On 13/09/2019 13:37, Tore Anderson wrote:
> * Tore Anderson
> 
>> Start out with the following /etc/dnsmasq.conf, replacing «wlp2s0» as 
>> appropriate:
>>
>> log-queries
>> no-hosts
>> no-resolv
>> server=1.1.1.1@wlp2s0
>>
>> Start Dnsmasq and send it a TCP query:
>>
>> $ src/dnsmasq -d -p 5333
> 
> Bisected:
> 
> 305ffb5ef0ba5ab1df32ef80f266a4c9e395ca13 is the first bad commit
> commit 305ffb5ef0ba5ab1df32ef80f266a4c9e395ca13
> Author: Simon Kelley <si...@thekelleys.org.uk>
> Date:   Sat Mar 16 18:17:17 2019 +0000
> 
>     Improve kernel-capability manipulation code under Linux.
>     
>     Dnsmasq now fails early if a required capability is not available,
>     and tries not to request capabilities not required by its
>     configuration.
> 
> :100644 100644 b942ec269cc6c1b7614a9d57cb0b9468507f031c 
> f2d38a0f9bb73b4f480cd323f49cd574fc3e2744 M      CHANGELOG
> :040000 040000 a4dd29e7fbdac449dd9b502e012beb2c25a47387 
> 7b0eb0f197c0cb857981c607be8b08d62cee9ff3 M      src
> 
> After some more debugging I realised that this is a heisenbug.
> 
> Starting Dnsmasq with the «-d» option does not accurately reproduce the 
> problem, since it will not drop privileges in debug mode.
> 
> To me it looks as if using a server specified with an interface requires root 
> privileges.
> 
> Thus, to trigger the actual bug, there are two options:
> 
> 1) Start Dnsmasq as non-root (broken on any version, at least since v2.80).
> 2) Start Dnsmasq as root (this works in v2.80, but is broken since 305ffb5 
> presumably because Dnsmasq now drops privileges it is going to need later on).
> 


Nicely analysed. My guess is that the code which determines (at startup)
if the process needs to keep CAP_NET_BIND_SERVICE when it drops root
fails in this case. If this is corrected, then starting dnsmasq with
this config as non-root should fail at startup.

Back in a mo.....


Simon.




> Tore
> 
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> 


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