Cygwin sshd broken by seemingly trivial network change

[Stephen Carrier]

On Wed, Dec 16, 2020 at 09:37:08AM -0600, worsafe@bellsouth.net wrote:
> While installing a new router, I changed my local network from 192.168.4.*
> to 192.168.50.*. This seems to have broken Cygwin sshd on both of my remote
> computers, but only for Cygwin; sshd works fine if I boot the remote
> computer from a linux thumb drive. I have noticed no other problems with the
> new network configuration.
> --------
> ssh -vvv does not give any messages that look useful to me. Authentication
> problems usually give some useful message, but this seems to fail before
> getting that far:
> 
> ssh -vvv $ASUS12
> OpenSSH_8.3p1, OpenSSL 1.1.1f 31 Mar 2020
> debug1: Reading configuration data /home/cdr/.ssh/config
> debug1: /home/cdr/.ssh/config line 1: Applying options for *
> debug1: Reading configuration data /etc/ssh_config
> debug2: resolve_canonicalize: hostname 192.168.50.105 is address
> debug2: ssh_connect_direct
> debug1: Connecting to 192.168.50.105 [192.168.50.105] port 22.
> debug1: connect to address 192.168.50.105 port 22: Connection timed out
> ssh: connect to host 192.168.50.105 port 22: Connection timed out
> -----------
> The server is running, as confirmed by cyrunsrv -Q sshd.
> /var/log/sshd.log is an empty file.
> -------
> nmap shows port 22 open on the remote server:
> nmap -p22 $ASUS12
> Nmap scan report for asus12 (192.168.50.105)
> Host is up (0.13s latency).
> PORT STATE SERVICE
> 22/tcp filtered ssh
> ----------
> However, telnet fails before returning the expected header string:
> telnet $ASUS12 22
> Connecting To 192.168.50.105...Could not open connection to the host, on
> port 22
> : Connect failed
> (For comparison, linux returns the string "SSH-2.0-OpenSSH_7.9p1
> Debian-10+deb10u2")
> -------

I think 'filtered' means there is something in the way.  Is all this
on the same LAN but with a new router?  Are the connections wireless?
I would try the telnet test from the server itself, both to its external
address and to localhost.  All in search of clues.  This seems like a 
networking issue or a specific configuration on the server that limits
client access or listening ports.

Stephen

> I can't think what to try short of reinstalling sshd.
> 
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