On Sun, May 25, 2025 at 5:09 PM NeilBrown <neil@xxxxxxxxxx> wrote:
>
> On Mon, 26 May 2025, Chuck Lever wrote:
> > On 5/20/25 9:20 AM, Chuck Lever wrote:
> > > Hiya Rick -
> > >
> > > On 5/19/25 9:44 PM, Rick Macklem wrote:
> > >
> > >> Do you also have some configurable settings for if/how the DNS
> > >> field in the client's X.509 cert is checked?
> > >> The range is, imho:
> > >> - Don't check it at all, so the client can have any IP/DNS name (a mobile
> > >> device). The least secure, but still pretty good, since the ert. verified.
> > >> - DNS matches a wildcard like *.umich.edu for the reverse DNS name for
> > >> the client's IP host address.
> > >> - DNS matches exactly what reverse DNS gets for the client's IP host address.
> > >
> > > I've been told repeatedly that certificate verification must not depend
> > > on DNS because DNS can be easily spoofed. To date, the Linux
> > > implementation of RPC-with-TLS depends on having the peer's IP address
> > > in the certificate's SAN.
> > >
> > > I recognize that tlshd will need to bend a little for clients that use
> > > a dynamically allocated IP address, but I haven't looked into it yet.
> > > Perhaps client certificates do not need to contain their peer IP
> > > address, but server certificates do, in order to enable mounting by IP
> > > instead of by hostname.
> > >
> > >
> > >> Wildcards are discouraged by some RFC, but are still supported by OpenSSL.
> > >
> > > I would prefer that we follow the guidance of RFCs where possible,
> > > rather than a particular implementation that might have historical
> > > reasons to permit a lack of security.
> >
> > Let me follow up on this.
> >
> > We have an open issue against tlshd that has suggested that, rather
> > than looking at DNS query results, the NFS server should authorize
> > access by looking at the client certificate's CN. The server's
> > administrator should be able to specify a list of one or more CN
> > wildcards that can be used to authorize access, much in the same way
> > that NFSD currently uses netgroups and hostnames per export.
> >
> > So, after validating the client's CA trust chain, an NFS server can
> > match the client certificate's CN against its list of authorized CNs,
> > and if the client's CN fails to match, fail the handshake (or whatever
> > we need to do).
> >
> > I favor this approach over using DNS labels, which are often
> > untrustworthy, and IP addresses, which can be dynamically reassigned.
> >
> > What do you think?
>
> I completely agree with this. IP address and DNS identity of the client
> is irrelevant when mTLS is used. What matters is whether the client has
> authority to act as one of the the names given when the filesystem was
> exported (e.g. in /etc/exports). His is exacly what you said.
Well, what happens when someone naughty copies the cert. to a different
system?
--> It is true that verification will show that the cert. is valid, but is it
valid for that client system?
(Not checking the reverse DNS name makes the check somewhat weaker,
imho. Now, is having the IP address in the cert. and checking
that stronger.
Well, maybe. The hassle is that the certs. all have to be replaced when
some network type decides to reconfigure the LANs or move the system
onto a different subnet for some reason.)
Another way a cert. can be protected from being moved to a different client
by someone naughty is adding a passphrase to it (the -aes256 option on
the openssl command line when creating the CSR). The hassle is that
someone has to type the passphrase in when the system is started/rebooted.
There is no perfect solution (or security is not a binary value, if
you prefer), so
I chose to provide as many options as I could, so that sysadmins could choose
what works for them. (Of course, they need to understand what is going on and
documenting that can be a challenge.)
rick
>
> Ideally it would be more than just the CN. We want to know both the
> domain in which the peer has authority (e.g. example.com) and the type
> of authority (e.g. serve-web-pages or proxy-file-access or
> act-as-neilb).
> I don't know internal details of certificates so I don't know if there
> is some other field that can say "This peer is authorised to proxy file
> access requests for all users in the given domain" or if we need a hack
> like exporting to nfs-client.example.com.
>
> But if the admin has full control of what names to export to, it is
> possible that the distinction doesn't matter. I wouldn't want the
> certificate used to authenticate my web server to have authority to
> access all files on my NFS server just because the same domain name
> applies to both.
>
> Thanks,
> NeilBrown
