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Secure Port Forwarder

This is a remote port forwarder based on SSH protocol (ssh -R). It only supports remote port forwarding, but not the rest of SSH features (no session/channel is supported), so it's safer than running a full featured SSH server on a public network.

With Secure Port Forwarder running on a public network, a server behind a firewall can be exposed to the public network using:

ssh -N -R https/www.example.com:localhost:8080 user@spf.example.com

Here assuming Secure Port Forwarder is running on spf.example.com on regular SSH port 22, and the server behind the firewall is running on localhost:8080. The ssh command asks Secure Port Forwarder to forward www.example.com:80 and rely it to localhost:8080.

However Secure Port Forwarder doesn't exposing the specified DNS and port to the public network. Instead, it only opens a random TCP port on localhost and forwards connections to the SSH client. User must provide an endpoint setup script for setting up www.example.com:80 on some proxy server (e.g. traefik).

Usage

Launch spfd without arguments to use default configurations:

  • -addr=:2022: listen on :2022 as SSH server address;
  • Use host keys from /etc/ssh;
  • Use ~/.ssh/authorized_keys for authorized keys;
  • -bind-addr=localhost: open random TCP port as requested on localhost;

In addition to that, specifying -setup-cmd=PROGRAM to use PROGRAM for setting up a DNS based reverse proxy.

For example, when using traefik, a shell script can be used to configure it for forwarding the request on a specific DNS to a localhost port. The PROGRAM is invoked as:

PROGRAM open|close tcp|sock ENDPOINT local-host:local-port
  • open is used to ask the script to start forwarding from ENDPOINT to local-host:local-port;
  • close is used to ask the script to stop forwarding from ENDPOINT.

ENDPOINT is defined for

  • tcp: public-host:public-port
  • sock: a unix socket path, it's recommended to be scheme/DNS, e.g. https/example.com.

According to -bind-address=A.B.C.D when launching spfd, and the SSH client command line, e.g.

ssh -N -R https/www.example.com:localhost:8080 user@spf.example.com

It will call the setup program as

PROGRAM open sock https/www.example.com A.B.C.D:port

The Client

spfc is a client to work with server-side spfd as an HTTP reverse proxy. It watches dynamic backend states and exposes/unexposes endpoints accordingly. The currently supported backend states providers are:

  • files: Watches a list of directories (not sub-directories) and loads backend configurations from text files;
  • k8s: Run spfc as a Kubernetes controller which watches Service resources with annotation spf.evo-cloud/endpoint and exposes the annotation value as the endpoint on the server side.

Files Provider

The format of a file containing backend states is:

ID ENDPOINT BACKEND-URL

E.g.

a https/a.example.com http://localhost:8080

Empty lines and lines starting with # are ignored. All the lines from all discovered files are merged to create the final list of backend states. Use the following command line flags to enable this provider:

  • --files-dirs: a semi-colon separated list of directory paths to watch for files and this must be specified to enable this provider;
  • --files-glob: a pattern to filter file names. The default value is * (matching all files).

Kubernetes Provider

Simply put --k8s on the command line to enable this provider. It watches Service resource with annotation spf.evo-cloud/endpoint. The value of the annotation is used as ENDPOINT on the server side. The format should be SCHEME/DNS. SCHEME can be https or http.

The first Port in the Service will be used as backend server port. If no Port is present, default HTTP port 80 will be used.