
344d 0e00 0809 0a0b 0c0d 0e0f 1011 1213
1415 1617 1819 1a1b 1c1d 1e1f 2021 2223
2425 2627 2829 2a2b 2c2d 2e2f 3031 3233
3435 3637
11:39:57.937230 truncated-ip - 5026 bytes missing!0.84.34.40 > 0.0.64.1: (frag 32769:5104@63512+) [ttl 0]
1400 1400 8001 ff03 0021 4500 0054 2228
0000 4001 0941 9e98 1032 9e98 01de 0800
86d7 0a56 2000 7d4c a335 2d4d 0e00 0809
0a0b 0c0d 0e0f 1011 1213 1415 1617 1819
1a1b 1c1d 1e1f 2021 2223 2425 2627 2829
2a2b 2c2d 2e2f 3031 3233 3435 3637
11:39:57.967230 demon-du.demon.co.uk > axiom.demon.co.uk: icmp: echo reply
4500 0054 cb2a 0000 ff01 a13d 9e98 01de
9e98 1032 0000 8ed7 0a56 2000 7d4c a335
2d4d 0e00 0809 0a0b 0c0d 0e0f 1011 1213
1415 1617 1819 1a1b 1c1d 1e1f 2021 2223
2425 2627 2829 2a2b 2c2d 2e2f 3031 3233
3435 3637

---- ---- ----
David Woodhouse Dave@axiom.demon.co.uk (+44) 1223 812896
Software Engineer Dave@imladris.demon.co.uk (+44) 976 658355
Axiom (Cambridge) Ltd., Swaffham Bulbeck, Cambridge, CB5 0NA, UK.
finger dwmw2@ferret.lmh.ox.ac.uk for PGP key.

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