Eudev

eudev is a fork of systemd's udev with the goal of obtaining better compatibility with existing software such as OpenRC, Upstart, older kernels, various toolchains, and anything else required by (but not well supported by) udev.

Migrating from udev to eudev
Migrating from udev 216 to eudev 1.10-r2 (March 2015) is straight forward:

Keep classic 'eth0' naming
There are multiple options to keep the classic,  , etc. naming. The simplest ones are:

Alternatively add  to the kernel command line, change the default policy or create your own.

Use new 'predictable' naming
The network interface naming convention is not the same so interfaces will need to be re-linked. Use as a link target for whatever interface names need to be added. Be sure to replace  in the commands below with the Ethernet interface names that are present on the system. It is possible to discover which interfaces exist by running the command:

Create symbolic links for the existing interfaces in the and  directories:

Add the script(s) to the default runlevel to have the interface(s) start automatically:

udev 171-r10 to eudev 1.2-r1
See this post on the Gentoo forums.

Broken system switching from udev to eudev
See this post on the Gentoo forums.