Testing port connectivity with Telnet

Products

VMware Desktop Hypervisor VMware vCenter Server VMware vSphere ESXi

Issue/Introduction

For troubleshooting purposes, it may be necessary to test connectivity to the different ports on your servers. This article provides you with the steps to use the Telnet utility to test connectivity to different ports on your servers from Windows, Linux, or Mac OS X.

Resolution

Testing port connectivity with Telnet from Windows

Note: The Telnet client is not installed by default on Windows 7, Windows Server 2008 R2, Windows Vista, Windows Server 2008, Windows 8, and Windows Server 2012. For more information, see the Microsoft TechNet article, Install Telnet Client.

For windows 10 and 11 telnet may need to be enabled by doing the following:

Open “Control Panel“.
Open “Programs“.
Select the “Turn Windows features on or off ” option.
Check the “Telnet Client” box.
Click “OK“.
The Telnet client should now be enabled.

Once installed and enabled to initiate a Telnet test to a port from Windows:

Open a command prompt. For more information, see Opening a command or shell prompt.
In the command prompt window, type:

telnet server port

Where server is the hostname or IP address of the server, and port is the port that you want to connect to.
Press Enter.

Note: To exit the Telnet application, press Ctrl + ], then type quit.

Depending on the application that uses the port, you may only see a blank screen with a cursor in the corner; this is normal. Some common outputs of a successful connection attempt are:

Connecting to port 902 on an ESXi/ESX host:

C:\>telnet server 902 Connecting... 220 VMware Authentication Daemon Version 1.10: SSL Required, ServerDaemonProtocol:SOAP, MKSDisplayProtocol:VNC
Connecting to port 25 on a mail server:

C:\>telnet server 25 Connecting... 220 server ESMTP Sendmail 8.13.3/8.13.3;
Connecting to port 443 on the vCenter Server:

C:\>telnet server 443 Connecting... 220 VMware Authentication Daemon Version 1.10: SSL Required, ServerDaemonProtocol:SOAP, MKSDisplayProtocol:VNC

If Telnet is unable to connect to the port, the output is similar to:

C:\>telnet server 902
Connecting To server...

Could not open connection to the host, on port 902: Connect failed

If the connection is refused, a firewall may be blocking that port from your source to the destination server. For more information, see Required ports for configuring an external firewall to allow ESX and vCenter Server traffic.

Testing port connectivity with Telnet from Linux or Mac OS X

To initiate a Telnet test to a port from Linux or Mac OS X:

Open a shell prompt. For more information, see Opening a command or shell prompt.

Note: Several distributions of Linux do not have a Telnet client installed by default. Refer to the website of your distribution for details on whether one is available and how to install the package.
In the shell prompt window, type:

telnet server port

Where server is the hostname or IP address of the server, and port is the port that you want to connect to.
Press Enter.

Note: To exit the Telnet application, press Ctrl + ], then type quit.

Depending on the application that uses the port, you may only see a blank screen with a cursor in the corner; this is normal. Some common outputs of a successful connection attempt are:

Connecting to port 902 on an ESXi/ESX host:

[root@server]$ telnet server 902 Trying server... Connected to server. Escape character is '^]'. 220 VMware Authentication Daemon Version 1.10: SSL Required, ServerDaemonProtocol:SOAP, MKSDisplayProtocol:VNC
Connecting to port 25 on a mail server:

[root@server]$ telnet server 25 Trying server... Connected to server. Escape character is '^]'. 220 server ESMTP Sendmail 8.13.3/8.13.3;

If Telnet is unable to connect to the port, the output is similar to:

[root@server]$ telnet server 902
Trying server...
telnet: connect to address server: Connection refused

If the connection is refused, a firewall may be blocking that port from your source to the destination server. For more information, see Required ports for configuring an external firewall to allow ESX and vCenter Server traffic.

Additional Information

For more information on required ports for VMware products, see TCP and UDP Ports required to access vCenter Server, ESX hosts, and other network components.