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ghostunnel

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A simple TLS proxy with mutual authentication for securing non-TLS services.

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Ghostunnel
==========

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👻

Ghostunnel is a simple TLS proxy with mutual authentication support for
securing non-TLS backend applications.

Ghostunnel supports two modes, client mode and server mode. Ghostunnel in
server mode runs in front of a backend server and accepts TLS-secured
connections, which are then proxied to the (insecure) backend. A backend can be
a TCP host/port or a UNIX domain socket. Ghostunnel in client mode accepts
(insecure) connections through a TCP or UNIX domain socket and proxies them to
a TLS-secured service.

**Supported platforms**: Ghostunnel is developed & tested on Linux, macOS,
and Windows but also runs on most other UNIX systems that are supported
by Go such as FreeBSD, OpenBSD, and NetBSD.

Key Features
============

**[Authentication & Authorization](#access-control-flags)**: Enforces mutual
TLS authentication by requiring valid client certificates. Supports
fine-grained access control checks on certificate fields (CN, OU, DNS/URI
SAN), and declarative authorization policies via [Open Policy
Agent](https://www.openpolicyagent.org) (OPA).

**[Certificate Hotswapping](#certificate-hotswapping)**: Reload certificates
without restarting via SIGHUP/SIGUSR1 or timed reload intervals, enabling use
of short-lived certificates.

**[Flexible Certificate Sources](#certificates)**: Load certificates and keys
from PEM/PKCS#12 files, ACME (Let's Encrypt), hardware security modules
(PKCS#11), macOS Keychain, Windows Certificate Store, or the SPIFFE Workload
API.

**[Secure by Default](#landlock-support)**: Listeners and targets are
restricted to localhost and UNIX sockets unless explicitly overridden with
`--unsafe-listen` or `--unsafe-target`, preventing accidental exposure. On
Linux, Landlock sandboxing is enabled by default to limit process privileges.

**[Metrics & Profiling](#metrics--profiling)**: Built-in status port with JSON
and Prometheus metrics endpoints, plus optional pprof profiling.

Ghostunnel also supports UNIX domain sockets, PROXY protocol v2,
systemd/launchd socket activation, Windows service management (SCM), and more.

Getting Started
===============

To get started and play around with Ghostunnel you will need X.509 client
and server certificates. If you already maintain a PKI, you can use your
existing certificates. Otherwise, you can use tools like
[mkcert](https://github.com/FiloSottile/mkcert) or
[cloudflare/cfssl](https://github.com/cloudflare/cfssl) to build one.

For quick testing and development, you can also generate throwaway test
certificates using the built-in generator:

# Generate test certificates and keys
    go tool mage test:keys

This will create a `test-keys` directory with all the necessary certificates and keys
for testing. **Note: These are test certificates only and should NOT be used in production.**

### Install

Ghostunnel is available through [GitHub releases][rel] and through [Docker Hub][hub].

Please note that the official release binaries are best-effort, and are usually
built directly via GitHub Actions on the latest available images. If you need
compatibility for specific OS versions we recommend building yourself.

Ghostunnel uses the [mage][mage] build system, a make/rake-like build tool using
Go. Mage is available as a Go tool dependency (no separate install needed). You
can build Ghostunnel with the commands shown below.

# Compile binary
    go tool mage go:build

# Build containers
    go tool mage docker:build

You can also run `go tool mage -l` to view all build targets and add `-v` to
mage commands to get more verbose output.

[rel]: https://github.com/ghostunnel/ghostunnel/releases
[hub]: https://hub.docker.com/r/ghostunnel/ghostunnel
[mage]: https://magefile.org

### Develop

Ghostunnel has an extensive suite of integration tests. Our integration test
suite requires Python 3.

To run tests:

# Option 1: run unit & integration tests locally
    go tool mage test:all

# Option 2: run unit & integration tests in a Docker container
    # This also runs PKCS#11 integration tests using SoftHSM in the container
    go tool mage test:docker

# Open coverage information in browser
    go tool cover -html coverage/all.profile

For more information on how to contribute, please see the [CONTRIBUTING](CONTRIBUTING.md) file.

Usage
=====

To see available commands and flags, run `ghostunnel --help`. You can get more
information about a command by adding `--help` to the command, like `ghostunnel
server --help` or `ghostunnel client --help`. Man pages are also available for
[Linux](docs/reference/manpage-linux.md) and [macOS](docs/reference/manpage-darwin.md).

By default, Ghostunnel runs in the foreground and logs to stdout. You can set
`--syslog` to log to syslog instead of stdout. If you want to run Ghostunnel
in the background, we recommend using a service manager.

### Certificates

Ghostunnel accepts certificates in multiple different file formats.

The `--keystore` flag can take a PKCS#12 keystore or a combined PEM file with the
certificate chain and private key as input (format is auto-detected). JCEKS/JKS
keystores are also supported for legacy use cases. The `--cert` / `--key` flags can
be used to load a certificate chain and key from separate PEM files (instead of a
combined one).

