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🚀 Horizontally scalable reverse tunnel relay server for exposing services behind NAT/Firewalls without port forwarding (Self-hosted cloudflared/ngrok alternative).

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# exposr - dynamic reverse tunnel relay server

exposr is a self-hosted reverse tunnel relay server that allows you to securely expose devices and services
behind NATs or firewalls to the Internet through public URLs.

exposr can for example be used for development and previews or for exposing services behind NAT/firewalls
to the Internet without port-forwarding and risk of exposing your IP address.

Why another "reverse tunnel server"? exposr takes a slightly different approach than other servers
of the same type. exposr is designed to run as a container with horizontal elastic scaling properties,
and is well suited to run in container-orchestration systems like Kubernetes.

# Features

* Clustering support with horizontally scalability - more nodes can be added to increase capacity.
* Multi-client connection - each client can maintain multiple persistent connection per tunnel.
* No configuration files! - All configuration can be done as environment variables or command line options.
* Designed to run behind a load balancer (HTTP/TCP) (ex. nginx or HAProxy).
* Suitable to run in container-orchestration systems such as Kubernetes.
* Multiple transports - multiplexed websocket with custom client or SSH client forwarding.
* Multiple ingress methods - HTTP (with custom domain support) or SNI for TLS/TCP protocols.
* Custom client can forward to any host, does not require root privileges and only requires outbound HTTP(s) connections.
* Tunnel configuration through restful APIs.
* No passwords or e-mails - but still secure. An account number together with the tunnel identifier serves as credentials.

What it does *not* do
* Certificate provisioning.
* DNS provisioning.

This is on purpose as the server is designed to be stateless and to have elastic scaling
properties. Meaning these are more suitable to handle in other parts of the deployment stack, for
example at the load balancer.

## Demo

![](https://exposr.github.io/docs/img/demo/exposr-demo-20220301.svg)

## Supported transports
exposr supports two different transport methods. This is the way a tunnel client
connects and establishes the tunnel.

| Type       | Method                     | Endpoint   | Client support        |
| ---------- | -------------------------- |----------- | --------------------- |
| Websocket  | Custom multiplex websocket | HTTP       | [`exposr`](https://github.com/exposr/exposr) |
| SSH        | SSH TCP forwarding         | TCP        | Any SSH client        |

The Websocket transport endpoint can run behind a HTTP load balancer on the same port
as the API. The SSH transport endpoint requires a dedicated TCP port and requires
a TCP load balancer in multi-node setups.

## Supported ingress methods
The following ingress methods are supported. Ingress is the way clients (or users)
connect to the tunnel to connect to the exposed services.

| Type  | Method                   | Protocol support | Requirements                | Load balancer req. |
| ----- | ------------------------ | ---------------- | --------------------------- | ------------------ |
| HTTP  | Virtual host (subdomain) | HTTP             | Wildcard domain             | HTTP               |
| SNI   | SNI                      | TLS              | Wildcard certificate+domain | TCP                |

## Supported storage options

The following storage options are supported. The default is no persistence,
SQLite is recommended for single-node setups. Tunnel configuration and
accounts are written to persistent storage.

| Type       | Single/multi-node        | Note                 |
| ---------- | ------------------------ | -------------------- |
| Memory     | Single-node              | Data lost on restart |
| SQLite     | Single node              |                      |
| PostgreSQL | Multi-node               |                      |
| Redis      | Multi-node               |                      |

## Clustering support
exposr can be run in a multi-node setup, ingress connections are re-routed to the node
that have the tunnel established. This allows for horizontal load balancing in round-robin
fashion without the need for sticky sessions.

| Type        | Discovery methods                  | Note                         |
| ----------- | ---------------------------------- | ---------------------------- |
| Single-node | Single-node                        | No clustering                |
| UDP         | IP Multicast or Native Kubernetes  | K8S through headless service |
| Redis       | Redis                              |                              |

To run exposr in clustering mode you need to select a cluster mode, the default
is UDP with node discovery through IP multicast or through Kubernetes headless
service, exposr will try to auto-detect the best discovery method to use.
To use the UDP mode IP connectivity on UDP port 1025 (default) between nodes is required.

It is also possible to use a Redis cluster with pub/sub capabilities.

NB: Multi-node storage is required for clustering setup.

# Architecture
exposr have three core concepts; transports, endpoints and ingress.

A tunnel is composed of a transport and a connection endpoint.
The endpoint is used by the client to establish a tunnel connection.
The transport of the tunnel is the underlying data stream of the tunnel, it supports
multiple independent streams over one connection.

