FreeTDS Development Group

["James K. Lowden" <jklowden AT freetds.org>]

Peter Deacon wrote:
> 
> UDP has no concept of connection.  Each message stands alone.
> 
> If the server binds to all interfaces and there are multiple interfaces 
> capable of reaching the client the source address of messages from the 
> server should be concidered random/determined by the routing system.

Not only are you right, you're right.  I even read as much after posting
my message.

And I found a server that answers on a different interface!  :-)  And,
using a combination of techniques, I seem to have something that works.  

1.  If I connect only, I miss answers from other interfaces.  
2.  If I bind only, I don't get ECONNREFUSED, only a timeout.  Perhaps
that's me or my implementation. 

Therefore, I do both, using two sockets, of course.  select(2) tells me
which one is ready, and, mirabile dictu, I get what I want: success,
timeout, or  ECONNREFUSED.  


== code ==
        fd_set fds;
        FD_ZERO(&fds);
        FD_SET(s1, &fds);
        FD_SET(s2, &fds);

        struct timeval timeout;
        bzero(&timeout, sizeof(timeout));
        timeout.tv_sec = 5;
        timeout.tv_usec = 0;
        
        cout << "reading from " << hostname << ":" << port << " ... " << 
flush;
        
        if( (erc = select(s2 + 1, &fds, NULL, NULL, &timeout)) == -1 ) {
                perror("select(2) failed");
                return 1;
        }

        if( 0 == erc ) {
                cerr << "timed out, sorry.\n";
                return 1;
        }
        
        cout << erc << " fd ready on " << flush;
        
        int s;
        char buffer[2048];
        
        if( FD_ISSET(s1, &fds) ) {
                s = s1;
                cout << "connected ... " << flush;
        }
        if( FD_ISSET(s2, &fds) ) {
                s = s2;
                cout << "bound ... " << flush;
        }
        
        while( (erc = read(s, &buffer, sizeof(buffer))) == -1 ) {
                perror("could not read");
                return 1;
        }

        cout << "done.  (read " << erc << " bytes)\n" << flush;

== edoc ==

Data arrive on the bound port.  Errors are detected on the connected port:


== output ==
$ ./connect_udp -h db03 -p 1434 
address of db03 is 10.10.10.10
binding local UDP socket ... done
sending  to  db03:1434 ... done.  (sent 1)
connecting UDP socket to db03:1434 ... done
writing  to  db03:1434 ... done.  (wrote 1)
reading from db03:1434 ... 1 fd ready on bound ... done.  (read 411 bytes)

./connect_udp -h localhost -p 1434
address of localhost is 127.0.0.1
binding local UDP socket ... done
sending  to  localhost:1434 ... done.  (sent 1)
connecting UDP socket to localhost:1434 ... done
writing  to  localhost:1434 ... done.  (wrote 1)
reading from localhost:1434 ... 1 fd ready on connected ...
 could not read: Connection refused
== tuptou ==

Too bad none of this is standardized.  Standards?  We don't need no
stinking standards!  

That about wraps up the research.  I think I'll file a bug report and see
if the project maintaner will update FreeTDS.  Oh, wait....

Regards, 

--jkl

Attachment: connect_udp.C
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