172 lines
7 KiB
Markdown
172 lines
7 KiB
Markdown
Opal
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====
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Opal is a Gemini server written in Rust. It is meant to serve dynamic content
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through CGI and does not serve static files. In a way, it is a companion project
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to the [Agate][agate] Gemini server which only serves static files. The
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project's goals are:
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- Focus on a small set of features (around CGI) but do them correctly.
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- Be nice with old/stupid hardware (TLS 1.2 is OK, be efficient, etc).
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- Don't add features (see the roadmap at the end of this file).
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- Try to keep resources (binary size, memory, etc) under tight control.
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Opal uses the `openssl` Rust bindings, which work with OpenSSL and LibreSSL, so
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it should work properly on those platforms. I currently only support Linux
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systems but if there is interest in other platforms let's do this together!
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Opal is licensed as GPLv3.
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[agate]: https://github.com/mbrubeck/agate/
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Installation
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------------
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### Pre-compiled releases
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Binary releases for 64-bit Linux systems are available on [my Gitea][gitea-rel]
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and on [Github][gh-rel].
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[gitea-rel]: https://git.dece.space/Dece/Opal/releases
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[gh-rel]: https://github.com/Dece/Opal/releases
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### Compiling from sources
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Compiling Opal requires Cargo installed with the stable Rust toolchain.
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Usage
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-----
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Use `opal -h` to get a list of options. There is no config file, every setting
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can be configured from the command line.
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- `-a, --address <address>`: specify the address(es) to listen to.
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- `-c, --cert <cert>`: server certificate path.
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- `-k, --key <key>`: server private key path.
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- `-r, --root-path <root_path>`: path to CGI scripts root.
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- `-e, --env <key=value>`: additional environment variables for CGI scripts;
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this option can be used multiple times.
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You can specify multiple addresses to listen to by using several `-a` options.
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Note that if you just want to listen to both IPv4 and IPv6 on any interface,
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listening only on `[::]:1965` should suffice for systems with dual-stack
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enabled (default on many Linux systems, maybe not BSD).
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### Systemd
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I personally run Opal as a Systemd service. Here is an example unit file:
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``` ini
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[Unit]
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Description=Opal Gemini server
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[Service]
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WorkingDirectory=/home/gemini/opal
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User=gemini
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Group=gemini
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ExecStart=/usr/local/bin/opal -a "[::]:1966" -c certs/cert.pem -k certs/key.pem -r cgi -e STORAGE_ROOT=storage
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Restart=always
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RestartSec=1
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SyslogIdentifier=opal
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# Security options:
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NoNewPrivileges=yes
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ProtectSystem=full
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ProtectHome=tmpfs
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BindReadOnlyPaths="/home/gemini/opal"
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BindPaths="/home/gemini/opal/storage"
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[Install]
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WantedBy=multi-user.target
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```
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- Opal has been installed in `/usr/local/bin`
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- The directory `/home/gemini/opal` contains the directories `certs`, `cgi` and
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`storage`, for certificates, the CGI scripts and a storage path.
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- The `/home` directory is not readable, except for `/home/gemini/opal` which is
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read-only, except for the `storage` directory which is writeable.
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This is just an example, please do not mindlessly copy and paste it without
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understanding what the options stand for. It is also possible to use a chrooted
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environment or the Systemd equivalent option RootDirectory. Your choice!
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CGI support
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-----------
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Opal tries to implement [RFC 3875][rfc3875] (CGI 1.1) and provides all the
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required environment variables to processes. It also add a bunch of Gemini
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specific variables, like a lot of other servers (Gemserv, Gmid, Gmnisrv, …). The
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environment for the subprocess is cleaned and should only contain those
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variables.
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[rfc3875]: https://datatracker.ietf.org/doc/html/rfc3875
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| Presence | Variable | Description |
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|-------------|------------------------|------------------------------------------------------|
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| always | GATEWAY_INTERFACE | "CGI/1.1" |
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| always | REMOTE_ADDR | Peer IP address and port |
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| always | REMOTE_HOST | Same as REMOTE_ADDR |
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| always | REQUEST_METHOD | Empty string for compatibility |
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| always | SCRIPT_NAME | Script name part of the URL path |
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| always | SERVER_NAME | Hostname used for SNI |
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| always | SERVER_PORT | Port where the request has been received |
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| always | SERVER_PROTOCOL | "GEMINI" |
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| always | SERVER_SOFTWARE | "opal/version", e.g. "opal/0.1.0" |
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| always | GEMINI_DOCUMENT_ROOT | CGI root |
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| always | GEMINI_SCRIPT_FILENAME | CGI script that matched the URL path |
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| always | GEMINI_URL | Full URL, normalized |
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| always | GEMINI_URL_PATH | URL path, normalized |
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| always | TLS_VERSION | TLS version, e.g. "TLSv1.3" |
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| always | TLS_CIPHER | TLS cipher suite, e.g. "TLS_AES_256_GCM_SHA384" |
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| optional | PATH_INFO | Path passed to the CGI process after the script name |
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| optional | QUERY_STRING | Query string if provided, still URL-encoded |
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| client cert | AUTH_TYPE | "CERTIFICATE" if one is provided |
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| client cert | REMOTE_USER | Subject common name (empty if unavailable) |
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| client cert | TLS_CLIENT_ISSUER | Issuer common name (empty if unavailable) |
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| client cert | TLS_CLIENT_HASH | Digest of the DER reprensetation of the cert |
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| client cert | TLS_CLIENT_NOT_AFTER | Validity end date, RFC 3339 format |
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| client cert | TLS_CLIENT_NOT_BEFORE | Validity start date, RFC 3339 format |
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Opal does not provide `CONTENT_LENGTH`, `CONTENT_TYPE`, `REMOTE_IDENT` because
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they do not make much sense in Gemini. `PATH_TRANSLATED` is also not implemented
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by pure laziness.
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The `TLS_CLIENT_HASH` is a string that starts with the 7 bytes `SHA256:`
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followed by the SHA256 digest of the DER representation of the client
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certificate, as an uppercase hex-string.
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It can be a bit confusing which variable represent what data, especially those
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related to the URL and the path. Take the following request as example:
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`gemini://localhost/env/sub1/sub2?search=élément`. Suppose our CGI root, in
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`/cgi`, contains the executable script named `env`. The variables will be:
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```
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GEMINI_DOCUMENT_ROOT=/cgi
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GEMINI_SCRIPT_FILENAME=/cgi/env
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GEMINI_URL=gemini://localhost/env/sub1/sub2?search=%C3%A9l%C3%A9ment
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GEMINI_URL_PATH=/env/sub1/sub2
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SCRIPT_NAME=/env
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PATH_INFO=/sub1/sub2
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QUERY_STRING=search=%C3%A9l%C3%A9ment
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```
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Roadmap
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-------
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Things that might end up in Opal one day:
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- Support SCGI; a bit more complex but should save resources on smol hardware.
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Things that probably won't be considered:
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- Serve static files; so many other servers do that correctly already!
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- Any kind of security mechanism that is not properly motivated.
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- FastCGI; un-smol…
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