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ifconfig



Useful “ifconfig” Commands to Configure Network Interface in Linux


ifconfig in short “interface configuration” utility for system/network administration in Unix/Linux operating systems to configure, manage and query network interface parameters via command line interface or in a system configuration scripts.
The “ifconfig” command is used for displaying current network configuration information, setting up an ip address, netmask or broadcast address to an network interface, creating an alias for network interface, setting up hardware address and enable or disable network interfaces.

1. View All Network Setting

The “ifconfig” command with no arguments will display all the active interfaces details. The ifconfig command also used to check the assigned IP address of an server.
[root@tecmint ~]# ifconfig
eth0      Link encap:EthernetHWaddr 00:0B:CD:1C:18:5A
inet addr:172.16.25.126  Bcast:172.16.25.63  Mask:255.255.255.224
inet6 addr: fe80::20b:cdff:fe1c:185a/64 Scope:Link
UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
RX packets:2341604 errors:0 dropped:0 overruns:0 frame:0
TX packets:2217673 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:293460932 (279.8 MiB)  TX bytes:1042006549 (993.7 MiB)
Interrupt:185 Memory:f7fe0000-f7ff0000
lo        Link encap:Local Loopback
inet addr:127.0.0.1  Mask:255.0.0.0
inet6 addr: ::1/128 Scope:Host
UP LOOPBACK RUNNING  MTU:16436  Metric:1
RX packets:5019066 errors:0 dropped:0 overruns:0 frame:0
TX packets:5019066 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:2174522634 (2.0 GiB)  TX bytes:2174522634 (2.0 GiB)
tun0      Link encap:UNSPECHWaddr 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00
inet addr:10.1.1.1  P-t-P:10.1.1.2  Mask:255.255.255.255
UP POINTOPOINT RUNNING NOARP MULTICAST  MTU:1500  Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:100
RX bytes:0 (0.0 b)  TX bytes:0 (0.0 b)

2. Display Information of All Network Interfaces

The following ifconfig command with -a argument will display information of all active or inactive network interfaces on server. It displays the results for eth0, lo, sit0 and tun0.
[root@tecmint ~]# ifconfig -a
eth0      Link encap:EthernetHWaddr 00:0B:CD:1C:18:5A
inet addr:172.16.25.126  Bcast:172.16.25.63  Mask:255.255.255.224
inet6 addr: fe80::20b:cdff:fe1c:185a/64 Scope:Link
UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
RX packets:2344927 errors:0 dropped:0 overruns:0 frame:0
TX packets:2220777 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:293839516 (280.2 MiB)  TX bytes:1043722206 (995.3 MiB)
Interrupt:185 Memory:f7fe0000-f7ff0000
lo        Link encap:Local Loopback
inet addr:127.0.0.1  Mask:255.0.0.0
inet6 addr: ::1/128 Scope:Host
UP LOOPBACK RUNNING  MTU:16436  Metric:1
RX packets:5022927 errors:0 dropped:0 overruns:0 frame:0
TX packets:5022927 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:2175739488 (2.0 GiB)  TX bytes:2175739488 (2.0 GiB)
sit0      Link encap:IPv6-in-IPv4
NOARP  MTU:1480  Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:0 (0.0 b)  TX bytes:0 (0.0 b)
tun0      Link encap:UNSPECHWaddr 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00
inet addr:10.1.1.1  P-t-P:10.1.1.2  Mask:255.255.255.255
UP POINTOPOINT RUNNING NOARP MULTICAST  MTU:1500  Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:100
RX bytes:0 (0.0 b)  TX bytes:0 (0.0 b)

3. View Network Settings of Specific Interface

Using interface name (eth0) as an argument with “ifconfig” command will display details of specific network interface.
[root@tecmint ~]# ifconfig eth0
eth0      Link encap:EthernetHWaddr 00:0B:CD:1C:18:5A
inet addr:172.16.25.126  Bcast:172.16.25.63  Mask:255.255.255.224
inet6 addr: fe80::20b:cdff:fe1c:185a/64 Scope:Link
UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
RX packets:2345583 errors:0 dropped:0 overruns:0 frame:0
TX packets:2221421 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:293912265 (280.2 MiB)  TX bytes:1044100408 (995.7 MiB)
Interrupt:185 Memory:f7fe0000-f7ff0000

