[Linux 文章收集] Ifconfig: 5 Examples To Configure Network Interface

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Preface 
ifconfig command is used to configure network interfaces. ifconfig stands for interface configurator. Ifconfig is widely used to initialize the network interface and to enable or disable the interfaces. In this article, let us review 7 common usages of ifconfig command. 

1. View Network Settings of an Ethernet Adapter 
Ifconfig, when invoked with no arguments will display all the details of currently active interfaces. If you give the interface name as an argument, the details of that specific interface will be displayed. 

# ifconfig eth0
eth0 Link encap:Ethernet HWaddr 00:2D:32:3E:39:3B
inet addr:192.168.2.2 Bcast:192.168.2.255 Mask:255.255.255.0
inet6 addr: fe80::21d:92ff:fede:499b/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:977839669 errors:0 dropped:1990 overruns:0 frame:0
TX packets:1116825094 errors:8 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:2694625909 (2.5 GiB) TX bytes:4106931617 (3.8 GiB)
Interrupt:185 Base address:0xdc00

2. Disable/Enable an Interface 
When a network interface is active, it is able to send and receive data; when it is inactive, it is not able to transmit or receive. You can use ifconfig to change the status of a network interface from inactive to active, or vice-versa. 

For instance, if network interface eth1 is inactive, you can activate it with the command: 

# sudo ifconfig eth1 up

Similarly, you can disable an active network interface using the down keyword. For instance, to disable the wireless network interface wlan0, use the command: 

# sudo ifconfig wlan0 down

3. Configuring An Interface 
ifconfig can be used at the command line to configure (or re-configure) a network interface. This is often unnecessary, since this configuration is typically handled by a script when you boot the system. If you'd like to do so manually, you will need superuser privileges, so we'll use sudo again when running these commands. 

To assign a static IP address to an interface, specify the interface name and the IP address. For example, to assign the IP address 69.72.169.1 to the interface wlan0, use the command: 

# sudo ifconfig wlan0 69.72.169.1

To assign a network mask to an interface, use the keyword netmask and the netmask address. For instance, to configure the interface eth1 to use a network mask of 255.255.255.0, the command would be: 

# sudo ifconfig eth1 netmask 255.255.255.0

To assign a broadcast address to an interface, use the keyword broadcast and the broadcast address. For instance, to configure the interface wlan1 to use abroadcast address of 172.16.25.98, the command would be: 

# sudo ifconfig wlan1 broadcast 172.16.25.98

These configurations can combined in a single command. For instance, to configure interface eth0 to use the static IP address 192.168.2.5, the network mask 255.255.255.0, and the broadcast address 192.168.2.7, the command would be: 

# sudo ifconfig eth0 192.168.2.5 netmask 255.255.255.0 broadcast 192.168.2.7

4. Change MTU 
This will change the Maximum transmission unit (MTU) to XX. MTU is the maximum number of octets the interface is able to handle in one transaction. For Ethernet the Maximum transmission unit by default is 1500. 

# ifconfig eth0 mtu XX

5. Promiscuous mode 
By default when a network card receives a packet, it checks whether the packet belongs to itself. If not, the interface card normally drops the packet. But inpromiscuous mode, the card doesn’t drop the packet. Instead, it will accept all the packets which flows through the network card. 

Superuser privilege is required to set an interface in promiscuous mode. Most network monitor tools use the promiscuous mode to capture the packets and to analyze the network traffic. Following will put the interface in promiscuous mode. 

# ifconfig eth0 promisc

Following will put the interface in normal mode. 

# ifconfig eth0 -promisc

Supplement 
* 鳥哥 - 第五章、 Linux 常用網路指令