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⁠1.2. Interface Configuration Files

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.

⁠1.2.1. Ethernet Interfaces

One of the most common interface files is /etc/sysconfig/network-scripts/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-ethX 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
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
Below is a listing of the configurable parameters in an Ethernet interface configuration file:
BONDING_OPTS=parameters
sets the configuration parameters for the bonding device, and is used in /etc/sysconfig/network-scripts/ifcfg-bondN. These parameters are identical to those used for bonding devices in /sys/class/net/bonding_device/bonding, and the module parameters for the bonding driver as described in bonding Module Directives.
This configuration method is used so that multiple bonding devices can have different configurations. It is highly recommended to place all of your bonding options after the BONDING_OPTS directive in ifcfg-name. Do not specify options for the bonding device in /etc/modprobe.d/bonding.conf, or in the deprecated /etc/modprobe.conf 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 ipcalc.
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=name
where name is a short host name to be sent to the DHCP server. Use this option only if the DHCP server requires the client to specify a host name before receiving an IP address.
DHCPV6C=answer
where answer is one of the following:
  • yes — Use DHCP to obtain an IPv6 address for this interface.
  • no — Do not use DHCP to obtain an IPv6 address for this interface. This is the default value.
An IPv6 link-local address will still be assigned by default. The link-local address is based on the MAC address of the interface as per RFC 4862.
DHCPV6C_OPTIONS=answer
where answer is one of the following:
  • -P — Enable IPv6 prefix delegation.
  • -S — Use DHCP to obtain stateless configuration only, not addresses, for this interface.
  • -N — Restore normal operation after using the -T or -P options.
  • -T — Use DHCP to obtain a temporary IPv6 address for this interface.
  • -D — Override the default when selecting the type of DHCP Unique Identifier (DUID) to use.
    By default, the DHCPv6 client (dhclient) creates a DHCP Unique Identifier (DUID) based on the link-layer address (DUID-LL) if it is running in stateless mode (with the -S option, to not request an address), or it creates an identifier based on the link-layer address plus a timestamp (DUID-LLT) if it is running in stateful mode (without -S, requesting an address). The -D option overrides this default, with a value of either LL or LLT.
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"
Instead of a custom initscript, use ETHTOOL_OPTS to set the interface speed and duplex settings. Custom initscripts run outside of the network init script lead to unpredictable results during a post-boot network service restart.

