[Libguestfs] [PATCH nbdkit 0/4] Reflection plugin, peer name.

[Richard W.M. Jones]

This series is based on my blog posting here:
https://rwmj.wordpress.com/2019/09/13/nbdkit-supports-exportnames/

It depends on the fix for realloc:
https://www.redhat.com/archives/libguestfs/2019-September/thread.html#00103

This series adds a fun plugin, and also an semi-related feature I've
long thought to be desirable.  You can consider patches 1 & 4, and
patches 2 & 3 as forming standalone patch series (but they do depend
on each other).

The fun plugin is a reflection plugin which "reflects" client
information back to the client.  We have a few fun plugins like this
(hello there, nbdkit-full-plugin) and normally I would push these
without review, but in this particular case there's a specific danger
that sending back data under control of the client might lead to a
security problem.  I _believe_ I have avoided that pitfall, but my
belief isn't as good as having experts review it :-)

The new feature is nbdkit_peer_name() which returns the sockaddr of
the peer.  It's essentially a wrapper around getpeername(2).  This
would allow several features to be implemented in future:

 - Plugins could accept or reject connections based on IP address.

 - Plugins could change content based on client.  (The fourth patch in
   the series is a PoC of this implemented in the new reflection
   plugin.)  Be cautious about combining this feature with multi-conn
   as it's not obviously always safe to do.

 - Some filters could usefully modify their behaviour based on client
   address: The TODO file currently notes that the rate filter could
   be changed to limit traffic based on client IP.

Rich.