[Dnsmasq-discuss] Multiple interfaces responding to DHCP requests

[Geert Stappers]

On Thu, May 13, 2021 at 01:05:07PM +0100, Nick Howitt wrote:
> I am trying to help someone who has a set up with three LAN's, all on
> different subnets and all acting as DHCP servers. He is getting an odd
> result that when a device on the enp2s0 LAN requests an IP, both enp2s0 and
> enp3s0 respond with IP's. I've never seen this before and my own server does
> not act this way.
> 
> From an nmap scan from a device on the enp2s0 LAN:
> ubuntu-local at latitude-e7470:~$ sudo nmap --script=broadcast-dhcp-discover -e enp0s31f6
> Starting Nmap 7.91 ( https://nmap.org <https://nmap.org> ) at 2021-05-08 11:23 EDT
> Pre-scan script results:
> | broadcast-dhcp-discover:
> |   Response 1 of 2:
> |     Interface: enp0s31f6
> |     IP Offered: 192.168.1.214
> |     DHCP Message Type: DHCPOFFER
> |     Server Identifier: 192.168.1.1
> |     IP Address Lease Time: 2m00s
> |     Renewal Time Value: 1m00s
> |     Rebinding Time Value: 1m45s
> |     Domain Name: emdentalb.local
> |     Domain Name Server: 192.168.1.1
> |     Router: 192.168.1.1
> |     Broadcast Address: 192.168.1.255
> |     Subnet Mask: 255.255.255.0
> |   Response 2 of 2:
> |     Interface: enp0s31f6
> |     IP Offered: 192.168.168.215
> |     DHCP Message Type: DHCPOFFER
> |     Server Identifier: 192.168.168.1
> |     IP Address Lease Time: 2m00s
> |     Renewal Time Value: 1m00s
> |     Rebinding Time Value: 1m45s
> |     Domain Name: emdentalb.local
> |     Domain Name Server: 192.168.168.1
> |     Router: 192.168.168.1
> |     Broadcast Address: 192.168.168.255
> |_    Subnet Mask: 255.255.255.0
> WARNING: No targets were specified, so 0 hosts scanned.
> Nmap done: 0 IP addresses (0 hosts up) scanned in 10.29 seconds
> 
> From the dnsmasq log:
> May  8 11:23:39 dnsmasq-dhcp[7226]: DHCPDISCOVER(enp2s0) de:ad:c0:de:ca:fe
> May  8 11:23:39 dnsmasq-dhcp[7226]: DHCPOFFER(enp2s0) 192.168.1.214 de:ad:c0:de:ca:fe
> May  8 11:23:42 dnsmasq-dhcp[7226]: DHCPDISCOVER(enp3s0) de:ad:c0:de:ca:fe
> May  8 11:23:42 dnsmasq-dhcp[7226]: DHCPOFFER(enp3s0) 192.168.168.215 de:ad:c0:de:ca:fe
> 
> His current configs (so not at the time of the logs as they have been
> tweaked to troubleshoot):
> /etc/dnsmasq.conf:
> bogus-priv
> cache-size=5000
> conf-dir=/etc/dnsmasq.d
> dhcp-authoritative
> dhcp-lease-max=1000
> domain-needed
> domain=######.local
> expand-hosts
> log-facility=/var/log/dnsmasq
> no-negcache
> port=53
> read-ethers
> resolv-file=/etc/resolv-peerdns.conf
> strict-order
> user=nobody
> 
> /etc/dnsmasq.d/dhcp.conf:
> dhcp-option=enp2s0,1,255.255.255.0
> dhcp-option=enp2s0,28,192.168.1.255
> dhcp-option=enp2s0,3,192.168.1.1
> dhcp-option=enp2s0,6,192.168.1.250
> dhcp-range=enp2s0,192.168.1.100,192.168.1.199,infinite
> 
> dhcp-option=enp3s0,1,255.255.255.0
> dhcp-option=enp3s0,28,192.168.168.255
> dhcp-option=enp3s0,3,192.168.168.1
> dhcp-option=enp3s0,6,192.168.1.1,192.168.168.1
> dhcp-range=enp3s0,192.168.168.50,192.168.168.99,48h
> 
> dhcp-option=enp4s0,1,255.255.255.0
> dhcp-option=enp4s0,28,192.168.169.255
> dhcp-option=enp4s0,3,192.168.169.1
> dhcp-option=enp4s0,6,192.168.169.1
> dhcp-range=enp4s0,192.168.169.100,192.168.169.254,24h
> 
> The infinite leases was an attempt to get round the problem as the devices
> were picking up IP's from the wrong LAN.
> 
> Do you know what is wrong here?


> How can I troubleshoot?


> I have a similar dual LAN set up and it works as expected with each
> LAN only responding with its own LAN DHCP settings.

That means the "problem" is outside dnsmasq.


> Both of us are running dnsmasq-2.76-10.el7_7.1.x86_64.
> 
> Thanks,

Feel welcome to report back.


> Nick


Regards
Geert Stappers

Could (should?) have ignored the original posting.

Looking forward to a message like
  After a better configuration of the managed network switch,
  we haven't seen the problem again.
-- 
Silence is hard to parse