Telnet to a Specific Port for Testing Purposes

Chưa phân loại
Telnet is both a protocol allowing us to access a remote device to control it and the program used to connect through this protocol. The Telnet protocol is the “cheap” version of ssh, unencrypted, vulnerable to sniffing and Man In the Middle attacks, by default the Telnet port must be closed.

The telnet program, which is not the Telnet protocol, can be useful to test port states which is the functionality this tutorial explains. The reason why this protocol is useful to connect to different protocols such as POP is both protocols support plain text (which is their main problem and why such services should not be used).

Checking ports before starting:

Before starting with telnet, let’s check with Nmap some ports on the sample target (

# nmap

Getting started with Telnet to specific ports for testing purposes:

Once we learned about open ports, we can start launching tests, let’s try the port 22 (ssh), on the console write “telnet <target> <port>” as shown below:

# telnet 22

As you can see in the example below the output says I’m connected to, therefore the port is open.

Let’s try the same on the port 80 (http):

# telnet 80

The output is similar with port 80, now let’s try the port 161 which according to Nmap is filtered:

# telnet 161

As you see the filtered port didn’t allow the connection to be established returning a time out error.

Now let’s try Telnet against a closed (not filtered) port, for this example I will use the port 81. Since Nmap didn’t report on closed ports before proceeding I will confirm it is closed, by scanning the specific port using the -p flag:

# nmap -p 81

Once confirmed the port is closed, let’s test it with Telnet:

# telnet 81

As you can see the connection wasn’t established and the error is different than with the filtered port showing “Connection refused”.

To close an established connection, you can press CTRL+] and you will see the prompt:


Then type “quit” and press ENTER.

Under Linux you can easily write a little shell script to connect through telnet with different targets and ports.

Open nano and create a file called with the following content inside:

  #! /bin/bash  #The first uncommented line will connect to through port $  telnet 80  #The second uncommented line will connect to through ssh.  telnet 22  #The third uncommented line will connect to through ssh  telnet 22  

Connections only start after the previous was closed, you can close the connection by passing any character, In the example above I passed “q”.

Yet, if you want to test many ports and targets simultaneously Telnet isn’t the best option, for which you have Nmap and similar tools

About Telnet:

As said before, Telnet is an unencrypted protocol vulnerable to sniffers, any attacker can intercept the communication between the client and the server in plain text accessing sensible information such as passwords.

The lack of authentication methods also allow possible attackers to modify the packages sent between two nodes.

Because of this Telnet was rapidly replaced by SSH (Secure Shell) which provides a variety of authentication methods and also encrypts the whole communication between nodes.

Bonus: testing specific ports for possible vulnerabilities with Nmap:

With Nmap we can go far more than with Telnet, we can learn the version of the program running behind the port and we can even test it for vulnerabilities.

Scanning a specific port to find vulnerabilities on the service:

The following example shows a scan against the port 80 of calling Nmap NSE script vuln to test offensive scripts looking for vulnerabilities:

# nmap -v -p 80 –script vuln

As you can see, since it is server no vulnerabilities were found.

It is possible to scan a specific port for a specific vulnerability; the following example shows how to scan a port using Nmap to find DOS vulnerabilities:

# nmap -v -p 80 –script dos

As you can see Nmap found a possible vulnerability (it was a false positive in this case).

You can find a lot of high quality tutorials with different port scanning techniques at

I hope you found this tutorial on Telnet to a specific port for testing purposes useful. Keep following LinuxHint for more tips and updates on Linux and networking

Sandclock IDC thành lập vào năm 2012, là công ty chuyên nghiệp tại Việt Nam trong lĩnh vực cung cấp dịch vụ Hosting, VPS, máy chủ vật lý, dịch vụ Firewall Anti DDoS, SSL… Với 10 năm xây dựng và phát triển, ứng dụng nhiều công nghệ hiện đại, Sandclock IDC đã giúp hàng ngàn khách hàng tin tưởng lựa chọn, mang lại sự ổn định tuyệt đối cho website của khách hàng để thúc đẩy việc kinh doanh đạt được hiệu quả và thành công.
Bài viết liên quan

Tổng hợp 15 lệnh OpenVZ commands thông dụng

1, Command to list the running VPSs in a node # vzlist Example: # vzlist CTID NPROC STATUS IP_ADDR...

Install Apache Kafka on Ubuntu

Apache Kafka is a message broker service like ActiveMQ and RabbitMQ. You can learn more about Apache Kafka at  In...

Hướng dẫn cài đặt PHP 7.x trên CentOS7

Vì phiên bản PHP5.x đã sắp đến thời gian ngừng hỗ trợ, do vậy một số ứng cần yêu cầu PHP...
Bài Viết

Bài Viết Mới Cập Nhật

Hướng dẫn chuyển đổi windows server windows evaluation to standard và active windows server 2008 + 2012 + 2016 + 2019

How to Update Ubuntu Linux

Squid Proxy Manager cài đặt và quản lý Proxy Squid tự động trên ubuntu

Hướng dẫn cài đặt Apache CloudStack

Hướng dẫn ký file PDF bằng chữ ký số (chữ ký điện tử) và sửa lỗi mới nhất 2021 foxit reader