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Michele's Blog

Observing X11 protocol differences

Michele Baldessari

I was trying to understand some oddities going with an X11 legacy application showing bad artifacts in one environment and working flawlessly in another environment. Since wireshark does not have any support for diffing two pcaps, I came up with the following steps:

  • Dump both working.pcap and nonworking.pcap into text files with full headers:
~/Devel/wireshark/tshark -r working.pcap -T text -V -Y "x11" > working.txt
~/Devel/wireshark/tshark -r notworking.pcap -T text -V -Y "x11" > notworking.txt
  • Prune the two text files with a script like the following:
def clean(source, dest):
    f = open(source, "r")
    x11_state = False
    output = []
    for line in f.readlines():
        if x11_state:
            if line.startswith(' '):
                output.append(line)
            else:
                x11_state = False
        else:
            if line.startswith('X11') and \
                    "Property" in line:
                output.append(line)
                x11_state = True
            else:
                continue
    o = open(dest, "w")
    for i in output:
        o.write(i)

if __name__ == '__main__':
    clean("working.txt", "working-trimmed.txt")
    clean("notworking.txt", "notworking-trimmed.txt")
  • At this point we can easily vimdiff the outputs obtained from above via vimdiff working-trimmed.txt notworking-trimmed.txt: vimdiff