[Dnsmasq-discuss] Partial denial of service with dnsmasq on resource constrained systems

Gordon Shawn capcoding at gmail.com
Mon Apr 5 15:16:24 UTC 2021

> Date: Thu, 1 Apr 2021 22:11:17 -0400
> From: "Neal P. Murphy" <neal.p.murphy at alum.wpi.edu>
> To: dnsmasq-discuss at lists.thekelleys.org.uk
> Subject: Re: [Dnsmasq-discuss] Partial denial of service with dnsmasq
>         on resource constrained systems
> Message-ID: <20210401221117.47313352 at playground>
> Content-Type: text/plain; charset=US-ASCII
> On Thu, 1 Apr 2021 23:55:08 +0100
> Simon Kelley <simon at thekelleys.org.uk> wrote:
> > >
> > > One other thing I saw while testing with large blocklists was a
> noticeable
> > > latency increase, likely related to lookup times. I recall some
> discussion
> > > on the ML where you mentioned work on a hash/tree solution was in
> > > progress. Were those changes completed?
> > >
> >
> >
> > This seems to be the crucial aspect here: large blocklists. Is we move
> > the large blocklists to a subsystem designed to handle them, then the
> > problem goes away.
> >
> > I could do with a handle on exactly how people are configuring dnsmasq
> > to do ad blocking. It's not something I have much experience of.
> On Smoothwall Express, I've conf'ed dnsmasq to 'undefine' a large number
> of FQDNs using the form 'local=/8teenporno.com/' I pull the Shalla data
> and use the ads, pron, warez, and a few other categories.
> 768 000 FQDNs makes dnsmasq use around 100MiB of RAM. On an Atom N270
> running SWE, response time is generally in the range of 75 ms to 100 ms
> when there's no traffic. With the DL saturated (using speedtest.net),
> response times range from 500ms to 2s. Saturated UL doesn't seem to affect
> response time much.
> I've been satisfied with its operation; I see almost no ads and pretty
> much nothing in the other categories I use.
> N
> This is still about 130 bytes per FQDN, sounds like a lot for the RAM

The key issue to me with large lists, is that addn-hosts etc can be
reloaded by SIGHUP and dnsmasq can be restarted very quickly which is fine,
however when  you have large conf files with many --local or --address
lines, a full dnsmasq restart is required and it can take seconds or
minutes which is very bad when you need update the blocklists.

It will be nice for local/address/cname lines be parsed just like the
addn-host files, i.e. with a SIGHUP they can be quickly parsed and used
instead of being part of  the full blown dnsmasq.conf for a time consuming

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