Proxy ARP on an untrusted interface
At a glance
Base severity
Area
Platform
Evidence level
Benchmark reference
Reads the export, not the device
ConfigProof reads a configuration export. This control is evaluated against the objects in that file, and it reports what the file says. It does not connect to the device and sees no traffic, so a finding here describes the configuration, not what the firewall is doing right now.
Why it matters
Proxy ARP answers for addresses behind the firewall on the internet side, which helps mapping and some spoofing attacks.
How to fix it
Web interface
The steps below are written for the web interface. A finding raised on your own export carries the same steps with your object names filled in.
- In ASDM: Configuration > Firewall > NAT Rules, and on each NAT rule toward the interface set Disable Proxy ARP, or turn it off for the interface with the CLI line below. Apply and save.
- On the CLI: sysopt noproxyarp <nameif>, and write memory.
Command line
Cisco ASA CLI
configure terminalsysopt noproxyarp <nameif>endwrite memory
An example. Values in angle brackets are yours to supply; a finding on your own export fills in the names it read. Typed in configuration mode; write memory saves it, with no separate commit. Read every line first.
Read it back
show running-config sysoptValues to fill in1
- <nameif>
- The interface name (nameif)
Before you type it2
- Written from the ASA 9.x command reference and not run on a device: read each line before you paste it.
- In multiple context mode, type it in the context it applies to (changeto context <name>).
How to confirm the fix
- Run show running-config sysopt and confirm sysopt noproxyarp names the interface.
Limits
- Check that no NAT rule relies on proxy ARP to answer for its mapped addresses before turning it off.
Compliance references
CIS Cisco ASA 9.x Firewall Benchmark v1.1.0
Covers part of the recommendation
Where a control aligns with a recommendation of the CIS Cisco ASA 9.x Firewall Benchmark v1.1.0, that recommendation is cited as a reference. Only that version was checked. A reference is not a certification.
More in Interfaces and Segmentation
See it on your own firewall
One complete analysis of one firewall costs nothing. A card is required and nothing is charged.