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Notes on securing NUT
=====================
The NUT Team is very interested in providing the highest security level to its
users.
Many internal and external mechanisms exist to secure NUT. And several steps are
needed to ensure that your NUT setup meets your security requirements.
This chapter will present you these mechanisms, by increasing order of security
level. This means that the more security you need, the more mechanisms you will
have to apply.
NOTE: you may want to have a look at NUT Quality Assurance, since some
topics are related to NUT security and reliability.
[[verifySourceSig]]
How to verify the NUT source code signature
-------------------------------------------
In order to verify the NUT source code signature for releases, perform the following steps:
- Retrieve the link:http://www.networkupstools.org/download.html[NUT source code] (nut-X.Y.Z.tar.gz) and the matching signature (nut-X.Y.Z.tar.gz.sig)
- Retrieve the link:http://www.networkupstools.org/source/nut-key.gpg[NUT maintainer's signature]:
$ gpg --fetch-keys http://www.networkupstools.org/source/nut-key.gpg
- Launch the GPG checking using the following command:
$ gpg --verify nut-X.Y.Z.tar.gz.sig
- You should see a message mentioning a "Good signature", like:
gpg: Signature made Thu Jul 5 16:15:05 2007 CEST using DSA key ID 204DDF1B
gpg: Good signature from "Arnaud Quette ..."
...
System level privileges and ownership
-------------------------------------
All configuration files should be protected so that the world can't read them.
Use the following commands to accomplish this:
chown root:nut /etc/nut/*
chmod 640 /etc/nut/*
Finally, the <<StatePath,state path>> directory, which holds the
communication between the driver(s) and upsd, should also be secured.
chown root:nut /var/state/ups
chmod 0770 /var/state/ups
NUT level user privileges
-------------------------
Administrative commands such as setting variables and the instant commands
are powerful, and access to them needs to be restricted.
NUT provides an internal mechanism to do so, through
linkman:upsd.users[5].
This file defines who may access instant commands and settings, and what
is available.
During the initial
<<NUT_user_creation,NUT user creation>>, we have created a
monitoring user for upsmon.
You can also create an 'administrator' user with full power using:
[administrator]
password = mypass
actions = set
instcmds = all
For more information on how to restrict actions and instant commands, refer
to linkman:upsd.users[5] manual page.
NOTE: NUT administrative user definitions should be used in conjunction with
<<TCP_Wrappers,TCP Wrappers>>.
Network access control
----------------------
If you are not using NUT on a standalone setup, you will need to enforce
network access to upsd.
There are various ways to do so.
NUT LISTEN directive
~~~~~~~~~~~~~~~~~~~~
linkman:upsd.conf[5].
LISTEN interface port
Bind a listening port to the interface specified by its Internet address. This
may be useful on hosts with multiple interfaces. You should not rely
exclusively on this for security, as it can be subverted on many systems.
Listen on TCP port `port` instead of the default value which was compiled into
the code. This overrides any value you may have set with `configure
--with-port`. If you don't change it with configure or this value, `upsd` will
listen on port 3493 for this interface.
Multiple LISTEN addresses may be specified. The default is to bind to
127.0.0.1 if no LISTEN addresses are specified (and ::1 if IPv6 support is
compiled in).
LISTEN 127.0.0.1
LISTEN 192.168.50.1
LISTEN ::1
LISTEN 2001:0db8:1234:08d3:1319:8a2e:0370:7344
This parameter will only be read at startup. You'll need to restart (rather
than reload) `upsd` to apply any changes made here.
Firewall
~~~~~~~~
NUT has its own official IANA port: 3493/tcp.
The `upsmon` process on slave systems (as well as `upsc`) connects to the
`upsd` process on the master system via this TCP port. The `upsd` process does
not connect out.
You should use this to restrict network access.
[[TCP_Wrappers]]
TCP Wrappers
~~~~~~~~~~~~
If the server is build with tcp-wrappers support enabled, it will check if the
NUT username is allowed to connect from the client address through the
'/etc/hosts.allow' and '/etc/hosts.deny' files.
NOTE: this will only be done for commands that require the user to be logged
into the server.
`hosts.allow`:
ups : admin@127.0.0.1/32
ups : monslave@127.0.0.1/32 monslave@192.168.1.0/24
`hosts.deny`:
upsd : ALL
Further details are described in hosts_access(5).
Configuring SSL
---------------
SSL is available as a build option ('--with-ssl').
It encrypts sessions between upsd and clients, and can also be used to
authenticate servers.
This means that stealing port 3493 from upsd will no longer net you interesting
passwords.
Several things must happen before this will work, however. This chapter will
present these steps.
Install OpenSSL
~~~~~~~~~~~~~~~
Install link:http://www.openssl.org[OpenSSL] as usual, either from source
or binary packages.
Recompile and install NUT
~~~~~~~~~~~~~~~~~~~~~~~~~
Recompile NUT from source, starting with 'configure --with-ssl'.
