I would like to replace the default 2048 bit host key generated when installing OpenSSH or starting it the first time with one of 4096 bit length.This leads to two questions:Does OpenSSH allow running with a 4096 bit RSA host key (for SSH2 obviously)?How can ssh-keygen be convinced not to prompt for a passphrase?The version of OpenSSH I am running is 6.6p1.
Generate own (stronger) RSA host key for OpenSSH?
ssh;openssh
Whatever key-length is supported in ssh-keygen most likely would work with sshd as well.  Besides that, you should generate your host-keys with ssh-keygen -h anyways, so if ssh-keygen isn't totally dumb, it should inform you if the desired key-length is not supported for host keys.The passphrase could be specified with -N; if you don't want the key to be encrypted, just specify an empty one (thus just hitting Enter if it prompts you would have the same effect).