4. How to Enable an Network Interface

The “up” or “ifup” flag with interface name (eth0) activates an network interface, if it is not in active state and allowing to send and receive information. For example, “ifconfig eth0 up” or “ifup eth0” will activate the eth0 interface.
[root@tecmint ~]# ifconfig eth0 up
OR
[root@tecmint ~]# ifup eth0

5. How to Disable an Network Interface

The “down” or “ifdown” flag with interface name (eth0) deactivates the specified network interface. For example, “ifconfig eth0 down” or “ifdown eth0” command deactivates the eth0 interface, if it is in active state.
[root@tecmint ~]# ifconfig eth0 down
OR
[root@tecmint ~]# ifdown eth0

6. How to Assign a IP Address to Network Interface

To assign an IP address to an specific interface, use the following command with an interface name (eth0) and ip address that you want to set. For example, “ifconfig eth0 172.16.25.125” will set the IP address to interface eth0.
[root@tecmint ~]# ifconfig eth0 172.16.25.125

7. How to Assign a Netmask to Network Interface

Using the “ifconfig” command with “netmask” argument and interface name as (eth0) allows you to define an netmask to an given interface. For example, “ifconfig eth0 netmask 255.255.255.224” will set the network mask to an given interface eth0.
[root@tecmint ~]# ifconfig eth0 netmask 255.255.255.224

8. How to Assign a Broadcast to Network Interface

Using the “broadcast” argument with an interface name will set the broadcast address for the given interface. For example, “ifconfig eth0 broadcast 172.16.25.63” command sets the broadcast address to an interface eth0.
[root@tecmint ~]# ifconfig eth0 broadcast 172.16.25.63

9. How to Assign a IP, Netmask and Broadcast to Network Interface

To assign an IP address, Netmask address and Broadcast address all at once using “ifconfig” command with all arguments as given below.
[root@tecmint ~]# ifconfig eth0 172.16.25.125 netmask 255.255.255.224 broadcast 172.16.25.63

10. How to Change MTU for an Network Interface

The “mtu” argument set the maximum transmission unit to an interface. The MTU allows you to set the limit size of packets that are transmitted on an interface. The MTU able to handle maximum number of octets to an interface in one single transaction. For example, “ifconfig eth0 mtu 1000” will set the maximum transmission unit to given set (i.e. 1000). Not all network interfaces supports MTU settings.
[root@tecmint ~]# ifconfig eth0 mtu 1000

11. How to Enable Promiscuous Mode

What happens in normal mode, when a packet received by a network card, it verifies that the packet belongs to itself. If not, it drops the packet normally, but in the promiscuous mode is used to accept all the packets that flows through the network card.
Most of the today’s network tools uses the promiscuous mode to capture and analyze the packets that flows through the network interface. To set the promiscuous mode, use the following command.
[root@tecmint ~]# ifconfig eth0 promisc

12. How to Disable Promiscuous Mode

To disable promiscuous mode, use the “-promisc” switch that drops back the network interface in normal mode.
[root@tecmint ~]# ifconfig eth0 -promisc

13. How to Add New Alias to Network Interface

The ifconfig utility allows you to configure additional network interfaces using alias feature. To add alias network interface of eth0, use the following command. Please note that alias network address in same sub-net mask. For example, if your eth0 network ip address is 172.16.25.125, then alias ip address must be 172.16.25.127.
[root@tecmint ~]# ifconfig eth0:0 172.16.25.127
Next, verify the newly created alias network interface address, by using “ifconfig eth0:0” command.
[root@tecmint ~]# ifconfig eth0:0
eth0:0    Link encap:EthernetHWaddr 00:01:6C:99:14:68
inet addr:172.16.25.123  Bcast:172.16.25.63  Mask:255.255.255.240
UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
Interrupt:17

14. How to Remove Alias to Network Interface

If you no longer required an alias network interface or you incorrectly configured it, you can remove it by using the following command.
[root@tecmint ~]# ifconfig eth0:0 down

15. How to Change the MAC address of Network Interface

To change the MAC (Media Access Control) address of an eth0 network interface, use the following command with argument “hw ether“. For example, see below.
[root@tecmint ~]# ifconfig eth0 hw ether AA:BB:CC:DD:EE:FF

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