dhcpd: fix malformed link-layer address packet

[yamanq]

closes #5840

See issue for details on bug and approach for this fix.

[snachx]

I think we should also modify the Length field of the source link-layer address option based on the length of the return result of hwAddrToLinkLayerAddr, instead of always setting it to 1. https://github.com/AdguardTeam/AdGuardHome/blob/ae7bd79a6c2dba0061db4d5d46b9069e4bd3f22c/internal/dhcpd/routeradv.go#LL179C11-L179C11

[yamanq]

You're absolutely right, I forgot to change that part. I pushed a change that should fix it, but I am no longer using AdGuard as I found other issues with the DHCP server.

[snachx]

You are right, AdGuard Home's DHCP server does indeed have several other issues. For example, the parameters for Router Advertisement (RA) messages are not configurable, and the most severe problem is that the default interval for sending RA messages is 1 second, which is too short. The recommended maximum interval according to the RFC is 600 seconds, and the minimum interval is 200 seconds. Additionally, it lacks the functionality to respond in real-time to received Router Solicitation (RS) messages, among other things. There is still much work to be done to use AdGuard Home as a DHCPv6 server.