[teddyh]

The myth of DNS “propagation” needs to die. Changed DNS entries do not “propagate”. The old cached DNS entries in DNS resolvers simply expire, in an arbitrary order. DNS resolvers are not linked geographically; there is no “propagation”.

If this tool was querying a list of widely-used public (and/or private) DNS resolvers, it might be useful. But pretending that DNS entries propagate geographically does not do anyone any favors.

[toast0]

a) different DNS systems get the change out to all the authoritatives different ways. Some of them with much delay. Delays are hopefully minimal on modern systems, but I've worked with bad systems where you change dns in the api and it takes minutes and sometimes hours for the authoritatives to start returning new results; traditional notify/axfr based systems often have a queue of several seconds at least.

b) as resolver caches expire, new queries will hopefully get new answers (as long as the authoritatives get updated per a), and eventually you get the new results everywhere except for resolvers that do terrible things...

When the change is made and it takes time for the results to show up everywhere, I think propagate is a reasonable verb. You could use disperse or diffuse or something else, but you need a verb to let people know it's going to take time for your changes to be visible everywhere.

I don't know that propagate necessary implies the change becomes visible in an orderly way. 'Around the globe' doesn't really either, it's just observing from around the globe as resolvers get new data.

What verb do you prefer to use to describe how unsychronized caches obtain new values?

[jon-wood]

> Delays are hopefully minimal on modern systems

The delays are almost always self inflicted by people who've blindly put 36400 in the TTL field like some sort of magic charm rather than considering how long they want the cache expiry to be. Long ago I was one of them and then I had the company greybeard point out that DNS record updates are a thing we had control over - just drop the TTL to 30 seconds or whatever a day before a planned change of servers and (barring the odd stubborn resolver which thinks it knows better than you do) the time it takes for those caches to refresh goes from 24 hours to 30 seconds.

[bux93]

djbdns/tinydns can schedule a change and give out a lower and lower TTL the nearer the changeover time gets - a quick search doesn't show any newer implementations but this is surely a good way to do it.

[timwis]

My interpretation of the concern is that propagate implies that the new value is being pushed out to other resolvers, when in fact those other resolvers pull the new value (once their cached value expires).

[c22]

Percolate?

[MaulingMonkey]

> The myth of DNS “propagation” needs to die.

What's the actual issue? Are you being frustrated by people laboring under the assumption that DNS records are being sent by carrier pidgeon or something?

> There is no geographical connection whatsoever.

DNS censorship will presumably be based on geopolitical boundaries, which in turn are bound by geography. And I wouldn't be entirely suprised if poor network connections - including those potentially geographically bound (poor weather / flooding / tornados severing or degrading links or power) had some (minor, infrequent) impact on the rate stale cache entries are evicted in favor of fresh ones.

Granted, none of that means a DNS resolver halfway across the globe from the authoritative servers can't typically get updated results <200ms (≈light speed), which is safely ignorable / won't be visible as records propagating from geographic neighbor to geographic neighbor. And granted further, I'm both too boring to censor, and too smart to be on call for anything that would make me aware of global outage reports - so the map is admittedly useless to me beyond farming that hacker vibe aura.

But I imagine there's at least one or two dudes out there that'll see a red dot in, say, Australia - and that'll save them a few minutes, by giving them a shortcut to determining the root cause of some issue reported in Australia by letting them correctly guess/blame stale DNS records.

[teddyh]

Yes, I am constantly needing to disabuse people of their misconception that DNS changes start to apply gradually by geographic distance, instead of applying arbitrarily by pure chance of when each resolver happened to query the record previously.

[AndyMcConachie]

>> The myth of DNS “propagation” needs to die.

> What's the actual issue? Are you being frustrated by people laboring under the assumption that DNS records are being sent by carrier pidgeon or something?

The actual issue is that some people misunderstand how DNS works, and the notion that records propagate doesn't help people understand it better. It propagates a misunderstanding of DNS.

[phikappa]

DNSGlobe – Rust TUI to watch misunderstandings about DNS propagate around the world (github.com/514-labs)

There I fixed it for you.

[josteink]

But it's written in Rust so it must be good :D

[marysol5]

Memory safe, but idea unsafe

[walrus01]

> The old cached DNS entries in DNS resolvers simply expire, in an arbitrary order.

I really wish that was so. There's lots of weird resolvers out there used by weird ISPs that don't properly respect the TTL value. Setting TTL to some really low number a full day before making a major change is no guarantee that your zone will expire in a bunch of places. All kinds of weird ISPs try to do things with the dns results they present to their customers.

[teddyh]

It’s still arbitrary, with no geographical (or other) form of propagation.

[Callicles]

That's what the tool is doing - querying a bunch of public resolvers around the world to see the state of what they resolve to. Since end users usually use DNS servers close to their location, this gives an idea, around the world, of who sees what.

Agreed, this is a cache that expires and refreshes from the source DNS server. It just looks like a virus that propagates when the cache expires.

[teddyh]

> It just looks like a virus that propagates when the cache expires.

No it does not. The changes do not happen geographically. There is no geographical connection whatsoever. Calling the tool “DNSGlobe”, and displaying a map, only further reinforces the myth.

[Terretta]

dnspatter

(dns spatter + dns patter)

[marysol5]

Anycast goes BRRRRRRRRR

Just because YOU query a server, doesn't mean that a user somewhere else is querying the same server.

