I don't need to tell anyone that a lot of interesting command-and-control traffic is sailing through our Web proxies right now. I encourage decent logging for anyone using Web proxies. Below are three example entries from a Squid access.log. This is "squid" format with entries for user-agent and referer tacked to the end.

Incidentally here is a diff of my Squid configuration that shows how I set up Squid.

r200a# diff /usr/local/etc/squid/squid.conf /usr/local/etc/squid/squid.conf.orig 
632,633c632,633
< acl our_networks src 10.0.0.0/8 172.16.0.0/12 192.168.0.0/16
< http_access allow our_networks
---
> #acl our_networks src 192.168.1.0/24 192.168.2.0/24
> #http_access allow our_networks
936c936
< http_port 172.16.2.1:3128
---
> http_port 3128
1990,1992d1989
< logformat squid-extended  %ts.%03tu %6tr %>a %Ss/%03Hs %<st
 %rm %ru %un %Sh/%<A %mt  "%{Referer}>h" "%{User-Agent}>h"
< 
< 
2022c2019
< access_log /usr/local/squid/logs/access.log squid-extended
---
> access_log /usr/local/squid/logs/access.log squid
2216c2213
< strip_query_terms off
---
> # strip_query_terms on
3056d3052
< visible_hostname r200a.taosecurity.com

If you worry I'm exposing this to the world, don't worry too much. I find the value of having this information in a place I can find it outweighs the possibility someone will use this data to exploit me. There's much easier ways to do that, I think.

The first record shows a Google query for the term "dia", where the referer was a query for "fbi". The second record is a Firefox prefetch of the first record. The third record is a query for a .gif.

 1213383786.614    255 192.168.2.103 TCP_MISS/200 9263
 GET http://www.google.com/search?hl=en&client=firefox-a&rls=
com.ubuntu%3Aen-US%3Aofficial&hs=Hqt&q=dia&btnG=Search -
 DIRECT/64.233.169.103 text/html "http://www.google.com/search
?q=fbi&ie=utf-8&oe=utf-8&aq=t&rls=com.ubuntu:en-US:official&client=firefox-a"
 "Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.8.1.14) Gecko/20060601
 Firefox/2.0.0.14 (Ubuntu-edgy)"

 1213383786.704     76 192.168.2.103 TCP_MISS/200 2775
 GET http://www.google.com/pfetch/dchart?s=DIA - 
 DIRECT/64.233.169.147 image/gif
 "http://www.google.com/search?hl=en&client=firefox-a&rls=com.ubuntu%3A
 en-US%3Aofficial&hs=Hqt&q=dia&btnG=Search" "Mozilla/5.0 (X11; U; Linux
 i686; en-US; rv:1.8.1.14) Gecko/20060601 Firefox/2.0.0.14 (Ubuntu-edgy)"

 1213383786.717     81 192.168.2.103 TCP_MISS/200 1146
 GET http://www.google.com/images/blogsearch-onebox.gif -
 DIRECT/64.233.169.99 image/gif "http://www.google.com/search?hl=en
 &client=firefox-a&rls=com.ubuntu%3Aen-US%3Aofficial&hs=Hqt&q=dia&btnG=Search"
 "Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.8.1.14) Gecko/20060601
 Firefox/2.0.0.14 (Ubuntu-edgy)"

What if you're a security person who can't access Web logs, but you have a NSM sensor in the vicinity? You might use Bro to log this activity, but I found something last year that's much simpler by Jason Bittel: Httpry.

r200a# httpry -h
httpry version 0.1.3 -- HTTP logging and information retrieval tool
Copyright (c) 2005-2008 Jason Bittel 
Usage: httpry [ -dhpq ] [ -i device ] [ -n count ] [ -o file ] [ -r file ]
              [ -s format ] [ -u user ] [ 'expression' ]

   -d           run as daemon
   -h           print this help information
   -i device    listen on this interface
   -n count     set number of HTTP packets to parse
   -o file      write output to a file
   -p           disable promiscuous mode
   -q           suppress non-critical output
   -r file      read packets from input file
   -s format    specify output format string
   -u user      set process owner
   expression   specify a bpf-style capture filter

Additional information can be found at:
   http://dumpsterventures.com/jason/httpry

In the following example I run Httpry against a trace of the traffic taken when I visited the site shown in the Squid logs earlier.

r200a# httpry -i bge0 -o /tmp/httprytest3.txt -q -u richard
 -s timestamp,source-ip,x-forwarded-for,direction,dest-ip,method,host,
 request-uri,user-agent,referer,status-code,http-version,reason-phrase
 -r /tmp/test3.pcap
r200a# cat /tmp/httprytest3.txt

# httpry version 0.1.3
# Fields: timestamp,source-ip,x-forwarded-for,direction,dest-ip,method,host,
request-uri,user-agent,referer,status-code,http-version,reason-phrase

06/13/2008 15:03:06     68.48.240.186   -       >       64.233.169.103  
GET     www.google.com  /search?hl=en&client=firefox-a&rls=com.ubuntu
%3Aen-US%3Aofficial&hs=Hqt&q=dia&btnG=Search      Mozilla/5.0
 (X11; U; Linux i686; en-US; rv:1.8.1.14) Gecko/20060601 Firefox/2.0.0.14
 (Ubuntu-edgy)      http://www.google.com/search?q=fbi&ie=utf-8&
oe=utf-8&aq=t&rls=com.ubuntu:en-US:official&client=firefox-a       -       
 HTTP/1.0        -

06/13/2008 15:03:06     64.233.169.103  -       <       68.48.240.186  
 -       -       -       -       -       200     HTTP/1.0        OK

06/13/2008 15:03:06     68.48.240.186   192.168.2.103   >       64.233.169.147 
 GET     www.google.com  /pfetch/dchart?s=DIA    Mozilla/5.0
 (X11; U; Linux i686; en-US; rv:1.8.1.14) Gecko/20060601 Firefox/2.0.0.14
 (Ubuntu-edgy)     http://www.google.com/search?hl=en&client=
firefox-a&rls=com.ubuntu%3Aen-US%3Aofficial&hs=Hqt&q=dia&btnG=Search -      
 HTTP/1.0        -

06/13/2008 15:03:06     68.48.240.186   192.168.2.103   >       64.233.169.99  
 GET     www.google.com  /images/blogsearch-onebox.gif   Mozilla/5.0
 (X11; U; Linux i686; en-US; rv:1.8.1.14) Gecko/20060601 Firefox/2.0.0.14
 (Ubuntu-edgy)     http://www.google.com/search?hl=en&client=
firefox-a&rls=com.ubuntu%3Aen-US%3Aofficial&hs=Hqt&q=dia&btnG=Search -      
 HTTP/1.0        -

06/13/2008 15:03:06     64.233.169.147  -       <       68.48.240.186  
 -       -       -       -       -       200     HTTP/1.0        OK
06/13/2008 15:03:06     64.233.169.99   -       <       68.48.240.186  
 -       -       -       -       -       200     HTTP/1.0        OK

As you can see, the format here is request-reply, although the last four records are request,request,reply,reply.

Although I first tried Httpry straight from the source code, in this case I tested an upcoming FreeBSD port created by my friend WXS. If you give Httpry a try, let me know what you think and how you like to invoke it on the command line. I plan to daemonize it in production and run it against a live interface, not traces.