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IFCONFIG(8) Linux System Administrator's Manual IFCONFIG(8)

NAME

ifconfig - configure a network interface

SYNOPSIS

ifconfig [-v] [-a] [-s] [interface]
ifconfig [-v] interface [aftype] options | address ...

DESCRIPTION

Ifconfig is used to configure the kernel-resident network interfaces. It is used at boot time to set up interfaces as necessary. After that, it is usually only needed when debugging or when system tuning is needed.

If no arguments are given, ifconfig displays the status of the currently active interfaces. If a single interface argument is given, it displays the status of the given interface only; if a single -a argument is given, it displays the status of all interfaces, even those that are down. Otherwise, it configures an interface.

Address Families

If the first argument aftype after the interface name is recognized as the name of a supported address family, that address family is used for decoding and displaying all protocol addresses.
Currently supported address families include inet (TCP/IP, default), inet6 (IPv6), ax25 (AMPR Packet Radio), ddp (Appletalk Phase 2), ipx (Novell IPX) and netrom (AMPR Packet radio).

All numbers supplied as parts in IPv4 dotted decimal notation may be decimal, octal, or hexadecimal, as specified in the ISO C standard (that is, a leading '0x' or '0X' implies hexadecimal; otherwise, a leading '0' implies octal; otherwise, the number is interpreted as decimal). Use of hexadecimal and octal numbers is not RFC-compliant and therefore its use is discouraged.

OPTIONS

display all interfaces which are currently available, even if down
display a short list (like netstat -i)
be more verbose for some error conditions
The name of the interface. This is usually a driver name followed by a unit number, for example eth0 for the first Ethernet interface. If your kernel supports alias interfaces, you can specify them with syntax like eth0:0 for the first alias of eth0. You can use them to assign more addresses. To delete an alias interface use ifconfig eth0:0 down. Note: for every scope (i.e. same net with address/netmask combination) all aliases are deleted, if you delete the first (primary).
This flag causes the interface to be activated. It is implicitly specified if an address is assigned to the interface; you can suppress this behavior when using an alias interface by appending an - to the alias (e.g. eth0:0-). It is also suppressed when using the IPv4 0.0.0.0 address as the kernel will use this to implicitly delete alias interfaces.
This flag causes the driver for this interface to be shut down.
[-]arp
Enable or disable the use of the ARP protocol on this interface.
[-]promisc
Enable or disable the promiscuous mode of the interface. If selected, all packets on the network will be received by the interface.
[-]allmulti
Enable or disable all-multicast mode. If selected, all multicast packets on the network will be received by the interface.
This parameter sets the Maximum Transfer Unit (MTU) of an interface to M bytes.
Set the remote IP address for a point-to-point link (such as PPP). This keyword is now obsolete; use the pointopoint keyword instead.
Set the IP network mask for this interface. This value defaults to the usual class A, B or C network mask (as derived from the interface IP address), but it can be set to any value.
Add an address to an interface.
Remove an address from an interface.
Create a new SIT (IPv6-in-IPv4) device, tunnelling to the given destination.
Set the interrupt line used by this device. Not all devices can dynamically change their IRQ setting.
Set the start address in I/O space for this device. Only a few old devices need this.
Set the start address for shared memory used by this device. Only a few old devices need this.
Set the physical port or medium type to be used by the device. Not all devices can change this setting, and those that can vary in what values they support. Typical values for type are 10base2 (thin Ethernet), 10baseT (twisted-pair 10Mbps Ethernet), AUI (external transceiver) and so on. The special medium type of auto can be used to tell the driver to auto-sense the media. Again, not all drivers can do this.
[-]broadcast [addr]
If the address argument is given, set the protocol broadcast address for this interface. Otherwise, set (or clear) the IFF_BROADCAST flag for the interface.
[-]pointopoint [addr]
This keyword enables the point-to-point mode of an interface, meaning that it is a direct link between two machines with nobody else listening on it.
If the address argument is also given, set the protocol address of the other side of the link, just like the obsolete dstaddr keyword does. Otherwise, set or clear the IFF_POINTOPOINT flag for the interface.
Set the hardware address of this interface, if the device driver supports this operation. The keyword must be followed by the name of the hardware class hwclass and the printable ASCII equivalent of the hardware address.
Hardware classes currently supported include ether (Ethernet), ax25 (AMPR AX.25), ARCnet and netrom (AMPR NET/ROM).
Set the multicast flag on the interface. This should not normally be needed as the drivers set the flag correctly themselves.
Set the length of the transmit queue of the device. It is useful to set this to small values for slower devices with a high latency (modem links, ISDN) to prevent fast bulk transfers from disturbing interactive traffic like telnet too much.
Change the name of this interface to newname. The interface must be shut down first.
The IP address to be assigned to this interface.

OUTPUT

Interface Statistics Table (-s)

The table lists active (default) or all known (-a) kernel interfaces. With the -s option it is the same output as netstat -i.

> ifconfig -s enp2s0f0
Iface     MTU    RX-OK RX-ERR RX-DRP RX-OVR  TX-OK TX-ERR TX-DRP TX-OVR Flg
enp2s0f0 1500 18668761      0   1318 0    26038367      0      0      0 BMRU

The result table shows the following columns:

Name and alias prefix of the network interface.
The maximum transfer unit in bytes of this interface.
Number of successfully received packets since interface statistic was reset.
Total count of receive errors since statistic reset. This includes: rx_errors (general receive errors), rx_crc_errors (packets received with a CRC checksum failure), rx_frame_errors (frame alignment errors, corrupted).
Number of incoming packets that were dropped before reaching the protocol stack. Common causes: no buffer space in the driver, congestion, or resource limitations.
Number of packets dropped due to FIFO buffer overflows in the NIC or driver. Common causes: the hardware could not push frames fast enough to the system.

