[Dnsmasq-discuss] Re: resolver options

[Simon Kelley]

AJ Weber wrote:
> But, technically, isn't dnsmasq the "client" to the upstream DNS 
> servers?  Don't YOU have control over how long you wait for a reply?  
> Couldn't we have a config-param, similar to how I envisioned this resolv 
> option would work, where if dnsmasq has multiple upstream nameserver's, 
> we could specify the timeout for a reply in ms or seconds?  This isn't 
> for a "no such domain", this is for a no-reply (host is down or DNS is 
> not responding on that host, etc.).

That could be done, but it won't make the client which originated the 
DNS query time out any faster, for the reason a gave before. It would 
also be a large change to dnsmasq: currently dnsmasq can't retry a query 
after a timeout, because it doesn't have a copy of the query. This means 
that dnsmasq's memory requirements are small and bounded: it recieves a 
query, forwards it upstream, and saves just enough information to 
process the reply. The memory holding the query is then free for re-use.
Dnsmasq relies on the client retrying and sending another copy of the 
query for retries. This is a great advantage on small routers which are 
short of memory.
>  
> If you did that, we could even have an option to send the request to all 
> (or some fraction) upstream servers in parallel and just take the first 
> reply.  I don't know if they'd call us a bad Inet citizen if we did 
> that, though.  But, hey, we're making-up for it by caching the reply, 
> right? ;)

Actually it does this already. At startup, it sends the query in 
parallel to all servers, "running a race" to see which is fastest. 
Whichever server replies first becomes "it" and gets queries from then 
on. If any query times out and is retried, the race gets re-run to 
select a new prefered server. This seems to work pretty well in practise.


Cheers,

Simon.



>  
> -AJ
>  
> 
>     ----- Original Message -----
>     *From:* Simon Kelley <mailto:simon at thekelleys.org.uk>
>     *To:* AJ Weber <mailto:aweber at comcast.net>
>     *Cc:* dnsmasq-discuss at lists.thekelleys.org.uk
>     <mailto:dnsmasq-discuss at lists.thekelleys.org.uk>
>     *Sent:* Tuesday, February 06, 2007 4:34 PM
>     *Subject:* Re: resolver options
> 
>     AJ Weber wrote:
>      >
>      > Thank you for your quick reply...
>      > 
>      > RE: "options in the custom resolver file", should we have the
>     ability to
>      > propagate some of these options?  For example, timeout, attempts and
>      > rotate?  I think they could be useful in certain situations (like my
>      > own...I don't want to have to update all my clients resolv
>     interaction
>      > -- or can this be set with a dhcp option of sorts?).
> 
>     It's not really possible: don't forget that all this is happening via
>     UDP: a client sends a request which is a UDP packet, and gets back an
>     answer (another UDP packet) or times out. If, for instance, the client
>     timeout is 10 seconds, and the dnsmasq one 5, dnsmasq has no way to
>     tell
>     the client to give up after 5 seconds except by returning "no such
>     domain", which is a lie and could cause all sort of problems if the
>     client really believes it. Really the only way is to adjust the timeout
>     at the client end.
> 
>     "rotate" is normally irrelevant when the client only has one nameserver
>     - dnsmasq, and "attempts" should be at least 2 (the default). dnsmasq
>     has quite complex methods for chosing between multiple upstream servers
>     which somewhat depend on clients retrying at least once.
> 
>      > 
>      > RE: The existing leases...I knew the leases were cached in a
>     file, but I
>      > didn't know that when dnsmasq starts it actually reads the file and
>      > leverages that information.  Is that documented somewhere?
>      > 
>     I think it's just assumed that that's the function of that file - a
>     persistent lease database which survives dnsmasq restarts and reboots.
> 
>     As an aside, things work pretty well even without the persistent
>     database. If the ratio of available addresses to clients is reasonably
>     high then clients will always get given the same address anyway. The
>     main problem is that the client name disappears from DNS after a
>     restart
>     until the client renews  its lease.
> 
>      > I've said it before, and I'll say it again:  This is a fantastic
>     piece
>      > of software!
>     *blush*
> 
>      > 
>      > Thanks again,
>      > AJ
>      > 
>     Cheers,
> 
>     Simon.
> 
>      >
>      >     ----- Original Message -----
>      >     *From:* Simon Kelley <mailto:simon at thekelleys.org.uk>
>      >     *To:* AJ Weber <mailto:aweber at comcast.net>
>      >     *Cc:* dnsmasq-discuss at lists.thekelleys.org.uk
>     <mailto:dnsmasq-discuss at lists.thekelleys.org.uk>
>      >     <mailto:dnsmasq-discuss at lists.thekelleys.org.uk>
>      >     *Sent:* Tuesday, February 06, 2007 12:42 PM
>      >     *Subject:* Re: resolver options
>      >
>      >     AJ Weber wrote:
>      >      >
>      >      > If I use a "different" resolv.conf file (i.e. I use a
>      >      > resolv-file=/etc/resolv.dnsmasq), will an "option" have
>     the same
>      >     effect
>      >      > it normally would?
>      >      >
>      >      > For example, I'm considering changing timeout to 2
>     (instead of the
>      >      > default of 5), because I've had some issues with my ISP's DNS
>      >     servers
>      >      > recently and don't want to wait too long for the timeout. 
>     With the
>      >      > broadband connection, any of the nameservers in my list
>     consistenly
>      >      > returns in < 1 sec (when they're up, of course).
>      >      >
>      >      > Will this have the desired effect?  Any comments or
>     concerns with
>      >     that
>      >      > change in general?  Am I misunderstanding the purpose of the
>      >     "options
>      >      > timeout:" ?
>      >
>      >     The only lines which are read by dnsmasq are "nameserver" ones -
>      >     everything else is ignored, except "search" if the option
>     --domain=# id
>      >     used, when the first entry of the search string is used as
>     dnsmasq's
>      >     domain setting.
>      >
>      >     To get the effect of faster time-outs, it's necessary to
>     alter the
>      >     timeout in the clients, not in dnsmasq.
>      >      >
>      >      > ALSO...(DIFFERENT TOPIC) : Is there no way to tell dnsmasq to
>      >     dump it's
>      >      > current lease-info to a file that could be read on restart
>     so it
>      >     knows
>      >      > about all outstanding DHCP leases?  It's rare that I have
>     to restart
>      >      > dnsmasq or the linux box it's on, but when I do, it forces
>     me to
>      >     restart
>      >      > a lot of client machines in case I'd get a dhcp/IP address
>     mix-up
>      >     (where
>      >      > dnsmasq doesn't know an IP Address is already leased to a
>     running
>      >      > client, and a new client requests a lease so it hands-out
>     the same
>      >      > address).
>      >
>      >     Dnsmasq does this by default, in
>     /var/lib/misc/dnsmasq.leases. You can
>      >     change the location of the lease file using --dhcp-leasefile.
>      >
>      >     Cheers,
>      >
>      >     Simon.
>      >
>      >      >
>      >      > Thanks in advance,
>      >      > AJ