This happens because darcs tries to use -p for port when putty only supports -
P. I have come up with a workaround for this issue, but it would be nice if 
this problem was fixed. Probably what needs to happen is that darcs needs to 
detect whether ssh is openssh or putty. This can be done with plink but not 
sure how to do this with putty. (putty doesn't support -V for version, plink 
does).

Please see this blog post for details:
http://blog.patch-tag.com/2009/02/25/how-to-get-around-firewalls-using-patch-tag

Salvatore, would this be easy to fix?

I think one easy implementation is to extend Darcs/Global.hs with ssh version
detection and then modify Ssh.hs so that getSSHOnly passes -p to ssh and -P to
putty/plink (Matthew: I thought plink *was* putty?).  

Note that we already pass different flags depending on whether we're calling ssh
(-p) and scp (-P).  So extending this logi in Ssh.getSSHOnly should not be too
difficult.

Note also that no knowledge of Darcs code is needed to work on this, just dive in.

I'd make either a type synonym or data type for SSH version info.  Maybe in the
future we'll be even fancier and keep track of state without using unsafePerformIO

Here is early prototype (not in darcs yet):
 
https://patch-tag.com/r/dixiecko/detectSSH/snapshot/current/content/pretty/src/detectSSH.hs

Pending issues:
- error handling (interrupted blocked commands)
- detection for sftp/scp from ssh/plink
- testing on real environment (windows/linux, with ssh/plink)

and find the best way how to include that into Darcs/Globals.hs and use
that in Ssh.hs.

Is this right way how to solve -P/-p problem (challenge/response query
over command line) ?

Discussion on
http://lists.osuosl.org/pipermail/darcs-users/2010-May/024081.html