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Re: [Libguestfs] [PATCH nbdkit 4/4] reflection: Enhance plugin to support client address mode.



On Mon, Sep 16, 2019 at 11:01:49AM -0500, Eric Blake wrote:
> On 9/15/19 9:55 AM, Richard W.M. Jones wrote:
> > +Another use for the reflection plugin is to send back the client's IP
> > +address:
> > +
> > + $ nbdkit reflection mode=address
> > + $ nbdsh -u 'nbd://localhost' -c 'print(h.pread(h.get_size(), 0))'
> > +
> > +which will print something like:
> > +
> > + b'[::1]:58912'
> 
> Do we want a mode that attempts to do DNS lookup to convert an address
> back to a name, so that this could result in b'localhost:58912'?

I suppose this could reveal too much information about the server's
(DNS?) configuration?

> > +
> > +  case AF_UNIX:
> > +    /* We don't want to expose the socket path because it's a host
> > +     * filesystem name.  The client might not really be running on the
> > +     * same machine (eg. it using a proxy).  However it doesn't even
> 
> missing 'is'

Will fix.

> > +++ b/tests/test-reflection-address.sh
> > @@ -0,0 +1,63 @@
> 
> > +# Test the relection plugin with mode=address.
> 
> reflection (hmm, I missed that typo in patch 1, where this was copied from)

Oops - fixed in all 3 places.

> > +# Run nbdkit.
> > +start_nbdkit -P reflection-address.pid -U $sock \
> > +       reflection mode=address
> 
> Is it worth also trying to test a TCP socket (although we have to worry
> about finding a free port for the server, as well as heavily filtering
> the result as it might be [::1] or 127.0.0.1, and most certainly will
> have a different client port number per test run.

Yes it's because of this problem that I'm only testing a Unix domain
socket here.

We do have a few tests which use localhost, but they occasionally fail
in Koji especially because our algorithm to pick a free port is racy:

https://github.com/libguestfs/nbdkit/blob/03a2cc3d766edb17011dafe939e53d0f60f1c99b/tests/functions.sh.in#L128

As far as I know there's not a good way to fix this except to have
nbdkit choose a port, but that way is problematic in other ways.
Perhaps there is some way to have an external process hang on to a
port and pass it to nbdkit (using -s?)

> But overall, I think this is a useful thing to add to the plugin, and
> I'm not seeing any security holes in letting the client know the
> client's IP address as seen by the server.

OK I'll fix up all the things you have mentioned and push it, thanks.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
libguestfs lets you edit virtual machines.  Supports shell scripting,
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