systemd-activate socket activation for UDP daemons - sockets

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

This was recently added to systemd-activate: https://github.com/systemd/systemd/pull/2411, and will be part of systemd-229 when it is released.

Answer 2

I'm not sure that there is a way to do it with systemd-activate.
You may want to employ some .service unit file and a .socket unit file with dependencies. In a .socket unit you will describe ListenDatagram= option. See here for more details.

Answer 3

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);
}