How many A records can fit in a single DNS response?

Question

What are the size limits on DNS responses? For instance how many \'A\' resource records can be present in a single DNS response? The DNS response should still be cache-able.

Answer 1

The largest guaranteed supported DNS message size is 512 bytes.

Of those, 12 are used up by the header (see §4.1.1 of RFC 1035).

The Question Section appears next, but is of variable length - specifically it'll be:

• the domain name (in wire format)
• two bytes each for QTYPE and QCLASS

Hence the longer your domain name is, the less room you have left over for answers.

Assuming that label compression is used (§4.1.4), each A record will require:

• two bytes for the compression pointer
• two bytes each for TYPE and CLASS
• four bytes for the TTL
• two bytes for the RDLENGTH
• four bytes for the A record data itself

i.e. 16 bytes for each A record (§4.1.3).

You should if possible also include your NS records in the Authority Section.

Given all that, you might squeeze around 25 records into one response.

Answer 2

According to this RFC, the limit is based on the UDP message size limit, which is 512 octets. The EDNS standard supports a negotiated response with a virtually unlimited response size, but at the time of that writing (March 2011), only 65% of clients supported it (which means you can't really rely on it)