Situation

Server responded"Algorithm negotiation failed"Key exchange with the remote host failed. This can happen forexample computer does not support the selected  algorthms.

When ssh to a ubuntu system,putty can connect but ‘SSH Secure Shell Client’ is failed,according to the pop-up error message we can know that

Reason

Because i had past updated server, it’s that means i had updated the sshd algorithm negotiation on server meanwhile, but the sshd algorithm negotiation in client is the old, so server cannot compatible the algorithm negotiation from client.

The Way to resolve

1. Use a different system or the console to drop to a shell. Such as Putty
2. Chmod 777 & Edit the “/etc/ssh/sshd_config” file
3. Add all of the algorithm negotiation to sshd_config
4. Restart sshd sevice
   Add following code to sshd_config

Ciphers aes128-cbc,aes192-cbc,aes256-cbc,aes128-ctr,aes192-ctr,aes256-ctr,3des-cbc,arcfour128,arcfour256,arcfour,blowfish-cbc,cast128-cbcMACs hmac-md5,hmac-sha1,umac-64@openssh.com,hmac-ripemd160,hmac-sha1-96,hmac-md5-96KexAlgorithms diffie-hellman-group1-sha1,diffie-hellman-group14-sha1,diffie-hellman-group-exchange-sha1,diffie-hellman-group-exchange-sha256,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group1-sha1,curve25519-sha256@libssh.org