This is a short post on how to disable MD5-based HMAC algorithm’s for ssh on Linux.
1. Make sure you have updated openssh package to latest available version.
2. To change the ciphers/md5 in use requires modifying sshd_config file, you can append Ciphers & MACs with options as per the man page. For example:
# vi /etc/ssh/sshd_config Ciphers aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,arcfour MACs hmac-sha1,umac-64@openssh.com,hmac-ripemd160,hmac-sha2-256,hmac-sha2-512,hmac-ripemd160@openssh.com
From the man page of sshd_config:
# man sshd_config
Ciphers
Specifies the ciphers allowed for protocol version 2. Multiple ciphers must be comma-separated. The supported ciphers are
“3des-cbc”, “aes128-cbc”, “aes192-cbc”, “aes256-cbc”, “aes128-ctr”, “aes192-ctr”, “aes256-ctr”, “arcfour128”, “arcfour256”,
“arcfour”, “blowfish-cbc”, “rijndael-cbc@lysator.liu.se”, and “cast128-cbc”. The default is:
aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
aes256-cbc,arcfour,rijndael-cbc@lysator.liu.se
MACs Specifies the available MAC (message authentication code) algorithms. The MAC algorithm is used in protocol version 2 for data
integrity protection. Multiple algorithms must be comma-separated. The default is:
hmac-md5,hmac-sha1,umac-64@openssh.com,
hmac-ripemd160,hmac-sha1-96,hmac-md5-96,
hmac-sha2-256,hmac-sha2-512,hmac-ripemd160@openssh.com
3. Restart the sshd service.
# service sshd restart ### For CentOS/RHEL 6 # systemctl restart sshd ### For CentOS/RHEL 7