Re: sshd sessions hang after cygwin1.dll 3.0.7

[Takashi Yano <takashi.yano@nifty.ne.jp>]

On Thu, 30 Jan 2020 09:27:34 -0700
Bill Stewart wrote:
> I have created an OpenSSH installer for Windows users:
> 
> https://github.com/Bill-Stewart/Cygwin-OpenSSH
> 
> Basically it includes only the minimum files from Cygwin needed to run
> OpenSSH and has some additional conveniences (the foremost of which is
> to automatically install the service).
> 
> The problem:
> 
> When I use cygwin1.dll versions newer than 3.0.7, sshd.exe hangs
> whenever establishing a connection. Following is the output from 'sshd
> -d':
> 
> debug1: sshd version OpenSSH_8.1, OpenSSL 1.1.1d  10 Sep 2019
> debug1: private host key #0: ssh-rsa SHA256:...
> debug1: private host key #1: ecdsa-sha2-nistp256 SHA256:...
> debug1: private host key #2: ssh-ed25519 SHA256:...
> debug1: rexec_argv[0]='/usr/sbin/sshd'
> debug1: rexec_argv[1]='-d'
> debug1: Bind to port 22 on ::.
> Server listening on :: port 22.
> debug1: Bind to port 22 on 0.0.0.0.
> Server listening on 0.0.0.0 port 22.
> debug1: fd 5 clearing O_NONBLOCK
> debug1: Server will not fork when running in debugging mode.
> debug1: rexec start in 5 out 5 newsock 5 pipe -1 sock 8
> debug1: inetd sockets after dupping: 4, 4
> Connection from <remoteip> port 52466 on <localip> port 22
> debug1: Local version string SSH-2.0-OpenSSH_8.1
> debug1: Remote protocol version 2.0, remote software version OpenSSH_8.0
> debug1: match: OpenSSH_8.0 pat OpenSSH* compat 0x04000000
> debug1: permanently_set_uid: 197767/197121 [preauth]
> debug1: list_hostkey_types:
> rsa-sha2-512,rsa-sha2-256,ssh-rsa,ecdsa-sha2-nistp256,ssh-ed25519
> [preauth]
> debug1: SSH2_MSG_KEXINIT sent [preauth]
> debug1: SSH2_MSG_KEXINIT received [preauth]
> debug1: kex: algorithm: curve25519-sha256 [preauth]
> debug1: kex: host key algorithm: ecdsa-sha2-nistp256 [preauth]
> debug1: kex: client->server cipher: chacha20-poly1305@openssh.com MAC:
> <implicit> compression: none [preauth]
> debug1: kex: server->client cipher: chacha20-poly1305@openssh.com MAC:
> <implicit> compression: none [preauth]
> debug1: expecting SSH2_MSG_KEX_ECDH_INIT [preauth]
> debug1: rekey out after 134217728 blocks [preauth]
> debug1: SSH2_MSG_NEWKEYS sent [preauth]
> debug1: Sending SSH2_MSG_EXT_INFO [preauth]
> debug1: expecting SSH2_MSG_NEWKEYS [preauth]
> debug1: SSH2_MSG_NEWKEYS received [preauth]
> debug1: rekey in after 134217728 blocks [preauth]
> debug1: KEX done [preauth]
> debug1: userauth-request for user <username> service ssh-connection
> method none [preauth]
> debug1: attempt 0 failures 0 [preauth]
> debug1: user <username> matched 'User <username>' at line 142
> debug1: authentication methods list 0: password
> debug1: userauth_send_banner: sent [preauth]
> debug1: authentication methods list 0: password [preauth]
> debug1: userauth-request for user <username> service ssh-connection
> method password [preauth]
> debug1: attempt 1 failures 0 [preauth]
> Accepted password for <username> from <remoteip> port 52466 ssh2
> debug1: monitor_child_preauth: <username> has been authenticated by
> privileged process
> debug1: monitor_read_log: child log fd closed
> debug1: rekey in after 134217728 blocks
> debug1: rekey out after 134217728 blocks
> debug1: ssh_packet_set_postauth: called
> debug1: active: key options: agent-forwarding port-forwarding pty
> user-rc x11-forwarding
> debug1: Entering interactive session for SSH2.
> debug1: server_init_dispatch
> debug1: server_input_channel_open: ctype session rchan 0 win 1048576 max 16384
> debug1: input_session_request
> debug1: channel 0: new [server-session]
> debug1: session_new: session 0
> debug1: session_open: channel 0
> debug1: session_open: session 0: link with channel 0
> debug1: server_input_channel_open: confirm session
> debug1: server_input_global_request: rtype
> no-more-sessions@openssh.com want_reply 0
> debug1: server_input_channel_req: channel 0 request pty-req reply 1
> debug1: session_by_channel: session 0 channel 0
> debug1: session_input_channel_req: session 0 req pty-req
> debug1: Allocating pty.
> 
> The only resolution is to forcibly terminate the spawned copy of sshd
> (the one spawned by the 'sshd -d' process).
> 
> Server is running Windows 10 v1909.
> 
> I have tested, and the hang occurs in all versions of cygwin1.dll after 3.0.7.
> 
> When I revert back to cygwin1.dll 3.0.7, the problem does not occur,
> and the connection succeeds.
> 
> Any ideas?

You need cygwin-console-helper.exe for newer cygwin pty which
supports pseudo console.

I believe you do not need winpty anymore because newer cygwin
utilizes pseudo console in pty.

-- 
Takashi Yano <takashi.yano@nifty.ne.jp>

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