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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 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 and Darwin (macOS), although it should run on any UNIX system that exposes SO_REUSEPORT, including FreeBSD, OpenBSD and NetBSD. Ghostunnel also supports running on Windows, though with a reduced feature set.

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


Access control: Ghostunnel enforces mutual authentication by requiring a valid client certificate for all connections. 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. Certificate reloading can be triggered with a signal or on a regular time interval. This allows short-lived certificates to be used with ghostunnel, new certificates will get picked up transparently. And on platforms with SO_REUSEPORT support, restarts can be done with minimal downtime.

ACME Support: In server mode, Ghostunnel can optionally obtain and automatically renew a public TLS certificate via the ACME protocol.

Monitoring and metrics: Ghostunnel has a built-in status feature that can be used to collect metrics and monitor a running instance. Metrics can be fed into Graphite (or other systems) to see number of open connections, rate of new connections, connection lifetimes, timeouts, and other info.

Emphasis on security: We have put some thought into making ghostunnel secure by default and prevent accidental misconfiguration. For example, we always negotiate TLS v1.2 (or greater) and only use safe cipher suites. Ghostunnel also supports PKCS#11 which makes it possible to use Hardware Security Modules (HSMs) to protect private keys, and Square has a bug bounty that pays rewards for security findings.

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).


Ghostunnel is available through GitHub releases and through Docker Hub.

# Compile for local architecture
make ghostunnel

Note that ghostunnel requires Go 1.18 or later to build, and CGO is required for PKCS#11 support.


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

To run tests:

# Option 1: run unit & integration tests locally
make test

# Option 2: run unit & integration tests in a Docker container
GO_VERSION=1.18 make docker-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.


By default, ghostunnel runs in the foreground and logs to stderr. You can set --syslog to log to syslog instead of stderr. If you want to run ghostunnel in the background, we recommend using a service manager such as systemd or runit, or use a wrapper such as daemonize or dumb-init.


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). 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 or --allow-uri. 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 (see ACCESS-FLAGS for more information). 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-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.

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 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-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.

Advanced Features

Access Control Flags

Ghostunnel supports different types of access control flags in both client and server modes. All checks are made against the certificate of the client or server. Multiple flags are treated as a logical disjunction (OR), meaning clients can connect as long as any of the flags matches. Ghostunnel is compatible with SPIFFE X.509 SVIDs.

See ACCESS-FLAGS for details.

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 SIGUSR1 to the process or set a time-based reloading interval with the --timed-reload flag. This will cause ghostunnel to reload the certificate and private key from the files on disk. 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, Apple macOS, FreeBSD, NetBSD, OpenBSD and DragonflyBSD). 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 much match the private key that was loaded from the HSM previously, everything else works the same.

ACME Support

To have Ghostunnel automatically obtain and renew a public TLS certificate via ACME, use the --auto-acme-cert= flag (e.g. - You must also specify an email address so you will get notices from the CA about potentially important certificate lifecycle events. Specify the email address with the --auto-acme-email= flag. To use this feature, you must also specify the --auto-acme-agree-to-tos flag to indicate your explicit agreement with the CA's Terms of Service.

Ghostunnel defaults to using Let's Encrypt, but you can specify a different ACME CA URL using the --auto-acme-ca= flag. If you wish to test Ghostunnel's ACME features against a non-production ACME CA, use the --auto-acme-testca= flag. If --auto-acme-testca is specified, all ACME interaction will be with the specified test CA URL and the --auto-acme-ca= flag will be ignored.

ACME is only supported in server mode. Additionally, Ghostunnel must either be listening to a public interface on tcp/443, or somehow have a public tcp/443 listening interface forwarded to it (e.g. - systemd socket, iptables, etc.). Public DNS records must exist for a valid public DNS FQDN that resolves to the public listening interface IP.

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 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.

See HSM-PKCS11 for details.


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

See SPIFFE-WORKLOAD-API for details.

Socket Activation (experimental)

Ghostunnel supports socket activation via both systemd (on Linux) and launchd (on macOS). Socket activation is support 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 SOCKET-ACTIVATION for examples.

PROXY Protocol (experimental)

Ghostunnel in server mode supports signalling of transport connection information to the backend using the PROXY protocol (v2), just pass the --proxy-protocol flag on startup. Note that the backend must also support the PROXY protocol and must be configured to use it when setting this option.

MacOS Keychain Support (experimental)

Ghostunnel supports loading certificates from the macOS keychain. 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 SEP on newer Touch ID MacBooks. Certificates from the keychain can be loaded by selecting them based on the serial number, Common Name (CN) of the subject, or Common Name (CN) of the issuer.

For example, to load an identity based on subject name login keychain:

ghostunnel client \
    --keychain-identity <common-name-or-serial> \
    --listen unix:/path/to/unix/socket \
    --target \
    --cacert test-keys/cacert.pem

Or, if you'd like to load an identity by filtering on issuer name:

ghostunnel client \
    --keychain-issuer <issuer-common-name> \
    --listen unix:/path/to/unix/socket \
    --target \
    --cacert test-keys/cacert.pem

Both commands above launch a ghostunnel instance that uses the certificate and private key for the selected keychain identity to proxy plaintext connections from a given UNIX socket to Note that combining both the identity and issuer flags in one command will cause ghostunnel to select certificates where both attributes match (matching with AND on both subject name/issuer).