tcpdump: how to get grepable output?
For those like you who cannot use ngrep
, here's how to use awk
to make the tcpdump
output of packet contents grepable.
First some sample output as provided by tcpdump -x
, in order to present the task ahead:
$ tcpdump -xr dump.pcap 2>/dev/null
12:04:59.590664 IP 10.17.14.93.51009 > 239.194.1.9.51009: UDP, length 370
0x0000: 4500 018e 0000 4000 fa11 7625 0a11 0e5d
0x0010: efc2 0109 c741 c741 017a 6f28 1120 2020
0x0020: 3337 3030 3039 3031 3835 3635 3430 3130
...
And this is the copy-and-pastable awk
script you can pipe the output to
awk '{ if (match($0, /^[0-9]/, _)) { printf (NR == 1 ? "%s " : "\n%s "), $0; fflush() } else { sub(/^\s+0x[0-9a-z]+:\s+/, " "); gsub(" ", ""); printf "%s", $0 } } END { print ""; fflush() }'
in order to get the following, grepable output
12:04:59.590664 IP 10.17.14.93.51009 > 239.194.1.9.51009: UDP, length 370 4500018e00004000fa1176250a...
12:04:59.590798 IP 10.17.14.113.51011 > 239.194.1.11.51011: UDP, length 370 4500018e00004000fa11760f...
...
Below is a commented version of above script:
awk '{
# if this is a header line
if (match($0, /^[0-9]/, _)) {
# print the header, but:
# except for the first line,
# we need to insert a newline,
# as the preceding data lines
# have been stripped of theirs
# we also append a space to
# separate header info from the
# data that will get appended
printf (NR == 1 ? "%s " : "\n%s "), $0
# enforce line-buffering
fflush()
}
# otherwise it is a data line
else {
# remove the data address
sub(/^\s+0x[0-9a-z]+:\s+/, " ");
# remove all spaces
gsub(" ", "");
# print w/o newline
printf "%s", $0
}
}
END {
# print final newline, as
# the preceding data lines
# have been stripped of theirs
print ""
# enforce line-buffering
fflush()
}'
From the tcpdump
manpage:
-A Print each packet (minus its link level header) in ASCII. Handy
for capturing web pages.
Make sure you also use the -s 0
option to make sure the entire packet is displayed.