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UDP broadcast does not work to loopback device without network connection?

 
 
franklin.bowen@gmail.com
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      03-14-2006
While developing some Java tools that use UDP broadcasting, I found
that my laptop will not broadcast packets back to the loopback device
*IF* I do not have a network connection (IP address assigned to one of
my interfaces) even if i turn on broadcasting and share ports. Does
anyone know if this is expected (or maybe undefined?) behaviour in
networking in general, a Java specific issue, a bug, or some other
option I have not listed?

Thanks!

 
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Phil Frisbie, Jr.
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      03-14-2006
http://www.velocityreviews.com/forums/(E-Mail Removed) wrote:
> While developing some Java tools that use UDP broadcasting, I found
> that my laptop will not broadcast packets back to the loopback device
> *IF* I do not have a network connection (IP address assigned to one of
> my interfaces) even if i turn on broadcasting and share ports. Does
> anyone know if this is expected (or maybe undefined?) behaviour in
> networking in general, a Java specific issue, a bug, or some other
> option I have not listed?


I assume this is a Windows 2000 or XP system? If so, this is a very common
problem, and you can easily find the fix if you search for "disable windows
media sense" on your favorite search site. Media sense was another 'helpful'
Microsoft idea....

--
Phil Frisbie, Jr.
Hawk Software
http://www.hawksoft.com
 
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franklin.bowen@gmail.com
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      03-15-2006
Yes, the system is running XP. Thanks for the information. I got
"around" the problem by installing an ethernet PC card (USB would
probably work just as well) and plugging an ethernet cable between the
two NICs.

They auto-configured to different subnets (169.254.87.x and
169.254.104.x) but I just set the last two octets in my IPv4 broadcast
address to 0xFF and I can now continue my development work. Yay!

Thanks again for the useful reply! I appreciate it.

 
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googlegroups@marget.com
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      03-15-2006
(E-Mail Removed) wrote:
> Yes, the system is running XP. Thanks for the information. I got
> "around" the problem by installing an ethernet PC card (USB would
> probably work just as well) and plugging an ethernet cable between the
> two NICs.
>
> They auto-configured to different subnets (169.254.87.x and
> 169.254.104.x) but I just set the last two octets in my IPv4 broadcast
> address to 0xFF and I can now continue my development work. Yay!


I bet you'll find they're not in different subnets afterall. The mask
on those addresses should be a /16. You might want to put something
into your code so that it automagically figures out the correct
broadcast address to use.

I've had to do similar nonsense with my PowerBook: For some reason the
perl pcap libraries refuse to read from a capture file if my NIC
doesn't have a link. It makes no sense, but I haven't got to the
bottom of it yet. My workaround is to carry a PCMCIA ethernet card
(for which I don't even have drivers) and string that NIC over to my
onboard Ethernet.

Frustratingly, an Ethernet loopback plug like this one doesn't seem to
do the trick:

http://docs.sun.com/app/docs/doc/816...82opb3g?a=view

Maybe loopback plugs cannot work in a transceiver that supports
auto-crossover?

/chris

 
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