Set “autoneg off” before changing speed or duplex settings

Changing speed or duplex settings almost always requires disabling auto-negotiation with the autoneg off option. This option needs to be stated first, as the option entries are order-dependent.
Refer to Section 1.5, “Ethtool” for more ethtool options.
GATEWAY=address
where address is the IP address of the network router or gateway device (if any). Note that if this option is present in an interface configuration file it will take precedence over the GATEWAY option in the /etc/sysconfig/network file. It is therefore recommended that you chose between setting this option for global effect in /etc/sysconfig/network or per-interface.
HOTPLUG=answer
where answer is one of the following:
  • yes — This device should be activated when it is hot-plugged (this is the default option).
  • no — This device should not be activated when it is hot-plugged.
The HOTPLUG=no option can be used to prevent a channel bonding interface from being activated when a bonding kernel module is loaded.
HWADDR=MAC-address
where MAC-address is the hardware address of the Ethernet device in the form AA:BB:CC:DD:EE:FF. This directive must be used in machines containing more than one NIC 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. See the Fedora Networking Guide for more information on device naming.
IPADDR=address
where address is the IPv4 address.
IPV6ADDR=address
where address is the first static, or primary, IPv6 address on an interface.
The format is Address/Prefix-length. If no prefix length is specified, /64 is assumed. Note that this setting depends on IPV6INIT being enabled.
IPV6ADDR_SECONDARIES=address
where address is one or more, space separated, additional IPv6 addresses.
The format is Address/Prefix-length. If no prefix length is specified, /64 is assumed. Note that this setting depends on IPV6INIT being enabled.
IPV6INIT=answer
where answer is one of the following:
  • yes — Initialize this interface for IPv6 addressing.
  • no — Do not initialize this interface for IPv6 addressing. This is the default value.
    This setting is required for IPv6 static and DHCP assignment of IPv6 addresses. It does not affect IPv6 Stateless Address Autoconfiguration (SLAAC) as per RFC 4862.
    See Section 3.1.13, “/etc/sysconfig/network” for information on disabling IPv6.
IPV6_AUTOCONF=answer
where answer is one of the following:
  • yes — Enable IPv6 autoconf configuration for this interface.
  • no — Disable IPv6 autoconf configuration for this interface.
If enabled, an IPv6 address will be requested using Neighbor Discovery (ND) from a router running the radvd daemon.
Note that the default value of IPV6_AUTOCONF depends on IPV6FORWARDING as follows:
  • If IPV6FORWARDING=yes, then IPV6_AUTOCONF will default to no.
  • If IPV6FORWARDING=no, then IPV6_AUTOCONF will default to yes and IPV6_ROUTER has no effect.
IPV6_MTU=value
where value is an optional dedicated MTU for this interface.
IPV6_PRIVACY=rfc3041
where rfc3041 optionally sets this interface to support RFC 3041 Privacy Extensions for Stateless Address Autoconfiguration in IPv6. Note that this setting depends on IPV6INIT option being enabled.
The default is for RFC 3041 support to be disabled. Stateless Autoconfiguration will derive addresses based on the MAC address, when available, using the modified EUI-64 method. The address is appended to a prefix but as the address is normally derived from the MAC address it is globally unique even when the prefix changes. In the case of a link-local address the prefix is fe80::/64 as per RFC 2462 IPv6 Stateless Address Autoconfiguration.
LINKDELAY=time
where time is the number of seconds to wait for link negotiation before configuring the device.
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 the HWADDR directive.
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 ipcalc.
NM_CONTROLLED=answer
where answer is one of the following:
  • yes — NetworkManager is permitted to configure this device. This is the default behavior and can be omitted.
  • no — NetworkManager is not permitted to configure this device.

Note

The NM_CONTROLLED directive is dependent on the NM_BOND_VLAN_ENABLED directive in /etc/sysconfig/network. If and only if that directive is present and is one of yes, y, or true, will NetworkManager detect and manage bonding and VLAN interfaces.
NOZEROCONF=answer
where answer is one of the following:
  • yes — Do not set a route for dynamic IPv6 link-local addresses.
  • no — Allow setting a route for dynamic IPv6 link-local addresses.
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 DHCP, then yes is the default.
  • no — Do not modify /etc/resolv.conf.
SLAVE=answer
where answer 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.

⁠1.2.2. Specific ifcfg Options for Linux on System z

SUBCHANNELS=<read_device_bus_id>, <write_device_bus_id>, <data_device_bus_id>
where <read_device_bus_id>, <write_device_bus_id>, and <data_device_bus_id> are the three device bus IDs representing a network device.
PORTNAME=myname;
where myname is the Open Systems Adapter (OSA) portname or LAN Channel Station (LCS) portnumber.
CTCPROT=answer
where answer is one of the following:
  • 0 — Compatibility mode, TCP/IP for Virtual Machines (used with non-Linux peers other than IBM S/390 and IBM System z operating systems). This is the default mode.
  • 1 — Extended mode, used for Linux-to-Linux Peers.
  • 3 — Compatibility mode for S/390 and IBM System z operating systems.
This directive is used in conjunction with the NETTYPE directive. It specifies the CTC protocol for NETTYPE='ctc'. The default is 0.
OPTION='answer'
where 'answer' is a quoted string of any valid sysfs attributes and their value. The Fedora installer currently uses this to configure the layer mode, (layer2), and the relative port number, (portno), of QETH devices. For example: 
OPTIONS='layer2=1 portno=0'

⁠1.2.3. Required ifcfg Options for Linux on System z

NETTYPE=answer
where answer is one of the following:
  • ctc — Channel-to-Channel communication. For point-to-point TCP/IP or TTY.
  • lcs — LAN Channel Station (LCS).
  • qeth — QETH (QDIO Ethernet). This is the default network interface. It is the preferred installation method for supporting real or virtual OSA cards and HiperSockets devices.

