What is the relationship between DHCP and Multicast?

Question

linux

provider network. To filter garbage on the aggregator (all client vlans are listed on it and, accordingly, it is the default gateway and dhcp relay for them), we added several acl for filtering. Including deny ip any 224.0.0.0 15.255.255.255. After that, dhcp stopped working for about a hundred subscribers (in different vlans). After this rule was removed - dhcp earned. What is the connection???? dhcp servers - isc-dhcp, two pieces on Linux. I looked at the listening ports - there is no multicast there. And if multicast was so necessary for work, then it would fall off for everyone. Client equipment - I know for sure about one point at which they checked it - a kineticist. Changed poppy of the client - does not help. It is visible on the client access port, it is visible on the switch in front of the aggregator, it is not visible on the aggregator, it does not get into the fdb table. Removed multicast filtering - hit.
The search found only a mention of the 224.0.0.12 address here: stackoverflow.com/questions/10537917/discovering-d...
Who knows how dhcp relay and multicast are connected?
PS. I'm sorry, I figured out what the problem is. There is no connection between dhcp v4 and multicast.
When writing the ACL, they made a mistake, and instead of
deny ip any 224.0.0.0 15.255.255.255 they
wrote
deny ip any 224.0.0.0 31.255.255.255
. banned network 224.0.0.0/3, which includes 255.255.255.255
So we are our own evil Pinocchio.
The picture of the world has been restored, I apologize for the time taken.

Answer 1

No, seriously, do you work for an ISP?

Answer 2

There is such a protocol, FLIP . Although this is a draft, it has already been implemented somewhere. Most likely, in the implementation of some devices, it is hardwired and dhcp resets when there is no connection. Rather, they use it as an IP broadcast instead of FF.FF.FF.FF.
PS Why cut the multicast on the ACL? This is gemorno - you need to hang it on each subinterface, and it will also load the router once again. Usually this is done either on switches (if there is one vlan for several users), or they are wrapped in Null.
UPD: IANA is quite well definedreserved multicast addresses for dhcp requests. It can be assumed that any model or firmware of medium pop client devices adheres to these recommendations. In the end, whether or not the address is broadcast does not matter for the dhcp server, the main thing is that a request has arrived at it.

Answer 3

mirror the port and see what this "kineticist" is trying to ask
to compare with soap dishes normally receiving addresses

Answer 4

Request dumps are given in the comments to Pavel Vasterov 's answer . Turned on the vlan mirror, turned off the client port, turned on the client port. Traffic recorded.
Just in case, here is a traffic dump and a text file with the client's poppy.
https://yadi.sk/d/LbUwkUWLiDSHs

Answer 5

224.0.0.12 DHCP server / relay agent. RFC 1884