====== Commonly Used Network Commands / Tools ======
===== Define DNS Servers =====
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.

===== LISTEN address list =====
netstat Command to list LISTEN addresses (listening ports)
<code bash>
netstat -pnutl
netstat -pnutl | grep :80
</code>


Processes PID that opened tcp port 80 (run as root):
<code bash>
fuser 80/tcp
fuser -u -v 80/tcp
</code>
Sample output:
<code>
80/tcp:               1741  7842 11760 14459
</code>

Process name associated with PID #
<code bash>ls -l /proc/1741/exe</code>

===== Display Network Interface Statistics =====
<code bash>
netstat -i
</code>

===== Create a LISTEN address =====
nc Command to Create a service using bash script (to listen on port 8444 for example)
<code bash>
nc -k -l 8444
</code>
Keywords: Listener socket server bash bind TCP netcat\\
Resources: [[http://stackoverflow.com/questions/4739196/simple-socket-server-in-bash|Simple Socket Server in Bash]]


===== To send to a LISTEN address =====
Target IP is the IP address of the server running ''nc -k -l 8444'' and waiting on requests.
<code bash>
echo "test-hello" | nc <target-ip> 8444
</code>


===== Find systems without ping =====
Some servers/devices have ping (ICMP) disabled. In that case use arping to find them. Example use below.
<code bash>
arping -c1 -f -w1 192.168.1.1
</code>
==== arping usage help ====
<code>
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
</code>

===== Find open ports =====
nmap can be used to find open ports.  Simple example below:
<code bash>
nmap 192.168.1.1
</code>

Also ''nc'' can be used to find if a port is open
<code>
nc -zv 192.168.1.100 80
</code>

On Windows
<code bat>
PowerShell
Test-NetConnection -ComputerName 192.168.1.21 -Port 22
</code>