netdev - iproute2 - modifying routes in place

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Date:   Fri, 11 May 2018 07:42:58 -0400
From:   Ryan Whelan <rcwhelan@...il.com>
To:     netdev@...r.kernel.org
Subject: iproute2 - modifying routes in place

`ip route` has 2 subcommands that don't seem to work as expected and i'm
not sure if its a bug, or if i'm misunderstanding the semantics.

I am unable to modify a route 'in place', which from what i'm reading
online, I should be able to do with `ip route change` and/or `ip route
replace`.

After a route is created with either `ip route add` or `ip route replace`,
I am unable to use `change`, regardless of how I attempt to identify the
route i'm trying to alter.

  # ip -6 route show
...
fd9b:caee:ff93:ceef:3431:3831:3930:3032 dev internal0 src
fd9b:caee:ff93:ceef:3431:3831:3930:3031 metric 1000 pref medium
fe80::/64 dev enp0s3 proto kernel metric 256 pref medium
fe80::/64 dev enp0s8 proto kernel metric 256 pref medium
fe80::/64 dev internal0 proto kernel metric 256 pref medium

If i try to change the metric of the route that already exist via `ip route
change`, i get "No such file or directory.

# ip -6 route change fd9b:caee:ff93:ceef:3431:3831:3930:3032 dev internal0
src fd9b:caee:ff93:ceef:3431:3831:3930:3031 metric 100
RTNETLINK answers: No such file or directory

If i use `replace`, the command does not error, but creates another route
instead of replacing the current route.

# ip -6 route replace fd9b:caee:ff93:ceef:3431:3831:3930:3032 dev internal0
src fd9b:caee:ff93:ceef:3431:3831:3930:3031 metric 100

# ip -6 route show
...
fd9b:caee:ff93:ceef:3431:3831:3930:3032 dev internal0 src
fd9b:caee:ff93:ceef:3431:3831:3930:3031 metric 100 pref medium
fd9b:caee:ff93:ceef:3431:3831:3930:3032 dev internal0 src
fd9b:caee:ff93:ceef:3431:3831:3930:3031 metric 1000 pref medium
fe80::/64 dev enp0s3 proto kernel metric 256 pref medium
fe80::/64 dev enp0s8 proto kernel metric 256 pref medium
fe80::/64 dev internal0 proto kernel metric 256 pref medium

What am I missing or doing wrong?  Forgive me if i'm being dense, but i
could not find an answer online to explain this behavior.

If this is the wrong forum for this question, I apologize; please point me
in the right direction?

Linux: 4.16.6
iproute2: 4.15.0

Thank you!