systemd-activate socket activation for UDP daemons

Question

I like using systemd-activate(8) for testing socket-activated daemons during development, however, it seems it only listens for TCP connections:

% /usr/lib/systemd/systemd-activate -l 5700 ./prog
Listening on [::]:5700 as 3.

% netstat -nl |grep 5700
tcp6       0      0 :::5700             :::*             LISTEN 

I am using a program that handles datagrams (UDP). How can I make systemd-activate listen on a UDP port? Or is there a simple way to do this using other tools, without going to the trouble of crafting and installing a systemd unit file?

Answer 1

I ended up writing a simple C program to do this; code below (public domain).

The usage is:

./a.out <port-number> <prog> [<arg1> ...]

The program opens a UDP socket on <port-number>, sets the environment variables that systemd socket-activated daemons expect, then executes <prog> with whatever arguments follow.

#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <netinet/in.h>

int main(int argc, char **argv) {
    if (argc < 2) {
        printf("no port specified\n");
        return -1;
    }
    if (argc < 3) {
        printf("no program specified\n");
        return -1;
    }

    uint16_t port = htons((uint16_t) strtoul(argv[1], NULL, 10));
    if (port == 0 || errno) {
        printf("failed to parse port: %s\n", argv[1]);
        return -1;
    }

    /* create datagram socket */
    int fd = socket(AF_INET, SOCK_DGRAM, 0);
    if (fd < 0) {
        printf("failed to open socket; errno: %d\n", errno);
        return -1;
    }

    struct sockaddr_in sa;
    sa.sin_family = AF_INET;
    sa.sin_port = port;
    sa.sin_addr.s_addr = INADDR_ANY;

    /* bind socket to port */
    int r = bind(fd, (struct sockaddr *) &sa, sizeof(struct sockaddr_in));
    if (r < 0) {
        printf("bind failed; errno: %d\n", errno);
        return -1;
    }

    /* execute subprocess */
    setenv("LISTEN_FDS", "1", 0);
    execvp(argv[2], argv + 2);
}