[muppetman]

Thank you. I have this discussion all the time. The argument I get back "Right, but it seems like the change takes awhile because of the DNS cache expiring, so it's the same as propagating". My counter to that is to say "If I punch you in the mouth, would you just tell people I'd asked you be to be quiet, or would you use the right explanation as to what'd happened?"

(It's a stupid counterargument, btw, but it ends the discussion)

[timwis]

Just FYI, those discussions might be ending because your analogy comes across as a threat to punch them in the mouth, rather than because it convinces them.

[tgv]

But is there a functional difference between push and pull in this case? Is there something that cannot be explained in terms of propagation? Perhaps timing: there's no reason for propagation to wait an hour or a day.

BTW, only last week I explained a potential problem of a DNS change with caching, and it was understood. There doesn't seem to be a need to simplify it.

[jon-wood]

There is absolutely a reason for propagation to wait an hour or a day, and you've set that value in the TTL field for the record in question. It stands for Time To Live and specifies how long downstream resolvers should cache the last value they saw before querying the authoritative server again, if you set that value to a few seconds then almost every DNS resolver in the world will get updated values within a few seconds as they expire the cached value.

[tgv]

The discussion was about caching vs propagation. You're already mixing them.

[perching_aix]

There is no such myth that I'd be aware of, and I seriously doubt the author behind this would be operating under such a misunderstanding either. Maybe you used to at some point, but then that's a separate issue.

Propagation is just an incremental spread across a topology. Doesn't need to be a physical topology whatsoever.

It may seem like this program suggests a physical propagation as far as its elevator pitch goes, but one glance at the readme clears it up pretty quickly that that's not actually the case.

Changes do propagate, and seeing funny blinking lights on a world map is cool. Doesn't mean there'd be an intent to convey the process as a geographical spread.

[marysol5]

Not to mention 8.8.8.8 queried from the UK and from the US aren't the same place....

[browningstreet]

Ehh do you remember the defaults back in the day? And how long local vs intermediary vs backbone TTLs could be cached for, even above and beyond the set TTL?

The propagation part refers to how long it would take for all those cached requests to expire and when you could tell some random client they should be able to see the new value. Especially when you forgot to lower the TTL ahead of time.

It’s a term of art and it’s fine.

Oh that reminds me. I made a bunch of DNS changes a while ago and left all the TTLs set to 5 minutes. I should up them.

[teddyh]

> local vs intermediary vs backbone TTLs

This is an example of the myth in action. There are no such things. There is a single resolver, which you use, and a set of authoritative server, which that resolver will query when the TTL in the resolver’s cache times out. There is no chain of resolvers.

[inigyou]

What do you call it when an application queries systemd-resolved and systemd-resolved queries PiHole and PiHole queries your home router and your home router queries your ISP? What is that if not a chain?

[JdeBP]

Forwarding proxy DNS servers leading to a (yes) single resolving proxy DNS server at the end.

* https://jdebp.uk/FGA/dns-server-roles.html

[teddyh]

See this old thread: <https://news.ycombinator.com/item?id=19654515>

[toast0]

> There is no chain of resolvers.

There absolutely can be a chain of resolvers.

It's very common to run a caching recursive resolver on client nodes that sends its requests to an external recursive resolver (usually the network owner's suggested server). Nicer local caching resolvers will recurse themselves if/when the configured forwarding destination is unavailable. Some decades ago, it wasn't unusual for ISPs to run distributed recursive caching resolvers that would go through a centralized cache ... some of those systems may still exist?

If all of those servers respect and propagate (or pass on, whatever) the authoritative TTL as I think RFCs suggest, then you should see all the caches behind a centralized resolver expire at the same time and then get refreshed on their next request. Of course, some caches don't respect TTLs and some might return the TTL they received (or modified to) in responses from cache and who knows what other garbage they do.

That's why people want to know approximately how much of the population can see my changed records. Sometimes where in the world can people see my changed records ... not because the change is organized geographically, but because showing where people see one value vs the other is more visually stimulating than a % line... and also because that one dumb ISP server that has no cache expiration is likely to cause problems for users in one geography and not worldwide, so it's handy to know that users in that country will have problems when you turn off old servers that you took out of dns weeks ago.

Maybe propagate isn't the best verb for DNS changes to make their way from your source of truth to everyone else's resolutions, but you're welcome to suggest another.

Merriam webster includes these definitions of propagate that I think fit [1]:

> (transitive verb) 3a to cause to spread out and affect a greater number or greater area

> (intransitive verb) to travel through space or a material

Maybe you don't think the first one fits because of 'cause' in there. When you start returning new results, you don't do anything specific to cause the caches to fetch a new result, but... the new results do affect a greater number of caches over time.

We could agree to use percolate though, it was suggested elsewhere in the thread and it's a fun word and easier to spell. It will take time for this change to percolate through our distributed conciousness though. :p

[1] https://www.merriam-webster.com/dictionary/propagate

[teddyh]

> We could agree to use percolate though, it was suggested elsewhere in the thread and it's a fun word and easier to spell.

A more accurate term would be to talk about waiting until old DNS entries “age out”, or “expire”.

[teddyh]

> There absolutely can be a chain of resolvers.

A chain of proxies does not change how we should view the situation. Proxies, by definition, should not affect anything. See this old thread: <https://news.ycombinator.com/item?id=19654515>