Thanks for the info Simon, that's cleared that up for me. And thanks to the
others contributing to this thread.
I think that a utility to test a connection and provide an explicit diagnosis (rather than the clues provided by the
client just hanging in various places) would be rather nice to have.
On 01/04/11 12:59, Simon Slavin wrote:
On 1 Apr 2011, at 12:43am, Justin Clark-Casey wrote:
Some people on Stack Overflow think that IPAddress.Any means listen on all NICs
(http://stackoverflow.com/questions/1777629/how-to-listen-on-multiple-ip-addresses).
But my reading of the MS SDK reference above means that it only binds to one.
Anybody able to comment on this?
Stack Overflow is right. My reading of that SDK page is that it's wrong, and
should be corrected, but other MS documentation is clearer on what '::Any'
means, for example
http://msdn.microsoft.com/en-us/library/system.net.ipaddress.any.aspx
Returning to the standards, IP address 0.0.0.0 is reserved for specific
purposes for both sending and receiving. It's called the 'anonymous' address
(for historical reasons) or the 'broadcast' address (for current reasons).
If a computer SENDS a packet to 0.0.0.0 then it is multibroadcasting: sending
one message to every computer that can hear it. This is done most often to
announce the (un)availability of a service, for instance that a printer service
has come online. Sending to address 0.0.0.0 is done by, for example, DHCP and
zeroconf (what Apple calls 'Bonjour'). Things like routers are usually set up
to drop packets SENT to 0.0.0.0 so that you don't announce to the entire world
what address your printer can be found on.
When a computer LISTENS to the network interface bound to 0.0.0.0 then it is
telling its TCP stack that it doesn't care which network interface a message
comes in on, it wants it anyway. Almost every Internet application does this,
especially now many have both Ethernet sockets and WiFi capabilities: an app
doesn't care what its user is using right now, it just wants to 'use the
internet'. Under normal circumstances the only programs which /don't/ listen
on 0.0.0.0 are techie programs like network utilities, or a web server on a
gateway computer which needs to present a web site to internal users and make
sure it isn't available to external users.
I tried to find an RFC to point to as reference but nothing seems to spell this
out. The nearest thing i could find was RFC950.
Simon.
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Justin Clark-Casey (justincc)
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