systemfd 0.2.0

A convenient helper for passing sockets into another process. Best to be combined with listenfd and cargo-watch.
systemfd-0.2.0 is not a library.


systemfd is the 1% of systemd that's useful for development. It's a tiny process that opens a bunch of sockets and passes them to another process so that that process can then restart itself without dropping connections. For that it uses ths systemd socket passing protocol (LISTEN_FDS + LISTEN_PID) environment variables. Currently this only supports unix systems.

Teaser when combined with catch-watch you can get automatically reloading development servers:

$ systemfd --no-pid -s http::5000 -- cargo watch -x run

The --no-pid flag disables passing the LISTEN_PID variable. This makes listenfd skip the pid check which would fail with cargo watch otherwise. To see how to implement a server ready for systemfd see below.

This program was inspired by catflap but follows systemd semantics and supports multiple sockets.


You can get systemfd by installing it with cargo:

$ cargo install systemfd


systemfd can create one or multiple sockets as specified on the command line and then invokes another application. Each time you pass the -s (or --socket) parameter a new socket is created. The value for the parameter is a socket specification in the format [TYPE::]VALUE where TYPE defaults to tcp or unix depending on the value. The following types exist:

  • tcp: creates a tcp listener
  • http: creates a tcp listener for http usage (prints a http url in the info output)
  • https: creates a tcp listener for https usage (prints a https url in the info output)
  • unix: creates a unix listener socket
  • udp: creates a udp socket

VALUE depends on the socket type. The following formats are supported:

  • <port>: an integer port value, assumes as host
  • <host>:<port>: binds to a specific network interface and port
  • <unix-path>: requests a specific unix socket


$ systemfd -s http::5000 -- my-server-executable
$ systemfd -s http::5000 -s https::5443 -- my-server-executable
$ systemfd -s 5000 -- my-server-executable
$ systemfd -s udp::1567 -- my-game-server-executable

When systemfd starts it will print out the socket it created. This can be disabled by passing -q. Additionally if a port is set to 0 a random port is picked.

Usage with actix-web / cargo-watch and listenfd

And here is an example actix-web server that supports this by using the listenfd crate:

use listenfd::ListenFd;
use actix_web::{server, App, Path};

fn index(info: Path<(String, u32)>) -> String {
   format!("Hello {}! id:{}", info.0, info.1)

fn main() {
    let mut listenfd = ListenFd::from_env();
    let mut server = server::new(
        || App::new()
            .resource("/{name}/{id}/index.html", |r| r.with(index)));
    server = if let Some(listener) = listenfd.take_tcp_listener(0)? {
    } else {

And then run it (don't forget --no-pid):

$ systemfd --no-pid -s http::5000 -- cargo watch -x run