It's not the only reason. There are also delegations to inaccessible DNS servers for internal-only zones.
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I was referring to "failed closed." Using a TXT record may have been better, but it wouldn't have been sufficient to permit fail closed behavior.
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The question is what to do if the DNS query fails (e.g. timeout, SERVFAIL). "Fail closed" means forbid issuance when that happens, but too many DNS servers are busted for that to be viable. A successful response indicating no CAA record would still permit issuance.
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Two types of busted DNS: 1. CAA intolerance: DNS server drops CAA queries or replies with SERVFAIL or NOTIMP instead of a successful empty response. TXT record would solve this. 2. http://internal.example.com 's NS server is e.g. 10.1.1.10. TXT record wouldn't fix.
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Currently, the BRs require fail close if the zone is DNSSEC-signed. This has been a huge fiasco, because no off-the-shelf DNS resolver makes it easy to tell if a zone is DNSSEC-signed. CAs came up with a variety of creative solutions.
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