2022
https://te.legra.ph/Proverka-portov-03-14

packages:
iputils
  ping, tracepath

ping (ICMP)
http://www.unixmen.com/all-about-ping-command/

  RTT -  Round Trip Time - it takes a packet to reach a particular server / host.
  -s 100
    us heavy packets of size 100 (32/64 by default)
  -i 3
    wait for 3 sec (instead of 1) between sending ...
    can use 0.2 also
  -f
    flood - for testing network performance
  -q
    summary only
  -w 6
    ping timeout


mtr, tracepath, traceroute, tcptraceroute, ...
http://s1n1ster.blogspot.com.by/2013/02/tcptraceroute.html

    tracing is needed to recognize a way to the dst
    mtr - does not stop with 3 packets
      uses ICMP datagrams
      hop - jumps
      TTL - TCP header, number of routers (0, 1, 2, 3)
    traceroute
      uses UDP ... (but can be changes via -I)
    tracepath
      between them...

nc/telnet
    nc can as set-up connection and listen it also
    nc <host> <port>
        USER
        USER <username>
        PASS <pass>
        ...
        QUIT
    nc -l <port>
    ...
    echo Hello world | nc localhost 8080
        
nmap:
https://blog.urfix.com/10-cool-nmap-tricks-techniques/

    check open ports at the remote <host>
    by sending TCP packets

    nmap -sT -O <host>

tcpdump/tshark
    tcpdump analyzes headers only
    tcpdump -n -i lo [port 80]
        scan the lo i-face