Then install everything as usual.
Create a certificate and key for upsd
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
openssl (the program) should be in your PATH, unless you installed it from
source yourself, in which case it may be in /usr/local/ssl/bin.
Use the following command to create the certificate:
openssl req -new -x509 -nodes -out upsd.crt -keyout upsd.key
You can also put a '-days nnn' in there to set the expiration. If
you skip this, it may default to 30 days. This is probably not what
you want.
It will ask several questions. What you put in there doesn't matter a whole
lot, since nobody is going to see it for now. Future versions of the
clients may present data from it, so you might use this opportunity to
identify each server somehow.
Figure out the hash for the key
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Use the following command to determine the hash of the certificate:
openssl x509 -hash -noout -in upsd.crt
You'll get back a single line with 8 hex characters. This is the
hash of the certificate, which is used for naming the client-side
certificate. For the purposes of this example the hash is *0123abcd*.
Install the client-side certificate
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Use the following commands to install the client-side certificate:
mkdir <certpath>
chmod 0755 <certpath>
cp upsd.crt <certpath>/<hash>.0
Example:
mkdir /usr/local/ups/etc/certs
chmod 0755 /usr/local/ups/etc/certs
cp upsd.crt /usr/local/ups/etc/certs/0123abcd.0
If you already have a file with that name in there, increment the
0 until you get a unique filename that works.
If you have multiple client systems (like upsmon slaves), be sure
to install this file on them as well.
We recommend making a directory under your existing confpath to
keep everything in the same place. Remember the path you created,
since you will need to put it in upsmon.conf later.
It must not be writable by unprivileged users, since someone could
insert a new client certificate and fool upsmon into trusting a
fake upsd.
Create the combined file for upsd
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
To do so, use the below commands:
cat upsd.crt upsd.key > upsd.pem
chown root:nut upsd.pem
chmod 0640 upsd.pem
This file must be kept secure, since anyone possessing it could
pretend to be upsd and harvest authentication data if they get a
hold of port 3493.
Having it be owned by 'root' and readable by group 'nut' allows upsd
to read the file without being able to change the contents. This
is done to minimize the impact if someone should break into upsd.
Note on certification authorities (CAs) and signed keys
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
There are probably other ways to handle this, involving keys which have
been signed by a CA you recognize. Contact your local SSL guru.
Install the server-side certificate
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Install the certificate with the following command:
mv upsd.pem <upsd certfile path>
Example:
mv upsd.pem /usr/local/ups/etc/upsd.pem
After that, edit your upsd.conf and tell it where to find it:
CERTFILE /usr/local/ups/etc/upsd.pem
Clean up the temporary files
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
rm -f upsd.crt upsd.key
Restart upsd
~~~~~~~~~~~~
It should come back up without any complaints. If it says something
about keys or certificates, then you probably missed a step.
If you run upsd as a separate user id (like nutsrv), make sure that
user can read the upsd.pem file.
Point upsmon at the certificates
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Edit your upsmon.conf, and tell it where the CERTPATH is:
CERTPATH <path>
Example:
CERTPATH /usr/local/ups/etc/certs
Recommended: make upsmon verify all connections with certificates
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Put this in upsmon.conf:
CERTVERIFY 1
Without this, there is no guarantee that the upsd is the right host.
Enabling this greatly reduces the risk of man in the middle attacks.
This effectively forces the use of SSL, so don't use this unless
all of your upsd hosts are ready for SSL and have their certificates
in order.
Recommended: force upsmon to use SSL
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Again in upsmon.conf:
FORCESSL 1
If you don't use 'CERTVERIFY 1', then this will at least make sure
that nobody can sniff your sessions without a large effort. Setting
this will make upsmon drop connections if the remote upsd doesn't
support SSL, so don't use it unless all of them have it running.
Restart upsmon
~~~~~~~~~~~~~~
You should see something like this in the syslog from upsd:
foo upsd[1234]: Client mon@localhost logged in to UPS [myups] (SSL)
If upsd or upsmon give any error messages, or the (SSL) is missing,
then something isn't right.
If in doubt about upsmon, start it with -D so it will stay in
the foreground and print debug messages. It should print something
like this every couple of seconds:
polling ups: myups@localhost [SSL]
Obviously, if the '[SSL]' isn't there, something's broken.
Recommended: sniff the connection to see it for yourself
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Using tcpdump, Wireshark (Ethereal), or another network sniffer tool,
tell it to monitor port 3493/tcp and see what happens. You should only
see 'STARTTLS' go out, 'OK STARTTLS' come back, and the rest will be
certificate data and then seemingly random characters.
If you see any plaintext besides that (USERNAME, PASSWORD, etc.)
then something is not working.
