Network configuration and troubleshooting are the essential part of Linux administration. Even for a developer who works with Linux Systems, knowledge about Linux network commands is an added advantage. Few Linux networking command people use today is deprecated. In this post, I will explain about the important networking commands and its new versions.
List of Linux Networking and Troubleshooting Commands
Let’s have a look at the list of commands you can use to configure and troubleshoot networking in Linux
hostname ping ip ss (netstat) traceroute dig route
1. hostname
Hostname command is used to view the hostname of the machine and to set the hostname.
[email protected]:~$ hostname devopscube.com [email protected]:~$
You can use the hostname command to set a new hostname for the machine.
[email protected]:~$ sudo hostname temp.com [email protected]:~$ hostname temp.com [email protected]:~$
If you set the hostname using “hostname” command, when you restart the machine, the hostname will change to the name specified in the hostname file ( eg: /etc/hostname). So if you want to change the hostname permanently, you can use the /etc/hosts file or relevant hostname file present on the server.
For ubuntu machines, you can change it in the /etc/hostname file.
For RHEL, CentOS and Fedora you can change it in the /etc/sysconfig/network file.
Also read: List of Linux Commands Every Developer Should Know
2. ping
The ping command is used to check if the remote server is reachable or not. Ping command has the following syntax.
ping
ping using DNS name
[email protected]:~$ ping devopscube.com PING devopscube.com (208.91.198.132) 56(84) bytes of data. 64 bytes from cp-18.webhostbox.net (208.91.198.132): icmp_seq=1 ttl=51 time=64.8 ms 64 bytes from cp-18.webhostbox.net (208.91.198.132): icmp_seq=2 ttl=51 time=65.3 ms
ping using the IP address
[email protected]:~$ ping 8.8.8.8 PING 8.8.8.8 (8.8.8.8) 56(84) bytes of data. 64 bytes from 8.8.8.8: icmp_seq=1 ttl=54 time=8.47 ms 64 bytes from 8.8.8.8: icmp_seq=2 ttl=54 time=8.68 ms
Limit the packets to send
If you want to limit the ping output without using ctrl+c, then you can use the “-c” flag with a number as shown below.
[email protected]:~$ ping -c 1 devopscube.com PING devopscube.com (208.91.198.132) 56(84) bytes of data. 64 bytes from cp-18.webhostbox.net (208.91.198.132): icmp_seq=1 ttl=51 time=64.8 ms --- devopscube.com ping statistics --- 1 packets transmitted, 1 received, 0% packet loss, time 0ms rtt min/avg/max/mdev = 64.885/64.885/64.885/0.000 ms [email protected]:~$
3. ip (ifconfig)
ip command is used to display and manipulate routes and network interfaces. ip command is the newer version of ifconfig. ifconfig works in all the systems, but it is better to use ip command instead of ifconfig. Let’s have a look at few examples of ip command.
Display network devices and configuration
ip a or ip addr
[email protected]:~$ ip a 1: lo: <loopback,up,lower_up> mtu 65536 qdisc noqueue state UNKNOWN group default 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: eth0: <broadcast,multicast,up,lower_up> mtu 9001 qdisc pfifo_fast state UP group default qlen 1000 link/ether 06:e1:76:a0:eb:c9 brd ff:ff:ff:ff:ff:ff inet 172.31.18.184/20 brd 172.31.31.255 scope global eth0 valid_lft forever preferred_lft forever inet6 fe80::4e1:76ff:fea0:ebc9/64 scope link valid_lft forever preferred_lft forever
You can use this command with pipes and grep to get more granular output like IP address of eth0 interface. The following command gets the IP address of eth0 network interface.
ip a | grep eth0 | grep "inet" | awk -F" " '{print $2}'[email protected]:~$ ip a | grep eth0 | grep "inet" | awk -F" " '{print $2}' 172.31.18.184/20 [email protected]:~$
Get details of a specific interface
ip a show eth0
[email protected]:~$ ip a show eth0 2: eth0: <broadcast,multicast,up,lower_up> mtu 9001 qdisc pfifo_fast state UP group default qlen 1000 link/ether 06:e1:76:a0:eb:c9 brd ff:ff:ff:ff:ff:ff inet 172.31.18.184/20 brd 172.31.31.255 scope global eth0 valid_lft forever preferred_lft forever inet6 fe80::4e1:76ff:fea0:ebc9/64 scope link valid_lft forever preferred_lft forever [email protected]:~$
Enable and disable a network interface
You can enable and disable a particular network interface using IP command.
Syntax: ip link set
[email protected]:~$ sudo ip link set docker0 down [email protected]:~$ sudo ip link set docker0 up [email protected]:~$
4. ss (netstat)
ss command is a replacement for netstat. You can still use netstat command on all systems. Using ss command, you can get more information than netstat command. ss command is fast because it gets all the information from the kernel userspace. Now let’s have a look at few usages of ss command.
Listing all connections
The “ss” command will list all the TCP, UDP and Unix socket connections on your machine.
