A tcpdump tutorial and primer

[tptacek]

I. Love. Tcpdump. Once in a blue moon I'll boot up Wireshark, but it's pretty rare that there's a protocol question I need to answer that I can't answer faster with tcpdump -A.

[jswanson]

If you're on a server that doesn't have an X environment set up for wireshark, you can use tcpdump to spit to a file:

   -w     Write the raw packets to file rather than parsing and printing them out.  They can later be printed with the -r option.  Standard output is used if file is ``-''.  See pcap-savefile(5) for a description of the file format.

--

You can then open this file in wireshark on your desktop for easier analysis if you wish.

[est]

Hmm, how about display TCP payload only, capture SSL without ssldump, filter packets by process id?

[coolsunglasses]

tcpdump is one of my pet responses when people ask me why I still use terminal-based tooling.

[suprjami]

I guess it depends what you are trying to do. As someone who uses packet captures almost every day to solve other people's problems, all I want to see is an unfiltered binary capture file on the right interface, which I'll then go thru with tshark or Wireshark.

I think learning to build effective display filters in those tools is more useful than learning to use a capture tool in complex ways.

This can apply to troubleshooting your own problems too, as you can then inspect things in your own time and gather evidence to present later, and even use a capture file to replay traffic.

[jweir]

If you want to program with tcpdump check out its libary, pcap.

http://www.tcpdump.org/pcap.html

Lately I have been playing with a Go interface to it, https://github.com/miekg/pcap

Great tool, and lots of fun.

[hoggle]

Also for node programmers there is https://github.com/mranney/node_pcap I remember doing some pcap C programming some years ago but node with its streaming nature and powerful js derived abstractions is just so much more satisfying to me, simply less boilerplate and tons of fun - don't fall too far down the well.. then again it's Saturday tomorrow! ;)

[lightblade]

Which one came first? Tcpdump or pcap?

[Thaxll]

To send tcpdump output to your local Wireshark:

ssh -c arcfour root@myserver tcpdump -nn -U -s0 -w - 'not port 22' | wireshark -k -i -

[e12e]

I'd think you'd do well to stay away from RC4 for ssh as well as SSL/TLS?

[irickt]

Previous discussion: https://news.ycombinator.com/item?id=4744427

[lightblade]

Awesome! I always wanted a command-line alternative to wireshark. I learned about tcpdump about a year ago, but the amount of options is a little off putting. I'm glad this guide came along.

[MichaelGG]

Command-line alternative to wireshark is tshark. tshark is much more capable, since you can use all the well-made Wireshark protocol dissectors.

[gwu78]

Correct me if I'm wrong, but hasn't tcpdump had several buffer overflows in the past?

Personally I prefer multilog + pflogd + some other tool to examine the pcap file.

My old favorite is nc-data -d. The entire program fits on one page.

od or xxd -c1 |cut -d: -f2 will work too.

ngrep is fussy about interface types but I use that too.

Filters for nc-data output can be written in lex, sed, awk, lua, whatever.

I've even experimented with snobol4 and spitbol on packets since the output format of nc-data is so simple.

[danielweber]

Who else remembers when tcpdump kept on refusing to put a "see raw packet" option because you might use it to steal passwords?

Those were the days. Now all I can complain about is that you need root permissions on OpenBSD merely to read a capture file.

[minimax]

you need root permissions on OpenBSD merely to read a capture file.

How does that even work? It seems like if you can read the file there is nothing stoping you from trying to parse its contents.

[joeblau]

This is a great tool for getting free wi-fi if you know what you're doing and can change your MAC address.

[bananas]

Just what I needed. Been writing an SSDP stack with the aid of wireshark and it's been painful. I'm on OSX so it's been a case of frigging around with XQuartz as much as anything else. tcpdump is just pleasure and power compared to wireshark.

[anth1y]

I use this tutorial all the time when I'm trying to debug MTA issues.

[purple_horse]

One of my favs:

tcpdump -i int -n -w - -l -s 1500 | strings

[jacksoncage]

tcpdump for the win!