Bug 2745

Created attachment 3015 [details]
patch to support versionaddendum in client

A few years back, there was discussion on the mailing list about adding SNI support to SSH by fiddling with the version banner exchange at the very beginning of the protocol exchange, before the encrypted channel is created. Daniel Gillmor at the time suggested that if this was to be done, using the VersionAddendum mechanism might be a good way to do it:

https://lists.mindrot.org/pipermail/openssh-unix-dev/2013-November/031811.html

This patch does that. 

For example, a user might want to do
ssh -oVersionAddendum=X-Host:realhost.site.com sharedIpAddr.site.com

With the patch, someone who wanted to put in place a quick proxy running on port 22 of sharedIpAddr.site.com that looked for a string like
X-Host:realhost.site.com 
could then proxy the connection to realhost.site.com. 

The patch supports '%h' per Daniel's suggestion, so you can do:
ssh -oVersionAddendum=Host:%h server.example.net

or just put VersionAddendum in your configuration file. 

The caveats that Daniel warned are still true - this may not be something an admin wants to turn on by default so you're not leaking the actual host you're connecting to over the unencrypted banner exchange (though if you have a shared secret with the proxy you could encrypt the host string). Also, because the banner strings are used as part of the key exchange if the proxy sends something different than the actual end host sends key exchange will fail. 

I largely copied the code from how the server side handles VersionAddendum. The big change is that I passed around the 'host' variable in ssh_connect.c so the %h expansion would work when the banner string is actually constructed - if there's a global I could read from I wouldn't need to change the calls up the stack. Other expansions don't work, like %u or %p, because the data for those are not passed along right now.

I didn't add anything to ssh_api.c - it doesn't look like that file uses the config settings so I don't think VersionAddendum would get picked up there, but maybe I'm not reading it right. 

Subscriptions to the mailing list are not working right now so I haven't posed this to the list, but hopefully after the server migration stuff is worked out I'll be able to subscribe and send this there.

If it's easier for folks to read or use, this patch is in the client_version_addendum branch here:
https://github.com/epaulson/openssh-portable/commit/69daef3b8a99d6c85f357f200c4aaa06fe28eaff

Thanks!