RA is used by auto-configuration. What is it auto-configuration ? It's a solution who allows devices to choose their own address.
How it works ?
Autoconfiguration is a two-step process:
- Host has to obtain prefix information to configure its address. To do this, it sends a Router Solicitation (RS). It's a multicast frame (ICMPv6 RS) for each router.
- Router advertisement (RA) is sent back by the router. These RAs are also sent periodically by the router. In our case, it's an instantaneous response.
How it works ?
Autoconfiguration is a two-step process:
- Host has to obtain prefix information to configure its address. To do this, it sends a Router Solicitation (RS). It's a multicast frame (ICMPv6 RS) for each router.
- Router advertisement (RA) is sent back by the router. These RAs are also sent periodically by the router. In our case, it's an instantaneous response.
It's a helpful solution for administrator but it's also a good way for hacker to shunt data flows. A hacker can send a wrong RA to a device. In this case, there are 2 issues, denial of service or man in the middle attack. If the hacker sends his own address (gateway information in the RA) all the flows can pass through him. Another error can occur with a misconfigured device, the host should obtain a wrong prefix. To protect against this kind of attack you can use this simple configuration (Port ACL) to stop untrusted RA. The access-list stop RA on all the untrusted interface. Just keep free interface where the router is plugged:
ipv6 access-list filter_RA
remark Block Rogue RA
deny icmp any any router-advertisement
permit any any
!
interface gigabitethernet 1/0/10
ipv6 traffic-filter filter_RA in
On the Catalyst 6500 and 4500 it's possible to use a macro to configure this kind of PACL:
interface gigabitethernet 1/0/10
ipv6 nd raguard
It's also possible to filter DHCPv6 with a PACL and just authorize the interface of the server to send response:
ipv6 access-list filter_DHCP
remark Block traffic from DHCP to client
deny udp any eq 547 any eq 546
permit any any
!
interface gigabitethernet 1/0/10
ipv6 traffic-filter filter_DHCP in
