[9front] [PATCH] ip/ipconfig: don't add default routes for ipv6 prefixes with a validlt of 0

[9front] [PATCH] ip/ipconfig: don't add default routes for ipv6 prefixes with a validlt of 0

[Arne Meyer]

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Some cpe routers announce invalidated ipv6 prefixes with a preflt and validlt of 0
for some time after they get a new prefix. the default routes for these prefixes are
getting continually removed and added with every ra we receive, so just don't bother.

At least my fritzbox behaves that way. rfc 6204 states that in this case preflt should be 0
and validlt should be 2 hours or it's current value and then counting down. But here we are.

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From: Arne Meyer <meyer.arne83@netcologne.de>
Date: Mon, 10 Jun 2024 15:45:31 +0000
Subject: [PATCH] ip/ipconfig: don't add default routes for ipv6 prefixes with a validlt of 0


at least some cpe routers announce invalidated ipv6 prefixes with a preflt and validlt of 0
for some time after they get a new prefix. the default routes for these prefixes are
getting continually removed and added with every ra we receive, so just don't bother.
---
diff 0dc37f9794c63827005b67a39a8ad749e138eda9 822a065672ccb69fd4003c6f35cc462fe4765edf
--- a/sys/src/cmd/ip/ipconfig/ipv6.c
+++ b/sys/src/cmd/ip/ipconfig/ipv6.c
@@ -725,6 +725,7 @@
 		memmove(r->hash, hash, SHA1dlen);
 
 		if(conf.routerlt == 0
+		|| conf.validlt == 0
 		|| isula(conf.laddr)
 		|| ipcmp(conf.gaddr, conf.laddr) == 0
 		|| ipcmp(conf.gaddr, conf.lladdr) == 0)

Re: [9front] [PATCH] ip/ipconfig: don't add default routes for ipv6 prefixes with a validlt of 0

[hiro]

it might not be the fritz!box's fault:
against all sanity and against the working mechanism described in the
RFC, the ISPs invalidate old prefixes immediately after they give you
a new PPP session. sadly this is the most common scenario now for end
users. in this case it makes sense that the fritze sets the same 0
lifetime in RAs for the old prefixes.
tbh there's no other good reasons for prefixes changing continuously
like that anyways, apart from this ISP-generated sabotage edge-case.

Re: [9front] [PATCH] ip/ipconfig: don't add default routes for ipv6 prefixes with a validlt of 0

[Arne Meyer]

you're right, the fritzbox is not to blame. after sending my mail i found out there is rfc7084 which 
invalidates rfc6204 and then there is rfc9096 which annotates the former and describes this exact behavior.
so it is to be expected from newer cpe routers.