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Opal
====
Opal is a Gemini server written in Rust. It is meant to serve dynamic content
through CGI and does not serve static files. In a way, it is a companion project
to the [Agate][agate] Gemini server which only serves static files. The
project's goals are:
- Focus on a small set of features (around CGI) but do them correctly.
- Be nice with old/stupid hardware (TLS 1.2 is OK, be efficient, etc).
- Don't add features unless someone actively wants them in.
- Try to keep resources (binary size, memory, etc) under tight control.
Opal uses the `openssl` Rust bindings, which work with OpenSSL and LibreSSL, so
it should work properly on those platforms. I currently only support Linux
systems but if there is interest in other platforms let's do this together!
Opal is licensed as GPLv3.
[agate]: https://github.com/mbrubeck/agate/
Usage
-----
Use `opal -h` to get a list of options. There is no config file, every setting
can be configured from the command line.
- `-a, --address <address>`: specify the address to listen to.
- `-c, --cert <cert>`: server certificate path.
- `-k, --key <key>`: server private key path.
- `-r, --root-path <root_path>`: path to CGI scripts root.
- `-e, --env <key=value>`: additional environment variables for CGI scripts;
this option can be used multiple times.
CGI support
-----------
Opal tries to implement [RFC 3875][rfc3875] (CGI 1.1) and provides all the
required environment variables to processes. It also add a bunch of Gemini
specific variables, like a lot of other servers (Gemserv, Gmid, Gmnisrv, …). The
environment for the subprocess is cleaned and should only contain those
variables.
[rfc3875]: https://datatracker.ietf.org/doc/html/rfc3875
| Presence | Variable | Description |
|-------------|------------------------|------------------------------------------------------|
| always | GATEWAY_INTERFACE | "CGI/1.1" |
| always | REMOTE_ADDR | Peer IP address and port |
| always | REMOTE_HOST | Same as REMOTE_ADDR |
| always | REQUEST_METHOD | Empty string for compatibility |
| always | SCRIPT_NAME | Script name part of the URL path |
| always | SERVER_NAME | Hostname used for SNI |
| always | SERVER_PORT | Port where the request has been received |
| always | SERVER_PROTOCOL | "GEMINI" |
| always | SERVER_SOFTWARE | "opal/version", e.g. "opal/0.1.0" |
| always | GEMINI_DOCUMENT_ROOT | CGI root |
| always | GEMINI_SCRIPT_FILENAME | CGI script that matched the URL path |
| always | GEMINI_URL | Full URL, normalized |
| always | GEMINI_URL_PATH | URL path, normalized |
| always | TLS_VERSION | TLS version, e.g. "TLSv1.3" |
| always | TLS_CIPHER | TLS cipher suite, e.g. "TLS_AES_256_GCM_SHA384" |
| optional | PATH_INFO | Path passed to the CGI process after the script name |
| optional | QUERY_STRING | Query string if provided, still URL-encoded |
| client cert | AUTH_TYPE | "CERTIFICATE" if one is provided |
| client cert | REMOTE_USER | Subject common name (empty if unavailable) |
| client cert | TLS_CLIENT_ISSUER | Issuer common name (empty if unavailable) |
| client cert | TLS_CLIENT_HASH | Digest of the DER reprensetation of the cert |
| client cert | TLS_CLIENT_NOT_AFTER | Validity end date, RFC 3339 format |
| client cert | TLS_CLIENT_NOT_BEFORE | Validity start date, RFC 3339 format |
Opal does not provide `CONTENT_LENGTH`, `CONTENT_TYPE`, `REMOTE_IDENT` because
they do not make much sense in Gemini. `PATH_TRANSLATED` is also not implemented
by pure laziness.
The `TLS_CLIENT_HASH` is a string that starts with "SHA256:" followed by the
SHA256 digest of the DER representation of the client certificate, as an
uppercase hex-string.
It can be a bit confusing which variable represent what data, especially those
related to the URL and the path. Take the following request as example:
`gemini://localhost/env/sub1/sub2?search=élément`. Suppose our CGI root, in
`/cgi`, contains the executable script named `env`. The variables will be:
GEMINI_DOCUMENT_ROOT=/cgi
GEMINI_SCRIPT_FILENAME=/cgi/env
GEMINI_URL=gemini://localhost/env/sub1/sub2?search=%C3%A9l%C3%A9ment
GEMINI_URL_PATH=/env/sub1/sub2
SCRIPT_NAME=/env
PATH_INFO=/sub1/sub2
QUERY_STRING=search=%C3%A9l%C3%A9ment
Roadmap
-------
Things to consider:
- Multiple listening addresses, at least so we can easily listen on both IPv4
and IPv6.
- Support SCGI; a bit more complex but should save resources on smol hardware.
Things that probably won't be considered:
- Serve static files; so many other servers to that correctly already!
- Any kind of security mechanism that is not properly motivated.
- FastCGI; come on…