ARP (proxy?) issue on SonicWALL Pro5060 firewall

Posted on 2008-10-14
Last Modified: 2013-11-29
In trying to chase down another issue on a network I discovered what seems to be an anomaly or at least something which I don't believe should happen and/or is unnecessary, causing (possibly) certain network problems.

I have observed that whenever I ping a host on this LAN (from another host on the same LAN) the SonicWALL will always respond to the initial ARP request with its own MAC address.  This seems to be happening at the beginning of other network communications as well although I haven't dug as deep as I have with the following scenarios.

Please see the respective figures below (attached file) which correspond with the following (these are all on the same LAN - no VLAN's or anything):

Note: initiator (pinger) is Broadcom. . . / in each case.

1) Ping live host.  Initial arp request is answered by SonicWALL, then by actual host.  Subsequent ping request/reply is normal between only pinger and pingee.

2) Ping live host.  Initial arp request is answered by actual host, then by SonicWALL (arp response reverse of scenario 1).  Subsequent ping request/reply exchange works except that two ping requests are sent for each reply.  The first request is sent to the SonicWALL's MAC, the next is sent to the correct host MAC.  It is apparent that the initiating host cached the MAC address of both replies and is sending requests in the order the arp responses were received.

3) Ping non-existent host.  SonicWALL responds to arp request with its own MAC, then proceeds to send three arp requests to the (non-existent) destination host.  Since the initiating host received a reply to its initial arp request it sends specified number of ping requests to the SonicWALL's MAC.  Of course the SonicWALL doesn't answer and the result is request timed out.


1) Why does SonicWALL do this?

2) Can it be disabled?

3) Is this arp proxying?

Question by:powercram
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Expert Comment

ID: 22717008
Yeah, that looks like proxy arp.

Difficult to tell why without knowing more about the configuration on the SonicWall. Is it set to "Transparent Mode" by any chance?

Author Comment

ID: 22717442
Are you talking about Send gratuitous ARP to DMZ or LAN on transparent mode while HA failover in Hardware Failover Settings?  The answer is no, this is not set.

Accepted Solution

larsga earned 500 total points
ID: 22723241
The only reason I can think of for the SonicWall to behave the way it does is that it thinks (part of) 10.0.81 is sitting on a different interface (possibly a VPN?) and thus needs to do proxy arp for that ip range. Perhaps a VPN connection with wrong netmask/range, or "transparent mode" bridging between interfaces.

I'm unfortunately not that familiar with SonicWall configuration. Is it possible to dump it as a text file so that we can take a look at it? (With any potential security sensitive information anonymised of course).
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Author Comment

ID: 22725761
The SonicWALL is setup to provide an IP address from the local network (on X0) to VPN clients.  It makes some sense then that it needs to know the MAC address of all hosts with IP addresses from that network so it can forward packets, if necessary, to "remote" VPN hosts.

I have posed this question to SonicWALL support to see what they come back with.  I'm not too optimistic though as I'm sure one of the level-one guys will try to give a scripted answer, and it would be like pulling teeth to talk to a higher-level tech.  If I get anywhere with them I'll post it here.

If the assumption is correct about the VPN thing it helps to clear the waters a bit.  But now I'm back chasing the original issue which led me down this path.  Once I get my thoughts together and can form some logical questions I will ask them.

Expert Comment

ID: 22726647
It should really only do proxy arp for the IP addresses that the VPN clients are currently using, it should not do proxy arp for the entire 10.0.181.x range. Could you look in the VPN settings and see if it is possible to narrow down the range of addresses that are given to VPN clients, preferably parts of .181.x that are not going to be used by PCs/other devices on the local lan.

How are IP addresses assigned on the local lan, btw? If it is by a DHCP server, you might for example configure it to give out addresses from 181.10 - 181.200. And then in the VPN config on the SonicWall set it to use 181.201 - 181.250 as the pool of addresses given to VPN clients.

Author Comment

ID: 22731142
I think you are on to something here.

I have a W2K3 server running DHCP with a range of about 50 addresses.  The SonicWALL is setup to DHCP proxy from the LAN to the VPN clients.  I like your idea of defining a range just for VPN clients from the SonicWALL.  I'll give that a try and see if this changes the behavior.

Author Closing Comment

ID: 31506088
This interaction got me going in the right direction & I believe I found what I was after with it.  Thanks for the help.

Expert Comment

ID: 36421294
Hi Powercram,

I actually have the same problem here on or Sonicwall NSA 3500; Can you tell me what you ended up doing?

With kind regards,

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