GO Remote Desktop Gateway

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RDPGW is an implementation of the Remote Desktop Gateway protocol. This allows you to connect with the official Microsoft clients to remote desktops over HTTPS. These desktops could be, for example, XRDP desktops running in containers on Kubernetes.

AIM

RDPGW aims to provide a full open source replacement for MS Remote Desktop Gateway, including access policies.

Security requirements

Several security requirements are stipulated by the client that is connecting to it and some are enforced by the gateway. The client requires that the server's TLS certificate is valid and that it is signed by a trusted authority. In addition, the common name in the certificate needs to match the DNS hostname of the gateway. If these requirements are not met the client will refuse to connect.

The gateway has several security phases. In the authentication phase the client's credentials are verified. Depending the authentication mechanism used, the client's credentials are verified against an OpenID Connect provider, Kerberos, a local PAM service or a local database.

If OpenID Connect is used the user will need to connect to a webpage provided by the gateway to authenticate, which in turn will redirect the user to the OpenID Connect provider. If the authentication is successful the browser will download a RDP file with temporary credentials that allow the user to connect to the gateway by using a remote desktop client.

If Kerberos is used the client will need to have a valid ticket granting ticket (TGT). The gateway will proxy the TGT request to the KDC. Therefore, the gateway needs to be able to connect to the KDC and a krb5.conf file needs to be provided. The proxy works without the need for an RDP file and thus the client can connect directly to the gateway.

If local authentication is used the client will need to provide a username and password that is verified against PAM. This requires, to ensure privilege separation, that rdpgw-auth is also running and a valid PAM configuration is provided per typical configuration.

If NTLM authentication is used, the allowed user credentials for the gateway should be configured in the configuration file of rdpgw-auth.

Finally, RDP hosts that the client wants to connect to are verified against what was provided by / allowed by the server. Next to that the client's ip address needs to match the one it obtained the gateway token with if using OpenID Connect. Due to proxies and NAT this is not always possible and thus can be disabled. However, this is a security risk.

Configuration

The configuration is done through a YAML file. The configuration file is read from rdpgw.yaml by default. At the bottom of this README is an example configuration file. In these sections you will find the most important settings.

Authentication

RDPGW wants to be secure when you set it up from the start. It supports several authentication mechanisms such as OpenID Connect, Kerberos, PAM or NTLM.

Technically, cookies are encrypted and signed on the client side relying on Gorilla Sessions. PAA tokens (gateway access tokens) are generated and signed according to the JWT spec by using jwt-go signed with a 256 bit HMAC.

Multi Factor Authentication (MFA)

RDPGW provides multi-factor authentication out of the box with OpenID Connect integration. Thus you can integrate your remote desktops with Keycloak, Okta, Google, Azure, Apple or Facebook if you want.

Mixing authentication mechanisms

It is technically possible to mix authentication mechanisms. Currently, you can mix local with Kerberos or NTLM. If you enable OpenID Connect it is not possible to mix it with local or Kerberos at the moment.

Open ID Connect

To use OpenID Connect make sure you have properly configured your OpenID Connect provider, and you have a client id and secret. The client id and secret are used to authenticate the gateway to the OpenID Connect provider. The provider will then authenticate the user and provide the gateway with a token. The gateway will then use this token to generate a PAA token that is used to connect to the RDP host.

To enable OpenID Connect make sure to set the following variables in the configuration file.

Server:
  Authentication: 
    - openid
OpenId:
    ProviderUrl: http://<provider_url>
    ClientId: <your client id>
    ClientSecret: <your-secret>
Caps:
  TokenAuth: true

As you can see in the flow diagram when using OpenID Connect the user will use a browser to connect to the gateway first at https://your-gateway/connect. If authentication is successful the browser will download a RDP file with temporary credentials that allow the user to connect to the gateway by using a remote desktop client.

Kerberos

NOTE: Kerberos is heavily reliant on DNS (forward and reverse). Make sure that your DNS is properly configured. Next to that, its errors are not always very descriptive. It is beyond the scope of this project to provide a full Kerberos tutorial.

