David Lamparter [off-list ref] writes:

On Sat, May 07, 2011 at 07:18:44AM -0700, Eric W. Biederman wrote:

quoted

You can read the processes network namespace by opening
/proc/<pid>/ns/net.  Unfortunately comparing the network
namespaces for identity is another matter.  You will probably
be better off simply forcing the routing daemon to start
in the desired network namespace in it's initscript.

For purposes of clarity please have a look at my work in
progress patch for iproute2.  This demonstrates how I expect
userspace to work in a multi-network namespace world.

[...]

quoted

Subject: [PATCH] iproute2:  Add processless netnwork namespace support.

[...]

quoted

Configuration specific to a network namespace that
would ordinarily be stored under /etc/ is stored under
/etc/netns/<name>.  For example if the dns server
configuration is different for your vpn you would
create a file /etc/netns/myvpn/resolv.conf.

File descriptors that can be used to manipulate a
network namespace can be created by opening
/var/run/netns/<NAME>.

This adds the following commands to iproute.
ip netns add NAME
ip netns delete NAME
ip netns monitor
ip netns list
ip netns exec NAME cmd ....
ip link set DEV netns NAME

funny, this is almost exactly what my code does - though you're probably
doing it better and have more features ;)

Well if it has more features it is only because I have managed to keep
everything simple enough that adding features was easy.  I ignored all
of the hard bits.

http://git.spaceboyz.net/equinox/vrf-tools.git/
git://spaceboyz.net/equinox/vrf-tools.git

It currently forks off a daemon to keep the namespace open; attaching is
not possible yet, but opening a socket in a different namespace is.

I went the round of keeping a daemon open, saw how much code that
takes and how fragile that can be in the corner cases and decided to
patch the kernel to make the interfaces better.

Most of the actual management (mounting things & co.) I offloaded to
some shell scripts; I use it together with GNU screen (which makes it
very nice to grab one of the namespaces and start/stop/manage/...
things).

That does sound like a nice way of doing the management.

I also have patches for OpenVPN and pptpd floating around that make it
possible to 'cross' namespace boundaries, i.e. the VPN servers listen in
one namespace and have their devices in another.

For openvpn I have managed to get away with simply using an up script. 
Mostly the script is:

ip netns add $NSNAME || true
ip netns exec $NSNAME ip link set lo up
ip link set $dev netns $NSNAME
ip netns exec $NSNAME ip link set $dev up
ip netns exec $NSNAME ifconfig $dev $ifconfig_local netmask $ifconfig_netmask broadcast $ifconfig_broadcast

With a few extra bits for dns options and routes.  If I had an openvpn
built with the iproute option I expect I could get away by just wrapping
iproute.  Not that I would mind a patched openvpn.

Personally I think using a vpn in a network namespace seems like a
killer feature.

Eric