How to check the SSH version in Linux system

Secure Shell (SSH) allows remote login or remote execution of commands through an encrypted secure communication channel. SSH is designed to replace insecure plaintext protocols such as telnet, rsh, and rlogin. SSH provides a number of required features such as authentication, encryption, data integrity, authorization and forwarding/tunneling.

There are some minor version differences in the SSH protocol specification, but there are two main major versions: SSH1 (version 1.XX) and SSH2 (version 2.00).

In fact, SSH1 and SSH2 are two completely different and incompatible protocols. SSH2 significantly improves many aspects of SSH1. First of all, SSH is a macro design. Several different functions (such as authentication, transmission, connection) are packaged into a single protocol. SSH2 brings more powerful security features than SSH1, such as MAC-based integrity check, flexible session key updates, fully negotiated encryption algorithms, public key certificates, and more.

SSH2 is standardized by the IETF, and its implementation is widely deployed and accepted in the industry. Due to the popularity and encryption advantages of SSH2 over SSH1, many products have dropped support for SSH1. At the time of writing this article, OpenSSH still supports SSH1 and SSH2, however in all modern Linux distributions, OpenSSH servers disable SSH1 by default.

If you want to check the SSH protocol version supported by the local OpenSSH server, you can refer to the /etc/ssh/sshd_config file. Open /etc/ssh/sshd_config with a text editor and look at the "Protocol" field.

If it is displayed as follows, it means that the server only supports SSH2.

Protocol 2

If it is displayed as follows, it means that the server supports both SSH1 and SSH2.

Protocol 1

If you cannot access /etc/ssh/sshd_config because the OpenSSH service is running on a remote server. You can use an SSH client called ssh to check the supported protocols. Specifically, it forces ssh to use a specific SSH protocol, and then we check the response of the SSH server.

The following command forces ssh to use SSH1:

 ssh -1 user@remote_server

The following command forces ssh to use SSH2:

 ssh -2 user@remote_server

If the remote SSH server only supports SSH2, then the first option with "-1" will appear like the following error message:

Protocol major versions differ: 1 vs. 2

If the SSH server supports both SSH1 and SSH2, then both commands are valid.

Another way to check the version is to run an SSH scanning tool called scanssh. This command line tool is useful when you want to check a group of IP addresses or an entire local network to upgrade an SSH1-compatible SSH server.

The following is the basic SSH version scanning syntax.

sudo scanssh -s ssh -n [ports] [IP addresses or CIDR prefix] 

The "-n" option can specify the SSH port to scan. You can use Duhao separation to scan multiple ports. Without this option, scanssh will scan port 22 by default.

Use the following command to discover the SSH server in the 192.168.1.0/24 local network and check the SSH protocol v version:

 sudo scan -s ssh 192.168.1.0/24 

If scanssh reports "SSH-1.XX-XXXX" for a specific IP address, this implies that the minimum version supported by the relevant SSH server is SSH1. If the remote server only supports SSH2, scanssh will display "SSH-2.0-XXXX" ".

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