Date: Mon, 26 Apr 2021 06:50:10 -0700 (PDT) From: "Rodney W. Grimes" <freebsd-rwg@gndrsh.dnsmgr.net> To: Poul-Henning Kamp <phk@phk.freebsd.dk> Cc: Zhenlei Huang <zlei.huang@gmail.com>, "Rodney W. Grimes" <freebsd-rwg@gndrsh.dnsmgr.net>, freebsd-hackers@freebsd.org Subject: Re: Are there any RFCs for address selection for IPv4 Message-ID: <202104261350.13QDoA0E097896@gndrsh.dnsmgr.net> In-Reply-To: <8834.1619373361@critter.freebsd.dk>
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> Zhenlei Huang writes: > > > Reading RFC 3927 2.7, it states link-local addresses are not routable. The router shall > > discard those packets from or to link-local addresses. Then it make no sense for a host > > to select link-local address as source address when it initialize a connection, except for > > an edge case that the destination is also link-local address. > > As I understand it, it only makes sense let the kernel select a LL address under two > conditions: I take it you mean for IPv4 only? > A) The destination is also LL *or* multicast Agree, this is pretty clear from specs. > > B) There is only one "UP" interfaces with an LLA. This I am not clear on. RFC 3927 does bring up some of the problems, and possible solutions, for a multihomed situation with LL addresses. Section 3.2 > > Implementing the second criteria runs into us putting a (IPv6)LL > on the loopback interface. Huh? We already do that. > > Does anybody know why we put a (ipv6)LL on loopback interfaces ? I believe someplace in the bowls of all the IPv6 specs this is a requirement. I could not find it quickly though. Question: Should we allow a route to have a next hop of a LL(ipv4)? Reason: RFC3927 2.6.2: The host MUST NOT send a packet with an IPv4 Link-Local destination address to any router for forwarding. So, arguably, it is a violation to allow the default route to have a LL next hop for ipv4. For that matter, it is a violation to allow ANY ipv4 LL address to be the next hop in the routing table(s). -- Rod Grimes rgrimes@freebsd.org
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