[Rd] Reaching R-forge - tunnel ssh through ssh?

[Douglas Bates]

This question is only related to R-devel in that I need to work out
how to reach R-forge.R-project.org to update and commit files of R
packages for which I am a developer.

My desktop computer at work runs Ubuntu 8.04.  Because this is not a
supported operating system this computer is behind a firewall.  I can
use a proxy server for http, https and rsync connections.  However,
the proxy server does not allow outgoing (or incoming) ssh
connections.  I can use ssh to log in to a local computer that is not
behind the firewall and then use svn+ssh to contact R-forge.  I have
managed to get myself into a bind with that scheme, however.  I added
and deleted files on the checked out copy on the shared file system
using my desktop and then could not check in the changes from the
"visible" machine because of svn version skew.  The "visible" machine
has an out-of-date version of svn.

Does anyone know how to tunnel ssh over ssh and, further, how I would
specify that in the svn checkin?  If my desktop computer is A, the
"visible" computer is B and the R-forge server is C then I want to use
ssh to set up a tunnel between A and B so that A can reach C for an
svn+ssh checkin.

Off-list replies would be welcome.