Interface configuration files control the software interfaces for individual network devices. As the system boots, it uses these files to determine what interfaces to bring up and how to configure them. These files are usually named ifcfg-<name>, where <name> refers to the name of the device that the configuration file controls.

Ethernet Interfaces

One of the most common interface files is ifcfg-eth0, which controls the first Ethernet network interface card or NIC in the system. In a system with multiple NICs, there are multiple ifcfg-eth<x> files (where <x> is a unique number corresponding to a specific interface). Because each device has its own configuration file, an administrator can control how each interface functions individually.

The following is a sample ifcfg-eth0 file for a system using a fixed IP address:

DEVICE=eth0 
BOOTPROTO=none 
ONBOOT=yes 
NETWORK=10.0.1.0 
NETMASK=255.255.255.0 
IPADDR=10.0.1.27 
USERCTL=no

The values required in an interface configuration file can change based on other values. For example, the ifcfg-eth0 file for an interface using DHCP looks different because IP information is provided by the DHCP server:

DEVICE=eth0 
BOOTPROTO=dhcp 
ONBOOT=yes

The Network Administration Tool (system-config-network) is an easy way to make changes to the various network interface configuration files.

However, it is also possible to manually edit the configuration files for a given network interface.

Below is a listing of the configurable parameters in an Ethernet interface configuration file:

BOOTPROTO=<protocol>

Where <protocol> is one of the following:

• none - No boot-time protocol should be used.
• bootp - The BOOTP protocol should be used.
• dhcp - The DHCP protocol should be used.

BROADCAST=<address>

Where <address> is the broadcast address. This directive is deprecated, as the value is calculated automatically with ifcalc.

DEVICE=<name>

Where <name> is the name of the physical device (except for dynamically-allocated PPP devices where it is the logical name).

DHCP_HOSTNAME

Use this option only if the DHCP server requires the client to specify a hostname before receiving an IP address.

DNS{1,2}=<address>

Where <address> is a name server address to be placed in /etc/resolv.conf if the PEERDNS directive is set to yes.

ETHTOOL_OPTS=<options>

Where <options> are any device-specific options supported by ethtool. For example, if you wanted to force 100Mb, full duplex:

ETHTOOL_OPTS="autoneg off speed 100 duplex full"

NOTE: changing speed or duplex settings almost always requires disabling autonegotiation with the autoneg off option. This needs to be stated first, as the option entries are order-dependent.

GATEWAY=<address>

Where <address> is the IP address of the network router or gateway device (if any).

HWADDR=<mac -address>

Where is the hardware address of the Ethernet device in the form AA:BB:CC:DD:EE:FF. This directive is useful for machines with multiple NICs to ensure that the interfaces are assigned the correct device names regardless of the configured load order for each NIC's module. This directive should not be used in conjunction with MACADDR.

IPADDR=<address>

Where <address> is the IP address.

MACADDR=<mac -address>

Where <mac -address> is the hardware address of the Ethernet device in the form AA:BB:CC:DD:EE:FF. This directive is used to assign a MAC address to an interface, overriding the one assigned to the physical NIC. This directive should not be used in conjunction with HWADDR.

MASTER=<bond -interface>

Where <bond -interface> is the channel bonding interface to which the Ethernet interface is linked. This directive is used in conjunction with the SLAVE directive.

NETMASK=<mask>

Where <mask> is the netmask value.

NETWORK=<address>

Where <address> is the network address. This directive is deprecated, as the value is calculated automatically with ifcalc.

ONBOOT=<answer>

Where is one of the following:

• yes — This device should be activated at boot-time.
• no — This device should not be activated at boot-time.

PEERDNS=<answer>

Where is one of the following:

• yes — Modify /etc/resolv.conf if the DNS directive is set. If using DHCP, then yes is the default.
• no — Do not modify /etc/resolv.conf.

SLAVE=<bond -interface>

Where is one of the following:

• yes — This device is controlled by the channel bonding interface specified in the MASTER directive.
• no — This device is not controlled by the channel bonding interface specified in the MASTER directive.

This directive is used in conjunction with the MASTER directive.

SRCADDR=<address>

Where <address> is the specified source IP address for outgoing packets.

USERCTL=<answer>

Where <answer> is one of the following:

• yes — Non-root users are allowed to control this device.
• no — Non-root users are not allowed to control this device.