serve-expired-ttl & Redis

[mibere]

Let's assume unbound.conf contains

server:
  prefetch: yes
  serve-expired: yes
  serve-expired-ttl: 7200
  ...

cachedb:
  backend: "redis"
  redis-server-host: 127.0.0.1
  redis-server-port: 6379

It doesn't contain 'serve-expired-ttl-reset'.

A client visits a domain (it's TTL is 3600) for the first time: Unbound can't find an answer to this query in its built-in in-memory cache and also not in the backend (Redis). So Unbound has to resolve the query as usual (iterative DNS resolution) and stores the answer in it's cache and in Redis.

5 hours later...the client visits and requests the above domain again.
What happens now? Especially in regard to the configured 'serve-expired-ttl' in combination with Redis.

[gthess]

What should happen is:

• unbound looks at it's cache;
• if the record is there it will be ignored because serve-expired-ttl is enforced at 2hours;
• then redis will be searched and if the record is there it will also be ignored as per above.

By looking at the code I am not confident that the last point is correct.
Are you asking because you get unexpected results and can verify my suspicion?

[mibere]

then redis will be searched and if the record is there it will also be ignored as per above.

That's the reason for my question. I just wanted to know if also the entries in Redis will be ignored - at least that's what I would expect & hope.

By looking at the code I am not confident that the last point is correct.

Can you maybe take a closer look?

In the above scenario:

1. Unbound ignores the expired entry in it's internal cache
2. Unbound ignores the expired entry in Redis
3. Unbound resolve the query as usual
4. Unbound updates the expired entry in it's internal cache with the new result
5. Unbound updates the expired entry in Redis with the new result

[mibere]

@gthess any new information here?

[gthess]

It seems that cachedb (the module that communicates with Redis) does not respect the serve-expired-ttl option.

I am working on the serve-stale implementation on unbound and when revisiting the code for that I will make sure that serve-expired-ttl is honored there.

[mibere]

Ok, thanks for the response. Hope to see that cachedb honores serve-expired-ttl then.

[yeezylife]

Could you tell me whats the best setting for "serve-expired-ttl",and what would happen if I don't set“serve-expired-ttl”at all？ （Just “serve-expired: yes”）@gthess

[gthess]

Hi @yeezylife,
Such information can already be found in the unbound.conf man page. (Also online at https://www.nlnetlabs.nl/documentation/unbound/unbound.conf/#serve-expired-ttl).

If you are looking for Unbound experiences, general questions about Unbound, its operations and how you can achieve something, we have the Unbound community support mailing list, see https://nlnetlabs.nl/mailman/listinfo/unbound-users.