[Dnsmasq-discuss] dnsmasq stops receiving packets after network restart

[Simon Kelley]

On 24/09/18 19:12, Kristian Evensen wrote:
> Hello,
> 
> I have some routers running OpenWRT (latest nightly) and that I have
> to access remotely (using reverse SSH). When I restart networking
> (/etc/init.d/network restart), clients on the LAN can no longer obtain
> an IP address using DHCP. If I restart networking locally, DHCP works
> as expected after the network is back up.
> 
> In order to try and figure out what is going on, I have checked/tried
> the following:
> 
> * I started out by checking if dnsmasq has been restarted and if the
> DHCP socket has been created. I can always see the socket in netstat.
> * I then took a look at the firewall. I can see the DHCP packets in
> the INPUT chain in filter, which according to my understanding of
> Netfilter-internals is the last stop before a packet is delivered to a
> socket.
> * I then instrumented dnsmasq and added some logging in dhcp_packet()
> in dhcp.c. This function is never called, as none of my log-messages
> are written to syslog. I checked that the logging works by checking
> for my messages when DHCP is working.
> * Restarting dnsmasq makes DHCP work again. I can't see any difference
> in for example netstat-output.
> 
> Does anyone have any idea on what to try or where to look next? After
> having spent a couple of days on this issue, I am quickly starting to
> run out of ideas.
> 

I wonder if this is caused by dnsmasq using the BINDTODEVICE sockopt on
the DHCP socket. If the networking restart takes down and re-creates the
network interface, then that socket may be remain bound to the old
interface.

This comment in whichdevice() in dhcp-common.c decribes the condition
under which the binding happens.

 /* If we are doing DHCP on exactly one interface, and running linux, do
SO_BINDTODEVICE
     to that device. This is for the use case of  (eg) OpenStack, which
runs a new
     dnsmasq instance for each VLAN interface it creates. Without the
BINDTODEVICE,
     individual processes don't always see the packets they should.
     SO_BINDTODEVICE is only available Linux.

     Note that if wildcards are used in --interface, or --interface is
not used at all,
     or a configured interface doesn't yet exist, then more interfaces
may arrive later,
     so we can't safely assert there is only one interface and proceed.
*/

Simplest test is to make whichdevice always return NULL, and see if that
helps.


Cheers,

Simon.