[Dnsmasq-discuss] Multiple interfaces responding to DHCP requests

Nick Howitt nick at howitts.co.uk
Thu May 13 12:05:07 UTC 2021


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-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-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=enp2s0,192.168.1.100,192.168.1.199,infinite
dhcp-range=enp3s0,192.168.168.50,192.168.168.99,48h
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. Both of us are running 
dnsmasq-2.76-10.el7_7.1.x86_64.

Thanks,
Nick





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