10. 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.


you may want to have a look at NUT Quality Assurance, since some topics are related to NUT security and reliability.

10.1. How to verify the NUT source code signature

In order to verify the NUT source code signature for releases, perform the following steps:


As of NUT 2.8.0, a new release key is used, but the nut-key.gpg should be cumulative with older chain key files (includes them). You can view the key list in a downloaded copy of the URL above with:

$ gpg --with-colons --import-options import-show --dry-run --import < nut-key.gpg

…and as of this writing, it should contain two key sets for various identities of "Arnaud Quette" and one set of "Jim Klimov".

Just in case, the previous key file used since NUT 2.7.3 release is stored as NUT old maintainer’s signature for 2.7.3-2.7.4 releases

In order to verify an even older release, please use NUT old maintainer’s signature since 2002 until 2.7.3 release

  • 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", with formatting which depends on your gpg version, like:

    gpg: Signature made Thu Jun  1 00:10:16 2023 CEST
    gpg: Good signature from "Jim Klimov ..."
    Primary key fingerprint: B834 59F7 76B9 0224 988F  36C0 DE01 84DA 7043 DCF7


The previously used maintainer’s signatures would output (with markup of older gpg tools here):

gpg: Signature made Wed Apr 15 15:55:30 2015 CEST using RSA key ID 55CA5976
gpg: Good signature from "Arnaud Quette ..."


gpg: Signature made Thu Jul  5 16:15:05 2007 CEST using DSA key ID 204DDF1B
gpg: Good signature from "Arnaud Quette ..."

10.2. How to verify the NUT source code checksum

As a weaker but simpler alternative to verifying a signature, you can verify just the accompanying checksums of the source archive file. This is useful primarily to check against bit-rot in original storage or in transit. As far as disclaimers go: ideally, you should cover all provided algorithms — e.g. MD5 and SHA256 — to minimize the chance that intentional malicious tampering on the wire goes undetected. A myriad tools can check that on various platforms; some examples follow:

# Example original checksum to compare with, from NUT website:
$ cat nut-2.8.0.tar.gz.sha256
c3e5a708da797b7c70b653d37b1206a000fcb503b85519fe4cdf6353f792bfe5  nut-2.8.0.tar.gz
# Generate checksum of downloaded archive with perl (a NUT build dependency
# generally, though you may have to install Digest::SHA module from CPAN):
$ perl -MDigest::SHA=sha256_hex -le "print sha256_hex <>" nut-2.8.0.tar.gz
# Generate checksum of downloaded archive with openssl (another optional
# NUT build dependency):
$ openssl sha256 nut-2.8.0.tar.gz
SHA256(nut-2.8.0.tar.gz)= c3e5a708da797b7c70b653d37b1206a000fcb503b85519fe4cdf6353f792bfe5
# Generate checksum of downloaded archive with coreutils:
$ sha256sum nut-2.8.0.tar.gz
c3e5a708da797b7c70b653d37b1206a000fcb503b85519fe4cdf6353f792bfe5  nut-2.8.0.tar.gz
# Auto-check downloaded checksum against downloaded archive with coreutils:
$ sha256sum -c nut-2.8.0.tar.gz.sha256
nut-2.8.0.tar.gz: OK
# Generate checksum of downloaded archive with GPG:
$ gpg --print-md SHA256 nut-2.8.0.tar.gz
nut-2.8.0.tar.gz: C3E5A708 DA797B7C 70B653D3 7B1206A0
                  00FCB503 B85519FE 4CDF6353 F792BFE5

10.3. 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 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

10.4. 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 upsd.users(5).

This file defines who may access instant commands and settings, and what is available.

During the initial NUT user creation, we have created a monitoring user for upsmon.

You can also create an administrator user in NUT with full power using:

        password = mypass
        actions = set
        instcmds = all

For more information on how to restrict actions and instant commands, refer to upsd.users(5) manual page.


NUT administrative user definitions should be used in conjunction with TCP Wrappers.

10.5. 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


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 if no LISTEN addresses are specified (and ::1 if IPv6 support is compiled in).

LISTEN 2001:0db8:1234:08d3:1319:8a2e:0370:7344

As a special case, LISTEN * <port> (with an asterisk) will try to listen on "ANY" IP address for both IPv6 (::0) and IPv4 (, subject to upsd command-line arguments, or system configuration or support. Note that if the system supports IPv4-mapped IPv6 addressing per RFC-3493, and does not allow to disable this mode, then there may be one listening socket to handle both address families.

This parameter will only be read at startup. You’ll need to restart (rather than reload) upsd to apply any changes made here.


NUT has its own official IANA port: 3493/tcp.

The upsmon process on secondary systems, as well as any other NUT client (such as upsc, upscmd, upsrw, NUT-Monitor, …) connects to the upsd process on the system which manages the UPS, via this TCP port. Usually an upsmon process runs on the latter system in "primary" mode for the devices connected to it.

The upsd process does not initiate outgoing connections.

Certain NUT drivers (for network-managed devices) can initiate their own connections to various ports according to corresponding vendor protocol.

You should use this to restrict network access.

Uncomplicated Firewall (UFW) support

NUT can tightly integrate with Uncomplicated Firewall using the provided profile (nut.ufw.profile).

You must first install the profile on your system:

$ cp nut.ufw.profile /etc/ufw/applications.d/

To enable outside access to your local upsd, use:

$ ufw allow NUT

To restrict access to the network 192.168.X.Y, use:

$ ufw allow from to any app NUT

You can also use graphical frontends, such as gui-ufw (gufw), ufw-kde or ufw-frontends.

For more information, refer to:

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.


this will only be done for commands that require the user to be logged into the server.


upsd : admin@
upsd : observer@ observer@


upsd : ALL

Further details are described in hosts_access(5).

