[systemd-devel] Extend systemd-resolved service to override DNS response

[Barry Scott]

> On 15 Feb 2023, at 10:31, Aditya Sharma <aditya.sharma1128 at gmail.com> wrote:
> 
> Hi Kevin,
> 
> When the TTLs expire, those records in the cache become 'stale', and are normally purged. Your request is to have an option in systemd-resolved to *not* purge those records, but to continue serving them in case it is unable to communicate with the configured recursive resolver(s).
> 
> Sorry for the ambiguity.
> What I meant was to keep serving the record after the resolvers are not operational or during some outage.
> We were thinking of an approach where we keep on serving the last known good FQDNs even after the TTL has expired (only when we are unable to communicate with the resolvers). For that, we need to intercept the DNS calls and maintain some kind of local cache. So, wanted to understand how we can extend systemd-resolved service to serve the purpose.

I would be worried that breaking the TTL caching rules will create more problems then it solves.
Isn't the underlying issue that you have unreliable DNS servers that are important to your application?

Barry



> 
> Thanks
> Aditya
> 
> 
> On Tue, 14 Feb 2023 at 16:46, Kevin P. Fleming <lists.systemd-devel at kevin.km6g.us <mailto:lists.systemd-devel at kevin.km6g.us>> wrote:
>> On Tue, Feb 14, 2023, at 04:04, Aditya Sharma wrote:
>>> Hi Kevin,
>>> 
>>> If what you mean is that you want to serve 'stale' records from a cache when their TTLs have expired and the authoritative servers which provided them are not reachable, that's something that a number of existing recursive resolvers are able to do and it could be logical for systemd-resolved to offer it too.
>>> We are looking to prepare a solution similar to this, to serve back records for FQDNs in case of timeout from the DNS server.
>>> We want to understand how we can extend systemd-resolved to override response from DNS server in case of timeouts/failures.
>> 
>> Again, you need to be very specific in your request.
>> 
>> systemd-resolved communicates with one or more recursive resolvers (what you are calling "DNS server", but that term is ambiguous). If those resolvers are not operational, systemd-resolved will continue serving records from its cache (if the cache is enabled), until their TTLs expire.
>> 
>> When the TTLs expire, those records in the cache become 'stale', and are normally purged. Your request is to have an option in systemd-resolved to *not* purge those records, but to continue serving them in case it is unable to communicate with the configured recursive resolver(s).
>> 
>> In your original message you referred to a 'negative response' from the "DNS server", but that's a completely different situation.
>> 

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