Neighbor Discovery Protocol (NDP) for IPv6 performs functions similar to those provided by Address Resolution Protocol (ARP) for IPv4. RFC 4861 defines Neighbor Discovery for IP version 6 (IPv6). Hosts, routers, and firewalls use NDP to determine the link-layer addresses of neighbors on connected links, to keep track of which neighbors are reachable, and to update neighbors’ link-layer addresses that have changed. Peers advertise their own MAC address and IPv6 address, and they also solicit addresses from peers.
NDP also supports the concept of proxy, when a node has a neighboring device that is able to forward packets on behalf of the node. The device (firewall) performs the role of NDP Proxy.
Palo Alto Networks firewalls support NDP and NDP Proxy on their interfaces. When you configure the firewall to act as an NDP Proxy for addresses, it allows the firewall to send Neighbor Discovery (ND) advertisements and respond to ND solicitations from peers that are asking for MAC addresses of IPv6 prefixes assigned to devices behind the firewall. You can also configure addresses for which the firewall will not respond to proxy requests (negated addresses).
In fact, NDP is enabled by default, and you need to configure NDP Proxy when you configure NPTv6, for the following reasons:
- The stateless nature of NPTv6 requires a way to instruct the firewall to respond to ND packets sent to specified NDP Proxy addresses, and to not respond to negated NDP Proxy addresses.It is recommended that you negate your neighbors’ addresses in the NDP Proxy configuration, because NDP Proxy indicates the firewall will reach those addresses behind the firewall, but the neighbors are not behind the firewall.
- NDP causes the firewall to save the MAC addresses and IPv6 addresses of neighbors in its ND cache. (Refer to the figure in NPTv6 and NDP Proxy Example.) The firewall does not perform NPTv6 translation for addresses that it finds in its ND cache because doing so could introduce a conflict. If the host portion of an address in the cache happens to overlap with the host portion of a neighbor’s address, and the prefix in the cache is translated to the same prefix as that of the neighbor (because the egress interface on the firewall belongs to the same subnet as the neighbor), then you would have a translated address that is exactly the same as the legitimate IPv6 address of the neighbor, and a conflict occurs. (If an attempt to perform NPTv6 translation occurs on an address in the ND cache, an informational syslog message logs the event: NPTv6 Translation Failed.)
When an interface with NDP Proxy enabled receives an ND solicitation requesting a MAC address for an IPv6 address, the following sequence occurs:
- The firewall searches the ND cache to ensure the IPv6 address from the solicitation is not there. If the address is there, the firewall ignores the ND solicitation.
- If the source IPv6 address is 0, that means the packet is a Duplicate Address Detection packet, and the firewall ignores the ND solicitation.
- The firewall does a Longest Prefix Match search of the NDP Proxy addresses and finds the best match to the address in the solicitation. If the Negate field for the match is checked (in the NDP Proxy list), the firewall drops the ND solicitation.
- Only if the Longest Prefix Match search matches, and that matched address is not negated, will the NDP Proxy respond to the ND solicitation. The firewall responds with an ND packet, providing its own MAC address as the MAC address of the next hop toward the queried destination.
In order to successfully support NDP, the firewall does not perform NDP Proxy for the following:
- Duplicate Address Detection (DAD).
- Addresses in the ND cache (because such addresses do not belong to the firewall; they belong to discovered neighbors).
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