IPv6 routing requests ignore NLM_F_CREATE and NLM_F_REPLACE

Hi dee Ho!

I was enchancing an userspace application configuring IPv4 routes via
netlink sockets to support IPv6 route configuration too. While doing
this I noticed that NLM_F_* flags seemed to have no handling at IPv6
side. For example replacing a route to some destiantion, with route
having different pref_src (or metric or gateway or...) can be done by
having NLM_F_REPLACE flag specified in netlink request and leaving out
NLM_F_CREATE.

However with IPv6, if new route being requested has different properties
(like gateway or metric or..) the existing one will not be replaced.
Instead a new route will be created - even if NLM_F_CREATE was not
specified in request.

That causes some inconvenience when a route is being changed. Routes
need to be queried, and matching route needs to be explisitly deleted by
userspace application. Also creating new route even without NLM_F_CREATE
feels a bit strange to me.

I was wondering if this is a bug or wanted behaviour? I was thinking of
trying to write a patch to add support for replacing a route, but I feel
I'm a bit lost with the fib :) I guess the fib6_add_rt2node function
could be changed to inspect the NLM_F_ flags from nl_info pointer, and
to perform replace instead of returning -EEXIST / performing insertion.
Also returning error when NLM_F_CREATE is not specified, and existing
route is not found could propably be implemented.

Anyways, before I spend more time trying to understand the data
structures in fib6, I would like to ask if the handling of NLM_F_* flags
is dropped out in purpose?


Br. Matti Vaittinen

--

Theory:
Theoretical approach means that everything is well known, but still
nothing works.
Practice:
Practical approach means that everything works but no one knows why.

Thank God we have theory and practice balanced here. Nothing works, and
no one knows why...