If you want to configure the SSH daemon on a remote machine, you probably don't want to risk the chance of locking yourself out. Nowadays, properly configured machines can restart the SSH daemon while retaining the running connections. That's great, but if you don't want to rely on that, read on.
We want start a separate, temporary SSH daemon. Dropbear is great for that. We will do enough to run a temporary copy for the duration of configuring the regular SSH daemon installation. We won't install Dropbear permanently.
Download the latest release on the remote machine. In a user account, unpack, compile and make it:
remoteserver$ tar xfz dropbear-0.50.tar.gz remoteserver$ cd dropbear-0.50 remoteserver$ ./configure remoteserver$ make
Now generate a key for the server:
remoteserver$ ./dropbearkey -t rsa -f key.rsa
The server can be started and we'll use some high port so as not to get in the way of other services. Port 31337 is used below:
remoteserver$ sudo ./dropbear -p 31337 -r ./key.rsa
From your local machine, you should now be able to reach the server:
localmachine$ ssh -p 31337 remoteserver
Log in and configure the regularly installed SSH daemon. Restart it, do whatever you like. When you're done, exit and log in again as you'd normally do (i.e. not using the dropbear server but the regularly installed SSH server). If all is successful, kill the dropbear server and wipe out the temporarily compiled copy:
remoteserver$ sudo killall dropbear remoteserver$ rm -rf dropbear-0.50
Note: it's not necessary to start dropbear with sudo. However, dropbear then can't read the root-only files for successful authentication. The only authentication possible is key-based, with a key in ~/.ssh.