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Is there an automated way (either in tcpdump or via a helper app Out There) to generate a pcap file that contains only Ethernet, IP and Layer 4 (TCP in my case) headers, so that there is no payload/application data in the resulting pcap? I've found that since header sizes often vary, it's impossible to pick a capture size that won't catch any payload data.
930
tcpdump is a well known command line packet analyzer tool. Using tcpdump command we can capture the live TCP/IP packets and these packets can also be saved to a file.
The "-w" option lets you write the output of tcpdump to a file which you can save for further analysis. The "-r" option lets you read the output of a file. All you have to do is use the "-r" option with tcpdump command and specify the path of the file you want to read.
tcpdump also gives us an option to save captured packets in a file for future analysis. It saves the file in a pcap format, that can be viewed by tcpdump command or an open-source GUI-based tool called Wireshark (Network Protocol Analyzer) that reads tcpdump pcap format files.
Start Wireshark, then import the tcpdump captured session using File -> Open and browse for your file. You can also double-click the tcpdump capture file to open it in Wireshark, as long as it has the *. pcap file extension.
You can strip out the TCP payload very easily with Python's scapy module
BEFORE
[mpenning@hotcoffee tshark_wd]$ tcpdump -n -r sample.pcap
reading from file sample.pcap, link-type EN10MB (Ethernet)
00:25:42.443559 IP 192.168.12.237.1052 > 192.168.12.236.22: Flags [P.],
seq 2445372969:2445373021, ack 1889447842, win 63432, length 52
00:25:42.443607 IP 192.168.12.236.22 > 192.168.12.237.1052: Flags [.],
ack 52, win 65535, length 0
00:25:42.443980 IP 192.168.12.236.22 > 192.168.12.237.1052: Flags [P.],
seq 1:389, ack 52, win 65535, length 388
PAYLOAD STRIPPING
Running this as root in linux...
#!/usr/bin/env python
from scapy.all import *
INFILE = 'sample.pcap'
OUTFILE = 'stripped.pcap'
paks = rdpcap(INFILE)
for pak in paks:
pak[TCP].remove_payload()
wrpcap(OUTFILE, paks)
AFTER
[mpenning@hotcoffee tshark_wd]$ tcpdump -n -r stripped.pcap
reading from file sample.pcap, link-type EN10MB (Ethernet)
00:25:42.443559 IP truncated-ip - 52 bytes missing! 192.168.12.237.1052
> 192.168.12.236.22: Flags [P.], seq 2445372969:2445373021,
ack 1889447842, win 63432, length 52
00:25:42.443607 IP 192.168.12.236.22 > 192.168.12.237.1052: Flags [.],
ack 52, win 65535, length 0
00:25:42.443980 IP truncated-ip - 388 bytes missing! 192.168.12.236.22
> 192.168.12.237.1052: Flags [P.], seq 1:389,
ack 52, win 65535, length 388
In the tcpdump above, notice the "XX bytes missing!" messages. That is because we have removed the TCP payload.
53
If simple truncate would work for you, you could use:
tcpdump -i eth0 -s 96 -w test1.pcap
Later on you can analyze it with wireshark.
39
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