netcat and similar tools

| Wikipedia page about netcat |

netcat.traditional

The classic netcat, written by Hobbit. Installed on a default Debian Stretch and Buster.

| Ubuntu man page | stackexchange tunnel |

Install, on debian-like systems:

sudo apt install netcat-traditional

Check if port 80 is open: nc -zv google.com 80
Connection to google.com (142.251.143.78) 80 port [tcp/http] succeeded!

Relay server; redirect incoming (external) request on port 5060 to local port 8022:

nc.traditional -l -p 5060 -c "nc.traditional 127.0.0.1 8022"

nc - netcat (OpenBSD)

netcat-openbsd - the OpenBSD rewrite of netcat, including support for IPv6, proxies, and Unix sockets. Installed in a default Ubuntu 16.04.

Install: sudo apt install netcat-openbsd

Check (installed) package: apt-cache policy netcat-openbsd

Check if a port is open: nc -z -w 3 myhost 10026 && echo open || echo closed

ncat (by nmap)

ncat - the nmap rewrite of netcat

Install full nmap package with ncat, or just ncat, on debian-like systems:

sudo apt install nmap | sudo apt install ncat

Relay server; redirect incoming (external) request on port 5060 to local port 8022:

ncat -l -p 5060 -c "ncat 127.0.0.1 8022"

Proxy requests on port 8080 to 192.168.1.200:80:

mkfifo 2way

ncat -l 8080 0<2way | ncat 192.168.1.200 80 1>2way

socat

| socat | Ubuntu man page | Getting started |

Install socat:

sudo apt install socat

Test if a port is open: socat - TCP:myhost:10026 && echo open || echo closed

Create two connected serial ports:

socat -d -d pty,raw,echo=0 pty,raw,echo=0

2018/02/04 11:15:45 socat[10376] N PTY is /dev/pts/21
2018/02/04 11:15:45 socat[10376] N PTY is /dev/pts/22
2018/02/04 11:15:45 socat[10376] N starting data transfer loop with FDs [5,5] and [7,7]

Source: simple way to create a tunnel from one local port to another

Relay server; redirect incoming (external) request on port 5060 to local port 8022:

socat tcp-listen:5060,reuseaddr,fork tcp:localhost:8022

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