SPF Fail: too many DNS lookups
I’ve had a couple folks come to me recently for help troubleshooting SPF failures. The error messages said the SPF record was invalid, but by all checks it was valid.
Eventually, we tracked the issue down to how many include files were in the SPF record.
The SPF specification specifically limits the number of lookups that can happen during a SPF check.
SPF implementations MUST limit the number of mechanisms and modifiers that do DNS lookups to at most 10 per SPF check, including any lookups caused by the use of the “include” mechanism or the “redirect” modifier. If this number is exceeded during a check, a PermError MUST be returned. The “include”, “a”, “mx”, “ptr”, and “exists” mechanisms as well as the “redirect” modifier do count against this limit. The “all”, “ip4”, and “ip6” mechanisms do not require DNS lookups and therefore do not count against this limit. Processing limits
Some senders are using include: records that then have include: records that then have yet other include: records. Looking up all these include records caused the SPF lookup limits to be reached, thus causing the SPF lookups to fail (all the records can’t be pulled). Note, a lot of SPF checks don’t actually comply with the SPF spec in this area. Because of the spotty compliance, some too long SPF records won’t fail everywhere.
How can you fix this?
- Prune unnecessary :include files from the SPF record.
- Use different domains for mail from different places so you don’t need one SPF record to rule them all.
- Don’t publish SPF records, rely on DKIM for authentication. Even Hotmail is checking DKIM these days.
If you need to find out how many lookups your SPF record entails you can use the SPF checker at emailstuff.org. Enter in a domain in the box and hit go. Then click on the “DNS” tab to see the actual DNS lookups that are happening. Each red domain is a separate DNS lookup.
Secureserver.net has a lot of include: files and a lot of lookups. Because this is a checking tool, it doesn’t limit the number of queries (because that wouldn’t be useful). But in places that are implementing the spec correctly, and are refusing mail based on SPF failures, secureserver.net would experience mail failures.