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Access denied: Always check for protocol compatibility

Feb 23, 20222 mins
IT SkillsNetworking

It’s easy to overlook, but when devices can’t be accessed, make sure they’re both working with the same protocol versions.

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While working on a base-wide network cutover at a military installation recently, I was verifying configurations on core, distribution, and access-node devices. Using a local host computer on the network, I was connected to the distribution node via an Ethernet port in a separate room and successfully pinged the node to verify network connectivity.

Then I tried to access the node using PuTTY via SSH (port 22), the recommended and secure method, and received this error message: “Network Error: Connection Refused. The network connection PuTTY tried to make to your device/server was rejected by the server.” This error usually happens because the server does not provide the service which PuTTY is trying to access.

I was working in the same building that the distribution node was in, so I went to it and consoled in using a serial cable. I wanted to verify that the device’s SSH ACL statement allowed my host to access the node via SSH. It did.

I checked to make sure the SSH version my computer was using for the PuTTY session matched the version installed on the node. It didn’t. My computer was using SSHv1; the node used SSHv2, so I had to install a new version of SSH.

Since I had to upgrade the software anyway, I took the opportunity to switch from PuTTY to SecureCRT, which has a friendlier user interface. And I made sure it supported SSHv2.

Lesson learned: Always check for protocol compatibility when making network connections.


LaVar Johnson has 20 years’ IT experience, starting in radio communication in the US Navy. Now he fulfills government IT contracts as a Network Engineer with expertise in Layer 2 and 3 infrastructure.