Re: [PATCH] Allow userspace code to use flag IFA_F_SECONDARY to specify an ip address to be primary or secondary ip on an interface

[Julian Anastasov <ja@ssi.bg>]

	Hello,

On Tue, 24 Sep 2013, Vincent Li wrote:

> Thanks Julian for the comments, I imagined it would not be so simple
> as it changed old behavior with ip binary and some actions in
> __inet_del_ifa() that I am not fully aware of. my intention is to
> preserve the old behavior and extend the flexibility, I am unable to
> come up with a good patch to achieve the intended behavior.

...

> if someone can point me to the right patch directions or coming up
> with better patches, it is very much appreciated.

	My first idea was to use NLM_F_APPEND to implement
'ip addr prepend' and 'ip addr append' but the default
operation is 'append' without providing NLM_F_APPEND, so it
does not work.

	Another idea is to add new attribute IFA_PREFERENCE in
include/uapi/linux/if_addr.h just before __IFA_MAX, integer,
3 of the values are known. A preference for the used scope.

/* Add as last, default */
IFA_PREFERENCE_APPEND = 0,

/* Add as last primary, before any present primary in subnet */
IFA_PREFERENCE_PRIMARY = 128,

/* First for scope */
IFA_PREFERENCE_FIRST = 255,

	We should keep it in ifa as priority, for
sorting purposes. It can be 4-byte value, if user wants
to copy user-defined order into preference.

	Sorting order should be:

- all primaries sorted by decreasing scope, decreasing
priority and adding order

- then all secondaries (IFA_F_SECONDARY) sorted by decreasing
priority and adding order

	Usage:

ip addr add ... pref[erence] type_or_priority

# Add floating IP (append at priority 128)
# The primary mode is not guaranteed if another address from
# the same subnet is already using the same or higher priority.
ip addr add ... pref primary
# More preferred primary
ip addr add ... pref 129

# Add first IP for scope
ip addr add ... pref first

	The scope has similar 'sorting' property but not
for IPs in same subnet and it would be difficult to use
it for global routes.

	Thoughts?

Regards

--
Julian Anastasov <ja@ssi.bg>