http request and tcpdump

If you work with http reverse proxy one of the most frequent problem is that people working on the backend systems complain about things they expect but they don’t see coming from your frontend service.
Working with Tivoli Access Manager this happen to me every time I pass some value to the backend services like iv-user, iv-remote-address or LtpaToken… every time people open the browser, press F12 and expect to find those data into the http request exchanged with the browser… NO!!! It does not work like that! :\

In these moments the only way to close the case is sniff some packets and put them in front of their nose with a giant red arrow showing the damn data they are expecting and which is perfectly exchanged between TAM and backend servers.
You can do this in many ways, the fastest and simple imho is by one of the most important tool for problem solving and analysis, the swiss knife of every sysadmin: tcpdump.

In this case the syntax of tcpdump is a bit “esoteric”, here it is:

sudo tcpdump -nn -i <interface> -A -s 0 '<protocol> port <port> and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)'

For example: sudo tcpdump -nn -i eth0 -A -s 0 ‘tcp port 80 and (((ip[2:2] – ((ip[0]&0xf)<<2)) – ((tcp[12]&0xf0)>>2)) != 0)’

If you want to grab the dump and open with some other software (for example the great Wireshark) you must add “-w /path/to/dump/dump.dmp”

That’s all.

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