gmid

dead simple, zero configuration Gemini server

gmid is a simple and minimal Gemini server. It can run without configuration, so it's well suited for local development, but at the same time has a configuration file flexible enough to meet the requirements of most capsules.

gmid was initially written to serve static files, but can also optionally execute CGI scripts. It was also written with security in mind: on Linux, FreeBSD and OpenBSD is sandboxed via seccomp(2), capsicum(4)and pledge(2)+unveil(2) respectively.

Features

• IRI support (RFC3987)
• dual stack: can serve over both IPv4 and IPv6
• CGI scripts
• (very) low memory footprint
• small codebase, easily hackable
• virtual hosts
• sandboxed by default on OpenBSD, Linux and FreeBSD

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.

Building

gmid depends on a POSIX libc 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

make

To install execute:

make install

If you have trouble installing LibreSSL or libretls, as they aren't available as package on various Linux distribution, 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/cert.pem:...:ro \
    -v /path/to/key.pem:...:ro \
    -v /path/to/docs:/var/gemini \
    gmid -f -d /var/gemini -K ... -C ...

ellipses used for brevity.

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 restriction even in the presence of a sandbox.

On OpenBSD, the listener process runs with the stdio recvfd rpath inet pledges and has unveil(2)ed only the directories that it serves; the executor has stdio sendfd proc exec as pledges.

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

On Linux, a seccomp(2) filter is installed 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.