Comment 2 for bug 275288

Please disregard my findings above. Although that dhclient.conf syntax worked for me under Debian Sid, it's not the documented syntax for domain-list values as per dhcp-options(5) on Ubuntu.

With the following, I get the same behaviour as described by the original poster:

prepend domain-search "dom1.com dom2.com dom3.com dom4.com";

Ciao,
Sheldon.