Commonly Used Network Commands / Tools

Add/update DNS Server configurations in /etc/resolv.conf in the format nameserver <IP-Address>, one for each line. The /etc/resolve.conf file does get overwritten each time the system is rebooted. In Ububtu the /etc/resolvconf/resolv.conf.d/head is the source file and updating it keeps the changes permanent. In distros (such as Redhat) you can make the /etc/resolv.conf file read-only by chattr +i /etc/resolv.conf. Use the -i option to make the file read-write again. By default, the default gateway resolves DNS.

netstat Command to list LISTEN addresses (listening ports)

netstat -pnutl
netstat -pnutl | grep :80

Processes PID that opened tcp port 80 (run as root):

fuser 80/tcp
fuser -u -v 80/tcp

Sample output:

80/tcp:               1741  7842 11760 14459

Process name associated with PID #

ls -l /proc/1741/exe

netstat -i

nc Command to Create a service using bash script (to listen on port 8444 for example)

nc -k -l 8444

Keywords: Listener socket server bash bind TCP netcat
Resources: Simple Socket Server in Bash

Target IP is the IP address of the server running nc -k -l 8444 and waiting on requests.

echo "test-hello" | nc <target-ip> 8444

Some servers/devices have ping (ICMP) disabled. In that case use arping to find them. Example use below.

arping -c1 -f -w1 192.168.1.1

Usage: arping [-fqbDUAV] [-c count] [-w timeout] [-I device] [-s source] destination
  -f : quit on first reply
  -q : be quiet
  -b : keep broadcasting, don't go unicast
  -D : duplicate address detection mode
  -U : Unsolicited ARP mode, update your neighbours
  -A : ARP answer mode, update your neighbours
  -V : print version and exit
  -c count : how many packets to send
  -w timeout : how long to wait for a reply
  -I device : which ethernet device to use (eth0)
  -s source : source ip address
  destination : ask for what ip address

nmap can be used to find open ports. Simple example below:

nmap 192.168.1.1

Also nc can be used to find if a port is open

nc -zv 192.168.1.100 80

On Windows

PowerShell
Test-NetConnection -ComputerName 192.168.1.21 -Port 22