nanog mailing list archives
Re: MX Record Theories
From: David Conrad <drc () virtualized org>
Date: Thu, 28 May 2009 11:09:03 -1000
On May 28, 2009, at 5:04 AM, Bobby Mac wrote:
If you add enough recipients to an email, each domain within the send lineneeds to have an associated MX record.
Well, it needs to resolve to an A RR somehow, but for each domain name, you get a different query.
DNS by default starts with UDP which has a limit to the datagram size (64bit).
The UDP minimum datagram size that must be supported by DNS implementations is 512 bytes. The maximum is 64K bytes. Obviously if you try to send a 64K byte packet, it's going to fragment and as we all know, fragments are bad.
A flag is placed in the header which then requires the request to be sent via TCP (160bit V4).
If the response to a query won't fit in the UDP buffer (512 by default, although modern client implementations can advertise a larger buffer with EDNS0), the server will signal truncation in the response (with the TC bit), typically resulting in the client retransmitting the request via TCP.
Nowthat single query can be split up into many different packets providing that the request is more than the 160 bit and obviously IPV6 offers even moreinformation contained in a single packet.
IPv6 packets are a bit larger, but not that much. DNSSEC is where the fun starts.
Regards, -drc
Current thread:
- MX Record Theories gb10hkzo-nanog (May 26)
- Re: MX Record Theories Alex H. Ryu (May 26)
- Re: MX Record Theories Valdis . Kletnieks (May 26)
- Re: MX Record Theories Mark Andrews (May 26)
- Re: MX Record Theories Bobby Mac (May 28)
- Re: MX Record Theories David Conrad (May 28)
- Re: MX Record Theories Mark Andrews (May 28)
- Re: MX Record Theories William Herrin (May 26)
- Message not available
- Re: MX Record Theories gb10hkzo-nanog (May 26)
- <Possible follow-ups>
- Re: MX Record Theories gb10hkzo-nanog (May 27)
- Message not available
- Re: MX Record Theories gb10hkzo-nanog (May 28)
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