Examining network connections on Linux systems

Linux systems provide a lot of useful commands for reviewing network configuration and connections. Here's a look at a few, including ifquery, ifup, ifdown and ifconfig.

Examining network connections on Linux systems
Alan Levine (CC BY 2.0)

There are a lot of commands available on Linux for looking at network settings and connections. In today's post, we're going to run through some very handy commands and see how they work.

ifquery command

One very useful command is the ifquery command. This command should give you a quick list of network interfaces. However, you might only see something like this —showing only the loopback interface:

$ ifquery --list
lo

If this is the case, your /etc/network/interfaces file doesn't include information on network interfaces except for the loopback interface. You can add lines like the last two in the example below — assuming DHCP is used to assign addresses — if you'd like it to be more useful.

# interfaces(5) file used by ifup(8) and ifdown(8)
auto lo
iface lo inet loopback
auto eth0
iface eth0 inet dhcp

ifup and ifdown commands

The related ifup and ifdown commands can be used to bring network connections up and shut them down as needed provided this file has the required descriptive data. Just keep in mind that "if" means "interface" in these commands just as it does in the ifconfig command, not "if" as in "if I only had a brain".

ifconfig command

The ifconfig command, on the other hand, doesn't read the /etc/network/interfaces file at all and still provides quite a bit of useful information on network interfaces -- configuration data along with packet counts that tell you how busy each interface has been. The ifconfig command can also be used to shut down and restart network interfaces (e.g., ifconfig eth0 down).

$ ifconfig eth0
eth0      Link encap:Ethernet  HWaddr 00:1e:4f:c8:43:fc
          inet addr:192.168.0.6  Bcast:192.168.0.255  Mask:255.255.255.0
          inet6 addr: fe80::b44b:bdb6:2527:6ae9/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:60474 errors:0 dropped:0 overruns:0 frame:0
          TX packets:33463 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:43922053 (43.9 MB)  TX bytes:4000460 (4.0 MB)
          Interrupt:21 Memory:fe9e0000-fea00000

The RX and TX packet counts in this output are extremely low. In addition, no errors or packet collisions have been reported. The uptime command will likely confirm that this system has only recently been rebooted.

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