How to check your network connections on Linux

The ip command has a lot to tell you about the configuration and state of your network connections, but what do all those words and numbers mean? Let’s take a deep dive in and see what all the displayed values are trying to tell you.

When you use the ip a (or ip addr) command to get information on all the network interfaces on your system, you’re going to see something like this:

$ ip a
1: lo:  mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host
       valid_lft forever preferred_lft forever
2: enp0s25:  mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
    link/ether 00:1e:4f:c8:43:fc brd ff:ff:ff:ff:ff:ff
    inet 192.168.0.24/24 brd 192.168.0.255 scope global dynamic enp0s25
       valid_lft 57295sec preferred_lft 57295sec
    inet6 fe80::2c8e:1de0:a862:14fd/64 scope link
       valid_lft forever preferred_lft forever

The two interfaces on this system — the loopback (lo) and network (enp0s25) — are displayed along with a lot of stats. The “lo” interface is clearly the loopback. We can see the loopback IPv4 address (127.0.0.1) and the loopback IPv6 (::1) in the listing. The normal network interface is more interesting.

Why enp0s25 and not eth0

If you’re wondering why it’s called enp0s25 on this system instead of the likely more familiar eth0, a little explanation is in order.

The new naming scheme is referred to as the “Predictable Network Interface” naming. It’s been used on systemd-based Linux systems for some time. The interface name depends on the physical location of the hardware. The “en” simply means “ethernet” just like “eth” does for eth0. The “p” is the bus number of the ethernet card and the “s” is the slot number. So “enp0s25” tells us a lot about the hardware we’re working with.

The string of settings tell us that …

BROADCAST    the interface supports broadcasting
MULTICAST    the interface supports multicasting
UP           the network interface is enabled
LOWER_UP     the network cable is plugged in and device connected to network
mtu 1500     the maximum transfer unit (packet size) is 1,500 bytes

The other values listed also tell us a lot about the interface, but we need to know what words like “brd” and “qlen” represent. So, here’s a translation of the rest of the ip a shown above.

mtu 1500                      maximum transfer unit (packet size)
qdisc pfifo_fast              used for packet queueing
state UP                      network interface is up
group default                 interface group
qlen 1000                     transmission queue length
link/ether 00:1e:4f:c8:43:fc  MAC(hardware) address of the interface
brd ff:ff:ff:ff:ff:ff         broadcast address
inet 192.168.0.24/24          IPv4 address
brd 192.168.0.255             broadcast address
scope global                  valid everywhere
dynamic enp0s25               address is dynamically assigned
valid_lft 80866sec            valid lifetime for IPv4 address
preferred_lft 80866sec        preferred lifetime for IPv4 address
inet6 fe80::2c8e:1de0:a862:14fd/64		IPv6 address
scope link                    valid only on this device
valid_lft forever             valid lifetime for IPv6 address
preferred_lft forever         preferred lifetime for IPv6 address

You might have noticed that some of the information that the ifconfig command provides is not included in the ip a output — such as the stats on transmitted packets. If you want to see a list of the number of packets transmitted and received along with collisions, you can use this ip command:

$ ip -s link show enp0s25
2: enp0s25:  mtu 1500 qdisc pfifo_fast state UP mode DEFAULT group default qlen 1000
    link/ether 00:1e:4f:c8:43:fc brd ff:ff:ff:ff:ff:ff
    RX: bytes  packets  errors  dropped overrun mcast
    224258568  418718   0       0       0       84376
    TX: bytes  packets  errors  dropped carrier collsns
    6131373    78152    0       0       0       0

Another ip command provides information on a system’s routing table.

$ ip route show
default via 192.168.0.1 dev enp0s25 proto static metric 100
169.254.0.0/16 dev enp0s25 scope link metric 1000
192.168.0.0/24 dev enp0s25 proto kernel scope link src 192.168.0.24 metric 100

The ip command is extremely versatile. You can get a helpful cheat sheet on the ip command and its options from Red Hat.