005: RFC 4253

RFC 4253 is the primary document used to describe the core connection and key exchange mechanisms of SSH. I will be grabbing quotes from this document to help clarify parts that I found confusing.

The first sections we want to look at is 4.2. Protocol Version Exchange. This is states that once a client connects, both sides must send an identification string in the format of:

SSH-protoversion-softwareversion SP comments CR LF

In this case, protoversion should be 2.0, as that is the version being documented and used and the softwareversion can be any UTF-8 encoded string to identify the server, but it should not include a dash or space. Finally the SP means a space character and then comments can be an valid string. Finally the message must end in a carriage return and line feed, or CR/LF.

This means valid strings are:

SSH-2.0-billsSSH_3.6.3q3[CR][LF]
SSH-2.0-MySSHServer_1.0 Some Comment Allowed here[CR][LF]

So, our first task in our server is send our Protocol Version Exchange message to the client, and also read the Protocol Version Exchange from the client's socket.

Now, I have read the full RFC, and I know that most of the SSH data is sent as raw byte data and will require special processing, but until we should read from the socket for the first and save this value off. Once we have received that, we'll then need to start reading the socket as a binary data stream.