Number of packets successfully transmitted since interface statistic was reset.
Number of transmit errors. Includes collisions, carrier loss, and other transmission failures.
Number of packets dropped by the driver before being sent (e.g., due to congestion or lack of buffer space).
Number of packets lost due to transmit FIFO overflows in the hardware.
The flags for this interface, as listed below.

Interface Flags

The list of interface flags used in the short and detailed interface output. The names of the bit flag constants of the SIOCGIFFLAGS control are listed in netdevice(7).

Accepts all multicast packets (IFF_ALLMULTI).
Interface supports broadcast communication (IFF_BROADCAST).
Internal debugging for the interface enabled (IFF_DEBUG).
Interface is a loopback device (IFF_LOOPBACK).
Interface supports multicast communication (IFF_MULTICAST).
Address is dynamically set (e.g. by DHCP) (IFF_DYNAMIC).
Interface is in promiscuous mode (captures all packets) (IFF_PROMISC). This flag might not reliably show promisc mode.
Avoid use of trailers in packets (IFF_NOTRAILERS).
Interface does not use ARP (IFF_NOARP).
Interface is point-to-point (has a peer instead of broadcast) (IFF_POINTOPOINT).
Interface is part of a bonded device (IFF_SLAVE).
Interface controls a bonded device (IFF_MASTER).
Interface is operational and resources are allocated (IFF_RUNNING).
Interface is administratively up (IFF_UP).
[NO FLAGS], <>
If the bitmask for the interface status is 0.

Interface Details

Sample output for the details of a single interface:

> ifconfig enp2s0f0
enp2s0f0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500


        inet 203.0.113.9  netmask 255.255.255.0  broadcast 203.0.113.255


        inet6 fe80::a8bb:ccff:fedd:eeff  prefixlen 64  scopeid 0x20<link>


        inet6 2001:db8::a8bb:ccff:fedd:eeff  prefixlen 64  scopeid 0x0<global>


        ether aa:bb:cc:dd:ee:ff  txqueuelen 1000  (Ethernet)


        RX packets 18668507  bytes 9459465501 (8.8 GiB)


        RX compressed 0


        RX errors 0  dropped 1318  overruns 0  frame 0


        TX packets 26038199  bytes 16983080620 (15.8 GiB)


        TX compressed 0


        TX errors 0  dropped 0  overruns 0  carrier 0  collisions 0


        device interrupt 30

The output of the interface list is the same as netstat -i -e.

For each interface there is a block starting with the interface name the flags and the mtu, optional outfil and keepalive.

Then one line for each address, prefixed with the address type and its type specifc details.

In the example is one IPv4 (inet) address. it specifies netmask, optional broadcast. If the point-to-point flag is set if will show destination address.

The example follows with two IPv6 (inet6) addresses. Such a line specifies addresss, prefixlen, and scopeid:

IPv4 compatibility address.
A Global Unicast Address (UGA).
A site-unique address.
A link-local address.
A loopback address.

This is followed with a line for the hardware address family (ether in this case). This line contains txqueuelen if available.

If the device is configured for port selection, it has a media line.

After that the packets statistics for transmit (TX) and receive (RX) are shown (same as in the short format above, but the different error counters are shown seperate). Additionally the total number of bytes (frame sizes total) are shown (with a human friendly formatting as comment). The compressed packet counter lines are optional.

The final device line lists optional driver details, with some of the following keywords: interrupt, base, memory, and dma.

NOTES

Since kernel release 2.2 there are no explicit interface statistics for alias interfaces anymore. The statistics printed for the original address are shared with all alias addresses on the same device. If you want per-address statistics you should add explicit accounting rules for the address using the iptables(8) command.

Since net-tools 1.60-4 ifconfig is printing byte counters and human readable counters with IEC 60027-2 units. So 1 KiB are 2^10 byte. Note, the numbers are truncated to one decimal (which can by quite a large error if you consider 0.1 PiB is 112.589.990.684.262 bytes :)

Interrupt problems with Ethernet device drivers fail with EAGAIN (SIOCSIIFLAGS: Resource temporarily unavailable) it is most likely a interrupt conflict.

FILES

/proc/net/dev /proc/net/if_inet6

BUGS

Ifconfig uses the ioctl access method to get the full address information, which limits hardware addresses to 8 bytes. Because Infiniband hardware address has 20 bytes, only the first 8 bytes are displayed correctly. Please use ip link command from iproute2 package to display link layer informations including the hardware address.

While appletalk DDP and IPX addresses will be displayed they cannot be altered by this command.

SEE ALSO

Homepage of the net-tools project: https://net-tools.sourceforge.io

Prefixes for binary multiples (NIST): https://physics.nist.gov/cuu/Units/binary.html

route(8), netstat(8), arp(8), ip-link(8), iptables(8)
interfaces(5), ip(7), netdevice(7)

AUTHORS

Fred N. van Kempen <waltje@uwalt.nl.mugnet.org>,
Alan Cox <Alan.Cox@linux.org>, Andi Kleen,
Phil Blundell <Philip.Blundell@pobox.com>,
Bernd Eckenfels <net-tools@lina.inka.de>.

2025-09-10 net-tools