Ghostunnel also supports loading identities from the macOS keychain or the
SPIFFE Workload API and having private keys backed by PKCS#11 modules, see the
"Advanced Features" section below for more information.

### Server mode

This is an example for how to launch Ghostunnel in server mode, listening for
incoming TLS connections on `localhost:8443` and forwarding them to
`localhost:8080`. Note that while we use TCP sockets on `localhost` in this
example, both the listen and target flags can also accept paths to UNIX domain
sockets as their argument.

To set allowed clients, you must specify at least one of `--allow-all`,
`--allow-cn`, `--allow-ou`, `--allow-dns`, `--allow-uri`, or `--allow-policy`. All
checks are made against the certificate of the client. Multiple flags are
treated as a logical disjunction (OR), meaning clients can connect as long as
any of the flags matches. Alternatively, `--allow-spki-pin` authenticates clients by
out-of-band SPKI key pinning. See [ACCESS-FLAGS](docs/security/access-flags.md)
for more information. In this example, we assume that the CN of the client cert we want
to accept connections from is `client`.

**Note:** Before running the examples below, make sure you have generated the test
certificates by running `go tool mage test:keys` (see the [Getting Started](#getting-started)
section above).

Start a backend server:

nc -l localhost 8080

Start a Ghostunnel in server mode to proxy connections:

ghostunnel server \
        --listen localhost:8443 \
        --target localhost:8080 \
        --keystore test-keys/server-keystore.p12 \
        --cacert test-keys/cacert.pem \
        --allow-cn client

Verify that clients can connect with their client certificate:

openssl s_client \
        -connect localhost:8443 \
        -cert test-keys/client-combined.pem \
        -key test-keys/client-combined.pem \
        -CAfile test-keys/cacert.pem

Now we have a TLS proxy running for our backend service. We terminate TLS in
Ghostunnel and forward the connections to the insecure backend.

### Client mode

This is an example for how to launch Ghostunnel in client mode, listening on
`localhost:8080` and proxying requests to a TLS server on `localhost:8443`.

By default, Ghostunnel in client mode verifies targets based on the hostname.
Various access control flags exist to perform additional verification on top of
the regular hostname verification. See [ACCESS-FLAGS](docs/security/access-flags.md) for
more information.

Start a backend TLS server:

openssl s_server \
        -accept 8443 \
        -cert test-keys/server-combined.pem \
        -key test-keys/server-combined.pem \
        -CAfile test-keys/cacert.pem

Start a Ghostunnel with a client certificate to forward connections:

ghostunnel client \
        --listen localhost:8080 \
        --target localhost:8443 \
        --keystore test-keys/client-combined.pem \
        --cacert test-keys/cacert.pem

Verify that we can connect to `8080`:

nc -v localhost 8080

Now we have a TLS proxy running for our client. We take insecure local
connections, wrap them in TLS, and forward them to the secure backend.

### Full tunnel (client plus server)

We can combine the above two examples to get a full tunnel. Note that you can
start the tunnels in either order.

Start netcat on port `8001`:

nc -l localhost 8001

Start the Ghostunnel server:

ghostunnel server \
        --listen localhost:8002 \
        --target localhost:8001 \
        --keystore test-keys/server-combined.pem \
        --cacert test-keys/cacert.pem \
        --allow-cn client

Start the Ghostunnel client:

ghostunnel client \
        --listen localhost:8003 \
        --target localhost:8002 \
        --keystore test-keys/client-keystore.p12 \
        --cacert test-keys/cacert.pem

Verify that we can connect to `8003`:

nc -v localhost 8003

Now we have a full tunnel running. We take insecure client connections,
forward them to the server side of the tunnel via TLS, and finally terminate
and proxy the connection to the insecure backend.

Docker Images
=============

Docker images are published to [Docker Hub][hub] on each release. Three
variants are available:

| Image | Tag |
|-------|-----|
| Alpine | `ghostunnel/ghostunnel:latest-alpine`, `ghostunnel/ghostunnel:v1.x.x-alpine` |
| Debian | `ghostunnel/ghostunnel:latest-debian`, `ghostunnel/ghostunnel:v1.x.x-debian` |
| Distroless | `ghostunnel/ghostunnel:latest-distroless`, `ghostunnel/ghostunnel:v1.x.x-distroless` |

The `latest` tags always point to the most recent release. The bare
`ghostunnel/ghostunnel:latest` and `ghostunnel/ghostunnel:v1.x.x` tags are
aliases for the Alpine variant.

See [DOCKER](docs/deployment/docker.md) for details.

Advanced Features
=================

### Access Control Flags

Ghostunnel supports different types of access control flags in both client and
server modes to enforce authorization checks. Ghostunnel can check various
attributes of peer certificates directly, including a SPIFFE ID from a peer
using a [SPIFFE][spiffe] [X.509 SVIDs][svid]. In addition to this, Ghostunnel
also supports implementing authorization checks via [Open Policy Agent](https://www.openpolicyagent.org/)
(OPA) policies for maximum flexibility. Policies can be reloaded at runtime
much like certificates.

