Re: [lwip-users] Split DNS for multihomed setup

[Radouch, Zdenek]

> Can you pass a nameserver to use to resolving functions normally?

No. As you suspect the name server is never passed to these functions.
The reason for that is that in context of a DNS query the name server address
is static, independent of the query. 
Of course for debugging purposes, when something does not work and I need to
simulate the resolver work (by querying by hand from a command line), I make 
sure
to specify the name server address. Note that the name server address is the 
first
argument to "dig". But this is needed only because something does not work 
correctly.

> At least I have never used such functions but always relied on the OS to 
> select the correct namesever.

Well,  technically the OS does not select the server, nor does it select the 
"correct" server.
You, your admin, or the DHCP server configured by you, or by your admin, 
configure the DNS resolver
giving it the addresses of one or more name servers you want to be used later 
when you run your DNS query.
The resolver then iterates over the name servers which themselves may recurse 
on the query (contact
another name server) until the name is translated or until there is no more 
name server to try.
So there actually isn't a "correct" server, all you can say is the order in 
which the servers will be contacted.

Cheers,
-Z

________________________________________
From: address@hidden address@hidden on behalf of Simon Goldschmidt 
address@hidden
Sent: Saturday, August 16, 2014 1:55 PM
To: Mailing list for lwIP users
Subject: Re: [lwip-users] Split DNS for multihomed setup

Sergio R. Caprile wrote:

> This to me is the same source routing issue and you will
> face it for every application in which you need some specific
> destination associated to a specific source address/interface.

It is, but this one is an additional problem to make one application use the 
source based routing. And DNS is one of the few application level protocols 
that are shipped with core lwIP.

And no, I don't have a better idea than the suggested changes you made to 
dns.c. But is this anything like standard dns? Can you pass a nameserver to use 
to resolving functions normally? At least I have never used such functions but 
always relied on the OS to select the correct namesever.

Simon

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