tap(4) should go UP if opened

[Bruce M. Simpson]

Frank Behrens wrote:
> How does tun(4) handle this? tun(4) is also set to down, when closed. It is not set to up, when 
> ist is opened, but when an address is assigned by the user process. This is fine, because it 
> needs always an ip address. tap(4) as layer 2 tunnel device does not need an ip address, so 
> setting it up on open is IMHO the best solution.
>
>   
This isn't consistent with the other software cloneable interfaces which 
emulate certain layer 2 semantics, e.g. bridge, trunk, vlan; see below.
> Sound this reasonable or how should I handle the tap(4) open by an user process, when this 
> process does not run as root?
>   
I recently committed Landon Fuller's code which makes tap and tun 
cloneable interfaces which may then be created via 'ifconfig tap0 create'.

Automatically setting the interface to IFF_UP is not consistent with the 
semantics for other network interfaces; it requires specific privileges 
(usually super-user or PRIV_NET_SETIFFLAGS in -CURRENT) to do.

However, we also support the creation of tap/tun instances by 
non-super-users, so there is motivation for the change. Configuring a 
tap interface to up by a non-superuser should only be permitted if the 
interface itself was created by a non-superuser, and if 
net.link.tap.user_open is set to 1.

A more involved patch is needed to do this right for all cases -- we 
should not do this by default.

Regards,
BMS