gmid

gmid is a Gemini server written with security in mind. I initially wrote it to serve static files, but it has grown into a featureful server that can be used from either the command line to serve local directories

gmid docs  # serve the directory docs over gemini

or as a traditional daemon

gmid -c /etc/gmid.conf

Features

(random order)

• reconfiguration: reload the running configuration without interruption
• sandboxed by default on OpenBSD, Linux and FreeBSD
• IRI support (RFC3987)
• punycode support
• dual stack (IPv4 and IPv6)
• automatic certificate generation for config-less mode
• CGI scripts
• (very) low memory footprint
• small codebase, easily hackable
• virtual hosts
• per-location rules
• optional directory listings
• configurable mime types
• chroot support

Drawbacks

• not suited for very busy hosts. If you receive an high number of connection per-second you'd probably want to run multiple gmid instances behind relayd/haproxy or a different server.

Internationalisation (IRIs, UNICODE, punycode, all that stuff)

Even thought the current Gemini specification doesn't mention anything in this regard, I do think these are important things and so I tried to implement them in the most user-friendly way I could think of.

For starters, gmid has full support for IRI (RFC3987 — Internationalized Resource Identifiers). IRIs are a superset of URIs, so there aren't incompatibilities with URI-only clients.

There is full support also for punycode. In theory, the user doesn't even need to know that punycode is a thing. The hostname in the configuration file can (and must be) in the decoded form (e.g. naïve and not xn--nave-6pa), gmid will do the rest.

The only missing piece is UNICODE normalisation of the IRI path: gmid doesn't do that (yet).

Configuration

gmid has a rich configuration file, heavily inspired by OpenBSD' httpd. While you should definitely check the manpage because it documents every option in depth, here's an example of what gmid can do.

ipv6 on     # enable ipv6

server "example.com" {
    cert "/path/to/cert.pem"
    key  "/path/to/key.pem"
    root "/var/gemini/example.com"
    lang "it"
    cgi  "/cgi/*"

    location "/files/*" {
        auto index on
    }

    location "/repo/*" {
        # change the index file name
        index "README.gmi"
    }

    # redirect /cgi/man/... to man.example.com/...
    location "/cgi/man*" {
        strip 2
        block return 31 "gemini://man.example.com%p"
    }
}

server "man.example.com" {
    cert "..."
    key  "..."
    root "/var/gemini/man.example.com"

    # handle every request with the CGI script `man'
    entrypoint "man"
}

Building

gmid depends on a POSIX libc, OpenSSL/LibreSSL and libtls (provided either by LibreSSL or libretls). At build time, flex and yacc (or GNU bison) are also needed.

The build is as simple as

./configure
make

If the configure scripts fails to pick up something, please open an issue or notify me via email.

To install execute:

make install

Docker

If you have trouble installing LibreSSL or libretls, you can use Docker to build a gmid image with:

docker build -t gmid .

and then run it with something along the lines of

docker run --rm -it -p 1965:1965 \
    -v /path/to/gmid.conf:...:ro \
    -v /path/to/docs:/var/gemini \
    gmid -c .../gmid.conf

(ellipses used for brevity)

Local libretls

This is NOT recommended, please try to port LibreSSL/LibreTLS to your distribution of choice or use docker instead.

However, it's possible to statically-link gmid to locally-installed libretls quite easily. (It's how I test gmid on Fedora, for instance)

Let's say you have compiled and installed libretls in $LIBRETLS, then you can build gmid with

./configure CFLAGS="-I$LIBRETLS/include" \
            LDFLAGS="$LIBRETLS/lib/libtls.a -lssl -lcrypto -lpthread"
make

Testing

Execute

make regress

to start the suite. Keep in mind that the regression tests will create files inside the regress directory and bind the 10965 port.

Architecture/Security considerations

gmid is composed by two processes: a listener and an executor. The listener process is the only one that needs internet access and is sandboxed. When a CGI script needs to be executed, the executor (outside of the sandbox) sets up a pipe and gives one end to the listener, while the other is bound to the CGI script standard output. This way, is still possible to execute CGI scripts without restrictions even in the presence of a sandboxed network process.

On OpenBSD, the listener runs with the stdio recvfd rpath inet pledges, while the executor has stdio sendfd proc exec; both have unveiled only the served directories.

On FreeBSD, the executor process is sandboxed with capsicum(4).

On Linux, a seccomp(2) filter is installed in the listener to allow only certain syscalls, see sandbox.c for more information on the BPF program.

In any case, you are invited to run gmid inside some sort of container/jail/chroot.