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
including FreeBSD, OpenBSD and NetBSD. Ghostunnel also supports running on
Windows, though with a reduced feature set.
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 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
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.
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).
# Compile for local architecture make ghostunnel
Note that ghostunnel requires Go 1.19 or later to build, and CGO is required for PKCS#11 support.
To run tests:
# Option 1: run unit & integration tests locally make test # Option 2: run unit & integration tests in a Docker container GO_VERSION=1.19 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.
--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
--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.
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-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
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.
This is an example for how to launch ghostunnel in client mode, listening on
localhost:8080 and proxying requests to a TLS server on
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
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.
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
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
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.
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.
Ghostunnel also has experimental support for Open Policy Agent (OPA) policies.
See ACCESS-FLAGS for details.
You can silence specific types of log messages using the
--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
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
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
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.
To have Ghostunnel automatically obtain and renew a public TLS certificate via ACME,
--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
--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 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.
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
See METRICS for details.
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.
Ghostunnel supports socket activation via both systemd (on Linux) and launchd
(on macOS). Socket activation is support for the
flags, and can be used by passing an address of the form
<name> should be the name of the socket as defined in
your systemd/launchd configuration.
See SOCKET-ACTIVATION for examples.
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
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 example.com:443 \ --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 example.com:443 \ --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 example.com:443. 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).