[Dnsmasq-discuss] Different behaviour with "interface=" on two almost identical systems

[Chris Buechler]

On Sat, Mar 30, 2024 at 4:39 AM Chris Green <cl at isbd.net> wrote:
>
> On Fri, Mar 29, 2024 at 03:45:20PM -0500, Chris Buechler wrote:
> >    On Thu, Mar 28, 2024 at 11:40 AM Chris Green <[1]cl at isbd.net> wrote:
> >
> >      I did try stopping dnsmasq and then nothing was listening on port
> >      53.
> >      However, just to prove it:-
> >      root at backup:~# ss -tulpn sport = 53
> >          Netid State  Recv-Q Send-Q Local Address:Port  Peer Address:Port
> >      Process
> >          udp   UNCONN 0      0            [2]0.0.0.0:53
> >      0.0.0.0:*     users:(("dnsmasq",pid=1826,fd=4))
> >          udp   UNCONN 0      0               [::]:53            [::]:*
> >       users:(("dnsmasq",pid=1826,fd=6))
> >          tcp   LISTEN 0      32           [3]0.0.0.0:53
> >      0.0.0.0:*     users:(("dnsmasq",pid=1826,fd=5))
> >          tcp   LISTEN 0      32              [::]:53            [::]:*
> >       users:(("dnsmasq",pid=1826,fd=7))
> >
> >    That's the nature of how dnsmasq binds by default. You need two config
> >    lines to actually bind only to localhost:
> >    listen-address=127.0.0.1
> >    bind-interfaces
> >    The listen-address config will make it only reply to queries to
> >    localhost, but doesn't prevent wildcard binding. bind-interfaces makes
> >    it bind explicitly to the listen-address. See the bind-interfaces
> >    section of the man page for details.
> >    [4]https://thekelleys.org.uk/dnsmasq/docs/dnsmasq-man.html
> >
> OK, thank you, I did read those man page entries but found it rather
> confusing!  It has fixed the problem though! :-)
>
> I still don't quite understand why it seemed to work on one system but
> not on another almost identical one.

Glad that worked for you. The difference is likely either the other
system already had bind-interfaces in its config somewhere so it's not
binding wildcard, or otherwise has a host firewall blocking port 53 if
it is wildcard binding.