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