First. I know nothing about this. Have a read below and feel free to completely ignore everything

Looking at the manual page of inet:setopts/2 you can use the raw option to set options not explicitly supported by setopts

They have an example setting TCP_LINGER2 to 30 using this:

inet:setopts(Sock,[{raw,6,8,<<30:32/native>>}])
or in elixir
:inet.setopts(sock, [{:raw, 6, 8, <<30::32-native>>}])

Unfortunately I don’t know the protocol level or option number for SO_RCVTIMEO (it might be platform dependent?) and I don’t know how to format a timeval struct outside of C.

My best guess would be something like:

$ cat tempo.c
#include <sys/socket.h>
#include <stdio.h>

int main() {
    printf("SO_RCVTIMEO: %d\n", SO_RCVTIMEO);
    printf("SOL_SOCKET: %d\n", SOL_SOCKET); 
    printf("sizeof(time_t): %lu\n", sizeof(time_t));
    printf("sizeof(timeval): %lu\n", sizeof(struct timeval));
}
$ gcc tempo.c
$ ./a.out
SO_RCVTIMEO: 20
SOL_SOCKET: 1
sizeof(time_t): 8
sizeof(timeval): 16

And then use the above values to try to set the option.

:inet.setopts(socket, [{:raw, 1, 20, <<30::64-native-unsigned, 0::64-native-unsigned>>}]).

This might be very dangerous. Are 1 and 20 and the timeval sizes and format platform and machine architecture dependent? Is the option even set? I don’t know how to find out.

As for ranch you should be able to use Transport:setopts/2 which means either ranch_tcp or ranch_ssl depending on transport used. There might also be an option to pass the raw option in when you start a listener through the options.