[Kalium]

Based on the highly unscientific sample of the set of questions asked by my coworkers in my office today, the failure mode of end users receiving NX DOMAIN has happened much more than on most days.

I don't need to make sure no exploits are possible. However, it at all possible, I'd like to help ensure that things aren't accidentally made more dangerous. It's one thing to consider and make a tradeoff. It's quite another to be ignorant of what the price is.

[pyre]

Well, it obviously happens when the resolver is down, but that's the situation that this logic is being proposed to smooth over. The normal day-to-day does not see a high percentage of resolvers failing to respond, or else people would be getting NX DOMAIN for high profile domains much more often.

[Kalium]

I'm just trying to make sure we don't wind up making DNS poisoning nastier in an effort to be more user-friendly.

[robryk]

All the attacks mentioned here seem to be of the following shape:

1. Let's somehow get a record that points at a host controlled by us into many resolvers (by compromising a host or by actually inserting a record).

2. Let's prolong the time this record is visible to many people by denying access to authoritative name servers of a domain.

(1) is unrelated to caching-past-end-of-ttl, so you need to be able to do (1) already. (2) just prolongs the time (1) is effective and required you to be able to deny access to the correct DNS server. Is it really that much easier to deny access to a DNS server than it is to redirect traffic to that DNS server and supply bogus reponses?

[Kalium]

DNS cache poisoning is currently a very common sort of attack. The UDP-y nature of DNS makes it very easy. There are typically some severe limitations placed on the effectiveness of this attack by low TTLs. It does not require you to deny access to the authoritative server. This attack is also known as DNS spoofing: https://en.wikipedia.org/wiki/DNS_spoofing

Ignoring TTLs in favor of your own policy means poisoned DNS caches can persist much longer and be much more dangerous.

[robryk]

Right now, to keep a poisoned entry one must keep poisoning the cache.

In that world, one can still do that. One can also poison the entry once and then deny access to the real server. You seem to be arguing that this is easier than continuous poisoning. Do I understand you correctly?

[Kalium]

You are correct in your assessment of the current dangers of DNS poisoning.

I am in no way arguing about ease of any given attack over any other. I am arguing that a proposed change results in an increased level of danger from known attacks.

I'm arguing that the proposed change at hand, keeping DNS records past their TTLs, makes DNS poisoning attacks more dangerous because access to origin servers can be denied. Right now TTLs are a real defense against DNS cache poisoning, and the idea at hand removes that in the name of user-friendliness.