A copy of slave.jar can be found in WEB-INF/slave.jar inside hudson.war.
In a simple case, this could be something like "ssh hostname java -jar ~/bin/slave.jar". However, it is often a good idea to write a small shell script, like the following, on a slave so that you can control the location of Java and/or slave.jar, as well as set up any environment variables specific to this slave node, such as PATH.
#!/bin/sh exec java -jar ~/bin/slave.jar
You can use any command to run a process on the slave machine, such as RSH, as long as stdin/stdout of this process will be connected to "java -jar ~/bin/slave.jar" eventually.
In a larger deployment, It is also worth considering to load slave.jar from a NFS-mounted common location, so that you don't have to update this file every time you update Hudson.
Setting this to "ssh -v hostname" may be useful for debugging connectivity issue.