I've got a linux server running openssh. I can connect to it both from the local LAN and remotely. However, there is one client (a Windows 10 laptop) that can only connect to it locally. When I try to connect remotely, authentication is accepted, but the ssh client on the laptop hangs and must be killed with Process Explorer. I thought the problem might be:
- Windows Firewall - Nope. Turned it off, got same behavior.
- ssh client (cygwin) - Nope. Got the same behavior with putty.
- Windows 10 - Nope. I can successfully connect remotely from another Win10 machine.
I've tried a fresh install of both cygwin & putty.
I tried running ssh with several -v options, and comparing the output to the other Win10 machine that is able to connect. The output was identical, up to a point:
Authenticated to <<IP REMOVED>>. debug2: fd 5 setting O_NONBLOCK debug2: fd 6 setting O_NONBLOCK debug1: channel 0: new [client-session] debug3: ssh_session2_open: channel_new: 0 debug2: channel 0: send open debug3: send packet: type 90 debug1: Requesting firstname.lastname@example.org debug3: send packet: type 80 debug1: Entering interactive session. debug1: pledge: network debug3: receive packet: type 91 debug2: channel_input_open_confirmation: channel 0: callback start debug2: fd 3 setting TCP_NODELAY debug3: ssh_packet_set_tos: set IP_TOS 0x10 debug2: client_session2_setup: id 0 debug2: channel 0: request pty-req confirm 1 debug3: send packet: type 98 debug2: channel 0: request shell confirm 1 debug3: send packet: type 98 debug2: channel_input_open_confirmation: channel 0: callback done debug2: channel 0: open confirm rwindow 0 rmax 32768 >>> "bad" machine hangs here debug3: receive packet: type 99 debug2: channel_input_status_confirm: type 99 id 0 debug2: PTY allocation request accepted on channel 0 debug2: channel 0: rcvd adjust 2097152 debug3: receive packet: type 99 debug2: channel_input_status_confirm: type 99 id 0 debug2: shell request accepted on channel 0 Welcome to Linux Mint 17.3 Rosa (GNU/Linux 3.19.0-32-generic x86_64) Welcome to Linux Mint
On rare occasions, it has gotten further - once or twice even to the Welcome message - but the connection is never responsive to typing input.
I tried running sshd -d manually on the server, and comparing output between a "bad" remote session and a "good" one from another client. Output is identical.
To summarize: it doesn't seem to be Windows Firewall, or the client software, or Win10, or the port forwarding to the server, or DNS, or the server itself. The problem is only this one client machine, and only when connecting from outside the local LAN. It is authenticating successfully. And the client machine is running the same OS/ssh client as another machine that doesn't have the problem, and I can't see anything in the logs that distinguishes it, either.
EDIT: I should also mention, ssh connection to other remote servers works fine from all machines. It seems to be just this server/client pair, and only when connecting remotely.
UPDATE: See my comments immediately below for more info - the problem seems to be specific to the local net.
What further steps can I take to debug it?