Determine WAN DHCP Server and restrict via firewall rule

Hello there.

I recently performed some port scans on my OpenWrt from the WAN side. In total there was only one Port which was detected UDP Port 68. As of my knowledge this port is used to get send and receive DHCP requests so I can get an IP from my ISP. Although the Port was marked as "closed" the port scan was able to pick it up.

How can I get the IP address from my ISP? I've tried to get some information in the system log, but there wasn't a specific IP address.

Help would be appreciated.

This port is to receive solicited dhcp replies. What exactly is your concern?

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I would like that this port does not show op on port scans. I currently have a firewall rule which allows this port to be open, but I would like to restrict the incoming dhcp replies to a specific ip/ip range where the dhcp server from my isp is.

This would be a question for your ISP, not OpenWrt forum.

My question is if there is any way to see where the dhcp response packages are coming from? I've found the log entries but there wasn't mentioned any ip address from the dhcp server.

There is the packet capturing way, but I don't recommend it.

I already thought about that. If there's no other way...

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Aight, wireshark it is.

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