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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: <LOOPBACK,UP,LOWER_UP> 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: <BROADCAST,MULTICAST,UP,LOWER_UP> 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 <BROADCAST,MULTICAST,UP,LOWER_UP> 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: <BROADCAST,MULTICAST,UP,LOWER_UP> 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.
via: https://www.networkworld.com/article/3262045/linux/checking-your-network-connections-on-linux.html
作者：Sandra Henry-Stocker 译者：译者ID 校对：校对者ID