I'm trying to enter a 4028 bit DKIM key into DNS and it seems that I'm exceeding both the UDP 512 byte limit and also the maximum record size for a TXT record.

How does someone properly create a large key (with implied larger encoded size) and import it into DNS?

  • Need more detail; what is the DNS server software? – JGurtz Apr 4 '11 at 18:54
  • Have you carefully considered your key size? The RFC states: "Verifiers MUST be able to validate signatures with keys ranging from 512 bits to 2048 bits, and they MAY be able to validate signatures with larger keys." So your long key may not get verified. – HTTP500 Apr 4 '11 at 19:11
  • @JGurtz We are using "UltraDNS" it's a hosted service. – random65537 Apr 4 '11 at 19:48
  • @Jason - We can do 2048, but I think that bit length may exceed the UDP packet limit. – random65537 Apr 4 '11 at 19:48
  • Perhaps you should consider that the problems you create through the use of an unusually long key far outweigh the advantages you hope to gain from it. – John Gardeniers Apr 5 '11 at 0:45
up vote 20 down vote accepted

You need to split them in the text field. I believe that 2048 is the practical limit for key sizes. Split the text field into parts 255 characters or less. There is overhead for each split.

There are two formats for long fields.

TXT "part one" \ "part two"

TXT ( "part one" "part two" )

Both of which will combine as "part onepart two". More details from Zytrax.

To generate my dkim entry I insert my public key file and wrap it in quotation marks.
My public key file contains the following:


After editing the key in my dns zone file appears as follows:

dkim3._domainkey        IN      TXT     ("v=DKIM1; t=s; p=" 

DNS returns it as follow:

 bill:~$ host -t TXT dkim3._domainkey.systemajik.com
 dkim3._domainkey.systemajik.com descriptive text "v=DKIM1\; t=s\; p=" "MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQD78Ki2d0zmOlmjYNDC7eLG3af12KrjmPDeYRr3" "q9MGquKRkRFlY+Alq4vMxnp5pZ7lDaAXXwLYjN91YY7ARbCEpqapA9Asl854BCHMA7L+nvk9kgC0" "ovLlGvg+hhqIPqwLNI97VSRedE60eS+CwcShamHTMOXalq2pOUw7anuenQIDAQAB"

DNS treats it as one long string with no extra spaces where the lines are joined. All " " sequences are ignored.

  • Can you give a example? for this I can't figure out how to make it clear what part 1 and part 2 is.... #threadnecro – janw Jul 17 '13 at 11:28
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    @janw I've added one of my keys as an example. This key is only 1024 bits. – BillThor Jul 18 '13 at 0:34
  • 5
    Tnx for the quick reply. But this key fits in a 255 key. So I still don't get how you split it into multiple keys. – janw Jul 18 '13 at 7:30
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    It is not DNS that ignores the " " sequences and you actually prove that in your last quote. It is SPF RFC 4408 section 3.1.3 which defines that concatenation should be used by applications reading the DNS records for SPF validation. – Phil Mar 8 '16 at 8:29
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    @Alnitak The applicable RFC for this case is tools.ietf.org/html/rfc6376#section-3.6. Strings are concatenated without spaces between them. SPF and other formats follow the same rules allowing breaks to be placed at arbitrary locations. This can be done for readability or to limit record size. – BillThor May 10 '17 at 3:09

It's OK if the record is greater than the UDP 512-byte limit because DNS will use TCP.

This should be transparent to the user, but sometimes buggy firewall appliances (such as Cisco PIX/ASA) will filter/block these larger queries.

If you are using MySQL/MariaDB as your DNS backend, like PowerDNS you could resize your content column.

Default PowerDNS content length is VARCHAR(255)

So your DKIM signature will be trimmed off to 255 characters

to fix this

just change the content size via the MySQL CLI / MariaDB CLI

mysql -u root -p

USE powerdns;
alter table records modify column content text not null;

restart your DNS Service (eg PowerDNS)

service pdns restart

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