Ssh – Public Key Authentication Windows Port of OpenSSH

known-hostspowershellsshwindows-server-2012-r2

I have been attempting to get Public Key Authentication working with the PowerShell port of OpenSSH onto a VM running Windows Server 2012 R2.

I have faithfuly followed the installation instructions and have assured that my file permissions are correct for .ssh\authorized_keys. (Can't post link to the specific instructions in the Win32-OpenSSH wiki, since I'm too little to post more than two links, see comment below).

I am able to log in to the windows host from a linux host as expected with username/password. No luck with Key Authentication, however.

Local (linux host) Configuation

My local .ssh/config file contains:

Host remotehostname
    HostName remotehostname
    User remoteuser
    Port 22
    IdentityFile /home/myusername/.ssh/id_dsa

The permissions in the local .ssh directory appear correct:

[me@localhost.ssh]$ ls -ltrh
total 56K
-rw------- 1 cengadmin cengadmin 1.6K Sep 11 10:01 known_hosts
-r-------- 1 cengadmin cengadmin  672 Sep 11 10:06 id_dsa
-r-------- 1 cengadmin cengadmin  580 Sep 11 10:13 config

Remote (windows host) Configuration

The .ssh directory on my remote host is as follows:

 Directory of C:\Users\REMOTEUSER\.ssh

09/11/2017  10:07 AM    <DIR>          .
09/11/2017  10:07 AM    <DIR>          ..
09/11/2017  10:07 AM               623 authorized_keys
09/11/2017  10:05 AM               672 id_dsa
09/11/2017  10:05 AM               623 id_dsa.pub
               5 File(s)          4,012 bytes
               2 Dir(s)  10,752,004,096 bytes free

C:\Users\REMOTEUSER\.ssh>icacls authorized_keys
authorized_keys NT SERVICE\sshd:(R)
                NT AUTHORITY\SYSTEM:(F)
                BUILTIN\Administrators:(F)
                FOODOM1\REMOTEUSER:(F)


C:\Users\REMOTEUSER\.ssh>icacls id_dsa
id_dsa BUILTIN\Administrators:(F)
       NT AUTHORITY\SYSTEM:(F)
       DHDOM1\REMOTEUSER:(R,W)

My authorized_keys file contains only the output of type id_dsa.pub > authorized_keys.

C:\Users\REMOTEUSER\.ssh>fc id_dsa.pub authorized_keys
Comparing files id_dsa.pub and AUTHORIZED_KEYS
FC: no differences encountered

sshd_config has PubkeyAuthentication enabled

PubkeyAuthentication yes

The configuration and permissions appear sane to me. However, I get the ubiquitous missing begin marker error I always get when I botch permissions.

sshd.log

I see:
debug2: key not found

which generally means I have the wrong key in authorized_keys but I think the diff above disproves this problem.

Clues? Be gentle, I've not used Windows in anger in nearly 10 years.

ssh -v output

(note that I have a other rsa keys in this directory, not included above for clarity)

