[Dnsmasq-discuss] dnsmasq using 100% of cpu

[Geert Stappers]

On Mon, Feb 17, 2020 at 08:32:49PM +0000, Simon Kelley wrote:
> 
> 
> On 17/02/2020 13:31, Donald Sharp wrote:
> > Running:
> > 
> > sharpd at eva:~/dnsmasq$ /sbin/dnsmasq --version
> > Dnsmasq version 2.80  Copyright (c) 2000-2018 Simon Kelley
> > Compile time options: IPv6 GNU-getopt DBus i18n IDN DHCP DHCPv6 no-Lua
> > TFTP conntrack ipset auth DNSSEC loop-detect inotify dumpfile
> > ----
> > 
> > When I install several hundred thousand routes into the kernel and
> > remove them( or some variation thereof ), dnsmasq eventually ends up
> > running 100% cpu:
> > 
> > top - 18:45:18 up 1 day,  7:44,  1 user,  load average: 2.70, 2.65, 2.34
> > Tasks: 424 total,   3 running, 421 sleeping,   0 stopped,   0 zombie
> > %Cpu(s): 12.1 us,  6.9 sy,  0.0 ni, 80.2 id,  0.0 wa,  0.0 hi,  0.7 si,
> >  0.0 st
> > MiB Mem :  32131.3 total,  19483.6 free,   6620.3 used,   6027.4 buff/cache
> > MiB Swap:  32718.0 total,  31693.0 free,   1025.0 used.  24698.2 avail Mem
> > 
> >     PID USER      PR  NI    VIRT    RES    SHR S  %CPU  %MEM     TIME+
> > COMMAND                            
> >  293183 nobody    20   0   11040   2040   1688 R  99.7   0.0 148:48.40
> > dnsmasq        
> > 
> > strace output:
> > 
> > poll([{fd=3, events=POLLIN}, {fd=4, events=POLLIN}, {fd=5,
> > events=POLLIN}, {fd=6, events=POLLIN}, {fd=7, events=POLLIN}, {fd=8,
> > events=POLLIN}], 6, -1) = 1 ([{fd=4, revents=POLLERR}])
> > poll([{fd=3, events=POLLIN}, {fd=4, events=POLLIN}, {fd=5,
> > events=POLLIN}, {fd=6, events=POLLIN}, {fd=7, events=POLLIN}, {fd=8,
        ...
> > poll([{fd=3, events=POLLIN}, {fd=4, events=POLLIN}, {fd=5,
> > events=POLLIN}, {fd=6, events=POLLIN}, {fd=7, events=POLLIN}, {fd=8,
> > events=POLLIN}], 6, -1) = 1 ([{fd=4, revents=PO^Cstrace: Process 293183
> > detached
> > 
> > I can pretty much make this happen at will.  What can I provide to help
> > debug this?
> 
> The first thing I'd like to know is what file descriptor 4 is, providing
> us with the first (say) 500 or 1000 lines of strace output would help
> with that.
> 
> 
> > 
> > As a side note, I was not placing these routes into the default linux
> > routing table.  Does dnsmasq need to be paying attention to these routes?
> > 
> 
> 
> To save typing I've just pasted a comment from the code which explains
> why adding routes affects dnsmasq
> 
>      /* We arrange to receive netlink multicast messages whenever the
> network route is added.
>          If this happens and we still have a DNS packet in the buffer,
> we re-send it.
>          This helps on DoD links, where frequently the packet which
> triggers dialling is
>          a DNS query, which then gets lost. By re-sending, we can avoid
> the lookup
>          failing. */
> 
> 
> I suspect that  the solution to this is to restrict the above to the
> "main" routing table.
> 

Matching that with "[PATCH] Ignore routes in non-main tables"
 ( http://lists.thekelleys.org.uk/pipermail/dnsmasq-discuss/2020q1/013824.html )



Regards
Geert Stappers
-- 
Silence is hard to parse