[email protected]:~$ ss Netid State Recv-Q Send-Q Local Address:Port Peer Address:Port u_str ESTAB 0 0 * 7594 * 0 u_str ESTAB 0 0 @/com/ubuntu/upstart 7605 * 0 u_str ESTAB 0 0 * 29701 * 0 u_str ESTAB 0 0 /var/run/dbus/system_bus_socket 29702 * 0 tcp ESTAB 0 400 172.31.18.184:ssh 1.22.167.31:61808
The output of ss command will be big so you can use ” ss | less ” command to make the output scrollable.
Filtering out TCP, UDP and Unix sockets
If you want to filter out TCP , UDP or UNIX socket details, use “-t” “-u” and “-x” flag with the “ss” command. It will show all the established connections to the specific ports. If you want to list both connected and listening ports using “a” with the specific flag as shown below.
ss -ta ss -ua ss -xa
List all listening ports
To list all the listening ports, use “-l” flag with ss command. To list specific TCP, UDP or UNIX socket, use “-t”, “-u” and “-x” flag with “-l” as shown below.
[email protected]:~$ ss -lt State Recv-Q Send-Q Local Address:Port Peer Address:Port LISTEN 0 128 *:ssh *:* LISTEN 0 50 :::http-alt :::* LISTEN 0 50 :::55857 :::* LISTEN 0 128 :::ssh :::* LISTEN 0 50 :::53285 :::* [email protected]:~$
5. traceroute
traceroute is a network troubleshooting utility. Using traceroute you can find the number of hops required for a particular packet to reach the destination. If you do not have traceroute utility in your system or server, you can install it from the native repository.
syntax: traceroute traceroute google.com
traceroute to google.com (173.194.33.163), 30 hops max, 60 byte packets 1 ec2-50-112-0-84.us-west-2.compute.amazonaws.com (50.112.0.84) 1.974 ms 1.895 ms 1.899 ms 2 100.64.1.247 (100.64.1.247) 1.414 ms 100.64.1.137 (100.64.1.137) 1.127 ms 100.64.1.97 (100.64.1.97) 1.313 ms 3 100.64.0.198 (100.64.0.198) 1.443 ms 100.64.0.62 (100.64.0.62) 2.160 ms 100.64.0.60 (100.64.0.60) 2.116 ms 10 66.249.94.214 (66.249.94.214) 6.313 ms 7.104 ms 209.85.249.34 (209.85.249.34) 5.986 ms 11 209.85.244.65 (209.85.244.65) 6.157 ms 6.341 ms 6.574 m. . 12 sea09s18-in-f3.1e100.net (173.194.33.163) 6.302 ms 6.517 ms 6.071 ms [email protected]:~$
The above output shows the hop count (12) to reach google.com from devopscube AWS ec2 server.
6. dig
If you have any task related to DNS lookup, you can use “dig” command to query the DNS name servers. If you want to get the A record for the particular domain name, you can use the dig command with the domain name. From the output, in the answer section, you will find the A record of the domain name as shown below.
[email protected]:~$ dig twitter.com ; <<>> DiG 9.9.5-3-Ubuntu <<>> twitter.com ;; global options: +cmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 17052 ;; flags: qr rd ra; QUERY: 1, ANSWER: 4, AUTHORITY: 0, ADDITIONAL: 1 ;; OPT PSEUDOSECTION: ; EDNS: version: 0, flags:; udp: 4096 ;; QUESTION SECTION: ;twitter.com. IN A ;; ANSWER SECTION: twitter.com. 30 IN A 199.59.149.230 twitter.com. 30 IN A 199.59.150.7 ;; Query time: 9 msec ;; SERVER: 172.31.0.2#53(172.31.0.2) ;; WHEN: Mon Jan 26 06:17:56 UTC 2015 ;; MSG SIZE rcvd: 104 [email protected]:~$
7. route
“route” command is used to get the details of route table for your system and to manipulate it. Let us look at few examples for the route command.
Listing all routes
Execute the “route” command without any arguments to list all the existing routes in your system or server.
[email protected]:~$ route Kernel IP routing table Destination Gateway Genmask Flags Metric Ref Use Iface default ip-172-31-16-1. 0.0.0.0 UG 0 0 0 eth0 172.17.0.0 * 255.255.0.0 U 0 0 0 docker0 172.31.16.0 * 255.255.240.0 U 0 0 0 eth0 [email protected]:~$
If you want to get the full output in numerical form without any hostname, you can use “-n” flag with the route command.
[email protected]:~$ route -n Kernel IP routing table Destination Gateway Genmask Flags Metric Ref Use Iface 0.0.0.0 172.31.16.1 0.0.0.0 UG 0 0 0 eth0 172.17.0.0 0.0.0.0 255.255.0.0 U 0 0 0 docker0 172.31.16.0 0.0.0.0 255.255.240.0 U 0 0 0 eth0 [email protected]:~$
If you think we missed any important command, let us know in the comments section.
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