The config_eth0 variable is the heart of an interface configuration. It's a high level instruction list for configuring the interface (eth0 in this case). Each command in the instruction list is performed sequentially. The interface is deemed OK if at least one command works.

Here's a list of built-in instructions.

If a command fails, you can specify a fallback command. The fallback has to match the config structure exactly.

You can chain these commands together. Here are some real world examples.

# Adding three IPv4 addresses
config_eth0=(
  "192.168.0.2/24"
  "192.168.0.3/24"
  "192.168.0.4/24"
)

# Adding an IPv4 address and two IPv6 addresses
config_eth0=(
  "192.168.0.2/24"
  "4321:0:1:2:3:4:567:89ab"
  "4321:0:1:2:3:4:567:89ac"
)

# Keep our kernel assigned address, unless the interface goes
# down so assign another via DHCP. If DHCP fails then add a
# static address determined by APIPA
config_eth0=(
  "noop"
  "dhcp"
)
fallback_eth0=(
  "null"
  "apipa"
)

Init scripts in /etc/init.d can depend on a specific network interface or just net. All network interfaces in Gentoo's init system provide what is called net.

If, in /etc/rc.conf, rc_depend_strict="YES" is set, then all network interfaces that provide net must be active before a dependency on "net" is assumed to be met. In other words, if you have a net.eth0 and net.eth1 and an init script depends on "net", then both must be enabled.

On the other hand, if rc_depend_strict="NO" is set, then the "net" dependency is marked as resolved the moment at least one network interface is brought up.

But what about net.br0 depending on net.eth0 and net.eth1? net.eth1 may be a wireless or PPP device that needs configuration before it can be added to the bridge. This cannot be done in /etc/init.d/net.br0 as that's a symbolic link to net.lo.

The answer is defining an rc_need_ setting in /etc/conf.d/net.

rc_need_br0="net.eth0 net.eth1"

For a more detailed discussion about dependency, consult the section Writing Init Scripts in the Gentoo Handbook. More information about /etc/rc.conf is available as comments within that file.

Variable names are dynamic. They normally follow the structure of variable_${interface|mac|essid|apmac}. For example, the variable dhcpcd_eth0 holds the value for dhcpcd options for eth0 and dhcpcd_essid holds the value for dhcpcd options when any interface connects to the ESSID "essid".

However, there is no hard and fast rule that states interface names must be ethx. In fact, many wireless interfaces have names like wlanx, rax as well as ethx. Also, some user defined interfaces such as bridges can be given any name, such as foo. To make life more interesting, wireless Access Points can have names with non alpha-numeric characters in them - this is important because you can configure networking parameters per ESSID.

The downside of all this is that Gentoo uses bash variables for networking - and bash cannot use anything outside of English alpha-numerics. To get around this limitation we change every character that is not an English alpha-numeric into a _ character.

Another downside of bash is the content of variables - some characters need to be escaped. This can be achived by placing the \ character in front of the character that needs to be escaped. The following list of characters needs to be escaped in this way: ", ' and \.

In this example we use wireless ESSID as they can contain the widest scope of characters. We shall use the ESSID My "\ NET:

(This does work, but the domain is invalid)
dns_domain_My____NET="My \"\\ NET"

(The above sets the dns domain to My "\ NET when a wireless card
connects to an AP whose ESSID is My "\ NET)