Using putty to debug ssh through a firewall

I currently use ssh to access things outside of a firewall. The
firewall allows outbound traffic on port 23, so I run an ssh server on
that port (sshd for cygwin). I then connect with an ssh client on
that port, and can set up any port forwarding that I desire. It
worked beautifully until just the other day. Now, when I connect, I
the message exchange hangs. Putty doesn't give enough detail on what
the message exchange actually is, so here is some output from the
cygwin ssh implementation using maximum verbosity:


$ ssh -vvv -p 23 aaa.bbb.ccc.ddd
OpenSSH_4.6p1, OpenSSL 0.9.8e 23 Feb 2007
debug2: ssh_connect: needpriv 0
debug1: Connecting to aaa.bbb.ccc.ddd [aaa.bbb.ccc.ddd] port 23.
debug1: Connection established.
debug1: identity file /cygdrive/c/Documents and Settings/user/.ssh/
identity type -1
debug1: identity file /cygdrive/c/Documents and Settings/user/.ssh/
id_rsa type -1
debug1: identity file /cygdrive/c/Documents and Settings/user/.ssh/
id_dsa type -1
debug1: Remote protocol version 1.99, remote software version
OpenSSH_4.6
debug1: match: OpenSSH_4.6 pat OpenSSH*
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_4.6
debug2: fd 3 setting O_NONBLOCK
debug1: SSH2_MSG_KEXINIT sent



And it just hangs there until the connection times out. Any ideas on
what might be causing a hang at that stage?

.

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