I received the following e-mail from a user:
Jun 30 (1 day ago)
I followed the Gentoo UML guide (http://www.gentoo.org/doc/en/uml.xml) and was successful at getting a running UML instance. However, I struggled with the networking and have a tip that may save others some time:
if using iptables, make sure the FORWARD chain on the packet filtering is letting the packets through. My chain was empty but the policy was set to DROP. As soon as I changed the policy to ACCEPT I was able to ping www.gentoo.org.
Thanks again for the documentation!
Heh. I didn't realize anybody still used UML.
I have moved the UML guide to https://wiki.gentoo.org/wiki/User-mode_Linux/System_testing_with_UML