⁠1.2.4. Alias and Clone Files

Two lesser-used types of interface configuration files are alias and clone files. As the ip command of the iproute package now supports assigning multiple address to the same interface it is no longer necessary to use this method of binding multiple addresses to the same interface.

Note

At the time of writing, NetworkManager does not detect IP aliases in ifcfg files. For example, if ifcfg-eth0 and ifcfg-eth0:1 files are present, NetworkManager creates two connections, which will cause confusion.
For new installations, users should select the Manual method on the IPv4 or IPv6 tab in NetworkManager to assign multiple IP address to the same interface. For more information on using this tool, see the Fedora Networking Guide.
Alias interface configuration files, which are used to bind multiple addresses to a single interface, use the ifcfg-if-name:alias-value naming scheme. Do not confuse alias ifcfg file and interface names with VLAN ifcfg file and interface names which take the form: ifcfg-if-name.vlan-id and ethX.vlan-id respectively.
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. Under this configuration, eth0 is bound to a dynamic IP address, but the same physical network card can receive requests via the fixed, 10.0.0.2 IP address.

Warning

Alias interfaces do not support DHCP.
A clone interface configuration file should use the following naming convention: ifcfg-if-name-clone-name. While an alias file allows multiple addresses for an existing interface, a clone file is used to specify additional options for an interface. For example, a standard DHCP Ethernet interface called eth0, may look similar to this: 
DEVICE=eth0
ONBOOT=yes
BOOTPROTO=dhcp
Since the default value for the USERCTL directive is no if it is not specified, users cannot bring this interface up and down. To give users the ability to control the interface, create a clone by copying ifcfg-eth0 to ifcfg-eth0-user and add the following line to ifcfg-eth0-user: 
USERCTL=yes
This way a user can bring up the eth0 interface using the /sbin/ifup eth0-user command because 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.
It is no longer possible to create alias and clone interface configuration files using a graphical tool. However, as explained at the beginning of this section, it is no longer necessary to use this method as it is now possible to directly assign multiple IP address to the same interface. For new installations, users should select the Manual method on the IPv4 or IPv6 tab in NetworkManager to assign multiple IP address to the same interface. For more information on using this tool, see the Fedora Networking Guide.

⁠1.2.5. Dialup Interfaces

If you are 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-pppX
where X is a unique number corresponding to a specific interface.
The PPP interface configuration file is created automatically when wvdial, or Kppp is used to create a dialup account. It is also possible to create and edit this file manually.
The following is a typical /etc/sysconfig/network-scripts/ifcfg-ppp0 file: 
DEVICE=ppp0
NAME=test
WVDIALSECT=test
MODEMPORT=/dev/modem
LINESPEED=115200
PAPNAME=test
USERCTL=true
ONBOOT=no
PERSIST=no
DEFROUTE=yes
PEERDNS=yes
DEMAND=no
IDLETIMEOUT=600
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 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 allows 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 disconnects itself.
INITSTRING=string
where string is the initialization string passed to the modem device. This option is primarily used in conjunction with SLIP interfaces.
LINESPEED=value
where value is the baud rate of the device. Possible standard values 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 results 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 connections to a remote system.
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 IP address of the remote system. 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.

⁠1.2.6. Other Interfaces

Other common interface configuration files 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.

Do not manually edit the ifcfg-lo script

The loopback interface script, /etc/sysconfig/network-scripts/ifcfg-lo, should never be edited manually. 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 as an Ethernet device, except that it utilizes a parallel port.
Interface configuration files for Linux on System z include the following:
ifcfg-hsiN
A HiperSockets interface is an interface for high-speed TCP/IP communication within and across z/VM guest virtual machines and logical partitions (LPARs) on an IBM System z mainframe.