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In Linux and Networking Last updated: November 22, 2022
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Configuring, monitoring, and securing networks form an essential part of a Sysadmin’s job. When it comes to managing Linux networks, there are many commands and utilities available.

At times networked systems fail. You as an administrator are required to diagnose and resolve issues. Monitoring helps in detecting problems and fixing them before things get out of hand. Monitoring for security and performance also forms an essential part of an admin’s activities.

Here we discuss some commonly used commands to manage Linux networks.


The iproute2 package includes the IP command which is used for network and routing configuration. This replaces the traditional ifconfig and route commands.

ip takes a second argument that specifies the object on which you wish to execute a command and an action like add, delete, or show.

ip link

ip link is for configuring, adding, and deleting network interfaces. Use ip link show command to display all network interfaces on the system :

$ ip link show
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: enp0s3: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP mode DEFAULT group default qlen 1000
    link/ether 02:35:97:08:6b:2a brd ff:ff:ff:ff:ff:ff
3: enp0s8: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP mode DEFAULT group default qlen 1000
    link/ether 08:00:27:6f:60:ff brd ff:ff:ff:ff:ff:ff

You can see the man page for ip link with:

$ man  ip-link

ip address

Use ip address command to display addresses, bind new address or delete old ones. The man page ip address command is named as ip-address.

For example, the following command shows the IP address assigned to the network interface enp0s8:

ip address show dev enp0s8
3: enp0s8: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
    link/ether 08:00:27:6f:60:ff brd ff:ff:ff:ff:ff:ff
    inet brd scope global enp0s8

ip route

Use the IP route to print or display the routing table. The following command displays the contents of the routing table:

$ ip route show
default via dev enp0s3 dev enp0s8  proto kernel  scope link  src dev enp0s3  proto kernel  scope link  src


While Nmap had been used in many movies,  The Matrix Reloaded (Wikipedia, IMDB, Amazon) turned Nmap into a movie star!.

Nmap (“Network Mapper”) is a powerful utility used for network discovery, security auditing, and administration. Many system admins use it to determine which of their systems are online, and also for OS detection and service detection. 

The default Nmap scan shows the ports, their state (open/closed), and protocols. It sends a packet to 1000 most common ports and checks for the response.

$ nmap

Starting Nmap 7.01 ( ) at 2020-09-07 10:32 UTC
Nmap scan report for
Host is up (0.00077s latency).
Not shown: 997 filtered ports
22/tcp   open   ssh
80/tcp   open   http
3306/tcp closed mysql

To check which hosts on your network are up:

$ nmap -sn

Starting Nmap 7.01 ( ) at 2020-09-07 11:59 UTC
Nmap scan report for
Host is up (0.00084s latency).
Nmap scan report for
Host is up (0.0021s latency).
Nmap scan report for
Host is up (0.00026s latency).
Nmap done: 256 IP addresses (3 hosts up) scanned in 2.61 seconds

Use -O flag to identify which operating system a host is running.

$ sudo  nmap -O
Starting Nmap 7.01 ( ) at 2020-09-07 13:44 UTC
Nmap scan report for
Host is up (0.00053s latency).
Running: Linux 3.X
OS CPE: cpe:/o:linux:linux_kernel:3
OS details: Linux 3.10 - 3.19
Network Distance: 1 hop
OS detection performed. Please report any incorrect results at .
Nmap done: 1 IP address (1 host up) scanned in 21.95 seconds

A word of caution: Nobody appreciates their systems being scanned over the internet. So before you do so, seek permission.

You can also use Nmap on Windows, check out this installation guide.


Use ping to see if a host is alive. This super simple command helps you check the status of a host or a network segment. Ping command sends an ICMP ECHO_REQUEST packet to the target host and waits to see if it replies.

However, some hosts block ICMP echo requests with a firewall. Some sites on the internet may also do the same.

By default, ping runs in an infinite loop. To send a defined number of packets, use -c flag.

