Question

Proxy ARPs on ASAs

Asked by: question01

How do proxy arps work on the ASA?

If I have the following setup.

external host ---------- (outside) | ASA| (inside)---------- internal host

external host: 192.168.1.2
asa outside interface: 192.168.1.1
asa inside interface: 10.0.1.1
internal host: 10.0.1.2

I nat the internal host to 10.0.2.2 using a static nat
static (inside,outside) 10.0.1.2 10.0.2.2 netmask 255.255.255.255

My question, how does the proxy arp work.
If the external host is trying to access the internal host 10.0.1.2 through its nat address 10.0.2.2 does the ASA do a proxy arp for 10.0.1.2?

I have set up the lab as above and turned on sysopt noproxyarp outside, cleared the arp tables and can still access the inside host through its nat address.

Can anyone explain how the proxy arps work.

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Asked On
2009-10-28 at 22:54:56ID24853714
Tags

Cisco ASA

,

Proxy arp networking

Topic

Virtual Private Networking (VPN)

Participating Experts
2
Points
500
Comments
7

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Answers

 

by: benhansonPosted on 2009-10-29 at 00:13:37ID: 25691212

The internal host is 10.0.1.2/?  Are you using a class A inside?(10.0.0.0/8, 10.0.0.0 255.0.0.0)  At this point the scenario doesn't quite make sense.  I would expect the static to be from the 192.168.1.x subnet to the 10.x.x.x subnet:

static (inside,outside) 10.0.1.2 192.168.1.2 netmask 255.255.255.255

The proxyarp feature works as you are thinking, I'm assuming you expected the NAT to fail after you issued 'sysopt noproxyarp outside', and you are right.  It shouldn't be working.  If proxyarp is enabled, the the ASA responds to arps for any IP address that has a static/global/nat defined for it.  It responds with it's own external MAC address so the arp'ing client knows that it should send traffic the ASA's direction.  If you do 'sysopt noproxyarp outside', the ASA will only respond to ARP request for it's actual interfaces, and thus communication from the outside to the inside host should fail.

Can you reconfirm it is working, then run 'show running-config sysopt' and post the results?

 

by: benhansonPosted on 2009-10-29 at 01:18:32ID: 25691473

Unless you have a really special circumstance(like a read-only archive), the share permissions are mostly just an administrative headache.  It provides a second point to ensure that something didn't get missed on the NTFS side, but it also eliminates flexibility on the NTFS side.

Here is an MS page with some guidelines:

http://technet.microsoft.com/en-us/library/cc754178.aspx

One thing to consider, when profile folders are created automatically, only the User and System have permissions and they both have full control.  I've been told that the user needs to have Full Control for everything to work, but I don't have a concrete example of a failure.  If it is a requirement, then you would need to grant the user full control at the share level as well.

 

by: benhansonPosted on 2009-10-29 at 01:23:39ID: 25691502

Somehow that last post was sent to the wrong question, please ignore.

 

by: Texas_BillyPosted on 2009-10-29 at 07:30:30ID: 25694068

Your static needs to have outside address first. It's a little counterintuitive, but the syntax is that you list the higher security interface first, i.e., static (inside,outside), then you list the outside IP first.  So it should be

static (inside,outside) 192.168.1.1 10.0.1.2 netmask 255.255.255.255  

Turning off proxy arp won't kill the 1:1 static xlation or it's accessibility from the WAN, provided you have a valid access-list on the WAN interface allowing some traffic into the host at 192.168.1.1.  Here is a good article on proxyarp - it specifies router, but this works for any device that uses the proxyarp feature, ASA included:  
http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080094adb.shtml

Remember something about proxyarp and how it works with your NAT:  Proxyarp allows the device to respond to arp queries and fake the sender by pretending to be the recipient so that successive queries will know where to go.  But if you've created a 1:1 NAT xlation and allowed traffc in by some access list such as "access-list inbound permit tcp any host 192.168.1.1 eq 443", then ssl sessions will be allowed in whether proxyarp is enabled on the WAN interface or not.  Those sessions are hitting with 443, not arp, they'll still come through.  The way to stop traffic from coming into a 1:1 NAT is to remove the static, or remove the ACL that allows that traffic to come in.  --TX

 

by: question01Posted on 2009-10-29 at 15:46:04ID: 25698971

OK thanks for the explanation people.

TX, thanks for that. That article and your explanation helped alot. Looks like I was getting my wires a bit crossed.  

A proxy arp is simply a layer 3 devices acting as a proxy for an arp request for a machine that sits off another interface as the arp request broadcast would not normall make it through the layer 3 device.

Still one slight point of confusion. Why would you use a proxy arp rather than just routing the traffic through at layer 3?




 

by: question01Posted on 2009-10-29 at 16:03:36ID: 25699051

Hang on thing I got my answer for that previous comment.
from the article

"Proxy ARP must be used on the network where IP hosts are not configured with a default gateway or do not have any routing intelligence."

Can you give an example of where this would be used?



 

by: question01Posted on 2009-10-29 at 17:56:11ID: 25699490

20120131-EE-VQP-002

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