See [ACCESS-FLAGS](docs/security/access-flags.md) for details.

[spiffe]: https://spiffe.io/
[svid]: https://github.com/spiffe/spiffe/blob/main/standards/X509-SVID.md

### Logging Options

You can silence specific types of log messages using the `--quiet=...` flag,
such as `--quiet=conns` or `--quiet=handshake-errs`. You can pass this flag
repeatedly if you want to silence multiple different kinds of log messages.

Supported values are:
* `all`: silences **all** log messages
* `conns`: silences log messages about new and closed connections.
* `conn-errs`: silences log messages about connection errors encountered (post handshake).
* `handshake-errs`: silences log messages about failed handshakes.

In particular we recommend setting `--quiet=handshake-errs` if you are
running TCP health checks in Kubernetes on the listening port, and you
want to avoid seeing error messages from aborted connections on each health
check.

### Certificate Hotswapping

To trigger a reload, simply send `SIGHUP` (or `SIGUSR1`) to the process or set a time-based
reloading interval with the `--timed-reload` flag. This will cause Ghostunnel
to reload the certificate, private key, and CA bundle from the files on disk
(as well as OPA policy bundles, if configured). Once successful, the reloaded
certificate will be used for new connections going forward.

Additionally, Ghostunnel uses `SO_REUSEPORT` to bind the listening socket on
platforms where it is supported (Linux, macOS, FreeBSD, NetBSD, OpenBSD, and
DragonFly BSD). This means a new Ghostunnel can be started on the same host/port
before the old one is terminated, to minimize dropped connections (or avoid
them entirely depending on how the OS implements the `SO_REUSEPORT` feature).

Note that if you are using an HSM/PKCS#11 module, only the certificate will
be reloaded. It is assumed that the private key in the HSM remains the same.
This means the updated/reissued certificate must match the private key that
was loaded from the HSM previously, everything else works the same.

See [RELOADING](docs/certificates/reloading.md) for details.

### ACME Support

Ghostunnel in server mode supports the ACME protocol for automatically
obtaining and renewing a public certificate, assuming it's exposed publicly
on tcp/443 and there are valid public DNS FQDN records that resolve to the
listening interface IP.

See [ACME](docs/certificates/acme.md) for details.

### Metrics & Profiling

Ghostunnel has a notion of "status port", a TCP port (or UNIX socket) that can
be used to expose status and metrics information over HTTPS. The status port
feature can be controlled via the `--status` flag. Profiling endpoints on the
status port can be enabled with `--enable-pprof`.

See [METRICS](docs/networking/metrics.md) for details.

### HSM/PKCS#11 support

Ghostunnel has support for loading private keys from PKCS#11 modules, which
should work with any hardware security module that exposes a PKCS#11 interface,
including YubiKeys (via the YKCS11 module).

See [HSM-PKCS11](docs/certificates/hsm-pkcs11.md) for details, including a step-by-step
guide for using Ghostunnel with a YubiKey.

### Windows/macOS Keychain Support

Ghostunnel supports loading certificates from the Windows and macOS keychains.
This is useful if you have identities stored in your local keychain that you
want to use with Ghostunnel, e.g. if you want your private key(s) to be backed
by the Secure Enclave on newer Touch ID MacBooks.

See [KEYCHAIN](docs/certificates/keychain.md) for details.

### SPIFFE Workload API

Ghostunnel has support for maintaining up-to-date, frequently rotated
identities and trusted CA certificates from the SPIFFE Workload API.

See [SPIFFE-WORKLOAD-API](docs/certificates/spiffe-workload-api.md) for details.

### Socket Activation

Ghostunnel supports socket activation via both systemd (on Linux) and launchd
(on macOS). Socket activation is supported for the `--listen` and `--status`
flags, and can be used by passing an address of the form `systemd:<name>` or
`launchd:<name>`, where `<name>` should be the name of the socket as defined in
your systemd/launchd configuration.

See [SYSTEMD](docs/deployment/systemd.md) and [LAUNCHD](docs/deployment/launchd.md)
for examples.

### PROXY Protocol Support

Ghostunnel in server mode supports signaling of transport connection information
to the backend using the [PROXY protocol](https://www.haproxy.org/download/3.1/doc/proxy-protocol.txt)
(v2), just pass the `--proxy-protocol` flag on startup. Use `--proxy-protocol-mode`
to also include TLS metadata and/or client certificate details. Note that the
backend must support the PROXY protocol and must be configured to use it when
setting this option.

See [PROXY-PROTOCOL](docs/networking/proxy-protocol.md) for details on modes and TLV extensions.

### Landlock Support

Ghostunnel can use [Landlock](https://landlock.io) to limit process privileges
on Linux. Landlock is enabled by default in best-effort mode and can be
disabled using `--disable-landlock` if necessary (not recommended). When
enabled, Ghostunnel will limit its access to files and sockets based on the
flags passed at startup. Note that Landlock does not work with PKCS#11 modules
and is disabled if PKCS#11 is used (as PKCS#11 modules are opaque to us we
can't craft workable Landlock rules for them).

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