RRSet size limitation lower than predicted by RDLENGTH field size

[Danny Mayer]

Tom Byrnes wrote:
> We're pushing the limits of RRSet sizes for A records in the responses to
> queries for our lists, but we're finding that the practical limit is much
> lower than that predicted in the binary message format specs.
>  

What limits does you think you are pushing?

> The octets in the RDLENGTH param (16 bit unsigned = 65535) should allow
> 16384 A records in a single RRSET using TCP, but the behavior we are
> observing in BIND is a limitation of 4096 A records.
>  

Your calculations are wrong since there's additional information sent in 
a DNS packet. In any case DNS usually uses UDP and not TCP. If the query 
client supports it it will use EDNS0 to send the responses. In addition, 
since you apparently have too many addresses to fit in a UDP packet are 
exceeding the ability of the DNS to send it via UDP so it sends a 
truncated flag to indicate the the client that it needs to retry with TCP.

> We're using Bind 9.4.1-P1 on Gentoo.
>  
> Any ideas what's causing this, or how to fix it?
>  

Why do you have so many address records for a single name?

Danny

> Thanks in advance.
>  
> Tom Byrnes
> CTO
> ThreatSTOP
> 
> 
>