Linux red hat telnet

CentOS / RHEL 7 : How to install and configure telnet

It’s not recommended to use telnet as it is not secure. The passwords are transferred using a plain text and any packet sniffer can easily track you. Nevertheless, it’s sometimes required to install telnet anyways. To check if you have telnet package already installed on your system, use :

# rpm -qa | grep telnet telnet-server-0.17-59.el7.x86_64 telnet-0.17-59.el7.x86_64

In order to turn Telnet on make sure that you have the packages telnet-server and telnet installed:

Installing telnet packages

If the 2 required packages are not installed, install it using yum.

# yum install telnet-server telnet

Configuring/enabling telnet

1. Add the service to firewalld.

The built in firewalld blocks Telnet port 23 by default because the protocol is not considered secure. Please make sure that the port is open or if a non-default port is being used, that the port associated with Telnet is open for telnet traffic to pass through.

# firewall-cmd --add-service=telnet --zone=public

Run the rule again with the “–permanent” flag for it to persist across firewalld restarts.

# firewall-cmd --add-service=telnet --zone=public --permanent

2. Add the service to selinux.

You will have to also add the service to SELinux. This is required only in the case where SELinux is enabled on the system.

# semanage port -a -t telnetd_port_t -p tcp

3. Enable and start the telnet service.

Start the service using the systemctl command.

# systemctl start telnet.socket

Enable the telnet service to start at boot.

# systemctl enable telnet.socket

4. Verify

Once you are done with the configuration, verify if the telnet to a server works.

# telnet geeklab02 Trying 10.10.10.10. Connected to 10.10.10.10. Escape character is '^]'. Kernel 3.10.0-327.el7.x86_64 on an x86_64 geeklab02 login: testuser Password: Last login: Sat Jan 23 18:19:43 from geeklab [testuser@geeklab02 ~]$ hostname geeklab02

Источник

How to Install Telnet on Linux (CentOS, RHEL) using the YUM package manager

Although Telnet is not considered the most secure protocol for remote access, it can still be helpful for certain network administration tasks, such as testing and troubleshooting network services.

In this guide, we will specifically focus on how to install Telnet on Red Hat-based Linux distributions (e.g., CentOS, RHEL) using the YUM package manager. By the end of this guide, you will have successfully installed Telnet on your system and be ready to use it for your specific needs.

Step 1: Update Your System

Before installing Telnet, it is essential to update your system’s package list to ensure you are installing the latest version of the software. To update your system, open a terminal and run the following command:

Step 2: Install the Telnet Package

With your system updated, you can now install the Telnet package using YUM. Run the following command in your terminal:

YUM will automatically resolve any dependencies and proceed with the installation. Once the installation is complete, you can verify the Telnet installation by running the following command:

If you see the Telnet command prompt, the installation was successful.

Step 3: Connect to a Remote Server Using Telnet

To connect to a remote server using Telnet, enter the following command in the terminal, replacing “webhostinggeeks.com” with the server’s address and “port_number” with the appropriate port:

telnet webhostinggeeks.com port_number

Press Enter to establish the connection. You can now interact with the remote server using the Telnet protocol.

Commands and Their Functions:

• sudo yum update – Updates the system’s package list.
• sudo yum install telnet – Installs the Telnet package using YUM.
• telnet – Connects to a remote server using the Telnet protocol.

Conclusion

This guide has provided you with the necessary steps to install Telnet on a Red Hat-based Linux distribution using the YUM package manager. Now that Telnet is installed on your system, you can use it for network administration tasks, such as testing and troubleshooting network services. Keep in mind that Telnet is not a secure protocol, so consider using more secure alternatives like SSH for remote connections whenever possible.

We encourage you to leave comments and suggest improvements to this guide. Your feedback helps us improve our content and better serve our audience.

Dimitri Nek

Dimitri is a Linux-wielding geek from Newport Beach and a server optimization guru with over 20 years of experience taming web hosting beasts. Equipped with an arsenal of programming languages and an insatiable thirst for knowledge, Dimitri conquers website challenges and scales hosting mountains with unmatched expertise. His vast knowledge of industry-leading hosting providers allows him to make well-informed recommendations tailored to each client’s unique needs.

Источник

Using Telnet to troubleshoot your mail system

For this article, I use example.com as the domain name for mail servers and email addresses. In addition, I use the standard ports of 25 for SMTP and 110 for POP3.

Great Linux resources

The first thing you’ll need to do is install Telnet if it is not already present. (Telnet is not automatically installed on all systems in favor of the more secure SSH protocol.) To install Telnet on a Red Hat Enterprise Linux-based system, type:

Testing an SMTP mail server

Now that you have Telnet installed, you’re ready to start. Let’s first verify that your SMTP server is available by connecting to it via Telnet with the following command:

Once connected, you will receive a response similar to the following:

Initiate the connection with the SMTP service by using the following command:

The response looks like this:

You are now ready to send an email. Start by telling the system what email address the mail is coming from:

mail from: admin@example.com 

The system will then acknowledge that the information is in the correct syntax, in the same format used in the previous image.

