Monday, March 26, 2012

Tcpdump: Packet Filtering And Analysis with Examples

Below are some examples which will help you to understand packet filtering better. Again, a word of warning, before performing packet filtering, make sure that you do not run it in promiscuous mode and you have permissions from your employer or you run it in your own network, better if you could create your own network using virtual machines. Make sure that it is legal in your country.

Let us start.

1. Capture everything coming to your machine and write it to dump.pcap:
tcpdump -w dump.pcap

2. Capture only tcp packets and write it to dump.pcap:
tcpdump tcp -w dump.pcap
You can also use udp in place of tcp to capture udp packets

3. Capture all packets from port 8080 and write it to dump.pcap:
tcpdump port 8080 -w dump.pcap
You can use dst port and src port in place of port to specify the filter for destination and source port only.

4. Capture all udp packets with destination port 53 and write it to dump.pcap:
tcpdump udp and dst port 53 -w dump.pcap
Use and to connect two or more types of filters

5. Capture all the packets from interface eth0 only and write it to dump.pcap:
tcpdump -i eth0

6. Capture all the packets coming from
tcpdump src -w dump.pcap

7. Capture all the packets coming from or going to
tcpdump host -w dump.pcap

8. Capture tcp packets from interface wlan0 going to at port 80:
tcpdump -i wlan0 and tcp and dst and port 80 -w dump.pcap

9. Change the size of packet capture:
tcpdump -s 0 -w dump.pcap
Here -s signifies the snap length. You can specify the maximum length of packets to capture. Bigger packets will be captured in truncated manner. Do not specify the highest limit when you don't have to. Taking larger snapshots both increases the amount of time it takes to process packets and, effectively, decreases the amount of packet buffering. Maximum limit as of now is 65535 with 0 representing the same.

10. Capture only 500 packets:
tcpdump -c 500 -w dump.pcap

11. Capture all the packets except coming from or going to
tcpdump not host

12. Capture all the packets coming from and going to entire subnet:
tcpdump net
Next, you can use wireshark to analyse the packets. I wrote a post sometime ago, about wireshark which might help you.