To use Kerberos make sure you have a keytab and krb5.conf file. The keytab is used to authenticate the gateway to the KDC and the krb5.conf file is used to configure the KDC. The keytab needs to contain a valid principal for the gateway.

Use ktutil or a similar tool provided by your Kerberos server to create a keytab file for the newly created service principal. Place this keytab file in a secure location on the server and make sure that the file is only readable by the user that runs the gateway.

ktutil
addent -password -p HTTP/[email protected] -k 1 -e aes256-cts-hmac-sha1-96
wkt rdpgw.keytab

Then set the following in the configuration file.

Server:
  Authentication:
    - kerberos
Kerberos:
    Keytab: /etc/keytabs/rdpgw.keytab
    Krb5conf: /etc/krb5.conf
Caps:
  TokenAuth: false

The client can then connect directly to the gateway without the need for a RDP file.

PAM / Local (Basic Auth)

The gateway can also support authentication against PAM. Sometimes this is referred to as local or passwd authentication, but it also supports LDAP authentication or even Active Directory if you have the correct modules installed. Typically (for passwd), PAM requires that it is accessed as root. Therefore, the gateway comes with a small helper program called rdpgw-auth that is used to authenticate the user. This program needs to be run as root or setuid.

NOTE: The default windows client mstsc does not support basic auth. You will need to use a different client or switch to OpenID Connect, Kerberos or NTLM authentication.

NOTE: Using PAM for passwd (i.e. LDAP is fine) within a container is not recommended. It is better to use OpenID Connect or Kerberos. If you do want to use it within a container you can choose to run the helper program outside the container and have the socket available within. Alternatively, you can mount all what is needed into the container but PAM is quite sensitive to the environment.

Ensure you have a PAM service file for the gateway, /etc/pam.d/rdpgw. For authentication against local accounts on the host located in /etc/passwd and /etc/shadow you can use the following.

auth required pam_unix.so
account required pam_unix.so

Then set the following in the configuration file.

Server:
  Authentication:
    - local
AuthSocket: /tmp/rdpgw-auth.sock
Caps:
  TokenAuth: false

Make sure to run both the gateway and rdpgw-auth. The gateway will connect to the socket to authenticate the user.

# ./rdpgw-auth -n rdpgw -s /tmp/rdpgw-auth.sock

The client can then connect to the gateway directly by using a remote desktop client.

NTLM

The gateway can also support NTLM authentication. Currently, only the configuration file is supported as a database for credential lookup. In the future, support for real databases (e.g. sqlite) may be added.

NTLM authentication has the advantage that it is easy to setup, especially in case the gateway is used for a limited number of users. Unlike PAM / local, NTLM authentication supports the default windows client mstsc.

WARNING: The password is currently saved in plain text. So, you should keep the config file as secure as possible and avoid reusing the same password for other applications. The password is stored in plain text to support the NTLM authentication protocol.

To enable NTLM authentication make sure to set the following variables in the configuration file.

Configuration file for rdpgw:

Server:
  Authentication:
    - ntlm
Caps:
  TokenAuth: false

Configuration file for rdpgw-auth:

Users:
 - {Username: "my_username", Password: "my_secure_password"} # Modify this password!

The client can then connect to the gateway directly by using a remote desktop client using the gateway credentials configured in the YAML configuration file.

TLS

The gateway requires a valid TLS certificate. This means a certificate that is signed by a valid CA that is in the store of your clients. If this is not the case particularly Windows clients will fail to connect. You can either provide a certificate and key file or let the gateway obtain a certificate from letsencrypt. If you want to use letsencrypt make sure that the host is reachable on port 80 from the letsencrypt servers.

For letsencrypt:

Tls: auto

for your own certificate:

Tls: enable
CertFile: server.pem 
KeyFile: key.pem

NOTE: You can disable TLS on the gateway, but you will then need to make sure a proxy is run in front of it that does TLS termination.

