LinuxSA Mailing list archives

  From: Alan Kennington <akenning@dog.topology.org>
  To  : John Lehmann <jplehmann@home.com>
  Date: Sat, 21 Apr 2001 16:09:07 +0930

Re: socket programming - connection refused from recvfrom call

On Fri, Apr 20, 2001 at 01:03:06PM -0500, John Lehmann wrote:
> 
> I would expect that if X is not 5000, the program execution would block at
> the recvfrom and wait until a message arrives, however i get the error
> connection refused. (if you take out the send, the receive does block as
> expected)  On solaris, this code works as expected though.


John,

This isn't all that likely to be relevant, but your situation seems to
me to be similar to one that I used to find with solaris, but not with
linux, or vice versa.

I found that if you do a connect() on a UDP socket, a solaris machine
will generally send an ICMP packet to check is the other machine exists,
or if the remote socket is on the local host, it will check to see
if the remote UDP port is open. If this check fails, then the
UDP connect() fails.

That might not seem relevant.
But... the documentation for recvfrom says this:

--------------------------------------------------
       If from is not NULL, and the socket is not connection-ori­
       ented,  the  source  address  of the message is filled in.
       The argument fromlen is a value-result parameter, initial­
       ized  to  the size of the buffer associated with from, and
       modified on return to indicate  the  actual  size  of  the
       address stored there.

       The  recv call is normally used only on a connected socket
       (see connect(2)) and is identical to recvfrom with a  NULL
       from parameter.                       
--------------------------------------------------

It's just remotely possible that when you don't specify the 
from-address in the recvfrom() call, the OS is checking on the
remote host to see if it's connected, and if not, the implicit
connect() in your recvfrom() call is failing.

Thus if you specify a remote address in a sendto() function, the
OS may be remembering that you did this, and assuming that you
want to do the recvfrom() from that address or something.

To check this, try opening the remote UDP port first, before you
run your program, and see if that makes the UDP layer happy.
If so, then probably you are indeed seeing an implicit connect() call.

An alternative method of checking is to run "tcpdump" or ethereal
on the interface, and see if an ICMP is sent to the remote host,
and see if an error condition of some sort is returned
from the remote host when the UDP port is not currently open.

Cheers,
Alan Kennington.

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