apache web server logging - custom log files vs. general log file - linux

Question

When I don't specify a logfile in the virtual host sections of my conf-file the logs are written in the file specified in httpd.conf (=access_log).
A log-entry would look like this:
SOMEIP - - [22/Jan/2013:18:34:08 +0100] "GET / HTTP/1.1" 200 1752 "-" "Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.17 (KHTML, like Gecko) Chrome/SOMEIP Safari/537.17"
SOMEIP - - [22/Jan/2013:18:34:08 +0100] "GET /img/homepage_bg.png HTTP/1.1" 304 - "http://DOMAIN/" "Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.17 (KHTML, like Gecko) C$
But when I define a log file in the virtual host section the new log file contains different information:
SOMEIP - - [22/Jan/2013:18:33:34 +0100] "GET / HTTP/1.1" 200 1752
SOMEIP - - [22/Jan/2013:18:33:34 +0100] "GET /img/homepage_bg.png HTTP/1.1" 304 -
i define the log file like this:
CustomLog logs/DOMAIN-access_log common
Why does a custom log contain less information than the general log where all virtual hosts log in by default?

Answer 1

You need to define the alias "common" with a log format that includes the user-agent.
LogFormat "%h %l %u %t \"%r\" %>s %b "%{User-agent}i" common

Answer 2

You didn't say what flavour of Linux you're using. Any decently configured Apache (for example the Debian-based ones like Ubuntu, Mint, etc.) will already have a fitting LogFormat containing the user-agent in their configuration. Look for all the lines matching LogFormat. You should find something like this:
LogFormat "%v:%p %h %l %u %t \"%r\" %>s %b \"%{Referer}i\" \"%{User-Agent}i\"" vhost_combined
LogFormat "%h %l %u %t \"%r\" %>s %b \"%{Referer}i\" \"%{User-Agent}i\"" combined
LogFormat "%h %l %u %t \"%r\" %>s %b" common
LogFormat "%{Referer}i -> %U" referer
LogFormat "%{User-agent}i" agent
Just use the combined or even the vhost_combined parameter for your logfile:
CustomLog logs/DOMAIN-access_log combined
You should also look at the documentation for the Custom Log Formats.