Was doing some debugging the other day on a legacy system with an older version of tcpdump installed. When I imported to packets to Wireshark in order to better interpret the results - I saw the familiar message "Packet size limited during capture":
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| You never realize there's a problem with a packet capture until after you've finished and shipped it somewhere else for analysis |
It's then I remembered that older versions of tcpdump default to a snaplen of 68 bytes. In order to correct this you need to manually specify a longer snaplen. From the manpage of tcpdump 3.9.4:
-s Snarf snaplen bytes of data from each packet rather than the default of 68 (with SunOS's NIT, the minimum is actually 96). 68 bytes is adequate for IP, ICMP, TCP and UDP but may truncate protocol information from name server and NFS packets (see below). Packets truncated because of a limited snapshot are indicated in the output with ''[|proto]'', where proto is the name of the protocol level at which the truncation has occurred. Note that taking larger snapshots both increases the amount of time it takes to process packets and, effectively, decreases the amount of packet buffering. This may cause packets to be lost. You should limit snaplen to the smallest number that will capture the protocol information you're interested in. Setting snaplen to 0 means use the required length to catch whole packets.
So in order to capture the whole packet, you need to set a snaplen of 0 using the option "-s 0":
~]# tcpdump -i eth0 -n -s 0 host 192.168.1.1
This would capture traffic (-i eth0) on the eth0 interface, (-n) not converting host addresses to names, (-s 0) capturing the entire packet, (host 192.168.1.1) where the packet is to/from a host with the IP 192.168.1.1.