$ ping -c 3 
PING ( 56 data bytes
64 bytes from icmp_seq=0 ttl=118 time=7.898 ms
64 bytes from icmp_seq=1 ttl=118 time=7.960 ms
64 bytes from icmp_seq=2 ttl=118 time=6.247 ms

--- ping statistics ---
3 packets transmitted, 3 packets received, 0.0% packet loss
round-trip min/avg/max/stddev = 6.247/7.368/7.960/0.793 ms

With -o flag ping exits successfully after receiving one reply packet.

$ ping -o
PING ( 56 data bytes
64 bytes from icmp_seq=0 ttl=118 time=7.540 ms

--- ping statistics ---
1 packets transmitted, 1 packets received, 0.0% packet loss
round-trip min/avg/max/stddev = 7.540/7.540/7.540/0.000 ms

You can use -n flag to avoid reverse DNS lookups. The ICMP sequence number is particularly important. A Break in sequence numbers indicates lost packets.

A failed ping could be due to

  • network failure
  • host being not alive
  • firewall blocking ICMP ECHO requests

You can also perform an online ping test to check the connectivity from different parts of the world.


While ping verifies the availability of a host, iPerf helps analyze and measure network performance between two hosts. With iPerf, you open a connection between two hosts and send some data. iPerf then shows the bandwidth available between the two hosts.

You can install an iPerf using your distribution package manager. For example on Ubuntu-based distributions you can install like this:

$ sudo apt install iperf -y

Once you have installed iPerf on both the machines, start the iPerf server on one of them. The following example starts the iPerf server on a host with IP address

$ iperf -s
Server listening on TCP port 5001
TCP window size: 85.3 KByte (default)

On the second machine start iPerf with the -c flag. This connects with the server and sends some data.

$ iperf -c
Client connecting to, TCP port 5001
TCP window size: 85.0 KByte (default)
[  3] local port 42177 connected with port 5001
[ ID] Interval       Transfer     Bandwidth
[  3]  0.0-10.0 sec  1.13 GBytes   972 Mbits/sec

iPerf returns with the bandwidth results in a few seconds.


If ping shows missing packets, you should use traceroute to see what route the packets are taking.  Traceroute shows the sequence of gateways through which the packets travel to reach their destination. For example, traceroute from my machine to shows the following:

$ traceroute
traceroute to (, 64 hops max, 52 byte packets
 1  dlinkrouter.dlink (  5.376 ms  2.076 ms  1.932 ms
 2 (  5.190 ms  5.125 ms  4.989 ms
 3 (  7.165 ms  5.749 ms  5.755 ms
 4 (  5.918 ms *  8.483 ms
 9 (  6.359 ms (  5.448 ms (  6.400 ms

Line 4 in this output shows a * in the round trip times. This indicates no response was received. This can be due to many reasons – as the traceroute ICMP packets are low-priority, these may be dropped by a router. Or there could be simply congestion.  If you see a * in all the time fields for a given gateway, then possibly the gateway is down.

Many web-based route tracing tools allow you to do a reverse traceroute, that is, from a website to your host. You can check these at or Geekflare Traceroute.


tcpdump is a packet sniffing tool and can be of great help when resolving network issues. It listens to the network traffic and prints packet information based on the criteria you define.

For example, you can examine all packets sent to or from a particular host, Ubuntu18 in this example:

$ sudo tcpdump host ubuntu18 -n -c 5
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 262144 bytes
14:12:11.509092 IP > Flags [P.], seq 2991049004:2991049112, ack 2956233368, win 501, options [nop,nop,TS val 292041322 ecr 405604219], length 108
14:12:11.509146 IP > Flags [P.], seq 108:252, ack 1, win 501, options [nop,nop,TS val 292041322 ecr 405604219], length 144
14:12:11.509218 IP > Flags [P.], seq 252:288, ack 1, win 501, options [nop,nop,TS val 292041322 ecr 405604219], length 36
14:12:11.509259 IP > Flags [P.], seq 288:500, ack 1, win 501, options [nop,nop,TS val 292041322 ecr 405604219], length 212
14:12:11.509331 IP > Flags [P.], seq 500:768, ack 1, win 501, options [nop,nop,TS val 292041322 ecr 405604219], length 268
5 packets captured
6 packets received by filter
0 packets dropped by kernel

By default, tcpdump resolves IP addresses to hostnames. Use -n flag, if you do not want tcpdump to perform name lookups.

tcpdump output prints one line for each packet. Use -c flag to limit output, 5 in the example above.

tcpdump is useful for solving network problems and also identifying potential problems. It is a good idea to run a tcpdump on your network occasionally to verify everything is in order.