Example configuration file for Open ID Connect

# web server configuration. 
Server:
 # can be set to openid, kerberos, local and ntlm. If openid is used rdpgw expects
 # a configured openid provider, make sure to set caps.tokenauth to true. If local
 # rdpgw connects to rdpgw-auth over a socket to verify users and password. Note:
 # rdpgw-auth needs to be run as root or setuid in order to work. If kerberos is
 # used a keytab and krb5conf need to be supplied. local can be stacked with 
 # kerberos or ntlm authentication, so that the clients selects what it wants.
 Authentication:
  # - kerberos
  # - local
  - openid
  # - ntlm
 # The socket to connect to if using local auth. Ensure rdpgw auth is configured to
 # use the same socket.
 # AuthSocket: /tmp/rdpgw-auth.sock
 # Basic auth timeout (in seconds). Useful if you're planning on waiting for MFA
 BasicAuthTimeout: 5
 # The default option 'auto' uses a certificate file if provided and found otherwise
 # it uses letsencrypt to obtain a certificate, the latter requires that the host is reachable
 # from letsencrypt servers. If TLS termination happens somewhere else (e.g. a load balancer)
 # set this option to 'disable'. This is mutually exclusive with 'authentication: local'
 # Note: rdp connections over a gateway require TLS
 Tls: auto
 # gateway address advertised in the rdp files and browser
 GatewayAddress: localhost
 # port to listen on (change to 80 or equivalent if not using TLS)
 Port: 443
 # list of acceptable desktop hosts to connect to
 Hosts:
  - localhost:3389
  - my-{{ preferred_username }}-host:3389
 # if true the server randomly selects a host to connect to
 # valid options are: 
 #  - roundrobin, which selects a random host from the list (default)
 #  - signed, a listed host specified in the signed query parameter
 #  - unsigned, a listed host specified in the query parameter
 #  - any, insecurely allow any host specified in the query parameter
 HostSelection: roundrobin 
 # a random strings of at least 32 characters to secure cookies on the client
 # make sure to share this across the different pods
 SessionKey: thisisasessionkeyreplacethisjetzt
 SessionEncryptionKey: thisisasessionkeyreplacethisnunu!
  # where to store session details. This can be either file or cookie (default: cookie)
  # if a file store is chosen, it is required to have clients 'keep state' to the rdpgw
  # instance they are connected to.
 SessionStore: cookie
  # tries to set the receive / send buffer of the connections to the client
 # in case of high latency high bandwidth the defaults set by the OS might
 # be to low for a good experience
 # ReceiveBuf: 12582912
 # SendBuf: 12582912 
# Open ID Connect specific settings
OpenId:
 ProviderUrl: http://keycloak/auth/realms/test
 ClientId: rdpgw
 ClientSecret: your-secret
# Kerberos:
#  Keytab: /etc/keytabs/rdpgw.keytab
#  Krb5conf: /etc/krb5.conf
#  enabled / disabled capabilities
Caps:
 SmartCardAuth: false
 # required for openid connect
 TokenAuth: true
 # connection timeout in minutes, 0 is limitless
 IdleTimeout: 10
 EnablePrinter: true
 EnablePort: true
 EnablePnp: true
 EnableDrive: true
 EnableClipboard: true
Client:
  # template rdp file to use for clients
  # rdp file settings and their defaults see here: 
  # https://docs.microsoft.com/en-us/windows-server/remote/remote-desktop-services/clients/rdp-files
  defaults: /etc/rdpgw/default.rdp
  # this is a go string templated with {{ username }} and {{ token }}
  # the example below uses the ASCII field separator to distinguish
  # between user and token 
  UsernameTemplate: "{{ username }}@bla.com\x1f{{ token }}"
  # If true puts splits "[email protected]" into the user and domain component so that
  # domain gets set in the rdp file and the domain name is stripped from the username
  SplitUserDomain: false
  # If true, removes "username" (and "domain" if SplitUserDomain is true) from RDP file.
  # NoUsername: true
Security:
  # a random string of 32 characters to secure cookies on the client
  # make sure to share this amongst different pods
  PAATokenSigningKey: thisisasessionkeyreplacethisjetzt
  # PAATokenEncryptionKey: thisisasessionkeyreplacethisjetzt
  # a random string of 32 characters to secure cookies on the client
  UserTokenEncryptionKey: thisisasessionkeyreplacethisjetzt
  # Signing makes the token bigger and we are limited to 511 characters
  # UserTokenSigningKey: thisisasessionkeyreplacethisjetzt
  # if you want to enable token generation for the user
  # if true the username will be set to a jwt with the username embedded into it
  EnableUserToken: true
  # Verifies if the ip used to connect to download the rdp file equals from where the
  # connection is opened.
  VerifyClientIp: true