$ ssh -v -i .ssh/id_dsa myhostname
OpenSSH_6.6.1, OpenSSL 1.0.1e-fips 11 Feb 2013
debug1: Reading configuration data /home/localuser/.ssh/config
debug1: /home/localuser/.ssh/config line 21: Applying options for raleys-etl
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: /etc/ssh/ssh_config line 56: Applying options for *
debug1: Hostname has changed; re-reading configuration
debug1: Reading configuration data /home/localuser/.ssh/config
debug1: /home/localuser/.ssh/config line 15: Applying options for remotehostname
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: /etc/ssh/ssh_config line 56: Applying options for *
debug1: Connecting to remotehostname [00:00:00:00] port 22.
debug1: Connection established.
debug1: identity file /home/localuser/.ssh/id_dsa type -1
debug1: identity file /home/localuser/.ssh/id_dsa-cert type -1
debug1: identity file /home/localuser/.ssh/ssis_rsa type -1
debug1: identity file /home/localuser/.ssh/ssis_rsa-cert type -1
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_6.6.1
debug1: Remote protocol version 2.0, remote software version OpenSSH_7.5
debug1: match: OpenSSH_7.5 pat OpenSSH* compat 0x04000000
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: server->client aes128-ctr hmac-sha1-etm@openssh.com none
debug1: kex: client->server aes128-ctr hmac-sha1-etm@openssh.com none
debug1: kex: curve25519-sha256@libssh.org need=20 dh_need=20
debug1: kex: curve25519-sha256@libssh.org need=20 dh_need=20
debug1: sending SSH2_MSG_KEX_ECDH_INIT
debug1: expecting SSH2_MSG_KEX_ECDH_REPLY
debug1: Server host key: ECDSA e7:aa:c8:d4:8b:02:58:da:64:e6:18:26:d3:be:6a:b2
debug1: Host 'remotehostname' is known and matches the ECDSA host key.
debug1: Found key in /home/localuser/.ssh/known_hosts:5
debug1: ssh_ecdsa_verify: signature correct
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: SSH2_MSG_NEWKEYS received
debug1: SSH2_MSG_SERVICE_REQUEST sent
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug1: Authentications that can continue: publickey,password,keyboard-interactive
debug1: Next authentication method: publickey
debug1: Offering RSA public key: localuser@localhost.localdomain
debug1: Authentications that can continue: publickey,password,keyboard-interactive
debug1: Offering RSA public key: localuser@localhost.localdomain
debug1: Authentications that can continue: publickey,password,keyboard-interactive
debug1: Offering RSA public key: localuser@localhost.localdomain
debug1: Authentications that can continue: publickey,password,keyboard-interactive
debug1: Trying private key: /home/localuser/.ssh/id_dsa
debug1: key_parse_private2: missing begin marker
debug1: read PEM private key done: type DSA
debug1: Authentications that can continue: publickey,password,keyboard-interactive
debug1: Next authentication method: keyboard-interactive
Received disconnect from 00:00:00:00: 2: Too many authentication failures

Best Answer

Wow. Just spent a couple of hours debugging this.

So, turn logging for the ssh server:

  • Edit /ProgramData/ssh/sshd_config
    • Ensure you have SyslogFacility LOCAL0
    • Ensure you have LogLevel DEBUG3
  • Restart the OpenSSH SSH Server in Services
    • CMD:
      C:> net stop sshd
      C:> net start sshd
    • GUI: a quick way to get to Services is to press the Windows+R key combo, and enter services.msc in the resulting Run dialog.

Now you will find full debug info is being written to /ProgramData/ssh/logs/sshd.log. Just look in the log file after you have attempted to ssh into the machine.

I had two problems:

Problem 1: The correct authorized_keys file

The debug log said:

2019-03-08 … debug1: trying public key file __PROGRAMDATA__/ssh/administrators_authorized_keys

Ah, so not .ssh/authorized_keys then. I am in the Administrators group, and sshd_config has a special stanza for us folks. I copied the contents of my .ssh/authorized_keys file to /ProgramData/ssh/administrators_authorized_keys, and restarted the server.

Problem 2: Loose permissions

Now I had

2019-03-08 … debug3: Bad permissions. Try removing permissions for user: S-1-9-22 on file C:/ProgramData/ssh/administrators_authorized_keys.

icacls said

C:\ProgramData\ssh> icacls administrators_authorized_keys
administrators_authorized_keys NT AUTHORITY\SYSTEM:(F)
                           BUILTIN\Administrators:(F)
                           NT AUTHORITY\SYSTEM:(I)(F)
                           BUILTIN\Administrators:(I)(F)
                           NT AUTHORITY\Authenticated Users:(I)(RX)

There's a lot of permissions inherited from the folder and above (that's what (I) signifies). Remove the inheritance. /inheritance:r is your friend here.

C:\ProgramData\ssh> icacls administrators_authorized_keys /inheritance:r
processed file: administrators_authorized_keys
Successfully processed 1 files; Failed processing 0 files

Looks good now:

C:\ProgramData\ssh> icacls administrators_authorized_keys
administrators_authorized_keys NT AUTHORITY\SYSTEM:(F)
                           BUILTIN\Administrators:(F)

So I restarted the server, and it's working. Sheesh.

Don't forget to undo your changes to LogLevel and SyslogFacility in sshd_config.

Edit

Of course, none of this detective work was needed if only I knew where the docs were. See

https://docs.microsoft.com/en-us/windows-server/administration/openssh/openssh_server_configuration

Sigh.

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