OpenSSH is almost completely compatible with the commercial SSH 1.2.x. There are, however, a few exceptions that you will need to bear in mind while upgrading: 1. OpenSSH does not support any patented transport algorithms. Only 3DES and Blowfish can be selected. This difference may manifest itself in the ssh command refusing to read its config files. Solution: Edit /etc/ssh/ssh_config and select a different "Cipher" option ("3des" or "blowfish"). 2. Old versions of commercial SSH encrypt host keys with IDEA The old versions of SSH used a patented algorithm to encrypt their /etc/ssh/ssh_host_key This problem will manifest as sshd not being able to read its host key. Solution: You will need to run the *commercial* version of ssh-keygen on the host's private key: ssh-keygen -u /etc/ssh/ssh_host_key 3. Incompatible changes to sshd_config format. OpenSSH extends the sshd_config file format in a number of ways. There is currently one change which is incompatible with the old. Commercial SSH controlled logging using the "QuietMode" and "FascistLogging" directives. OpenSSH introduces a more general set of logging options "SyslogFacility" and "LogLevel". See the sshd manual page for details. 4. Warning messages about key lengths Commercial SSH's ssh-keygen program contained a bug which caused it to occasionally generate RSA keys which had their Most Significant Bit (MSB) unset. Such keys were advertised as being full-length, but are actually only half as secure. OpenSSH will print warning messages when it encounters such keys. To rid yourself of these message, edit you known_hosts files and replace the incorrect key length (usually "1024") with the correct key length (usually "1023"). 5. Spurious PAM authentication messages in logfiles OpenSSH will generate spurious authentication failures at every login, similar to "authentication failure; (uid=0) -> root for sshd service". These are generated because OpenSSH first tries to determine whether a user needs authentication to login (e.g. empty password). Unfortunatly PAM likes to log all authentication events, this one included. If it annoys you too much, set "PermitEmptyPasswords no" in sshd_config. This will quiet the error message at the expense of disabling logins to accounts with no password set. This is the default if you use the supplied sshd_config file. 6. Empty passwords not allowed with PAM authentication To enable empty passwords with a version of OpenSSH built with PAM you must add the flag "nullok" to the end of the password checking module in the /etc/pam.d/sshd file. For example: auth required/lib/security/pam_unix.so shadow nodelay nullok This must be done in addtion to setting "PermitEmptyPasswords yes" in the sshd_config file. There is one caveat when using empty passwords with PAM authentication: PAM will allow _any_ password when authenticating an account with an empty password. This breaks the check that sshd uses to determined whether an account has no password set and grant users access to the account regardless of the policy specified by "PermitEmptyPasswords". For this reason, it is recommended that you do not add the "nullok" directive to your PAM configuration file unless you specifically wish to allow empty passwords. 7. Rhosts authentication does not work Make sure that ssh is installed with the setuid bit set. Note that the Makefile does not do this by default. 8. X11 and/or agent forwarding does not work Check your ssh_config and sshd_config. The default configuration files disable authentication agent and X11 forwarding. 9. ssh takes a long time to connect with Linux/glibc 2.1 The glibc shipped with Redhat 6.1 appears to take a long time to resolve "IPv6 or IPv4" addresses from domain names. This can be kludged around with the --with-ipv4-default configure option. This instructs OpenSSH to use IPv4-only address resolution. (IPv6 lookups may still be made by specifying the -6 option).