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 on x86-64 platforms, although it should run on any UNIX system that exposes SO_REUSEPORT, including Darwin (macOS), FreeBSD, OpenBSD and NetBSD. Ghostunnel also supports running on Windows, though with a reduced feature set. We recommend running on x86-64 to benefit from constant-time implementations of cryptographic algorithms that are not available on other platforms.
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. To trigger a reload, simply send
SIGUSR1 to the process (or set a time-based reloading interval). This will
cause ghostunnel to reload the certificate/key. Once successful, the reloaded
certificate will be used for new connections going forward.
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 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.
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).
Binaries can be built from source as follows (cross-compile requires Docker and xgo):
# Compile for local architecture make ghostunnel # Cross-compile release binaries make -f Makefile.dist dist
Note that ghostunnel requires Go 1.10 or later to build, and CGO is required for PKCS#11 support. See also CROSS-COMPILE.md for instructions on how to cross-compile a custom build with CGO enabled.
To run tests:
# Option 1: run unit & integration tests locally make test # Option 2: run unit & integration tests in a Docker container GO_VERSION=1.10 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 two formats of keystores, either a PKCS#12 keystore or a
combined PEM file that contains both the certificate chain and private key.
Both formats can be used with the
--keystore flag, ghostunnel will
automatically detect the file format and handle it appropriately. If you are
using a PKCS#12 keystore protected by a password, you will also need to pass
--storepass flag. If you want to use ghostunnel with a PKCS#11 module,
see the section on PKCS#11 below.
This is an example for how to launch ghostunnel in server mode, listening for
incoming TLS connections on
localhost:8443 and forwarding them to
To set allowed clients, you must specify at least one of
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
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.
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
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.
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
The X.509 certificate on the status port will be the same as the certificate used for proxying (either the client or server certificate). This means you can use the status port to inspect/verify the certificate that is being used, which can be useful for orchestration systems.
Example invocation with status port enabled:
ghostunnel client \ --listen localhost:8080 \ --target localhost:8443 \ --keystore test-keys/client-keystore.p12 \ --cacert test-keys/cacert.pem \ --status localhost:6060
Note that we set the status port to "localhost:6060". Ghostunnel will start an internal HTTPS server and listen for connections on the given host/port. You can also specify a UNIX socket instead of a TCP port.
How to check status and read connection metrics:
# Status information (produces JSON output) curl --cacert test-keys/cacert.pem https://localhost:6060/_status # Metrics information (produces JSON output) curl --cacert test-keys/cacert.pem https://localhost:6060/_metrics
How to use profiling endpoints, if
--enable-pprof is set:
# Human-readable goroutine dump curl --cacert test-keys/cacert.pem 'https://localhost:6060/debug/pprof/goroutine?debug=1' # Analyze execution trace using pprof tool go tool pprof -seconds 5 https+insecure://localhost:6060/debug/pprof/profile
go tool pprof does not support setting CA certificates at the
moment, hence the use of the
https+insecure scheme in the last example. You
can use the standard
https scheme if your ghostunnel is using a certificate
trusted by your system (c.f. golang/go#20939). For more
information on profiling via pprof, see the
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. An easy way to test the PKCS#11 interface for development purposes is with SoftHSM. Note that CGO is required in order for PKCS#11 support to work (see CROSS-COMPILE.md for instructions to cross-compile with CGO enabled).
To import the server test key into SoftHSM, for example:
softhsm2-util --init-token \ --slot 0 \ --label ghostunnel-server \ --so-pin 1234 \ --pin 1234 softhsm2-util --id 01 \ --token ghostunnel-server \ --label ghostunnel-server \ --import test-keys/server-pkcs8.pem \ --so-pin 1234 \ --pin 1234
To launch ghostunnel with the SoftHSM-backed PKCS11 key (on macOS):
ghostunnel server \ --keystore test-keys/server-cert.pem \ --pkcs11-module /usr/local/Cellar/softhsm/2.4.0/lib/softhsm/libsofthsm2.so \ --pkcs11-token-label ghostunnel-server \ --pkcs11-pin 1234 \ --listen localhost:8443 \ --target localhost:8080 \ --cacert test-keys/cacert.pem \ --allow-cn client
--pkcs11-pin flags can be
used to select the private key to be used the PKCS11 module. It's also possible
to use environment variables to set PKCS11 options instead of flags (via
PKCS11_PIN), useful if you don't
want to show the PIN on the command line.
--keystore needs to point to the certificate chain that corresponds
to the private key in the PKCS#11 module, with the leaf certificate being the
first certificate in the chain. Ghostunnel doesn't have the ability to read
the certificate chain directly from the module at this point in time.
If you need to inspect the state of a PKCS11 module/token, we recommend the
pkcs11-tool utility from OpenSC. For example, it can be used
to list slots or read certificate(s) from a module:
# List slots on a module pkcs11-tool --module $MODULE -L # Show certificates (if any) available pkcs11-tool --module $MODULE -O -y cert # Read certificate chain given a label pkcs11-tool --module $MODULE --label $LABEL --read-object -y cert
macOS keychain support (experimental)
If ghostunnel has been compiled with build tag
certstore (off by default,
requires macOS 10.12+) a new flag will be available that allows for 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
Common Name (CN) of the subject.
For example, if you have an identity with CN 'example' in your login keychain:
ghostunnel client \ --keychain-identity example \ --listen localhost:8080 \ --target example.com:443 \ --cacert test-keys/cacert.pem
The command above launches a ghostunnel instance that uses the certificate and private key with Common Name 'example' from your login keychain to proxy plaintext connections from localhost:8080 to example.com:443.