How to use pcap file as input instead of interface

[moolinet]

Hello,

I have a tcpdump capture of erspan traffic.
I would like to pass it through gulp to decapsulte the erspan headers
and output the result back into a pcap file for further analysis with wireshark.

How can I do that?

Thanks,

[jmakov]

I believe there's a -d flag for that - " -d decapsulate Cisco ERSPAN GRE packets (sets -f value)".

[moolinet]

Yes I plan on using the -d flag to decapsulate ERSPAN (GRE) packets
but I was looking for a way to use a pcap file as source instead of eth0

for example:
instead of gulp -i eth0 -d
something like gulp -i capture.pcap -d

according to usage() I should be able to read from stdin:

cat capture.pcap | gulp -i - -d -o .
gulp:	Sending raw pcap data to a terminal is not a good idea.
	If you really want to do that, pipe gulp through cat but you
	probably want to redirect stdout to a file or another program instead.
	Perhaps you meant to pipe into 'tcpdump -r-' or 'ngrep -I-' ?

I finally got it working. requires root and when reading from stdin we need to specify file output -o

$ cat capture.pcap | sudo gulp  -d -i - -o . -n out

9857 packets captured
ring buffer use: 1.3% of 100 MB

which creates file pcap000.pcap

unfortunately the result pcap file is filled with invalid L2 frames. as if payload has been truncated.

viewing the original capture.pcap file with wireshark correctly show the GRE packets with complete payload.

passing the same file through gulp without -d results in a new pcap file with the same encapsulated packets as the source.

[jmakov]

Thanks for the more verbose description of your situation - now I understand your problem. I don't have a good answer yet how to solve your problem - it would need further investigation and funding for the project.

If that's not an option for you, what I can suggest is trying to contact one of the original authors and copy-paste your question there - he seems to be still active on his blog: https://blog.crox.net/archives/72-gulp-tcpdump-alternative-for-lossless-capture-on-Linux.html. We can then update the docs or the code with his solution here (if he doesn't decide to maintain his own project on GitHub as it's currently the case).