Netstat command is used to examine network connections, routing tables, and various network settings and statistics.

Use -i flag to list the network interfaces on your system.

Here is an example:

$ netstat -i
Kernel Interface table
eth0       1500 0      4001      0      0 0          2283      0      0      0 BMRU
eth1       1500 0     27154      0      0 0        838962      0      0      0 BMRU
lo        65536 0         0      0      0 0             0      0      0      0 LRU

Using -r flag will display the routing table. This shows the path configured for sending network packets.

$ netstat -r
Kernel IP routing table
Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface
default         UG        0 0          0 eth0        *        U         0 0          0 eth1        *        U         0 0          0 eth0

An asterisk in the last two lines indicates that no gateway is required to send packets to any host on these networks. This host is directly connected to the networks and

In the first line, the destination is the default, which means any packet destined for a network not listed in this table is handled by the router 10.0.2,2.

netstat command without any options displays a list of open sockets. Use -l flag to show only listening sockets, which by default, are not shown. You can use -a flag to show listening and non-listening sockets. Here is an example:

$ netstat -a
Active Internet connections (servers and established)
Proto Recv-Q Send-Q Local Address           Foreign Address         State      
tcp        0      0 *:ssh                   *:*                     LISTEN     
tcp        0     36           ESTABLISHED
tcp6       0      0 [::]:ssh                [::]:*                  LISTEN     
udp        0      0 *:bootpc                *:*                                
Active UNIX domain sockets (servers and established)
Proto RefCnt Flags       Type       State         I-Node   Path
unix  3      [ ]         DGRAM                    8186     /run/systemd/notify

More Netstat command example here


Linux installations have a lot of services running by default. These should be disabled or preferably removed, as this helps in reducing the attack surface. You can see what services are running with the netstat command. While netstat is still available, most Linux distributions are transitioning to ss command.

use ss command with -t and -a flags to list all TCP sockets. This displays both listening and non-listening sockets.

$ ss -t -a
State       Recv-Q Send-Q            Local Address:Port                Peer Address:Port   
LISTEN      0      128                           *:sunrpc                         *:*       
LISTEN      0      128                           *:http                           *:*       
LISTEN      0      128                           *:ssh                            *:*       
LISTEN      0      128                           *:60031                          *:*       
ESTAB       0      0                         
ESTAB       0      0                         
LISTEN      0      128                          :::sunrpc                        :::*       
LISTEN      0      128                          :::http                          :::*       
LISTEN      0      128                          :::ssh                           :::*       
LISTEN      0      128                          :::54715                         :::*

To display only TCP connections with state established:

ss -a -t -o state established
Recv-Q Send-Q                 Local Address:Port                     Peer Address:Port   
0      0                                    timer:(keepalive,23min,0)
0      0                                    timer:(keepalive,114min,0)


ssh enables you to connect securely with remote hosts over the internet. Earlier rlogin and telnet were used to connect to and administer remote hosts. However, both suffer from a fundamental flaw, that is, they send all information including login names and passwords in cleartext.

ssh enables secure communication over the internet with the following two features :

  • It confirms that the remote host is, who it says it is.
  • It encrypts all communication between the hosts.

To connect to a remote host you need to have an OpenSSH server running on the remote host. You can install it using your distribution package manager. For example on Ubuntu you can install it like this:

$ sudo apt install openssh-server

Here is an example showing how you can connect to the remote host using the ssh command:

me@ubuntu-xenial:~$ ssh
The authenticity of host ' (' can't be established.
ECDSA key fingerprint is SHA256:s2tNJQa/C1/W0SevGm7Rt3xoBZG1QL5yT3ff/+PMpnY.
Are you sure you want to continue connecting (yes/no)? yes

You get a message saying that the authenticity of the host cannot be established, this is because it’s the first time a connection is being made with (server) and the ssh client has never seen this remote host before. Enter yes to continue connecting. Once the connection has been established, you are prompted for a password:

Warning: Permanently added '' (ECDSA) to the list of known hosts.
me@'s password:

After you enter the correct password, you are logged into the remote host.

