network cisco ccna gns3 certification arteq

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Monday, December 12, 2011

DNS goes both ways...

DNS (port 53) uses both TCP and UDP...  you've got it memorized, those bastards won't trick you on that one... you know damn well that TCP is connection oriented and that means reliable transport among other things...  you also know UDP doesn't care, just keeps sending those 512 bytes (it can go higher, but 512 is the safe bet) whether the receiver likes it or not... look at TFTP; your IOS is counting on getting to your gear and so are you, but UDP blithely leaves any semblance of reliability up to the application layer...

so why does DNS use both?

DNS uses TCP for zone transfers because its reliability keeps the zone databases consistent.

DNS uses UDP for DNS queries... if it doesn't get a response, it will simply re-transmit after 3-5 seconds

UDP just doesn't give a shit...

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