Next, tell the system the recipients’ email address or addresses with the following command:

The system acknowledges that the information is in the correct syntax, as follows:

Tell the system that you are beginning the body of your email by typing data as follows:

The system will respond with a single dot ( . ) on its own line, which ends the email headers:

Now, set the subject and write the email’s content. Then, tell the system to send the email by using the dot ( . ) as follows:

subject: test Test that your email addresses are working properly. Thanks, SysAdmin . 

The system then reports that the email was accepted for delivery:

Exit the session by typing quit , which disconnects us as shown here:

Testing a POP mail server

Now, check the configuration of your POP3 service, which is provided by Dovecot. The first step is to connect to the server over POP3, which is served on port 110:

$ telnet mail.example.com 110 

Once connected, you will receive a response similar to the following:

Next, provide the information for the user account you’re accessing. In this case, verify that user1 received the email you just sent them:

You see that the username is correct with:

Next, use the following command to authenticate using the user’s password:

If the user and password are correct, the authentication is confirmed as follows:

To check and see what mail is available, use the list command:

This command returns a list of all email for that user, and the size of that email, as shown below (it does not return any other information).

To read an email, we need to use the retr command, which is short for retrieve:

In this case, you asked to retrieve email number one, which as you can see is the email you sent directly via SMTP through Telnet:

As this was a test email, delete it from the system using the dele command, as follows:

Here you can see that the system responds that the mail is marked as deleted:

Run the list command again. You can see that there is no longer any email:

As with the SMTP services, we can type quit to exit:

Wrapping up

You can tell a lot by using Telnet to perform basic email server testing. Whether you’re using POP or SMTP, experiment with this process when things are going well so you can see when things go wrong.

What to read next

Two popular tools for testing network services on a basic level are telnet and Netcat. Here’s how to use them.

Источник

Basic troubleshooting with telnet and netcat

Two popular tools for testing network services on a basic level are telnet and Netcat. Here’s how to use them.

In the early years of computing, telnet was used to connect to the command line on remote systems. SSH has replaced telnet for remote access needs, and these days when you hear about telnet , it is usually when somebody is using the client as a generic network troubleshooting tool.

That’s because, in troubleshooting sessions, sysadmins turn to telnet and netcat to test connectivity to service offerings.

Telnet

See what this process looks like for telnet :

[dminnich@dminnichlt tmp]$ telnet -4 www.redhat.com 80 Trying 104.117.5.18. Connected to www.redhat.com. Escape character is '^]'. 

Here is a breakdown of the parameters:

• -4 means to use IPV4. This flag is not necessary but it made my logs prettier.
• www.redhat.com is the hostname/IP address to connect to.
• 80 is the port to connect to.

Great Linux resources

That Connected to www.redhat.com line states that the connection was successful. This result means that the server is operational and there is nothing on the network (or any client or server machine) blocking this connection from happening.

You can exit out of telnet by pressing Ctrl+] and then typing quit :

^] telnet> quit Connection closed. 

Here is an example of a failed connection in telnet :

[dminnich@dminnichlt tmp]$ telnet -4 www.redhat.com 21 Trying 23.1.49.220. telnet: connect to address 23.1.49.220: Connection timed out 

Depending on how the remote network is configured, you may see a Connection refused message immediately, or you may have to wait a while to get the Connection timed out error. Alternatively, if telnet sits there for a few seconds without any output, it is usually safe to assume that the connection will time out, so you can stop the connection attempt by doing a Ctrl+C.

When you see errors like this, it means that any of the following things are wrong:

• The server daemon isn’t running.
• The server itself isn’t up.
• A firewall rule is blocking the connection.
• There is no network route to the destination.

Netcat

Now, look at this same process with Netcat ( ncat on Red Hat Enterprise Linux 8 and related distributions, abbreviated nc ). Here is an example of a successful connection using Netcat (Ctrl+C will exit the Netcat session:)

[dminnich@dminnichlt tmp]$ nc -w3 -4 -v www.redhat.com 80 Ncat: Version 7.70 ( https://nmap.org/ncat ) Ncat: Connected to 104.117.5.18:80. 

That -w3 says wait three seconds and then give up, which is a nice Netcat-native feature telnet is missing.

Here is what a failed connection looks like in Netcat:

[dminnich@dminnichlt tmp]$ nc -w3 -4 -v www.redhat.com 21 Ncat: Version 7.70 ( https://nmap.org/ncat ) Ncat: Connection timed out. 

Netcat also supports listening on ports for incoming connections, as well as basic port scanning and some other niceties. These features and the fact that lots of operating systems install Netcat but not telnet by default are why some sysadmins are starting to use Netcat instead of telnet for their troubleshooting needs.

Interacting with services

One final thing: Both of these tools can interact with the service offerings they connect to. If you type syntactically correct protocol messages and hit Enter, you will receive responses from the service. Here is an example:

[dminnich@dminnichlt tmp]$ nc -4 -w3 -v google.com 80 Ncat: Version 7.70 ( https://nmap.org/ncat ) Ncat: Connected to 172.217.5.238:80. GET / .  

You may have to hit Enter a few times in the above example. Interacting with service offerings in this fashion gets complicated fast, especially when encryption comes into play, but if you need to test the internals of something—or if you don’t have a better protocol-specific tool like curl around—you can make it work.

Источник