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mandrei99
Post  Post subject: Tcpdump: How to to capture only ICMP Fragmentation needed notifications  |  Posted: Thu Aug 22, 2013 6:50 am

Joined: Tue Aug 04, 2009 9:16 am
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Tcpdump: How to to capture only ICMP Fragmentation needed notifications

How to capture only ICMP Destination unreachable - Fragmentation required and DF bit set in tcpdump.

Man tcpdump quote:
Quote:
Quote:
Some offsets and field values may be expressed as names
rather than as numeric values. The following protocol
header field offsets are available: icmptype (ICMP type
field), icmpcode (ICMP code field), and tcpflags (TCP
flags field).



As shows on ICMP wiki page http://en.wikipedia.org/wiki/Internet_C ... e_Protocol, ICMP Destination unreachable is type 3 and Fragmentation needed is code 4.

Code:
# tcpdump -nni vlan111 -e icmp[icmptype] == 3 && icmp[icmpcode] == 4
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on vlan111, link-type EN10MB (Ethernet), capture size 65535 bytes
12:46:41.500646 00:10:db:ff:10:02 > 00:07:e9:a5:9b:fa, ethertype IPv4 (0x0800), length 70: 10.1.111.1 > 10.1.111.10: ICMP 10.0.0.3 unreachable - need to frag (mtu 1382), length 36


The tcpdump filter "icmp[icmptype] == 3 && icmp[icmpcode] == 4" does the job.





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