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.
8.2.1. 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.
Below is a sample ifcfg-eth0 file for a system using a fixed IP
address:
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 quite a bit different, because IP
information is provided by the DHCP server:
DEVICE=eth0
BOOTPROTO=dhcp
ONBOOT=yes
The Network Administration Tool
(redhat-config-network) is an easy way to make
changes to the various network interface configuration files (see the
chapter titled Network Configuration in the
Red Hat Linux Customization Guide for detailed instructions on using
this tool).
However, it is also possible to edit the configuration files for a given network
interface by hand.
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.
DEVICE=<name>,
where <name>
is the name of the physical device (except for
dynamically-allocated PPP devices where it is the
logical name).
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.
IPADDR=<address>,
where <address> is
the IP address.
NETMASK=<mask>,
where <mask> is
the netmask value.
NETWORK=<address>,
where <address> is
the network address. This directive is deprecated.
ONBOOT=<answer>,
where <answer> 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 <answer> is
one of the following:
yes — Modify
/etc/resolv.conf if the DNS directive is
set. If using DCHP, then yes is the
default.
no — Do not modify
/etc/resolv.conf.
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.
8.2.2. Dialup Interfaces
If connecting to the Internet via a dialup connection, a configuration
file is necessary for the interface.
PPP interface files are named using the following format
ifcfg-ppp<X>
(where <X> is a unique number
corresponding to a specific interface).
The PPP interface configuration file is created automatically when
wvdial, the
Network Administration Tool or
Kppp is used to create a dialup
account. The Red Hat Linux Getting Started Guide contains instructions for
using these GUI-based dialup connection tools. It is also possible to
create and edit this file manually.
Serial Line Internet Protocol (SLIP) is another
dialup interface, although it is used less frequently. SLIP files have
interface configuration file names such as
ifcfg-sl0.
Other options, not already discussed, that may be used in these files
include:
DEFROUTE=<answer>,
where <answer>
is one of the following:
yes — Set this interface as the
default route.
no — Do not set this interface as
the default route.
DEMAND=<answer>,
where <answer>
is one of the following:
yes — This interface will allow
pppd to initiate a connection when someone
attempts to use it.
no — A connection must be manually
established for this interface.
IDLETIMEOUT=<value>,
where <value> is
the number of seconds of idle activity before the interface will
disconnect itself.
INITSTRING=<string>,
where <string>
is the initialization string passed to the modem device. This option is
primarily used with SLIP interfaces.
LINESPEED=<value>,
where <value> is
the baud rate of the device. Possible standard values here include
57600, 38400,
19200, and 9600.
MODEMPORT=<device>,
where <device>
is the name of the serial device that is used to establish the connection
for the interface.
MTU=<value>,
where <value>
is the Maximum Transfer Unit (MTU) setting
for the interface. The MTU refers to the largest number of bytes
of data a frame can carry, not counting its header information. In
some dialup situations, setting this to a value of
576 will result in fewer packets dropped and a
slight improvement to the throughput for a connection.
NAME=<name>,
where <name> is the
reference to the title given to a collection of dialup connection
configurations.
PAPNAME=<name>,
where <name> is
the username given during the Password Authentication
Protocol (PAP) exchange that occurs to allow you to
connect to a remote system.
PEERDNS=<answer>,
where <answer>
is one of the following:
yes — Modify the
system's /etc/resolv.conf file entries to use
the DNS servers provided by the remote system when a connection
is established.
no — The
/etc/resolv.conf file will not be
modified.
PERSIST=<answer>,
where <answer>
is one of the following:
yes — This interface should be kept
active at all times, even if deactivated after a modem hang
up.
no — This interface should not be
kept active at all times.
REMIP=<address>,
where <address>
is the remote system's IP address. This is usually left
unspecified.
WVDIALSECT=<name>,
where <name>
associates this interface with a dialer configuration in
/etc/wvdial.conf. This file contains the
phone number to be dialed and other important information for the
interface.
8.2.3. Other Interfaces
Other common interface configuration files that use these options
include the following:
ifcfg-lo — A local
loopback interface is often used in
testing, as well as being used in a variety of applications that
require an IP address pointing back to the same system. Any data
sent to the loopback device is immediately returned to the host's
network layer.
Warning
Never edit the loopback interface script,
/etc/sysconfig/network-scripts/ifcfg-lo, by
hand. Doing so can prevent the system from operating correctly.
ifcfg-irlan0 — An
infrared interface allows information
between devices, such as a laptop and a printer, to flow over an
infrared link. This works in a similar way to an Ethernet device
except that it commonly occurs over a peer-to-peer connection.
ifcfg-plip0 — A Parallel
Line Interface Protocol (PLIP) connection works much
the same way, except that it utilizes a parallel port.
ifcfg-tr0 — Token
Ring topologies are not as common on
Local Area Networks
(LANs) as they once were, having been
eclipsed by Ethernet.
8.2.4. Alias and Clone Files
Two lesser-used types of interface configuration files found in the
/etc/sysconfig/network-scripts/ directory are
alias and clone files.
Alias interface configuration files take names in the format of
ifcfg-<if-name>:<alias-value>,
and they allow an alias to point to an interface. For example, an
ifcfg-eth0:0 file could be configured to specify
DEVICE=eth0:0 and a static IP address
of 10.0.0.2, serving as an alias of an Ethernet interface already
configured to receive its IP information via DHCP in
ifcfg-eth0. At that point, the
eth0 device is bound to a dynamic IP address, but
it can always be referred to on that system via the fixed 10.0.0.2 IP
address.
A clone interface configuration file should follow this naming
convention,
ifcfg-<if-name>-<clone-name>.
While an alias file is another way to refer to an existing interface
configuration file, a clone file is used to specify additional options
when specifying an interface. For example, if you have a standard DHCP
Ethernet interface called eth0, it may look
similar to this:
DEVICE=eth0
ONBOOT=yes
BOOTPROTO=dhcp
Since USERCTL is set to no if it is
not specified, users cannot bring this interface up and down. To give
users this ability, create a clone by copying
ifcfg-eth0 to
ifcfg-eth0-user and add the following line:
USERCTL=yes
When a user brings up the eth0 interface
with the ifup eth0-user command, the configuration
options from ifcfg-eth0 and
ifcfg-eth0-user are combined. While this is
a very basic example, this method can be used with a variety of
options and interfaces.
The easiest way to create alias and clone interface configuration
files is to use the graphical
Network Administration Tool. For more on using this
tool, see the chapter called Network
Configuration in the Red Hat Linux Customization Guide.
