Man Interfaces

Last changed: 2009/02/07 17:20 



Alejo Duque

/etc/network/interfaces contains network interface configuration information for the ifup(8) and ifdown(8) commands. This is where you configure how your system is connected to the network. Lines starting with `#' are ignored. Note that end-of-line comments are NOT supported, comments must be on a line of their own. A line may be extended across multiple lines by making the last character a backslash. The file consists of zero or more "iface", "mapping", "auto" and "allow-" stanzas. Here is an example.

auto lo eth0

allow-hotplug eth1

iface lo inet loopback

mapping eth0

script /usr/local/sbin/map-scheme

map HOME eth0-home

map WORK eth0-work

iface eth0-home inet static



up flush-mail

iface eth0-work inet dhcp

iface eth1 inet dhcp

Lines beginning with the word "auto" are used to identify the physical interfaces to be brought up when ifup is run with the -a option. (This option is used by the system boot scripts.) Physical interface names should follow the word "auto" on the same line. There can be multiple "auto" stanzas. ifup brings the named interfaces up in the order listed. Lines beginning with "allow-" are used to identify interfaces that should be brought up automatically by various subsytems. This may be done using a command such as "ifup —allow=hotplug eth0 eth1", which will only bring up eth0 or eth1 if it is listed in an "allow-hotplug" line. Note that "allow-auto" and "auto" are synonyms. Stanzas beginning with the word "mapping" are used to determine how a logical interface name is chosen for a physical interface that is to be brought up. The first line of a mapping stanza consists of the word "mapping" followed by a pattern in shell glob syntax. Each mapping stanza must contain a script definition. The named script is run with the physical interface name as its argument and with the contents of all following "map" lines (without the leading "map") in the stanza provided to it on its standard input. The script must print a string on its standard output before exiting. See /usr/share/doc/ifupdown/examples for examples of what the script must print. Mapping a name consists of searching the remaining mapping patterns and running the script corresponding to the first match; the script outputs the name to which the original is mapped. ifup is normally given a physical interface name as its first non-option argument. ifup also uses this name as the initial logical name for the interface unless it is accompanied by a suffix of the form =LOGICAL, in which case ifup chooses LOGICAL as the initial logical name for the interface. It then maps this name, possibly more than once according to successive mapping specifications, until no further mappings are possible. If the resulting name is the name of some defined logical interface then ifup attempts to bring up the physical interface as that logical interface. Otherwise ifup exits with an error. Stanzas defining logical interfaces start with a line consisting of the word "iface" followed by the name of the logical interface. In simple configurations without mapping stanzas this name should simply be the name of the physical interface to which it is to be applied. (The default mapping script is, in effect, the echo command.) The interface name is followed by the name of the address family that the interface uses. This will be "inet" for TCP/IP networking, but there is also some support for IPX networking ("ipx"), and IPv6 networking ("inet6"). Following that is the name of the method used to configure the interface. Additional options can be given on subsequent lines in the stanza. Which options are available depends on the family and method, as described below. Additional options can be made available by other Debian packages. For example, the wireless-tools package makes available a number of options prefixed with "wireless-" which can be used to configure the interface using iwconfig(8). (See wireless(7) for details.) Options are usually indented for clarity (as in the example above) but are not required to be.

Creating stable names for network interfaces

With Udev and modular network drivers, the network interface numbering is not persistent across reboots by default, because the drivers are loaded in parallel and, thus, in random order. For example, on a computer having two network cards made by Intel and Realtek, the network card manufactured by Intel may become eth0 and the Realtek card becomes eth1. In some cases, after a reboot the cards get renumbered the other way around. To avoid this, Udev comes with a script and some rules to assign stable names to network cards based on their MAC address.

Pre-generate the rules to ensure the same names get assigned to the same devices at every boot, including the first:

for NIC in /sys/class/net/* ; do

INTERFACE=${NIC##*/} udevadm test —action=add —subsystem=net $NIC


Now, inspect the /etc/udev/rules.d/70-persistent-net.rules file, to find out which name was assigned to which network device:

cat /etc/udev/rules.d/70-persistent-net.rules

The file begins with a comment block followed by two lines for each NIC. The first line for each NIC is a commented description showing its hardware IDs (e.g. its PCI vendor and device IDs, if it's a PCI card), along with its driver in parentheses, if the driver can be found. Neither the hardware ID nor the driver is used to determine which name to give an interface; this information is only for reference. The second line is the Udev rule that matches this NIC and actually assigns it a name.

