[colug-432] SPF and other stuff for personal domains

[Rob Funk]

On Monday, February 13, 2017 10:26:12 AM EST, Rick Troth wrote:
> ip4:198.178.231.250 
> ip4:174.105.80.118 
> mean that 198.178.231.250 and 174.105.80.118 are explicitly okay as senders 
> And here I really should add some IP6 entries since I claim to 
> be so fond of IPv6. But how long can a TXT record be? Or can we 
> have multiple TXT records serving SPF? This particular mechanism 
> maybe doesn't scale as well as the others. 

The initial length limit on TXT records is that a single "string" in a TXT 
record is limited to 255 characters. However, multiple "strings" can be 
concatenated together, up to a 64k limit. The other major limit that 
applies (and is often a relevant factor for DKIM) is UDP; a DNS record 
longer than 512 bytes won't fit in a UDP packet, requiring a retry in TCP. 
Some DNS clients don't do TCP, and even among those that do, adding a TCP 
handshake adds time to the query.

> Question: can I use DKIM with Postfix? or even with Sendmail? 
> Or what MTA do y'all use? 

Yes you can, in fact with the same "milter" mechanism for both Postfix and 
Sendmail. You set up a daemon like opendkim that understands how to filter 
mail as a "milter", and tell Postfix/Sendmail to filter mail through it.


- Rob