The ingress is for traffic destined for the tunnel target, an ingress supports one
specific protocol and have a distinct way of identifying which tunnel the request
is bound for.

```
                          +-----------------------+
      +----------------+  |  +-----------------+  |
----->|    Ingress     +--|->|    Transport    +--|-----------+
      +----------------+  |  +-----------------+  |           v
                          |                       |   +----------------+     +--------------+
                          |        Tunnel         |   |    Client      +---->|    Target    |
                          |                       |   +-------+--------+     +--------------+
                          |  +-----------------+  |           |
                          |  |    Endpoint     |<-|-----------+
                          |  +-----------------+  |           |
                          +-----------------------+           |
                             +-----------------+              |
                             |      API        |<-------------+
                             +-----------------+
```

# Tunnels and accounts
A tunnel is identified by a string consisting of alphanumeric `(a-z, 0-9)` characters and dashes `(-)`.
Minimum 4 characters and maximum 63 characters. The tunnel identifier have to start with a alpha character.
This is chosen so that the tunnel identifier can be used as a DNS label.

Example

my-tunnel-identifier-14

An account number is a 16 character string selected from the case-insensitive alphabet `CDEFHJKMNPRTVWXY2345689`.
The number is formatted into 4 groups of 4 characters separated by a separator.
Dashes and spaces are accepted as separator, as well as no separator.

Example

MNF4-P6Y6-M2MR-RVCT
    MNF4 P6Y6 M2MR RVCT
    MNF4P6Y6M2MRRVCT

A tunnel is owned by one account, one account can have multiple tunnels.
There is no password or key associated with an account.

It's _not_ possible for a user to list all tunnels belonging to an account.
This makes it possible to use the account number together with the tunnel identifier as credentials as both
needs to be known in order to perform privileged operations on a tunnel.
# Running exposr

## Runtime artifacts

### Containers
Containers are available for deployments in container runtime environments.

Latest release is available with the `latest` tag, latest development (master branch) is available with the `unstable` tag.

## Quick start
You can quickly try out exposr without installing anything.

Run the server, the server will listen on port 8080 and the API will be exposed at `http://host.docker.internal:8080`.
HTTP ingress sub-domains will be allocated from `http://localhost:8080`.

docker run --rm -ti -p 8080:8080 ghcr.io/exposr/exposrd:latest --allow-registration --ingress-http-url http://localhost:8080

Start the client with, this will create a tunnel called `example` and connect it to `http://example.com`.
The tunnel will be available at `http://example.localhost:8080`.

docker run --rm -ti ghcr.io/exposr/exposr:latest -s http://host.docker.internal:8080/ tunnel connect example http://example.com

Try the tunnel

curl --resolve example.localhost:8080:127.0.0.1 http://example.localhost:8080

## Configuration

exposr needs to have at least one ingress and one transport method enabled. The default option enables
the HTTP ingress and the WS transport.

### Account creation
Account creation is disabled by default and needs to be enabled. It can be enabled in two ways, either through
the public API or by enabling the administration API. It's recommended to only use the admin API
for account creation.

To enable it through the public API start exposr with the flag `--allow-registration`.

> ⚠️ Warning: Enabling public account registration will allow anyone to register an account and to create tunnels on your server.

### Administration

#### Interface
The administration interface runs on a separate port from public API. By default it uses `8081`.
The interface can be enabled by passing the flag `--admin-enable true`.

The administration interface exposes a `/ping` endpoint for load balancer health checks.

#### API
The administration API runs on a separate port from public API. By default it uses `8081`.
The API can be enabled by passing the flag `--admin-api-enable true`.

To further enable the administration API an API key must be configured.

exposrd --admin-api-enable true --admin-api-key <insert key>

> ⚠️ Warning: The API key allows full privileged access to all accounts and tunnels.

### Configuring HTTP ingress
The HTTP ingress can be enabled by passing the flag `--ingress http`.
It uses the same port as the API port, and fully supports HTTP(s) including upgrade requests (ex. websockets).

The HTTP ingress uses subdomains and virtual hosts to determine the tunnel id and requires a
wildcard DNS entry to be configured and pointed to your server or load balancer.

*.example.com  IN A  10.0.0.1

The domain needs to be configured with `--ingress-http-url`.

exposrd --ingress http --ingress-http-url http://example.com

Each tunnel will be allocated a subdomain, ex. `http://my-tunnel.example.com`.

If you have a proxy or load balancer in-front of exposr that terminates HTTPS, pass the domain with
the `https` protocol instead. (`--ingress-http-url https://example.com`).