Welcome to Ubuntu 14.04.6 LTS (GNU/Linux 3.13.0-170-generic x86_64)
 * Documentation:

You can exit this remote shell with the exit command.

Also, you can easily execute a single command on the remote host using ssh. For example, to run df -h on the remote host:

$ ssh df -h
me@'s password: 
Filesystem      Size  Used Avail Use% Mounted on
udev            241M   12K  241M   1% /dev
tmpfs            49M  384K   49M   1% /run
/dev/sda1        40G  1.6G   37G   5% /
none            224G  113G  111G  51% /vagrant

scp and sftp

scp (secure copy) is very similar to cp command for copying files, with an addition – you can include remote hostnames in the source or destination pathnames. The hostname and the directory path are separated by a colon. This enables you to copy files securely over the network in an encrypted form. The following command copies a.txt from the local machine to :

me@ubuntu-xenial:~$ scp a.txt
me@'s password: 
a.txt                                           100%    0     0.0KB/s   00:00

sftp (secure ftp) is also a file copy program similar to ftp. However, it uses an SSH encrypted tunnel to copy files, instead of sending everything in cleartext. Also, you do not need an FTP server running on the remote host. You only need an ssh server. Here is an example session:

me@ubuntu-xenial:~$ sftp
me@'s password: 
Connected to
sftp> put kali-linux-2020.3-installer-netinst-i386.iso
Uploading kali-linux-2020.3-installer-netinst-i386.iso to /home/me/kali-linux-2020.3-installer-netinst-i386.iso
kali-linux-2020.3-installer-netinst-i386.iso    100%  435MB  27.2MB/s   00:16    
sftp> bye


Mostly we use ifconfig command to check the IP address assigned to the system.

[root@lab ~]# ifconfig
eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet  netmask  broadcast
        inet6 fe80::c035:b2ff:fe9d:72d5  prefixlen 64  scopeid 0x20<link>
        ether c2:35:b2:9d:72:d5  txqueuelen 1000  (Ethernet)
        RX packets 1333200  bytes 167143230 (159.4 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 979666  bytes 93582595 (89.2 MiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

lo: flags=73<UP,LOOPBACK,RUNNING>  mtu 65536
        inet  netmask
        inet6 ::1  prefixlen 128  scopeid 0x10<host>
        loop  txqueuelen 1000  (Local Loopback)
        RX packets 16  bytes 1392 (1.3 KiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 16  bytes 1392 (1.3 KiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

[root@lab ~]#


dig (Domain Information Groper) is a flexible tool for interrogating DNS name servers.

It performs DNS lookups and displays the answers that are returned from the name servers.

[root@lab ~]# dig

; <<>> DiG 9.11.13-RedHat-9.11.13-5.el8_2 <<>>
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 12310
;; flags: qr rd ra ad; QUERY: 1, ANSWER: 2, AUTHORITY: 0, ADDITIONAL: 1

; EDNS: version: 0, flags:; udp: 512
;			IN	A


;; Query time: 12 msec
;; WHEN: Wed Sep 16 17:58:45 UTC 2020
;; MSG SIZE  rcvd: 74

[root@lab ~]#


telnet connect destination’s host and port via a telnet protocol if a connection establishes means connectivity between two hosts is working fine.

[root@lab ~]# telnet 443
Connected to
Escape character is '^]'.


nslookup is a program to query domain name servers and resolving IP.

[root@lab ~]# nslookup

Non-authoritative answer:

[root@lab ~]#


Networking in Linux is a vast subject, with a large number of commands and utilities. In this article, we have discussed some commonly used commands which hopefully, will help you in managing and securing your network.

  • Vijay Khurana
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