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gmid

gmid is a fast Gemini server written with security in mind. I initially wrote it to serve static files, but it has grown into a featureful server.

Features

(random order)

  • sandboxed by default on OpenBSD, Linux and FreeBSD
  • reconfiguration: reload the running configuration without interruption
  • automatic redirect/error pages (see block return)
  • IRI support (RFC3987)
  • automatic certificate generation for config-less mode
  • CGI and FastCGI support
  • virtual hosts
  • location rules
  • event-based asynchronous I/O model
  • low memory footprint
  • small codebase, easily hackable

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, with every detail carefully documented in the manpage. Here's a minimal example of a config file:

server "example.com" {
    cert "/path/to/cert.pem"
    key  "/path/to/key.pem"
    root "/var/gemini/example.com"
}

and a slightly more complex one

ipv6 on     # enable ipv6

# define a macro
cert_root = "/path/to/keys"

server "example.com" {
    alias "foobar.com"

    cert $cert_root "/example.com.crt"
    key  $cert_root "/example.com.pem"
    root "/var/gemini/example.com"

    # lang for text/gemini files
    lang "en"

    # execute CGI scripts in /cgi/
    cgi "/cgi/*"

    # only for locations that matches /files/*
    location "/files/*" {
        # generate directory listings
        auto index on
    }

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

Building

gmid depends on a POSIX libc, libevent2, OpenSSL/LibreSSL and libtls (provided either by LibreSSL or libretls). At build time, yacc (or GNU bison) is also needed.

The build is as simple as

./configure
make

or make static to build a statically-linked executable.

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

To install execute:

make install

Please keep in mind that the master branch, from time to time, may be accidentally broken on some platforms. gmid is developed primarily on OpenBSD/amd64 and commits on the master branch don't get always tested in other OSes. Before tagging a release however, a comprehensive testing on various platform is done to ensure that everything is working as intended.

Docker

If you have trouble installing LibreSSL or libretls, in contrib there's a sample Dockerfile. See the contrib page for more information.

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 -levent"
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 four processes: the parent process, the logger, the listener and the executor. The parent process is the only one that doesn't drop privileges, but all it does is to wait for a SIGHUP to reload the configuration and spawn a new generation of children process. The logger process gathers the logs and prints 'em to stderr or syslog (for the time being.) The listener process is the only one that needs internet access and is sandboxed by default. The executor process exists only to fork and execute CGI scripts, and optionally to connect to FastCGI applications.

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

On FreeBSD, the listener and logger process are 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 about the BPF program. If available, landlock is used to limit the portion of the file system gmid can access (requires linux 5.13+.)

In any case, it's advisable to run gmid inside some sort of container/jail/chroot.