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101 lines
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101 lines
5.0 KiB
Markdown
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How to check your network connections on Linux
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======
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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.
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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:
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```
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$ ip a
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1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
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link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
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inet 127.0.0.1/8 scope host lo
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valid_lft forever preferred_lft forever
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inet6 ::1/128 scope host
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valid_lft forever preferred_lft forever
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2: enp0s25: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
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link/ether 00:1e:4f:c8:43:fc brd ff:ff:ff:ff:ff:ff
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inet 192.168.0.24/24 brd 192.168.0.255 scope global dynamic enp0s25
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valid_lft 57295sec preferred_lft 57295sec
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inet6 fe80::2c8e:1de0:a862:14fd/64 scope link
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valid_lft forever preferred_lft forever
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```
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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.
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### Why enp0s25 and not eth0
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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.
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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.
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The <BROADCAST,MULTICAST,UP,LOWER_UP> string of settings tell us that ...
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```
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BROADCAST the interface supports broadcasting
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MULTICAST the interface supports multicasting
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UP the network interface is enabled
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LOWER_UP the network cable is plugged in and device connected to network
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mtu 1500 the maximum transfer unit (packet size) is 1,500 bytes
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```
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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.
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```
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mtu 1500 maximum transfer unit (packet size)
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qdisc pfifo_fast used for packet queueing
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state UP network interface is up
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group default interface group
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qlen 1000 transmission queue length
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link/ether 00:1e:4f:c8:43:fc MAC(hardware) address of the interface
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brd ff:ff:ff:ff:ff:ff broadcast address
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inet 192.168.0.24/24 IPv4 address
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brd 192.168.0.255 broadcast address
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scope global valid everywhere
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dynamic enp0s25 address is dynamically assigned
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valid_lft 80866sec valid lifetime for IPv4 address
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preferred_lft 80866sec preferred lifetime for IPv4 address
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inet6 fe80::2c8e:1de0:a862:14fd/64 IPv6 address
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scope link valid only on this device
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valid_lft forever valid lifetime for IPv6 address
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preferred_lft forever preferred lifetime for IPv6 address
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```
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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:
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```
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$ ip -s link show enp0s25
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2: enp0s25: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP mode DEFAULT group default qlen 1000
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link/ether 00:1e:4f:c8:43:fc brd ff:ff:ff:ff:ff:ff
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RX: bytes packets errors dropped overrun mcast
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224258568 418718 0 0 0 84376
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TX: bytes packets errors dropped carrier collsns
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6131373 78152 0 0 0 0
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```
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Another **ip** command provides information on a system's routing table.
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```
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$ ip route show
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default via 192.168.0.1 dev enp0s25 proto static metric 100
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169.254.0.0/16 dev enp0s25 scope link metric 1000
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192.168.0.0/24 dev enp0s25 proto kernel scope link src 192.168.0.24 metric 100
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```
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The **ip** command is extremely versatile. You can get a helpful cheat sheet on the **ip** command and its options from [Red Hat][1].
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--------------------------------------------------------------------------------
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via: https://www.networkworld.com/article/3262045/linux/checking-your-network-connections-on-linux.html
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作者：[Sandra Henry-Stocker][a]
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译者：[译者ID](https://github.com/译者ID)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]:https://www.networkworld.com/author/Sandra-Henry_Stocker/
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[1]:https://access.redhat.com/sites/default/files/attachments/rh_ip_command_cheatsheet_1214_jcs_print.pdf
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