How to read and debug SSH verbose-mode?

Question

Let's say I use this command:

ssh -vvv user@server

I get an output similar to this:

send packet: type 21
ssh_set_newkeys: mode 1
receive packet: type 6
SSH2_MSG_SERVICE_ACCEPT received
receive packet: type 51
Permission denied (publickey,password)

• What are all these packet-types?
• Where can I read and learn about them? I always google something like ssh packet type51, but there must be a place where all the types are listed
• If you know how to understand this ssh verbose mode, where did you learn this?

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Thanks for your help :)

Answer 1

SSH is defined in IETF RFCs like

• RFC4253 - The Secure Shell (SSH) Transport Layer Protocol (note that there's errata and several updates)
• RFC4250 - The Secure Shell (SSH) Protocol Assigned Numbers

In general the places to find out how things work are IEEE standards and IETF RFCs. They're not the easiest read, the language takes a bit of getting used to.

To find resources of learning how to use/understand SSH you can search the internet. There's a multitude of websites for this. You can also buy a book from publishers like O'Reilly - those tend to be thick, but are also very comprehensive.

That gives you the theoretical background. In the end you gain practical knowledge only in practical situations. I.e. you learn how to interpret SSH logs by analyzing SSH logs :-)

See also IANA "SSH Parameters" -page with direct links to the applicable RFCs.

Techtarget's 5-part SSH tutorial

NOTE: These links are applicable for SSH-2, accepted as standard 2006. It's a complete re-write, and incompatible with original SSH-1, which was published 1996 and not much in use nowadays.