there were several of these entries every second during business hours. The problem was reported as windows users got a message telling their ip was already in use. Well, no wonder they were used, considering the number of dhcp requests. The DHCP scoop was simply filled, even though there was 240 addresses in the dhcp scoop, and only 50ish devices on site. There were several entries in dhcp lease table telling that mac-address 00:00:00:00:00:00, client ID [empty] is assigned to a bunch of ips, with the status "busy". Normally dhcp lease status is "bound".
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dhcp-lan client xx:xx:xx:xx:xx:xx declines IP adress 172.18.11.xx
Tracking down the mac-addresses revealed one macbook pro and a bunch of iphones.
The owner of the macbook and a iphone had a look at his home router log (an ubiquiti router) and found that his devices did the same thing at home.
So we requested all iphone owners to forget the lan-wlan, restart their devices, and connect to guest-wlan. So they did, and the problem went away.
Now we wanted to figure out what this was all about. We put 3 iphones on lan-wlan, and all looked good - for about 3 hours. Now the same problem occurred again. DHCP is flooded with requests, filling all available addresses.
The macbook is creating problems as soon as it is connected to lan. It doesn't matter if it uses wlan or cable - same problem. As soon as it connects to lan, it sends a lot of dhcp requests. It doesn't do this when connected to guest network - neither wlan or cable.
Lan-wlan and guest-wlan are two vlan. They uses the same switches, the same APs (6x wAP ac managed by capsman). Router is a rb3011. The two vlans are identically configured, except for the obvious (different vlan id, different ip range, and different switch ports for untagged traffic). In capsman the lan-wlan is primary configuration, while guest-wlan is secondary.
Any ideas how to debug this?
Has apple released some updates lately that could cause issues?
Edit: Arp is set to 'enable' on all interfaces. (I've read that proxy-arp may cause issues with apple devices and dhcp)