🚀 go-pugleaf

> We are using sockets with UDP in our application.
> The connect() system call binds our port number and sets up the
> remote port number and IP address.  Most of the time there is
> no peer at the other end, but the connect() and first write()
> do not return any errors.

Correct.  All connect() does on a UDP socket is store the peer's
IP address and port number, then return immediately.  All write()
does is package the data into a UDP/IP datagram and queue it for
the interface and return.  All's fine up to here, so no error is
returned.

> What does seem to happen is that a packet arrives at the socket
> after doing the write() and the subsequent read() that we perform
> returns error #239 - connection refused.  We only receive this
> error when the remote end is located at a different IP address.
>
> Is the following assumption about what is happening correct:
>  - there is no connection-related handshaking involved since 
>    UDP is selected as the protocol but the packet is being 
>    returned because it could not be delivered (port number 
>    was not assigned)

Correct.  If you used a diagnostic tool such as tcpdump you'd see
that an ICMP port unreachable is returned by the other end.  Figure
6.12 of my recent book "TCP/IP Illustrated" (Addison-Wesley) shows
that BSD systems convert this ICMP error into the Unix error
"Connection refused."

> If this is the case, I want to just ignore the "connection refused"
> error.  I don't remember getting this error when we were using
> sendto()/recvfrom() but we hadn't tried sending packets to a 
> different IP address either.

Depends what you mean by "ignore."  The ICMP error tells you that
no process is at the destination IP address with a UDP socket bound to
your destination port.  That tells you something.  Unless someone then
starts a process at that port, you'll continue receiving that error if
you continue writing to your connected socket.

You won't receive these ICMP errors at all unless you issue a connect().
Welcome to the bizarre world of connected vs. unconnected UDP sockets ...
If you want to see something truly bizarre: if your server is multihomed,
start the server and send a datagram to it from the client, *but* send the
datagram to the "wrong" interface on the server and watch with tcpdump ...

	Rich Stevens