# Linux Useful Tricks — telnet Alternatives


By [AnastasyP](https://paragraph.com/@anastasyp) · 2023-03-31

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Few tricks for DevOps operations in Linux

During networking troubleshooting, to check if a port on the remote server is open, we can use the following command to test telnet via port:

    $ telnet <hostname> 6663
    Trying xxx.xx.xx.xx…
    Connected to .
    Escape character is '^]'.
    ^CConnection closed by foreign host
    

But under some circumstances `telnet` is not installed on the server or we can’t use telnet (for internal reasons), here are the alternatives to check ports, as `telnet`:

ncat

`ncat` (or `nc`) is a versatile and powerful command-line utility in Linux that is used for reading from and writing to network connections using TCP or UDP. It is a part of the Nmap Project and is an improved version of the traditional `netcat` command. `ncat` is often referred to as the "Swiss Army knife" of networking tools due to its flexibility and numerous use cases.

It is designed to be a reliable back-end tool to instantly provide network connectivity to other applications and users. Ncat will not only work with IPv4 and IPv6 but provides the user with a virtually limitless number of potential uses.

    $ nc -zv <hostname> 443
    Ncat: Version 7.50 ( https://nmap.org/ncat )
    Ncat: Connected to xxx.xxx.xxx.xxx:443.
    Ncat: 0 bytes sent, 0 bytes received in 0.02 seconds.
    

nmap

`nmap` (Network Mapper) is a powerful and widely-used open-source tool for network scanning, security auditing, and network discovery. It is designed to efficiently discover hosts and services on a computer network, creating a "map" of the network. `nmap` is a versatile tool and is commonly used by network administrators, security professionals, and penetration testers.

> **_However in most organizations nmap is considered as a scaning tool and one can not use nmap without proper authorization, so use it with caution._**

    $ nmap -sT google.com
    Starting Nmap 7.70 ( https://nmap.org ) at 2022-11-04 21:01 GMT
    Nmap scan report for google.com (172.217.23.14)
    Host is up (0.12s latency).
    rDNS record for 172.217.23.14: lhr35s01-in-f14.1e100.net
    Not shown: 998 filtered ports
    PORT    STATE SERVICE
    80/tcp  open  http
    443/tcp open  https
    

curl

`curl` is a tool for transferring data from or to a server. It supports these protocols: DICT, FILE, FTP, FTPS, GOPHER, GOPHERS, HTTP, HTTPS, IMAP, IMAPS, LDAP, LDAPS, MQTT, POP3, POP3S, RTMP, RTMPS, RTSP, SCP, SFTP, SMB, SMBS, SMTP, SMTPS, TELNET, TFTP, WS and WSS.

    $ curl -vvv telnet://<hostname>:443
    * About to connect() to <hostname> port 443 (#0)
    *   Trying xxx.xxx.xxx.xxx...
    * Connected to <hostname> (xxx.xxx.xxx.xxx) port 443 (#0)
    

bash shell

If using Bash Shell, then you can use its feature to check if a port is open or closed:

    $ (timeout 1 bash -c '</dev/tcp/<hostname>/443' && echo PORT OPEN || echo PORT CLOSED) 2>/dev/null
    PORT OPEN
    

BASH often provides the support that we need using redirection from `/dev/proto/host/port`. The `/dev` directory contains entries for the physical devices that may or may not be present in the hardware. When executing a command on a `/dev/tcp/$host/$port` pseudo-device file, Bash opens a TCP connection to the associated socket.

python

You can use Python socket module to simply check if a port is open or not:

    import socket
    
    try:
        sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        sock.settimeout(4)
        result = sock.connect(('<hostname>', 443))
        print("Port is open")
    except socket.error as e:
        print("Port is not open")
    finally:
        sock.close()

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*Originally published on [AnastasyP](https://paragraph.com/@anastasyp/linux-useful-tricks-telnet-alternatives)*