How to build & install

NOTE: a docker image is available on docker hub, which removes the need for building and installing go.

Ensure that you have make (comes with standard build tools, like build-essential on Debian), go (version 1.19 or above), and development files for PAM (libpam0g-dev on Debian) installed.

Then clone the repo and issues the following.

cd rdpgw
make
make install

Testing locally

A convenience docker-compose allows you to test the RDPGW locally. It uses Keycloak and xrdp and exposes it services on port 9443. You will need to allow your browser to connect to localhost with and self signed security certificate. For chrome set chrome://flags/#allow-insecure-localhost. The username to login to both Keycloak and xrdp is admin as is the password.

NOTE: The redirecting relies on DNS. Make sure to add 127.0.0.1 keycloak to your /etc/hosts file to ensure that the redirect works.

NOTE: The local testing environment uses a self signed certificate. This works for MAC clients, but not for Windows. If you want to test it on Windows you will need to provide a valid certificate.

# with open id
cd dev/docker
docker-compose -f docker-compose.yml up

# or for arm64 with open id
docker-compose -f docker-compose-arm64.yml up

# or for local or pam
docker-compose -f docker-compose-local.yml up

You can then connect to the gateway at https://localhost:9443/connect for the OpenID connect flavors which will start the authentication flow. Or you can connect directly with the gateway set and the host set to xrdp if using the local flavor. You can login with 'admin/admin'. The RDP file will download and you can open it with a remote desktop client. Also for logging in 'admin/admin' will work.

Use

Point your browser to https://your-gateway/connect. After authentication and RDP file will download to your desktop. This file can be opened by one of the remote desktop clients and it will try to connect to the gateway and desktop host behind it.

Integration

The gateway exposes an endpoint for the verification of user tokens at https://yourserver/tokeninfo . The query parameter is 'access_token' so you can just do a GET to https://yourserver/tokeninfo?access_token= . It will return 200 OK with the decrypted token.

In this way you can integrate, for example, it with pam-jwt.

Client Caveats

The several clients that Microsoft provides come with their own caveats. The most important one is that the default client on Windows mstsc does not support basic authentication. This means you need to use either OpenID Connect, Kerberos or ntlm authentication.

In addition to that, mstsc, when configuring a gateway directly in the client requires you to either:

• "save the credentials" for the gateway
• or specify a (random) domain name in the username field (e.g. .\username) when prompted for the gateway credentials,

otherwise the client will not connect at all (it won't send any packages to the gateway) and it will keep on asking for new credentials.

Finally, mstsc requires a valid certificate on the gateway.

The Microsoft Remote Desktop Client from the Microsoft Store does not have these issues, but it requires that the username and password used for authentication are the same for both the gateway and the RDP host.

The Microsoft Remote Desktop Client for Mac does not have these issues and is the most flexible. It supports basic authentication, OpenID Connect and Kerberos and can use different credentials

The official Microsoft IOS and Android clients seem also more flexible.

Third party clients like FreeRDP might also provide more flexibility.

TODO

• Improve Web Interface

Acknowledgements

• This product includes software developed by the Thomson Reuters Global Resources. (go-ntlm - BSD-4 License)