DEV Community

loading...

tcpdump cheat sheet

dannypsnl profile image 林子篆 Originally published at dannypsnl.github.io on ・2 min read

tcpdump?

A command line tool for analyzing network packets.

How to get packets?

Using -i option, tcpdump -i <device-name>, you would get the packet through NIC(network interface card) named .

How to get <device-name>?

using ip link could get some, e.g.

$ ip link
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 16436 qdisc noqueue state UNKNOWN
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
    link/ether 08:01:37:d1:9c:bd brd ff:ff:ff:ff:ff:ff
$ tcpdump -i eth0
Enter fullscreen mode Exit fullscreen mode

Now I know how to get all packets through certain NIC, but I want to analysis specify connections, how can I get it?

Expression.

What Expression?

tcpdump expression.

What is tcpdump expression?

A set of combinable rules, for example src 10.0.0.10 means only the packets from 10.0.0.10 would show.Here is a list(incomplete) of tcpdump expression, the # leading comments:

host 10.0.0.10 # from or to 10.0.0.10
port 23 # contains port 23
dst 10.0.0.10 # as src, but for destination
# Protocols
arp # only ARP
icmp
tcp
udp
fddi
ether
# Protocol logic is simple, is not, for all
Enter fullscreen mode Exit fullscreen mode

You say they are combinable, how they combined?

Good question, they can be combined with or, and or with leading !(not); or is or, not and, for example, src 10.0.0.10 or src 10.0.0.11 is for packets from 10.0.0.10 or 10.0.0.11, obviously.

I found packets not showing all informations I want, what could I do?

Simple, tcpdump has -v for verbose output, -vv for more verbose, -vvv for even more verbose. -vvvv for… no, I’m kidding.

Ha, that’s funny, and the timestamp is not readable for me, how to make it more friendly?

Use -t, and not kidding, use -ttttt for maximally timestamp, and you have -kt, k <- 1..=5 for different level output.

I found the IP be replaced by host name but I don’t want it, how to remove it?

-n would tell tcpdump stop convert address.

What if I want to save my hard working result?

-w <filename> is what you’re looking for, it would write the raw packets to file named <filename> rather than parsing & printing. And remember you can record packets into a file, and use Wireshark to analysis it! For complex flow analyzing, I would do that. Now, the most important path already covered, you can always get more & fresher information about it from manpage of tcpdump, good lucks.

references

Discussion (1)

Forem Open with the Forem app