![]() ![]() ![]() Nmap offers a higher grade of automation than Netcat. Netcat is handy for TCP connectivity testing, but when it comes to a command-line tool with a powerful array of options, you cannot beat Nmap. There are other features and use cases for nc read the documentation to learn more. ![]() Now you can write and receive messages on both sides, like a bidirectional chat. ![]() On the receiving side, start a server: nc -l 2080 You can also use Netcat to start a server to help you test basic connectivity if you don't have a server handy. Ncat: 0 bytes sent, 0 bytes received in 0.12 seconds. Ncat: Host will be resolved by the proxy. For example, start a SOCKS proxy like this on port 2080: ssh -f -g -D 2080 -C -q -N access the servers behind your firewall through the proxy like this: $ nc -proxy 192.168.1.27:2080 -proxy-type socks5 -z -v -w 5 80 First, you can use a SOCKS (secure socket) proxy to scan servers with -x. Why would you use nc instead of the previous script written in Bash? There are a couple of reasons. If /usr/bin/nc -z -v -w 5 "$machine" "$port" > /dev/null 2>&1 thenĮcho "ERROR: Invalid or missing data file!" With that in mind, you can automate scanning a bunch of hosts using a Netcat wrapper: # Port check with Netcat Ncat: 0 bytes sent, 0 bytes received in 0.06 seconds. It also has several implementations.įor the sake of example, assume you want to check whether port 22 is open on host : $ nc -z -v -w 5 raspberrypi 22 Netcat is another versatile program that can use proxies to connect to other machines. It also doesn't deal well with filtered ports.įor example, what if you want to check connectivity with a host that is behind a firewall? It is faster than the Expect version because it doesn't require Telnet forking, but error handling is complicated. Next, you might decide it is OK to write a TCP port check in Bash: #!/bin/bash -e You also have to account for all the possible responses from the Telnet command, as well as subtle issues like your timeout being too small (if the port is being filtered, for example). This solution is not efficient, however, because you have to fork a telnet session for every port you want to check. When should you use this type of script? Expect is a good alternative if you need something quick, especially if you already have both Expect and Telnet installed on one of your machines. $ sudo dnf -y install expect telnetįor example, your output could look like this: $. "Connection refused" īefore running the script, ensure you have both Expect and Telnet installed. Send_user "Please provide the data file with machine port, one per line!" # Poor man TCP port scanner with Telnet and Expect With a bit of Expect magic, you could automate the process using this script: #!/usr/bin/env -S expect -f Say you have the following configuration file: 80 With Expect, you can read the list of hosts and ports from a file and use Telnet to check whether a TCP port is responding or not. Know what to expectĮxpect is an extension of the programming language Tcl, which can be used to automate external processes. The examples I use will focus on TCP connectivity testing rather than UDP, which is out of scope for this article.
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