UPD size in DNS

[Kevin Darcy]

Type your name here wrote:

> Hello,
>
> I have read that DNS can only handle UPD-packets of 512-bytes or less.
> Eventhrough the theoretical size is 65.535 bytes - 20 bytes IP-header - 8
> bytes UDP-header.
> Does the DNS-protocol have this limit? If yes, why?

The DNS protocol, as originally specified (see RFCs 1034 and 1035) has a
limit of 512 bytes on UDP packets. As I understand it, this was to prevent
fragmentation, regardless of the capabilities of the client, the server, or
any network node in between. When an answer doesn't fit in 512 bytes, it is
marked as truncated and the usual fallback is to retry the query using
TCP instead.

Note that EDNS0 (see RFC 2671) allows a UDP payload larger than 512 bytes to
be negotiated between a client and a server. But not all implementations
support EDNS0 yet...


- Kevin