Method and apparatus for detecting a router that improperly responds to ARP requests

1. A method for detecting a device that incorrectly responds to address-resolution-protocol (ARP) requests, comprising:

sending an ARP probe for a link-local address;

when a response is received to the ARP probe, sending an ARP request to the responding device asking for an Ethernet address associated with a reserved IP address, wherein the reserved IP address is an Ipv4 link-local broadcast address 169.254.255.255 or an IPv4 link-local address 169.254.0.0, which should not be assigned to any device; and

if a response is received from the device to the ARP request, placing the address of the device on a black list associated with a range of link-local IP addresses: and

ignoring subsequent ARP responses from source addresses in the black list for the link-local address range, so that subsequent ARP responses pertaining to that address range from the device will be ignored.

2. The method of claim 1 , wherein the ARP probe is sent by a requesting device that is in the process of assigning a candidate link-local IP address.

3. The method of claim 2 , wherein when no response is received to the ARP request with the reserved link-local IP address, the requesting device assigns the candidate link-local IP address.

4. The method of claim 1 , wherein the ARP request is a normal ARP request (not an ARP probe); and

wherein if no response is received to the ARP request, or if a response is received from a device and the source address for the device is placed on a black list, the method further comprises sending a subsequent ARP request for a valid link-local IP address to determine an Ethernet address associated with the valid link-local IP address.

5. A computer-readable storage medium storing instructions that when executed by a computer cause the computer to perform a method for detecting a device that incorrectly responds to address-resolution-protocol (ARP) requests, the method comprising:

sending an ARP probe for a link-local address;

when a response is received to the ARP probe, sending an ARP request to the responding device asking for an Ethernet address associated with a reserved IP address, wherein the reserved IP address is an IPv4 link-local broadcast address 169.254.255.255 or an IPv4 link-local address 169.254.0.0 , which should not be assigned to any device; and

if a response is received from the device to the ARP request, placing the address of the device on a black list associated with a range of link-local IP addresses; and

ignoring subsequent ARP responses from source addresses in the black list for the link-local address range, so that subsequent ARP responses pertaining to that address range from the device will be ignored.

6. The computer-readable storage medium of claim 5 , wherein the ARP probe is sent by a requesting device that is in the process of assigning a candidate link-local IP address.

7. The computer-readable storage medium of claim 6 , wherein when no response is received to the ARP request with the reserved link-local IP address, the requesting device assigns the candidate link-local IP address.

8. The computer-readable storage medium of claim 5 , wherein the ARP request is a normal ARP request (not an ARP probe); and

wherein if no response is received to the ARP request, or if a response is received from a device and the source address for the device is placed on a black list, the method further comprises sending a subsequent ARP request for a valid link-local IP address to determine an Ethernet address associated with the valid link-local IP address.

9. An apparatus that detects a device that incorrectly responds to address-resolution-protocol (ARP) requests, comprising:

a sending mechanism configured to sending an ARP probe for a link-local address;

wherein, when a response is received to the ARP probe, the sending mechanism is configured to send an ARP request asking for an Ethernet address associated with a reserved IP address, wherein the reserved IP address is an IPv4 link-local broadcast address 169.254.255.255 or an Ipv4 link-local address 169.254.0.0, which should not be assigned to any device; and

a black-listens mechanism, wherein if a response is received from the device, the black-listing mechanism is configured to:

place the address of the device on a black list associated with a range of link-local IP addresses: and to

ignore subsequent ARP responses from the source addresses in the black list for the link-local address range, so that subsequent ARP responses pertaining to the address range from the device will be ignored.

10. The apparatus of claim 9 , wherein the ARP probe is sent by a requesting device that is in the process of assigning a link-local IP address.

11. The apparatus of claim 10 , further comprising an assigning mechanism, wherein when no response is received to the ARP request with the reserved link-local IP address, the assigning mechanism is configured to assign the candidate link-local IP address.

12. The apparatus of claim 9 wherein the ARP request is a normal ARP request (not an ARP probe); and

wherein if no response is received to the ARP request, or if a response is received from a device and the source address for the device is placed on a black list, the sending mechanism is configured to send a subsequent ARP request for a valid link-local IP address to determine an Ethernet address associated with the valid link-local IP address.

13. A method for detecting a device that improperly responds to multicast and/or broadcast requests, comprising:

transmitting a first multicast or broadcast request to a plurality of recipients, wherein, when a response is received to the first request, transmitting a second multicast or broadcast request to the plurality of recipients, wherein the request is structured so that the request should not be responded to; and

if a response is received to the second request, from a device, piecing the address of the device on a black list; and

ignoring subsequent responses from the source addresses in the black list, so that subsequent responses from the device will be ignored.

14. The method of claim 13 , wherein the request is an ARP request.

15. A method for detecting a device that improperly responds to requests, comprising:

transmitting a first request to a plurality of recipients, wherein, when a response is received to the first request, transmitting a second request, wherein the second request is structured so that the request should not be responded to; and

if a device responds to the second request, placing the address of the device on a black list and:

ignoring subsequent responses from source addresses in the black list, so that subsequent responses from the device will be ignored.