how to rotate a tcpdump file

Matthew Seaman m.seaman at
Sun May 24 06:49:43 UTC 2009

Frank Shute wrote:
> On Sat, May 23, 2009 at 08:52:14PM +0100, Frank Shute wrote:

>> I was thinking of using the -C and -w options to tcpdump(1). From the
>> manpage:
>> -C     Before  writing  a  raw  packet to a savefile, check whether the
>>     file is currently larger than file_size and, if  so,  close  the
>>     current  savefile and open a new one.  Savefiles after the first
>>     savefile will have the name specified with the -w flag,  with  a
>>     number after it, starting at 1 and continuing upward.  The units
>>     of  file_size  are  millions  of  bytes  (1,000,000  bytes,  not
>>     1,048,576 bytes).
>> and now looking at it more closely, you don't even have to use
>> newsyslog. Just include the args: -C 10000000 -w my_tcpdump_log
> Oops! should be: -C 10 -w my_tcpdump_log
> I assume the OP is not too bothered whether it's megabytes or
> mebibytes or whatever the hell they call them (using base 10 rather
> than 2).

Hmmm... so when I said "tcpdump(1) doesn't have options to support rotating
dump files based on size" I was in fact *completely* wrong.  Memo to self:

Sorry for the noise folks.  Given it's a built-in function please ignore all
my blethering about shell scripts.



Dr Matthew J Seaman MA, D.Phil.                   7 Priory Courtyard
                                                  Flat 3
PGP:     Ramsgate
                                                  Kent, CT11 9PW

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