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kern/51450: tcptraceroute stopped working properly in 6.0 -> 7.0 transition
>Synopsis: tcptraceroute stopped working properly in 6.0 -> 7.0 transition
>Arrival-Date: Mon Aug 29 11:55:00 +0000 2016
>Release: NetBSD 7.0
System: NetBSD smistad.uninett.no 7.0 NetBSD 7.0 (GENERIC.201509250726Z) amd64
Record the traffic to a target of tcptraceroute with tcpdump.
Use wireshark to inspect the packet trace, notice that if the
sender runs NetBSD 7.0, wireshark notes that the TCP "Header
length is 0 (bogus, must be at least 20)".
Repeat the experiment with a machine which runs NetBSD 6.x,
and notice that tcptraceroute there works as intended, and
that the TCP header length field is set to 5 (x4-byte words = 20).
This causes machines which check for the TCP header length to
discard the probe packets emitted by a NetBSD 7.0-running
machine before detecting whether the port is open, and can
lead you to mistakenly conclude where the problem lies.
Sorry, don't know.
The diff to sys/netinet/raw_ip.c doesn't appear to shed much
light on what the problem might be, and neither does a similar
diff to ip_output.c.
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