Potential problems
~~~~~~~~~~~~~~~~~~
If you specify a certificate expiration date, you will eventually
see things like this in your syslog:
Oct 29 07:27:25 rktoy upsmon[3789]: Poll UPS [for750@rktoy] failed -
SSL error: error:14090086:SSL routines:SSL3_GET_SERVER_CERTIFICATE: certificate verify failed
You can verify that it is expired by using openssl to display the date:
openssl x509 -enddate -noout -in <certfile>
It'll display a date like this:
notAfter=Oct 28 20:05:32 2002 GMT
If that's after the current date, you need to generate another cert/key
pair using the procedure above.
Conclusion
~~~~~~~~~~
SSL support should be considered stable but purposely underdocumented
since various bits of the implementation or configuration may change in
the future. In other words, if you use this and it stops working after
an upgrade, come back to this file to find out what changed.
This is why the other documentation doesn't mention any of these
directives yet. SSL support is a treat for those of you that RTFM.
There are also potential licensing issues for people who ship binary
packages since NUT is GPL and OpenSSL is not compatible with it. You
can still build and use it yourself, but you can't distribute the
results of it. Or maybe you can. It depends on what you consider
"essential system software", and some other legal junk that we're not
going to touch.
Other packages have solved this by explicitly stating that an exception
has been granted. That is (purposely) impossible here, since NUT is the
combined effort of many people, and all of them would have to agree to a
license change. This is actually a feature, since it means nobody can
unilaterally run off with the source - not even the NUT team.
Note that the replacement of OpenSSL by Mozilla Network Security Services
(NSS) is scheduled in the future, to avoid the above licensing issues.
chrooting and other forms of paranoia
-------------------------------------
It has been possible to run the drivers and upsd in a chrooted jail for
some time, but it involved a number of evil hacks. From the 1.3 series,
a much saner chroot behavior exists, using BIND 9 as an inspiration.
The old way involved creating an entire tree, complete with libraries, a
shell (!), and many auxiliary files. This was hard to maintain and
could have become an interesting playground for an intruder. The new
way is minimal, and leaves little in the way of usable materials within
the jail.
This document assumes that you already have created at least one user
account for the software to use. If you're still letting it fall back
on "nobody", stop right here and go figure that out first. It also
assumes that you have everything else configured and running happily all
by itself.
Generalities
~~~~~~~~~~~~
Essentially, you need to create your configuration directory and state
path in their own little world, plus a special device or two.
For the purposes of this example, the chroot jail is /chroot/nut. The
programs have been built with the default prefix, so they are using
/usr/local/ups. First, create the confpath and bring over a few files.
mkdir -p /chroot/nut/usr/local/ups/etc
cd /chroot/nut/usr/local/ups/etc
cp -a /usr/local/ups/etc/upsd.users .
cp -a /usr/local/ups/etc/upsd.conf .
cp -a /usr/local/ups/etc/ups.conf .
We're using 'cp -a' to maintain the permissions on those files.
Now bring over your state path, maintaining the same permissions as
before.
mkdir -p /chroot/nut/var/state
cp -a /var/state/ups /chroot/nut/var/state
Next we must put /etc/localtime inside the jail, or you may get very
strange readings in your syslog. You'll know you have this problem if
upsd shows up as UTC in the syslog while the rest of the system doesn't.
mkdir -p /chroot/nut/etc
cp /etc/localtime /chroot/nut/etc
Note that this is not "cp -a", since we want to copy the *content*, not
the symlink that it may be on some systems.
Finally, create a tiny bit of /dev so the programs can enter the
background properly - they redirect fds into the bit bucket to make sure
nothing else grabs 0-2.
mkdir -p /chroot/nut/dev
cp -a /dev/null /chroot/nut/dev
Try to start your driver(s) and make sure everything fires up as before.
upsdrvctl -r /chroot/nut -u nutdev start
Once your drivers are running properly, try starting upsd.
upsd -r /chroot/nut -u nutsrv
Check your syslog. If nothing is complaining, try running clients like
upsc and upsmon. If they seem happy, then you're done.
symlinks
~~~~~~~~
After you do this, you will have two copies of many things, like the
confpath and the state path. I recommend deleting the 'real'
/var/state/ups, replacing it with a symlink to
/chroot/nut/var/state/ups. That will let other programs reference the
.pid files without a lot of hassle.
You can also do this with your confpath and point /usr/local/ups/etc at
/chroot/nut/usr/local/ups/etc unless you're worried about something
hurting the files inside that directory. In that case, you should
maintain a 'master' copy and push it into the chroot path after
making changes.
upsdrvctl itself does not chroot, so the ups.conf still needs to be in
the usual confpath.
upsmon
~~~~~~
This has not yet been applied to upsmon, since it can be quite
complicated when there are notifiers that need to be run. One
possibility would be for upsmon to have three instances:
- privileged root parent that listens for a shutdown command
- unprivileged child that listens for notify events
- unprivileged chrooted child that does network I/O
This one is messy, and may not happen for some time, if ever.
Config files
~~~~~~~~~~~~
You may now set chroot= and user= in the global section of ups.conf.
upsd chroots before opening any config files, so there is no way to
add support for that in upsd.conf at the present time.