Can't see user-agent header in access.log

[Alejandro <alejandritox@xxxxxxxxxxxxx>]

Dear all, I've a Squid proxy on a Debian Sarge box (I can't remember the 
squid version now). I want to filter by a "acl name browser" type in 
order to allow just Mozilla anc compatible web browsers. I made the 
appropriatte acl (acl webbrowsers browser Mozilla) and the filter rule 
(http_access deny !webbrowsers), but it doesn't work at all....Opera 
browsers can go through my Squid :(

So I went to the Squid's access.log file and I notice that user-agent 
header is not present at all.

What can I do to force the user-agent header to be present in the access 
log file in order to check its value ???

Special thanks

alejandro.-