#### BYOD (Bring Your Own Domain)

The HTTP ingress supports custom domain names to be assigned to a tunnel outside of the automatic one
allocated from the wildcard domain. Assigning a custom domain name to a tunnel will make exposr
recognize requests for the tunnel using this name.

To configure BYOD (altname) a CNAME for the domain must be created and pointing towards the FQDN
of the tunnel. For example, to use the name `example.net` for the tunnel `my-tunnel.example.com`
a CNAME should be configured for `example.net` pointing to `my-tunnel.example.com`.

example.net  IN CNAME  my-tunnel.example.com

Finally the altname needs to be enabled in exposr, this can be done through the cli.

exposr tunnel configure my-tunnel set ingress-http-altnames example.net

The request will be rejected unless the CNAME is properly configured.

Note that if you have a load balancer or proxy in front of exposr that terminates HTTPS
you need have a certificate that covers the altname.

### Configuring SNI ingress

To enable the SNI (Server Name Indication) ingress pass the flag `--ingress sni`.
The SNI ingress requires a dedicated TCP port, by default it uses 4430. The port can be changed with `--ingress-sni-port`.

The SNI ingress works by utilizing the SNI extension of TLS to get the tunnel from the hostname. Similar to
the HTTP ingress it requires a wildcard DNS entry (`*.example.com`), but also a wildcard certificate covering
the same domain name. It's compatible with any protocol that can run over TLS and a client that supports SNI.

exposr will monitor the provided certificate and key for changes and re-load the certificate on-the fly.

#### Certificate

The certificate must contain one wildcard entry, either as the common name (`CN`) or in the SAN list.
If there are multiple wildcard entries present, the first one will be used.

For production use, a real certificate should be used. Let's encrypt offers free wildcard certificates.
For testing a self-signed can be generated with openssl.

openssl req -x509 -newkey rsa:4096 -keyout private-key.pem -out certificate.pem -days 365 -nodes

#### Example

exposrd --ingress sni --ingress-sni-cert certificate.pem --ingress-sni-key private-key.pem

### Configuring SSH transport

To enable the SSH transport pass the flag `--transport ssh` to exposr.
By default it will use port 2200, it can be changed with `--transport-ssh-port`.
The base host name will by default use the API host, it can be overridden with `--transport-ssh-host`.

A new SSH host key will be generated at startup. If you run in a clustered setup it's recommended to provide
a static key so that clients always receive the same host key. The key can be specified either as a path or string
containing a SSH private key in PEM encoded OpenSSH format using `--transport-ssh-key`.

#### Example

Start the server with SSH transport enabled

> docker run --rm -ti -p 8080:8080 -p 2200:2200 ghcr.io/exposr/exposrd:latest --allow-registration --ingress-http-url http://localhost:8080 --transport ssh

Create and account and configure a tunnel

> docker run --rm -ti ghcr.io/exposr/exposr:latest -s http://host.docker.internal:8080/ account create
     ✔ 2022-02-24 19:00:00 +0100 - Creating account...success
    ✨ Created account DE94-JTNJ-FX5W-YWKY

> docker run --rm -ti ghcr.io/exposr/exposr:latest -s http://host.docker.internal:8080/ -a DE94-JTNJ-FX5W-YWKY tunnel create my-tunnel transport-ssh on ingress-http on
     ✔ 2022-02-24 19:00:10 +0100 - Creating tunnel...success (my-tunnel)
     ✔ 2022-02-24 19:00:20 +0100 - Setting transport-ssh to 'true'...done
     ✔ 2022-02-24 19:00:20 +0100 - Setting ingress-http to 'true'...done
    ✨ Created tunnel my-tunnel

Fetch the SSH endpoint URL

> docker run --rm -ti ghcr.io/exposr/exposr:latest -s http://host.docker.internal:8080/ -a MNF4-P6Y6-M2MR-RVC" tunnel info my-tunnel
    [...]
      Transports
        SSH: ssh://my-tunnel:kXBnFV6Z1YoZPhoVLmxn9UO-Cp2qh7R19CGRrA_ylYfiiZ32N-CR9LWyHtaHxXn8UXGPNSt5xXUxf-5DlZOvLg@localhost:2200

Establish the tunnel with SSH as normal

> ssh -o "StrictHostKeyChecking no" -o "UserKnownHostsFile /dev/null" -R example.com:80:example.com:80 ssh://my-tunnel:nfeflVuKGick0rD2C7Mqne6d-MDWPGCX6At7ygj0U8FTkgbLFi-XckuEUQ9-ipkJ0aRPkrxziKit4wWDisONXg@localhost:2200
    Warning: Permanently added '[localhost]:2200' (RSA) to the list of known hosts.
    exposr/v0.5.1
    Target URL: http://example.com/
    HTTP ingress: http://my-tunnel.localhost:8080/

The target can be configured with the `bind_address` part of the `-R` argument to ssh. If a target
has already been configured the left-hand part of -R can be left out, example `-R 0:example.com:80`.

