libifconfig: Add netlink based helper to bring the interface up/down

[saheed]

Summary

Adds ifconfig_set_up() to set and unset the IFF_UP
on a network interface using RTM_NEWLINK.

Sponsored by: Google LLC (GSoC)

[obiwac]

Looks good to me!

When you re-open D51871, you should set that as this revision's parent revision.

otherwise I have tested this out with the _nl_ifp_modify_generic fix and it all works :)

lib/libifconfig/libifconfig_nl.c

41

sort of off-topic for this change but I realize D51871 wouldn't have worked if we assumed programs could just set ifi_change to 0 or 0xFFFFFFFF. Maybe should look into how linux handles this, maybe similar to promisc

[kp]

Also not something that must be done in this commit, but we should modify sbin/ifconfig to the new functions. That'll remove a little more code from ifconfig and it'll mean we test this code (as part of any test that runs ifconfig foo up).

lib/libifconfig/libifconfig.h

393

I think that should be ifconfig_up() or even better ifconfig_set_up() rather than ifconfig_if_up().
We already have ifconfig_set_name(), ifconfig_set_metric(), ifconfig_set_description(), ... where we also don't include the _if_ part.

Given that this is libifconfig it's pretty much implicitly obvious that we'll be acting on a network interface so I don't think including _if_ adds anything.

lib/libifconfig/libifconfig_nl.c

30

Perhaps not something to fix in this commit, but we ought to do this in ifconfig_open().
That way library users can call ifconfig_open() as root and then drop privileges for the rest of their work.

(It's not just here, the carp libifconfig code does this wrong too.)

[saheed]

Respond to review comments

lib/libifconfig/libifconfig.h

393

You're right, I'll make it ifconfig_set_up().

lib/libifconfig/libifconfig_nl.c

30

I'll refactor it into ifconfig_open() soon in a new revision :)

41

I feel like on Linux the flags are not touched if both ifi_change and ifi_flags are 0 (see do_setlink and rtnl_configure_link). I'll look deep into it soon :)

[saheed]

Change ifconfig_if_up/down() to ifconfig_set_up()

[obiwac]

lib/libifconfig/libifconfig_nl.c

41

Either way, if we do change how _nl_modify_ifp_generic ups/downs interfaces to work correctly with 0xFFFFFFFF or 0, this patch will still work :)

[kp]

lib/libifconfig/libifconfig.h

390

Missing the 'up' param.

[mckusick]

Mentor approval.