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- A new per-interface knob IFF_ND6_NO_RADR and sysctl IPV6CTL_NO_RADR. This controls if accepting a route in an RA message as the default route. The default value for each interface can be set by net.inet6.ip6.no_radr. The system wide default value is 0. - A new sysctl: net.inet6.ip6.norbit_raif. This controls if setting R-bit in NA on RA accepting interfaces. The default is 0 (R-bit is set based on net.inet6.ip6.forwarding). Background: IPv6 host/router model suggests a router sends an RA and a host accepts it for router discovery. Because of that, KAME implementation does not allow accepting RAs when net.inet6.ip6.forwarding=1. Accepting RAs on a router can make the routing table confused since it can change the default router unintentionally. However, in practice there are cases where we cannot distinguish a host from a router clearly. For example, a customer edge router often works as a host against the ISP, and as a router against the LAN at the same time. Another example is a complex network configurations like an L2TP tunnel for IPv6 connection to Internet over an Ethernet link with another native IPv6 subnet. In this case, the physical interface for the native IPv6 subnet works as a host, and the pseudo-interface for L2TP works as the default IP forwarding route. Problem: Disabling processing RA messages when net.inet6.ip6.forwarding=1 and accepting them when net.inet6.ip6.forward=0 cause the following practical issues: - A router cannot perform SLAAC. It becomes a problem if a box has multiple interfaces and you want to use SLAAC on some of them, for example. A customer edge router for IPv6 Internet access service using an IPv6-over-IPv6 tunnel sometimes needs SLAAC on the physical interface for administration purpose; updating firmware and so on (link-local addresses can be used there, but GUAs by SLAAC are often used for scalability). - When a host has multiple IPv6 interfaces and it receives multiple RAs on them, controlling the default route is difficult. Router preferences defined in RFC 4191 works only when the routers on the links are under your control. Details of Implementation Changes: Router Advertisement messages will be accepted even when net.inet6.ip6.forwarding=1. More precisely, the conditions are as follow: (ACCEPT_RTADV && !NO_RADR && !ip6.forwarding) => Normal RA processing on that interface. (as IPv6 host) (ACCEPT_RTADV && (NO_RADR || ip6.forwarding)) => Accept RA but add the router to the defroute list with rtlifetime=0 unconditionally. This effectively prevents from setting the received router address as the box's default route. (!ACCEPT_RTADV) => No RA processing on that interface. ACCEPT_RTADV and NO_RADR are per-interface knob. In short, all interface are classified as "RA-accepting" or not. An RA-accepting interface always processes RA messages regardless of ip6.forwarding. The difference caused by NO_RADR or ip6.forwarding is whether the RA source address is considered as the default router or not. R-bit in NA on the RA accepting interfaces is set based on net.inet6.ip6.forwarding. While RFC 6204 W-1 rule (for CPE case) suggests a router should disable the R-bit completely even when the box has net.inet6.ip6.forwarding=1, I believe there is no technical reason with doing so. This behavior can be set by a new sysctl net.inet6.ip6.norbit_raif (the default is 0). Usage: # ifconfig fxp0 inet6 accept_rtadv => accept RA on fxp0 # ifconfig fxp0 inet6 accept_rtadv no_radr => accept RA on fxp0 but ignore default route information in it. # sysctl net.inet6.ip6.norbit_no_radr=1 => R-bit in NAs on RA accepting interfaces will always be set to 0.
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