[PD-dev] Requesting SVN commit access
IOhannes m zmoelnig
zmoelnig at iem.at
Wed Feb 26 15:41:40 CET 2014
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(taking this back to the list)
On 2014-02-26 14:55, Martin Peach wrote:
> On 2014-02-26 03:47, IOhannes m zmoelnig wrote:
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>> On 2014-02-26 04:57, Martin Peach wrote:
>> how come? how does DNS (after all, a central service in the
>> internet) work with this assumption?
>> UDP works fine with as a challenge/response system (less so as
>> client/server, given that there is no notion of a "connection"),
>> and for responses you need o be able to send messages somewhere.
>> for whatever reasons, many hardware manufacturers have the idea
>> that sending/receiving on the *same* port is a good idea.
> I see; but for DNS with [udpsend] and [udpreceive], it should work
> since one [udpsend] can send to port 53 of the DNS server and a
> separate [udpreceive 53] will receive its replies.
it seems i was a bit unclear here, and thus two separate things got
is a bi-directional protocol: you send a query (from a random port N)
to a DNS-server (on port 53), and the server sends a response from
port 53 to that port N.
like the idea that the receiving port is the same as the sending port.
DNS does not have this limitation of using the same port (thus you can
query a DNS-server on localhost without stepping on your own ports).
so having both [connect dns.server 53(->[udpsend] + [udpreceive 53]
is not sufficient to query a DNS-server.
> The problem seems to be that [udpsend] will assign itself a random
> port to which the remote device will send replies, but [udpsend]
> doesn't know its own port number.
well, but that's only half of the solution (for the DNS-problem):
even if [udpsend] had a notion of it's sending port, it would discard
all messages arriving on this port. but it still *listens* on (and
thus blocks) this port, so you cannot tell a [udpreceive] to (also)
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