ifconfig Command Reference

Last Updated on

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.

If you performed a minimal install you will need to install the net-tools package:

1
yum install -y net-tools
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
IFCONFIG(8)
Linux System Administrator's Manual

NAME
ifconfig - configure a network interface

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

NOTE
This program is obsolete! For replacement check ip addr and ip link. For statistics use ip -s link.

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 usu‐
ally 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 after the interface name is recognized as the name of a supported address family, that address family is used for decoding and dis‐
playing 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; oth‐
erwise, the number is interpreted as decimal). Use of hexadecimal and octal numbers is not RFC-compliant and therefore its use is discouraged.

OPTIONS
-a display all interfaces which are currently available, even if down

-s display a short list (like netstat -i)

-v be more verbose for some error conditions

interface
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 eth0:0 for the first alias of eth0. You can use them to assign a second address. 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).

up This flag causes the interface to be activated. It is implicitly specified if an address is assigned to the interface.

down 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.

mtu N This parameter sets the Maximum Transfer Unit (MTU) of an interface.

dstaddr addr
Set the remote IP address for a point-to-point link (such as PPP). This keyword is now obsolete; use the pointopoint keyword instead.

netmask addr
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 addr/prefixlen
Add an IPv6 address to an interface.

del addr/prefixlen
Remove an IPv6 address from an interface.

tunnel ::aa.bb.cc.dd
Create a new SIT (IPv6-in-IPv4) device, tunnelling to the given destination.

irq addr
Set the interrupt line used by this device. Not all devices can dynamically change their IRQ setting.

io_addr addr
Set the start address in I/O space for this device.

mem_start addr
Set the start address for shared memory used by this device. Only a few devices need this.

media type
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. Oth‐
erwise, set or clear the IFF_POINTOPOINT flag for the interface.

hw class address
Set the hardware address of this interface, if the device driver supports this operation. The keyword must be followed by the name of the hard‐
ware class 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).

multicast
Set the multicast flag on the interface. This should not normally be needed as the drivers set the flag correctly themselves.

address
The IP address to be assigned to this interface.

txqueuelen length
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.

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.
See http://www.scyld.com/expert/irq-conflict.html for more information.

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 informa‐
tions including the hardware address.

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

SEE ALSO
ip(8), iptables(8)
http://physics.nist.gov/cuu/Units/binary.html - Prefixes for binary multiples
Lima

About the author

Lima is the visual nautical indicator for "stop instantly."

Leave a Reply