On 13/12/2018 14:10, Craig Younkins wrote:
> First, thank you for dnsmasq!
> 
> I'm among a number of people[1][2][3][4] having trouble using dnsmasq
> for DHCP when it is running in a docker container. Everyone seems to get
> "no address range available for DHCP request via eth0" in their log
> unless they change to host networking mode.
> 
> The code path for that error message is at [5]. I'm having a little
> trouble understanding the 'contexts', but I think the problem is that
> the container is running in bridged networking mode, and thus the
> interface has an IP address outside the netmask range.
> 
> Is there a way to make this work without using host networking? Maybe
> adding the external IP to the container interface? Thank you for any
> suggestions!
> 


I'm not familiar with these docker "networking modes", can you explain
what they mean?


What's happening here is quite straightforward to understand.  A DHCP
request arrives at an interface which has the IP address 172.17.0.2 and
netmask 172.17.255.255. Dnsmasq tries to find a dhcp-range from which is
can allocate an address by looking for a DHCP range which covers the
same network. Since the only available DHCP range is
192.168.1.200,192.168.1.251 and that's not the same network, this fails.


Depending on exactly how docker is set up, something equivalent to the
ISC dhcpd's "shared-network" configuration might be the way to go. This
is a useful facility which I've considered adding before, essentially,
is allows you to tell dnsmasq that (in this case) 172.17.0.0/16 and
192.168.1.0/24 are both on the same network segment or broadcast domain.
That would allow dnsmasq to deduce that the request which comes from the
172.17.0.0/16 segment can be satisfied by a 192.168.1.0/24 address.

Note that there are other requirements needed to make this work.
Notably, a DHCP client that gets a 192.168.1.0/24 address has to have
suitable routing to allow it to route packets to 172.17.0.2, and the
reverse route is also needed.

To be clear: shared-network doesn't exist in released versions of
dnsmasq, I'm proposing new code.


Cheers,

Simon.


> Relevant sample configuration:
> addn-hosts=/etc/pihole/gravity.list
> addn-hosts=/etc/pihole/black.list
> addn-hosts=/etc/pihole/local.list
> localise-queries
> no-resolv
> cache-size=10000
> log-queries=extra
> log-facility=/var/log/pihole.log
> local-ttl=2
> log-async
> server=8.8.8.8
> server=8.8.4.4
> interface=eth0
> dhcp-authoritative
> dhcp-range=192.168.1.200,192.168.1.251,24h
> dhcp-option=option:router,192.168.1.1
> dhcp-leasefile=/etc/pihole/dhcp.leases
> domain=local
> 
> root@6082bda95199:/# ip a
> 1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN
> group default qlen 1000
>     link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
>     inet 127.0.0.1/8 <http://127.0.0.1/8> scope host lo
>        valid_lft forever preferred_lft forever
> 10: eth0@if11: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue
> state UP group default
>     link/ether 02:42:ac:11:00:02 brd ff:ff:ff:ff:ff:ff link-netnsid 0
>     inet *172.17.0.2/16 <http://172.17.0.2/16>* brd 172.17.255.255 scope
> global eth0
>        valid_lft forever preferred_lft forever
> 
> To reproduce, you can run something like what is in [6], then enabling
> the DHCP server through the non-ssl web interface. `docker exec -it
> pihole /bin/bash` to get into the container and `tail -f
> /var/log/pihole.log` for the log. 
> 
> [1] https://github.com/pi-hole/docker-pi-hole/issues/355
> [2] https://discourse.pi-hole.net/t/dhcp-not-working-docker/12593
> [3] 
> https://discourse.pi-hole.net/t/no-address-range-available-for-dhcp-request-via-eth0/14350
> [4] 
> https://serverfault.com/questions/825497/running-dnsmasq-in-docker-container-on-debian-check-dhcp-ignores-dnsmasq
> [5] 
> http://thekelleys.org.uk/gitweb/?p=dnsmasq.git;a=blob;f=src/rfc2131.c;h=56dc3d103741baeb68a730f0ce15a10338a2f885;hb=91421cb7575df7bb211dacc30dc7c7c715c38299#l345
> [6] https://github.com/pi-hole/docker-pi-hole/blob/master/docker_run.sh
> 
> Craig Younkins
> 
> _______________________________________________
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> 


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