Note that the connection token is only valid for one connection, and must be re-fetched for each connection.

#### Permanent SSH key
Generate an SSH key with (only the private key is required)

ssh-keygen -b 2048 -t rsa -f sshkey -q -N ""

The content of the file can be passed through environment variables

EXPOSR_TRANSPORT_SSH_KEY=$(<sshkey) exposrd [...]

You can also specify it as a path

exposrd [...] --transport-ssh-key /path/to/sshkey

### Storage setup
exposr supports persistance through SQLite, PostgreSQL or Redis.
To enable storage, pass the `--storage-url` option together with a connection string.

To configure PostgreSQL use `postgres://<connection-string>`.

exposrd --storage-url postgres://db_user:db_password@postgres-host/mydatabase

To configure Redis use `redis://<connection-string>`.

exposrd --storage-url redis://:redis_password@redis-host

To configure SQLite use `sqlite://<path>`.

exposrd --storage-url sqlite://exposr.sqlite

### Clustering setup
To run exposr in a clustering setup, the following is required;

* PostgreSQL or Redis for storage.
* IP connectivity between all nodes.
* Load balancer in-front of the nodes (ex. K8S, AWS ALB, GCP LB, nginx/haproxy/etc)

To run exposr in clustering mode the nodes needs IP connectivity between them, and
a pub/sub bus for messages. exposr supports pub/sub through a UDP or through Redis.

The clustering mode is configured using the `--cluster` option.

exposrd --cluster auto|udp|redis

To preserve the integrity of the message bus each message is signed using a per cluster signing key.
The message signature is validated by each node, and messages with invalid signatures are rejected.

You should configure a secret signing key using the `--cluster-key` option.

#### UDP
The UDP pub/sub bus is using a custom UDP protocol running on port 1025 (can be changed with `--cluster-udp-port`).
The UDP clustering mode supports two modes of node discovery. IP multicast and Kubernetes headless service discovery.

When using IP multicast, messages are sent to a IP multicast group, by default 239.0.0.1 (can be changed with `--cluster-udp-discovery-multicast-group`).
When using Kubernetes discovery, each Pod IP is discovered through DNS and messages are sent using unicast directly to each node.

exposr tries to auto-detect the environment and select the most appropriate discovery mode.
You can explicitly set the discovery mode using `--cluster-udp-discovery`.

exposrd --cluster udp --cluster-udp-discovery multicast|kubernetes

#### Redis
If using Redis as a storage option it may be convenient to use Redis as the pub/sub bus as well.
To do so you must pass a Redis connection string to the Redis cluster to use for pub/sub using the `--cluster-redis-url` option.

exposrd --cluster redis --cluster-redis-url redis//:redis-password@redis-host

### A note on scalability
Because of the persistent nature of the tunnel transport connections, the ingress of exposr does not
scale linear, but rather exhibits a sub-linear scaling.
When an ingress connection is made to a node that does not have a tunnel connected locally,
the connection is proxied internally by exposr to the node that have the tunnel connected.
This means that the ingress traffic will traverse two exposr nodes and with increased number of nodes
the probability of the ingress connection being mis-routed increases.

You can decrease the likelihood of miss-routing by allowing more client per-tunnel connections
using the `--transport-max-connections` option, at the expense of maintaining more connections per client.

### Using environment variables

Each option can be given as an environment variable instead of a command line option. The environment variable
is named the same as the command line option in upper case with `-` replaced with `_`, and prefixed with `EXPOSR_`.

For example the command line option `--ingress-http-url http://example.com` would be specified as `EXPOSR_INGRESS_HTTP_URL=http://example.com`.

Multiple value options are specified as comma separated values.
For example `--transport ws --transport ssh` would be specified as `EXPOSR_TRANSPORT=ws,ssh`

## Production deployment

### Kubernetes

exposr can be deployed to Kubernetes with helm.

Add the repository

helm repo add exposr https://exposr.github.io/helm-charts/
	helm repo update

Deploy with

helm install my-exposr exposr/exposr

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