[Dnsmasq-discuss] --server=/example/8.8.8.8 --server=/example/9.9.9.9 behaviour

[Simon Kelley]

On 16/04/2021 17:26, Petr Menšík wrote:
> Hi all,
> 
> I am trying to improve dnsmasq to handle better redirections of some
> given domains to multiple servers. Current implementation allows
> specifying multiple domains in single --server= statement, but only one
> server. It can be specified by multiple --server statements.
> 
> But some people would like domain specific forwards to choose outgoing
> forwarders in similar way to common forwarders. By common forwarders I
> mean multiple --server=<ip> without any domain specification, the same
> way /etc/resolv.conf parsing with at least two nameservers works.
> Current implementation chooses the best responding server (last_server)
> and only sometime tries also other servers.
> 
> I would like to implement similar logic not only for global forwarders,
> but also for any domain specific forwarders, where multiple forwarders
> are configured. Current behavior is to send queries to all
> domain-specific forwarders used for the domain. Even if three forwarders
> are configured and all are perfectly working, requests would be
> forwarded to all of them for every single query. It is bombarding all of
> them, but just the first reply would be forwarded back to client and be
> used. Is there a good reason for such behaviour? Are multiple servers
> for a domain considered exceptional?

There's no good reason. The excuse is that the current code is too poor
to make it easy. Hysterical Reasons!
> 
> --rev-server for example allows only single target IP specified, even if
> syntax would allow more easily. In DNS world a zone usually requires at
> least two servers handling it.
> 
> Current code walks multiple times daemon->servers list. First it finds
> longest domain match in search_servers() function.
> All servers are checked, when FORWARD_TEST queries were sent or
> FORWARD_TIME elapsed since last all servers check, whichever is sooner.
> Also on error responses. Then it walks the same list again, starting on
> daemon->last_server, sending forwarded queries. Unless
> forward->forwardall is set, only last_server is used. For domain
> specific forwarders, forwardall is always set.
> 
> It seems to me each domain should have at least a structure:
> struct server_domain {
>   char *domain;
>   struct server *last_server;
>   time_t forwardtime;
>   int forwardcount;
>   struct server_domain *next;
> };
> 
> Where it could store forwardcount, forwardtime and last_server for each
> different domain. I think it would make sense to add struct server
> *servers and int flags and iterate only servers for given domain on each
> forwarded query. But required changes for that seem to be huge, I failed
> to prepare working patch yet. Minimal change would use domain found by
> search_servers and just find server_domain structure with matching domain.
> 
> What do you think? Do you use multiple servers for custom domains?

This is definitely the area of dnsmasq most in need of re-writing. This
links in with another discussion on here, where people are using
thousands of

 local=/adserver.domain/

lines in their config, which runs slowly and uses lots of memory.



Cheers,

Simon.

> 
> Cheers,
> Petr
> 
> 
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