A simple SSL/TLS proxy with mutual authentication for securing non-TLS services
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Latest commit 0427a99 Jan 4, 2017 @csstaub csstaub committed on GitHub Merge pull request #103 from square/cs/deps
Update dependencies/simplify a bit of code



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Ghostunnel is a simple SSL/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 domain/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. In other words, ghostunnel is a replacement for stunnel.

Supported platforms: Ghostunnel is developed primarily for Linux on x86-64 platforms, although it should run on any UNIX system that exposes SO_REUSEPORT, including Darwin (macOS), FreeBSD, OpenBSD and NetBSD. We recommend running on x86-64 only, as Go (as of Go 1.7) doesn't have constant-time elliptic curve implementations for any other architectures.

See ghostunnel --help, ghostunnel server --help and ghostunnel client --help.


Authentication/access control: Ghostunnel enforces mutual authentication by always requiring a valid client certificate. We also support access control via checks on the subject (or subject alternative names) of a client certificate. This is useful for restricting access to services that don't have native access control.

Certificate hotswapping: Ghostunnel can reload certificates at runtime without dropping existing connections. To trigger a reload, simply send SIGUSR1 to the process. This will cause ghostunnel to reload the keystore files. Once successful, the reloaded certificate will be used for new connections going forward.

Automatic reloading: Ghostunnel can be configured to automatically reload certificates. You can specify an interval with the --timed-reload flag. If the timed reload flag is enabled, ghostunnel will reload the files periodically and check for changes. If a change is detected, it will attempt to reload the listener with the new certificates/private key.

Emphasis on security: We have put some thought into making ghostunnel secure by default. In server mode, the target backend must live on localhost or be a UNIX socket (unless --unsafe-target is specified). In a similar way, in client mode the listening socket must live on localhost or be a UNIX socket (unless --unsafe-listen is specified). Ghostunnel negotiates TLSv1.2 and uses safe ciphers.

Getting started

To get started and play around with the implementation, you will need to generate some test certificates. If you want to bootstrap a full PKI, one good way to get started is to use a package like square/certstrap. If you only need some test certificates for playing around with the tunnel, you can find some pre-generated ones in the test-keys directory (alongside instructions on how to generate new ones with OpenSSL).

Note that by default ghostunnel logs to stderr and runs in the foreground. You can set --syslog to log to syslog. For daemonization, we recommend using a utility such as daemonize. For an example on how to use ghostunnel in a Docker container, see the docker subdirectory.


To install ghostunnel, simply use:

go get -u github.com/square/ghostunnel

Note that ghostunnel requires Go 1.7 or later to build.

If you want to pull a specific version, check the releases tab in Github.


Ghostunnel has an extensive suite of integration tests. Our integration test suite requires Python 3.5 (or later) and gocovmerge to run. We use gvt for managing vendored dependencies.

To run tests:

# Run unit & integration tests
make test

# Open coverage information in browser
go tool cover -html coverage-merged.out

For more information on how to contribute, please see the CONTRIBUTING file.

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.

To set allowed clients, you must specify at least one of --allow-all, --allow-cn, --allow-ou, --allow-dns-san, or --allow-ip-san. It's possible to use these together or to specify them repeatedly to allow multiple clients. In this example, we assume that the CN of the client cert we want to accept connections from is client.

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.p12 \
    --cacert test-keys/root.crt \
    --allow-cn client

Verify that clients can connect with their client certificate:

openssl s_client \
    -connect localhost:8443 \
    -cert test-keys/client.crt \
    -key test-keys/client.key \
    -CAfile test-keys/root.crt

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.

Start a backend TLS server:

openssl s_server \
    -accept 8443 \
    -cert test-keys/server.crt \
    -key test-keys/server.key \
    -CAfile test-keys/root.crt

Start a ghostunnel with a client certificate to forward connections:

ghostunnel client \
    --listen localhost:8080 \
    --target localhost:8443 \
    --keystore test-keys/client.p12 \
    --cacert test-keys/root.crt

Verify that we can connect to 8080:

nc -v localhost 8080

Now we have a TLS proxy running for our client. We take the insecure local connection, 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 ghostunnels 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.p12 \
    --cacert test-keys/root.crt \
    --allow-cn client

Start the ghostunnel client:

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

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.