How can ARP be forwarded?

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MonarchX
DD-WRT User


Joined: 26 Sep 2009
Posts: 118

PostPosted: Wed Oct 27, 2021 10:01    Post subject: How can ARP be forwarded? Reply with quote
ARP frame (Layer 2) can only be sent/received from one MAC address to another. When router forwards a packet (Layer 3) from WAN host to LAN client, the packet reaches LAN client, but MAC address of WAN host is discarded by router and LAN client receives WAN host packet with router ARP frame. Is that correct or not?

So if LAN DNS server is not router and IP forwarding on LAN DNS server is disabled via SysCTL and IPTables, then LAN DNS server only needs INPUT and OUTPUT of ARP requests from LAN clients and router. It doesn't need/use any ARP forwarding, does it?
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Per Yngve Berg
DD-WRT Guru


Joined: 13 Aug 2013
Posts: 6248
Location: Romerike, Norway

PostPosted: Sat Oct 30, 2021 11:02    Post subject: Reply with quote
A DNS server does not use any ARP. A client need to know the IP address of the DNS server.

ARP is used by the IP stack to find the MAC address of another node that the netmask shows is on the same Ethernet segment. If the IP belongs to another sub-net, the packet is sent to the gateway.

Perhaps you are thinking of a DHCP server.
tedm
DD-WRT User


Joined: 13 Mar 2009
Posts: 463

PostPosted: Thu Nov 04, 2021 19:39    Post subject: Reply with quote
Broadcast forwarding can be done in Linux with

sysctl -w net.ipv4.conf.eth1.bc_forwarding=1

but it is Kernel version 5 or later. dd-wrt can't do it. (not yet, at any rate)

Of course, it's pretty nonsensical. But I suppose someone needed it for some weird application.
eugene1973
DD-WRT User


Joined: 21 May 2017
Posts: 164

PostPosted: Sat Nov 20, 2021 9:49    Post subject: Reply with quote
word of advice is to control all kinds of spoofing.
DNS is part of everything, even arp. arp spoofing
should help. generally you need to improve performance.
arp forwards itself. It already does. To improve
its performance you need to pay attention to router
discovery and neighbor discovery.

sysctl -w net.ipv6.conf.all.proxy_ndp=1
sysctl -w net.ipv4.conf.all.proxy_arp=1

you must do more though.
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