On 16/05/2022 17:27, Brad Silva wrote:
I ran into an issue with DHCP on DNSmsaq 2.85.
It's running on a host with two IP addresses on the same network. The
primary IP is .12 and the secondary is .10. DNSmasq was put in place to
replace a missing server (long story), so it was setup to listen on the
missing server's IP of .10.
The DHCP Relay device was sending requests to DNSmasq at .10, but
DNSmasq was replying to the request using the .12 IP address, which the
DHCP relay device promptly ignored.
I tried fixing the problem using various combinations of "interface="
and "bind-interface", but the reply was always being sent using the
primary IP (.12). I finally reversed the primary and secondary IP
addresses and the problem is fixed.
However, is there a proper way to fix this?
Or, is this a bug/limitation?
I think bug/limitation is the best way to describe it :)
The DHCP code doesn't bother about the source address in replies at all
(unlike the DNS code, which takes great care to use the destination
address of the request as the source address of the reply) It just lets
the kernel use its default, which is the primary address, as you found.
Most DHCP software ignores the header source address: the address of the
server to talk back to is in the "server address" field inside the DHCP
packet anyway, and that's what gets used as the ID of the server. Since
requests may be broadcast, there's not always a good value to set it to
anyway.
TL;DR I think your relay is not abiding by Postel's Law, but arguably
neither is dnsmasq.
It should be possible to make dnsmasq do the obvious better thing, but
it's not totally trivial since is uses bits of the API which are not
standard between Linux and BSD-derived systems.
Simon.
Config file and tcpdump capture below.
Thanks,
Brad
15:18:22.216760 IP 172.28.141.1.bootps > 172.28.8.10.bootps: BOOTP/DHCP,
Request from xx:xx:xx:xx:xx:xx (oui Unknown), length 266
15:18:23.578420 IP 172.28.141.1.bootps > 172.28.8.10.bootps: BOOTP/DHCP,
Request from xx:xx:xx:xx:xx:xx (oui Unknown), length 266
15:18:25.310501 IP 172.28.8.12.bootps > 172.28.141.1.bootps: BOOTP/DHCP,
Reply, length 300
15:18:25.311037 IP 172.28.8.12.bootps > 172.28.141.1.bootps: BOOTP/DHCP,
Reply, length 300
15:18:26.137923 IP 172.28.141.1.bootps > 172.28.8.10.bootps: BOOTP/DHCP,
Request from xx:xx:xx:xx:xx:xx (oui Unknown), length 308
15:18:29.142546 IP 172.28.8.12.bootps > 172.28.141.1.bootps: BOOTP/DHCP,
Reply, length 300
15:18:29.155150 IP 172.28.141.1.bootps > 172.28.8.10.bootps: BOOTP/DHCP,
Request from xx:xx:xx:xx:xx:xx (oui Unknown), length 266
15:18:29.155816 IP 172.28.8.12.bootps > 172.28.141.1.bootps: BOOTP/DHCP,
Reply, length 300
15:18:35.169175 IP 172.28.141.1.bootps > 172.28.8.10.bootps: BOOTP/DHCP,
Request from xx:xx:xx:xx:xx:xx (oui Unknown), length 266
15:18:35.169933 IP 172.28.8.12.bootps > 172.28.141.1.bootps: BOOTP/DHCP,
Reply, length 300
listen-address=127.0.0.1,172.28.8.10
#auto-expand simple queries, ones with no domain
expand-hosts
interface=eth0:1,lo
bind-interfaces
domain=myhouse.net <http://myhouse.net>
server=172.28.10.205
dhcp-range=set:tag0,172.28.8.100,172.28.8.200,255.255.255.0,168h
dhcp-option=tag:tag0,option:router,172.28.8.1
dhcp-option=tag:tag0,6,172.28.8.172.28..1.10.205
dhcp-range=set:tag1,172.28.16.100,172.28.16.200,255.255.255.0,168h
dhcp-option=tag:tag1,option:router,172.28.16.1
dhcp-option=tag:tag1,6,172.28.8.172.28..1.10.205
dhcp-range=set:tag2,172.28.140.100,172.28.140.200,255.255.255.0,1h
dhcp-option=tag:tag2,option:router,172.28.140.1
dhcp-option=tag:tag2,6,8.8.8.8,8.8.4.4
dhcp-range=set:tag3,172.28.141.100,172.28.141.200,255.255.255.0,1h
dhcp-option=tag:tag3,option:router,172.28.141.1
dhcp-option=tag:tag3,6,8.8.8.8,8.8.4.4
_______________________________________________
Dnsmasq-discuss mailing list
Dnsmasq-discuss@lists.thekelleys.org.uk
https://lists.thekelleys.org.uk/cgi-bin/mailman/listinfo/dnsmasq-discuss
_______________________________________________
Dnsmasq-discuss mailing list
Dnsmasq-discuss@lists.thekelleys.org.uk
https://lists.thekelleys.org.uk/cgi-bin/mailman/listinfo/dnsmasq-discuss
