ssh2proxy

The sshproxy allows to select different ssh backend hosts based on the ssh username. This allows users of multiple sshservers to access the ssh service through a single endpoint.

The proxy implements the client-side and server-side connections using the api of the libssh.a provided by the openssh project. This allows the proxy code to stay in sync with openssh updates and bugfixes.

The current version is built on top of openssh-8.2p1.

The proxy supports ssh2 password & pubkey authentication.

To allow transparent operation for a client, the ssh2proxy has to use the same hostkeys as the backend ssh servers. With different hostkeys a client with existing known_hosts entries would notice the proxy as a man-in-the-middle.

The backend host for each user can be configured in the sshproxy config file. For more complex setups it is possible to add a special switch module to the implementation.

For public key authentication the ssh2proxy has to use a different authentication scheme for the backend connection. The current implementation allows to switch to hostbased authentication for the backend. More info below.

Configuration

The example configuration file sshproxy.conf.example shows the basic configuration of the sshproxy.

• the sshproxy server listens on the configured bindaddr and proxies to the given default_server if nothing else is configured.

• the hostkey entry is used to configure private hostkeys for the connection to the client, and public hostkeys for the proxy-connections to the backend servers.

• with switch_target = <user> <host> entries, it is possible to direct ssh-users to hosts different from the default_server.

• the private key required for hostbased authentication can be configured via a hostkey_auth = <key> entry.

• the sshproxy can be tested via: sshproxy -c sshproxy.conf.example &

sshproxy protocol handling

1. In the first step the ssh protocol negotiation and the ssh key-exchange take place only between sshclient and sshproxy.

   The hostkey configured for the sshproxy is used for the key-exchange, to generate the session-id (priv) and is also transmitted in the proposal (pub).

2. After the key-exchange the sshproxy waits for a USERAUTH-requests with the 'username' for 'password' or 'pubkey' authentication.

3. Depending on the 'username' the sshproxy switch-module decides to which backend sshserver the proxy will connect.

4. The sshproxy connects to the backend sshserver und performs another key-exchange.

5. Now the USERAUTH-request is forwarded to the backend using the hostkey of the backend sshserver.

Password:

6. If the backend sshserver response to the USERAUTH-request is SUCCESS, then the proxy-authentication is complete. All subsequent ssh-packets can be passed transparently between sshclient and sshserver.

   For 'password'-requests the session-id is used only for the protocol handling of the ssh_packet_write() & ssh_packet_read() calls.

   So for 'password'-authentication it is possible to use different hostkeys for the proxy- and backend-connections.

Public Key Authentication:

7. When the sshclient uses 'pubkey'-authentication with a sshkey which is secured by a passphrase, then the sshproxy is first asked with the unsigned sshkey if the key is available on the server side.

   • this USERAUTH-request is forwarded to the sshserver as in point (5), and the response tells if the sshkey is allowed.

   • the sshproxy does not need access to the webspace of the user. The public sshkey has just to be configured in the .ssh/authorized_keys file of the user-account on the backend sshserver.

8. Now the sshclient authenticates again with a signed sshkey. This request is answered directly by the sshproxy after it validated the sshkey.

   • it is not possible to pass this USERAUTH-request on to the sshserver, since this request uses the hash from the key-exchange. And the hash is different for the proxy- and backend-connections. (see for example kexgex/kexdh->dh_gen_key()).

   For this reason it is necessary to use a different authentication like 'hostbased' for the Backend-Connection at this place. (see the example for hostbased configuration below)

9. For sshkeys without passphrase the protocol is slightly different. In this case already the first USERAUTH-request from the sshclient includes a signed sshkey. The sshproxy will pass this sshkey without signature on to the sshserver and then continues at point (8).

ssh2 states for public key authentication

For sshkey with passphrase and 'hostbased' authentication for the backend:

  0) exchange of SSH2_MSG_USERAUTH_REQUEST 'none'
  1)           proxy <- client SSH2_MSG_USERAUTH_REQUEST 'pubkey' no-sig
  2) server <- proxy           SSH2_MSG_USERAUTH_REQUEST 'pubkey' no-sig
  3) server -> proxy           SSH2_MSG_USERAUTH_PK_OK (server knows key)
  4)           proxy -> client SSH2_MSG_USERAUTH_PK_OK
  5)           proxy <- client SSH2_MSG_USERAUTH_REQUEST 'pubkey' sig
                               (check signature -> FAIL if not match)
  6) server <- proxy           SSH2_MSG_USERAUTH_REQUEST 'hostbased'
  7) server -> proxy           SSH2_MSG_REQUEST_SUCCESS (for: hostbased)
  8)           proxy -> client SSH2_MSG_REQUEST_SUCCESS (for: pubkey (5))

For sshkey without passphrase:

  0) exchange of SSH2_MSG_USERAUTH_REQUEST 'none'
  1)           proxy <- client SSH2_MSG_USERAUTH_REQUEST 'pubkey' sig
                               (check signature -> FAIL if not match)
  2) server <- proxy           SSH2_MSG_USERAUTH_REQUEST 'pubkey' no-sig
  3) server -> proxy           SSH2_MSG_USERAUTH_PK_OK (server knows key)
  4) server <- proxy           SSH2_MSG_USERAUTH_REQUEST 'hostbased'
  5) server -> proxy           SSH2_MSG_REQUEST_SUCCESS (for: hostbased)
  6)           proxy -> client SSH2_MSG_REQUEST_SUCCESS (for: pubkey (1))

Hostbased authentication configuration

For a hostbased backend connection the sshproxy uses the key configured via the 'hostkey_auth' config setting. To get this working, the backend sshserver needs to allow hostbased authentication and the key must be configured in the SSHDIR/etc/ssh_known_hosts file.

sshserver configuration for hostbased sshproxy access:

• sshserver SSHDIR/etc/sshd_config:

  HostbasedAuthentication yes
  IgnoreUserKnownHosts yes
  IgnoreRhosts yes
  

• sshserver SSHDIR/etc/shosts.equiv: Add hostline: <clienthost>

• sshserver SSHDIR/etc/ssh_known_hosts:

  Get proxy public hostkey:

  # cat <hostkey_proxy> | ssh-keygen -y -f /dev/stdin
    ssh-rsa <hostkey_proxy_pub>
  

  Add hostline: <host>[,<ip>] ssh-rsa <hostkey_proxy_pub>

sshserver configuration for hostbased connection test with normal ssh client:

• sshserver SSHDIR/etc/ssh_known_hosts: Add client public hostkey (for test):

  # ssh-keyscan -t rsa <clienthost>
  

• sshserver SSHDIR/etc/shosts.equiv: Add hostline: <clienthost> <clientuser>

• sshclient SSHDIR/etc/ssh_config: (uses libexec/ssh-keysign tool for authentication)

  EnableSSHKeysign yes
  

• sshclient connection test: (will try all hostkeys without HostbasedKeyTypes option)

  clientuser@clienthost # ssh -v -o PubkeyAuthentication=no -o PasswordAuthentication=no \
          -o HostbasedAuthentication=yes -o HostbasedKeyTypes=ssh-rsa <user>@<sshserver>