Sat Apr 30, 2022 2:35 pm
ARP, Address Resolution Protocol, permit to obtain MAC address from one IP.
Why?
The IP protocol work at layer 3, but the IP packet, from example to one ethernet to another, must be send with at least right destination MAC address.
The interface or the OS do not know what are the MAC address of remote device, but at least know the IP.
ARP permit to the OS to know what MAC have the remote device, using the IP.
If on the remote device the interface has ARP disabled, it does not communicate which MAC it has,
and for communicate with that device, a static MAC/IP corrispondence must be set on both remote and local devices.
This an be used for "hide" some device or for security settings...
If the ARP is enabled, MACs are discovered automatically and are added dynamic entries (with some timeout) added to the ARP table.
This is the normal behavior
If the interface act as Proxy ARP, acts as a transparent ARP proxy between different networks that the same interface can reach.
For example if you do a VPN between your PC and the office, you can browse remote LAN like is local, maintaining the same IP remote address pool
(no NAT, no Routes, etc.)
If the interface have reply-only ARP, reply only to his own MAC/IP but do not collect and do not request other MAC/IP
This device for reach other devices must have static ARP entry, but is reachable from other devices on LAN.
For example this can be used if the device must reply only to some specific static ARP entries, but must be visible from everyone.
If the interface have local proxy ARP set, it act like a proxy ARP, but the difference is: everytime reply with is own MAC address instead of the remote MAC
(act like a Acccess Point with Universal Repeater active, all device on DHCP have same MAC but differents IPs...)
With this settings all traffic pass inside "CPU" and is possible to do more checks and apply rules, the interface act as local MAC proxy and direct devices communication are impossible.
Probably my description is full of errors, but I hope help you to understand better.