udp broadcast

oha has asked for the wisdom of the Perl Monks concerning the following question:

dear Monks,

I have a device which sends udp broadcasts as shown in the tcpdump below:

10:23:48.240377 IP 192.168.0.50.49153 > 192.168.0.255.4210: UDP, lengt
+h 28
[download]

and I wrote the following:

use strict;
use warnings;

use IO::Socket;

my $sock = IO::Socket::INET->new(
    Type => IO::Socket::SOCK_DGRAM,
    Proto => 'udp',
    PeerAddr  => '192.168.0.255:4210',
    Broadcast => 1,
    ReuseAddr => 1,
);
print inet_ntoa($sock->sockaddr()), ":", $sock->sockport(), "\n";
print inet_ntoa($sock->peeraddr()), ":", $sock->peerport(), "\n";

my $data;
$sock->recv($data, 100) or die "$!";
print "Ť$datať\n";
[download]

which shows:

192.168.0.49:37550
192.168.0.255:4210
[download]

and then blocks forever, even tho tcpdump shows activity.

What am i doing wrong?

Comment on udp broadcast Select or Download Code

Replies are listed 'Best First'.
Re: udp broadcast by haukex (Archbishop) on May 13, 2020 at 09:08 UTC
I think you don't need to set up all those options the way you did, the following is enough for me to recieve UDP broadcasts. `use IO::Socket::INET; my $sock = IO::Socket::INET->new( Proto => 'udp', LocalPort => 4210 ); $sock->recv(my $data, 100) or die $!; print "<<$data>>\n";` [download] Test via `echo "Hello World" \| socat - UDP-DATAGRAM:255.255.255.255:4210,broadcast` Of course, if you've got a firewall, it has to allow incoming UDP packets on that port.	[reply] [d/l] [select]
Re^2: udp broadcast by oha (Friar) on May 13, 2020 at 09:26 UTC
great! thanks! I'm still confused why this work this way, since 4210 is not the local port, but the broadcast port. nevertheless it works and i'm happy :)	[reply]
Re^3: udp broadcast by haukex (Archbishop) on May 13, 2020 at 09:35 UTC
Glad to help! I'm still confused why this work this way, since 4210 is not the local port, but the broadcast port. It's the port you're listening on, on the local machine. (The sender is not necessarily sending from their port 4210, they may be sending from a random port, but their packets are addressed to port 4210 on the receiving side. Update: You can see this in the tcpdump output you showed: "`192.168.0.50.49153 > 192.168.0.255.4210`") BTW, if you want to get the address from which the packet was sent, you can call `$sock->peerhost` immediately after the `recv`.	[reply] [d/l] [select]


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