I am a bit stuck with an experiment of mine.

I have a network looking somewhat like this.

        | Internet |
             |
    ----  |Switch| ---- 
    |                  |
Server w/pub IP       | DD-WRT router 192.168.1.1 |
                           |
                         RFC1918 clients 192.168.1.0/24

What I want is for the RFC1918 clients to speak directly with each others.

On the server with the public IP I have this route: 192.168.1.0/24 dev eth0 scope link and can see that packets are infact reaching the dd-wrt router for 192.168.1.1, even though if I get no answer.

Trying to reach one of the RFC1918 clients from the public IP server will get no result, as the dd-wrt router is not announcing that network on to its external interface (arp who-has 192.168.1.107 tell xxx.xxx.xxx.xxx, but no answer).

The router being an WLAN dd-wrt router has of course a load of routes, VLANs and interfaces:

xxx.xxx.xxx.1 dev vlan2  scope link 
192.168.1.0/24 dev br0  proto kernel  scope link  src 192.168.1.1 
192.168.1.0/24 dev eth1  proto kernel  scope link  src 192.168.1.244 
84.215.64.0/18 dev vlan2  proto kernel  scope link  src xxx.xxx.xxx.xxx 
169.254.0.0/16 dev br0  proto kernel  scope link  src 169.254.255.1 
127.0.0.0/8 dev lo  scope link 
0.0.0.0 via xxx.xxx.xxx.1 dev vlan2

xxx.xxx.xxx.xxx being the public IP, and xxx.xxx.xxx.1 being the default route for the public IP.

I am not sure where to continue with this. I would recon that I both need routing on the dd-wrt router, as well as some iptables magic?

Why do something this complex? Why not ;)

Also, do not mind that "Internet" can get RFC1918 traffic, it wont go outside of the walls.

EDIT 1: Following the tip from stew I do indeed get the correct ARP flowing. And adding an iptables rule for allowing traffic from that specific public IPd machine I get traffic between the systems!

Oddly enough though, the speed I get from Server w/pub IP -> RFC1918 clients are the same as if the traffic were routed out onto the Internet and back.

Edit 2: Ok, disconnecting the external Internet connection will still give the same, crappy transfer speed. So it has to be something else.

Edit 3: Ok, I guess there are other reasons for this crappy speed. Case closed. :)