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What happens when a remote server doesn't understand EDNS0?
Author: ISC Support Reference Number: AA-00510 Views: 4697 Created: 2011-10-26 14:06 Last Updated: 2012-07-04 08:21 100 Rating/ 1 Voters

Question:

What are the situations (timeouts, FORMERR .. etc)  that mark a server as unable to speak EDNS0?  What happens afterwards?

Answer:

named tries to send a query with EDNS(0);

If the query fails, it will try again with EDNS(0) but with the packet size limited to 512 bytes; and if that fails, it will try again without EDNS(0).  If at that point it succeeds, then named memorizes the formula that worked: it won't try to use EDNS(0) again with that server for the duration of the server's TTL, or one day, whichever is shorter.

Failure in this context, is anything that prevents named from receiving a query response from the authoritative server, so timeout (no response) or invalid response (FORMERR).

To remove this internal setting for a server that has incorrectly been remembered as EDNS0-incapable,  flush the name from cache:

rndc flushname <name>
Flushing the name may not always work

If the record for the server is keyed by IP address rather than name, then it cannot be cleared from cache using rndc flushname. In that case, you will have to wait for the status to timeout (5 minutes) or flush the entire cache (rndc flush).  Clearing the remembered EDNS setting is only going to be useful if the problem that caused it to be set that way originally has been addressed.




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