Way to disable outgoing ARP requests on the wan interface on a sonicwall TZ 210

If you read the document that Evan referenced in his post you'll see that one of the components that will issue a reset is in fact, a Cache Cleanup function related to expired connection timers. This is the "idle session" clean up that I'm referring to. That's my bet as to the cause of the problem.

What you want is actually possible, but requires a pretty recent Linux kernel (>= 2.6.34, or a backport).

The option you need is /proc/sys/net/ipv4/conf/*/proxy_arp_pvlan:

proxy_arp_pvlan - BOOLEAN
    Private VLAN proxy arp.
    Basically allow proxy arp replies back to the same interface
    (from which the ARP request/solicitation was received).

    This is done to support (ethernet) switch features, like RFC
    3069, where the individual ports are NOT allowed to
    communicate with each other, but they are allowed to talk to
    the upstream router.  As described in RFC 3069, it is possible
    to allow these hosts to communicate through the upstream
    router by proxy_arp'ing. Don't need to be used together with
    proxy_arp.

    This technology is known by different names:
      In RFC 3069 it is called VLAN Aggregation.
      Cisco and Allied Telesyn call it Private VLAN.
      Hewlett-Packard call it Source-Port filtering or port-isolation.
      Ericsson call it MAC-Forced Forwarding (RFC Draft).

The upstream commit adding this support is 65324144b50bc7022cc9b6ca8f4a536a957019e3.