[Dnsmasq-discuss] /etc/resolv.conf polling limitation

[Simon Kelley]

On 22/03/2022 01:42, Preston Crow wrote:
> I just ran into an interesting issue:
> 
> In my config, I set: resolv-file=/etc/resolv.conf.upstream
> 
> If I edit that file, I instantly see dnsmasq reload things as expected 
> with various log messages.  That's good.
> 
> However, in my case, /etc/resolv.conf.upstream is a symlink.  If I 
> change the symlink to point to a different file, dnsmasq doesn't 
> notice.  I suspect it's using inotify to have the kernel tell it if the 
> file is changed, and it isn't checking for a symlink that also needs to 
> be watched.

Looking at the code, dnsmasq actually sets an inotify on the _directory_ 
containing the file. It is aware of symlinks, so if 
/etc/resolv.conf.upstream is a symlink to /conf/resolv.conf then it will 
watch the contents of /conf

However, all this is set up at startup time, so if you
1) start dnsmasq
2) change the link to /conf2/resolv.conf

then the inotify on /conf2 will not be in place until the next time 
dnsmasq starts.

If you'd moved the link to another file in /conf, it would have worked.

Similar, but subtly  different.

Simon.
> >
> I'm not sure if this qualifies as a bug.  If this is to be defined as 
> the expected behavior, then I would suggest adding a note to the 
> documentation on the polling pointing out that changes to symlinks won't 
> be noticed.
> 
> 
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