All Udev rules are made up of several keys, separated by commas and optional whitespace. This rule's keys and an explanation of each of them are as follows:

SUBSYSTEM=="net" - This tells Udev to ignore devices that are not network cards.

ACTION=="add" - This tells Udev to ignore this rule for a uevent that isn't an add ("remove" and "change" uevents also happen, but don't need to rename network interfaces).

DRIVERS=="?*" - This exists so that Udev will ignore VLAN or bridge sub-interfaces (because these sub-interfaces do not have drivers). These sub-interfaces are skipped because the name that would be assigned would collide with their parent devices.

ATTR{address} - The value of this key is the NIC's MAC address.

ATTR{type}=="1" - This ensures the rule only matches the primary interface in the case of certain wireless drivers, which create multiple virtual interfaces. The secondary interfaces are skipped for the same reason that VLAN and bridge sub-interfaces are skipped: there would be a name collision otherwise.

KERNEL=="eth*" - This key was added to the Udev rule generator to handle machines that have multiple network interfaces, all with the same MAC address (the PS3 is one such machine). If the independent interfaces have different basenames, this key will allow Udev to tell them apart. This is generally not necessary for most Linux From Scratch users, but does not hurt.

NAME - The value of this key is the name that Udev will assign to this interface.

The value of NAME is the important part. Make sure you know which name has been assigned to each of your network cards before proceeding, and be sure to use that NAME value when creating your configuration files below.

7.13.2. Creating Network Interface Configuration Files

Which interfaces are brought up and down by the network script depends on the files and directories in the /etc/sysconfig/network-devices hierarchy. This directory should contain a sub-directory for each interface to be configured, such as, where “xyz” is a network interface name. Inside this directory would be files defining the attributes to this interface, such as its IP address(es), subnet masks, and so forth.

The following command creates a sample ipv4 file for the eth0 device:

cd /etc/sysconfig/network-devices

mkdir -v ifconfig.eth0

cat > ifconfig.eth0/ipv4 << "EOF"








The values of these variables must be changed in every file to match the proper setup. If the ONBOOT variable is set to “yes” the network script will bring up the Network Interface Card (NIC) during booting of the system. If set to anything but “yes” the NIC will be ignored by the network script and not be brought up.

The SERVICE variable defines the method used for obtaining the IP address. The LFS-Bootscripts package has a modular IP assignment format, and creating additional files in the /etc/sysconfig/network-devices/services directory allows other IP assignment methods. This is commonly used for Dynamic Host Configuration Protocol (DHCP), which is addressed in the BLFS book.

The GATEWAY variable should contain the default gateway IP address, if one is present. If not, then comment out the variable entirely.

The PREFIX variable needs to contain the number of bits used in the subnet. Each octet in an IP address is 8 bits. If the subnet's netmask is, then it is using the first three octets (24 bits) to specify the network number. If the netmask is, it would be using the first 28 bits. Prefixes longer than 24 bits are commonly used by DSL and cable-based Internet Service Providers (ISPs). In this example (PREFIX=24), the netmask is Adjust the PREFIX variable according to your specific subnet.

7.13.3. Creating the /etc/resolv.conf File

If the system is going to be connected to the Internet, it will need some means of Domain Name Service (DNS) name resolution to resolve Internet domain names to IP addresses, and vice versa. This is best achieved by placing the IP address of the DNS server, available from the ISP or network administrator, into /etc/resolv.conf. Create the file by running the following:

cat > /etc/resolv.conf << "EOF"

  1. Begin /etc/resolv.conf

domain <Your Domain Name>

nameserver <IP address of your primary nameserver>

nameserver <IP address of your secondary nameserver>

  1. End /etc/resolv.conf


Replace <IP address of the nameserver> with the IP address of the DNS most appropriate for the setup. There will often be more than one entry (requirements demand secondary servers for fallback capability). If you only need or want one DNS server, remove the second nameserver line from the file. The IP address may also be a router on the local network.