IP Logging with Tethereal

Hi everybody
I have some internet points with Linux o.s. (RH8, RH9, Fedora1)
configured as Gateways.
A few PCs in the lans use these gws to browse over the internet.
For a log of the connections, with some content decode, I use tethereal
launched in the rc.local like this:

tethereal -i eth1 -q -n -p -t ad -R "http.request==TRUE or
(tcp.flags.syn==1 and tcp.flags.ack==0)" >`date '+%y%m%d%H%M%S'`.ethlog
&

It stay running and It creates a log file with the date/time as
filename and .ethlog as extension.
Well, it does work fine everywhere, except on a Fedora1 machine where
every 1/2 days I found it closed.
I have much traffic over that machine, but I have to notice that on
other machines the file reaches even 10 times bigger dimensions without
closing.
Can someone explain a reason why, and suggest a solution?
I thought about a daemon (I have one made by myself for another service
that doesn't have any problem) that launches tethereal and makes out
with a SIGNAL when tethereal closes, to re-launch it again. What about?
Do I risk something goes wrong and the daemon doesn't notice the
failure?
I could also think of an alternate solution: tcpdump doesn't allow me
the same filter and decoding as tethereal. Maybe does it exist some
other (better and safer) log sw?
Ty very much, I really need to solve this problem.
Marco 'Osolemio'