Command tlstun implements basic VPN over TLS (both client and server).

Client part is expected to be run locally and used by local services as a
socks5 proxy. Client establishes TLS session to the server running on some
remote host that performs outgoing requests on behalf of requests made to
the client.

Client and server authenticate each other with certificates which can be
created with openssl or https://github.com/artyom/gencert


Usage example

Generate server and client side certificates, they can be signed by the same
CA and saved using PEM encoding into a single file. Certificate keys should
also be saved as a separate PEM-encoded files. With gencert tool from
https://github.com/artyom/gencert this can be done as:

    gencert -hosts my.domain.tld

This produces five files in the current directory: client certificate + key
pair, another pair for the server and a certificate authority certificate.
Note that my.domain.tld should point to the host you plan running server
part of tlstun.

Now configure tlstun to run on the server that could be reached at
my.domain.tld like this:

    tlstun server -addr=:9000 -ca=ca.pem \
    	-cert=server-cert.pem -key=server-key.pem

The client part is expected to be running locally (on a laptop/workstation,
etc.):

    tlstun client -addr=127.0.0.1:1080 -ca=ca.pem \
    	-cert=client-cert.pem -key=client-key.pem \
    	my.domain.tld:9000

Client is now listening on localhost port 1080 and local software can be
configured to use this endpoint as a socks5 proxy.

Client may be configured to work with multiple servers:

    tlstun client -addr=127.0.0.1:1080 -ca=ca.pem \
    	-cert=client-cert.pem -key=client-key.pem \
    	my.domain.tld:9000 other.domain.tld:9000

By default client will periodically evaluate all endpoints and prefer one
which replies first. This check may be disabled with -no-check flag.

Instead of listing endpoints directly, client may discover them from SRV DNS
records. If -discover=domain.tld flag is set, client ignores any endpoints
given on the command line and instead looks up SRV record
_tlstun._tcp.domain.tld, which must follow standard format to specify
host/port pairs.


HTTPS compatible mode

Server may be configured to automatically issue certificate from ACME
provider (currently is's Let's Encrypt). For this to work, server must be
exposed over port 443 and it also doubles as an https endpoint. By default
it replies with 404 Not Found, but can optionally be configured to serve
static files from file system directory:

    tlstun server -addr=:443 -ca=ca.pem -acme \
    	-acme-domain=domain.tld [email protected] \
    	-acme-cache=/var/cache/acme \
    	-httproot=/var/www/domain.tld

Server may also be run with -dns-tls flag, which enables resolving via
DNS-over-TLS providers (currently Cloudflare and Quad9).