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tech:linux:useful_network_tools [2019/10/30 11:27] (current) – created - external edit 127.0.0.1
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 +====== 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>
  

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