Re: [Nbd] nbd-server working easily in cygwin in XP
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- Subject: Re: [Nbd] nbd-server working easily in cygwin in XP
- From: Wouter Verhelst <w@...112...>
- Date: Tue, 12 Aug 2008 18:53:58 -0300
- Message-id: <20080812215358.GC10784@...172...>
- In-reply-to: <20080812202951.GB10784@...172...>
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On Tue, Aug 12, 2008 at 05:29:51PM -0300, Wouter Verhelst wrote:
> On Tue, Aug 12, 2008 at 11:10:14AM -0700, qm <Brad Allen wrote:
> > couldn't as easily steal or modify runnable data. To make it
> > really secure, nbd-server ought to have a password settable in the
> > config file that nbd-client must use, which it requires upon
> > negotiation.
> I completely agree. It's been on my TODO list for a few years (with
> dreams of adding Kerberos support, too), but just hasn't happened yet.
> There're a few reasons for that:
> - First, I want to keep nbd-client as small as possible, so that it'll
> fit on embedded devices and in initrds and such; so using a (possibly
> huge) library to implement password authentication is out, unless it
> can optionally be disabled; this rules out things like SASL and
> Kerberos if they're the only thing we're going to use.
> - Second, I do /not/ want to send passwords across the wire in the clear
> in any case. Doing this will only provide a false sense of security,
> and I don't want to do that in any case.
> - Third, so far I just haven't had the time to properly work out a
> solution that satisfies both of the above constraints.
Actually, come to think about it, one advantage of being at a conference
like Debconf is that you can ask people around you to help
What we came up with is this:
- Server sends a random number as a way to challenge the client for a
- Client constructs something based on the IP address, password, and the
random number the server sent, pumps it through a secure hash
algorithm, and sends that back.
- Server constructs the same thing and pumps it through the same
algorithm. If the output matches, we're authenticated; if it doesn't
match, we're not.
Doing a checksum isn't usually an insanely expensive operation, and can
probably be done in a reasonably small amount of code, thereby not
enlarging nbd-client too much. Anyone writing this will have to learn
such an algorithm first, obviously; or alternatively, if we can find a
small library somewhere that will do such checksums _only_, we may
consider including that as a build requirement. I'm specifically not
considering openssl here, which takes about half a meg and may be too
much for embedded things or initramfses on memory-constrained systems.
This principle is reasonably future-proof; if the particular checksum
protocol we choose ever goes the way of MD5 (which people are now able
to construct a collision for in minutes on a laptop, according to
wikipedia), we can replace the checksum algorithm but keep the same
The IP address requirement is included as a simple MITM protection. Of
course this doesn't make NBD safe for eavesdroppers (no encryption, so
anyone with a sniffer between client and server can map out the entire
contents of the block device, given time), but at the very least it will
prevent unauthenticated access.
<Lo-lan-do> Home is where you have to wash the dishes.
-- #debian-devel, Freenode, 2004-09-22