10.6. 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.

SSL is available via two back-end libraries : NSS and OpenSSL (historically). You can choose to use one of them by specifying it with a build option (--with-nss or --with-openssl). If neither is specified, the configure script will try to detect one of them, with a precedence for OpenSSL.

OpenSSL backend usage

This section describes how to enable NUT SSL support using OpenSSL.

Install OpenSSL

Install OpenSSL as usual, either from source or binary packages. If using binary packages, be sure to include the developer libraries.

Recompile and install NUT

Recompile NUT from source, starting with configure --with-openssl.

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


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 part until you get a unique filename that works.

If you have multiple client systems (like upsmon instances in secondary mode), 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 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. NUT reads the key and certificate files after dropping privileges and forking.

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>


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 /usr/local/ups/etc/certs
Recommended: make upsmon verify all connections with certificates

Put this in upsmon.conf:


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:


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.

NSS backend usage

This section describes how to enable NUT SSL support using Mozilla NSS.

Install NSS

Install Mozilla NSS as usual, either from source or binary packages. If using binary packages, be sure to include the developer libraries, and nss-tools (for certutil).

Recompile and install NUT

Recompile NUT from source, starting with configure --with-nss.

Then install everything as usual.

Create certificate and key for the host

NSS (package generally called libnss3-tools) will install a tool called certutil. It will be used to generate certificates and manage certificate database.

Certificates should be signed by a certification authorities (CAs). Following commands are typical samples, contact your SSL guru or security officer to follow your company procedures.

Generate a server certificate for upsd:

  • Create a directory where store the certificate database: mkdir cert_db
  • Create the certificate database : certutil -N -d cert_db
  • Import the CA certificate: certutil -A -d cert_db -n "My Root CA" -t "TC,," -a -i rootca.crt
  • Create a server certificate request (here called "My nut server"): certutil -R -d cert_db -s "CN=My nut server,O=MyCompany,ST=MyState,C=US" -a -o server.req
  • Make your CA sign the certificate (produces server.crt)
  • Import the signed certificate into server database: certutil -A -d cert_db -n "My nut server" -a -i server.crt -t ",,"
  • Display the content of certificate server: certutil -L -d cert_db

Clients and servers in the same host could share the same certificate to authenticate them or use different ones in same or different databases. The same operation can be done in same or different databases to generate other certificates.

Create a self-signed CA certificate

NSS provides a way to create self-signed certificate which can acting as CA certificate, and to sign other certificates with this CA certificate. This method can be used to provide a CA certification chain without using an "official" certificate authority.

Generate a self-signed CA certificate:

  • Create a directory where store the CA certificate database: mkdir CA_db
  • Create the certificate database: certutil -N -d CA_db
  • Generate a certificate for CA: certutil -S -d CA_db -n "My Root CA" -s "CN=My CA,O=MyCompany,ST=MyState,C=US" -t "CT,," -x -2 (Do not forget to answer Yes to the question "Is this a CA certificate [y/N]?")
  • Extract the CA certificate to be able to import it in upsd (or upsmon) certificate database: certutil -L -d CA_db -n "My Root CA" -a -o rootca.crt
  • Sign a certificate request with the CA certificate (simulate a real CA signature): certutil -C -d CA_db -c "My Root CA" -a -i server.req -o server.crt -2 -6
Install the server-side certificate

Just copy the database directory (just the directory and included 3 database .db files) to the right place, such as /usr/local/ups/etc/:

mv cert_db /usr/local/ups/etc/
upsd (required): certificate database and self certificate

Edit the upsd.conf to tell where find the certificate database:

CERTPATH /usr/local/ups/etc/cert_db

Also tell which is the certificate to send to clients to authenticate itself and the password to decrypt private key associated to certificate:

CERTIDENT "certificate name" "database password"


Generally, the certificate name is the server domain name, but is not a hard rule. The certificate can be named as useful.

upsd (optional): client authentication


This functionality is disabled by default. To activate it, recompile NUT with WITH_CLIENT_CERTIFICATE_VALIDATION defined:


UPSD can accept three levels of client authentication. Just specify it with the directive CERTREQUEST with the corresponding value in the upsd.conf file:

  • NO: no client authentication.
  • REQUEST: a certificate is request to the client but it is not strictly validated. If the client does not send any certificate, the connection is closed.
  • REQUIRE: a certificate is requested to the client and if it is not valid (no validation chain) the connection is closed.

Like CA certificates, you can add many "trusted" client and CA certificates in server’s certificate databases.

upsmon (required): upsd authentication

In order for upsmon to securely connect to upsd, it must authenticate it. You must associate an upsd host name to security rules in upsmon.conf with the directive CERTHOST.

CERTHOST associates a hostname to a certificate name. It also determines whether a SSL connection is mandatory, and if the server certificate must be validated.

CERTHOST "hostname" "certificate name" "certverify" "forcessl"

If the flag forcessl is set to 1, and upsd answers that it can not connect with SSL, the connection closes.

If the flag certverify is set to 1 and the connection is done in SSL, upsd's certificate is verified and its name must be the specified "certificate name".

To prevent security leaks, you should set all certverify and forcessl flags to 1 (force SSL connection and validate all certificates for all peers).

You can specify CERTVERIFY and FORCESSL directive (to 1 or 0) to define a default security rule to apply to all host not specified with a dedicated CERTHOST directive.

If a host is not specified in a CERTHOST directive, its expected certificate name is its hostname.

upsmon (optional): certificate database and self certificate

Like upsd, upsmon may need to authenticate itself (upsd's CERTREQUEST directive set to REQUEST or REQUIRE).

It must access to a certificate (and its private key) in a certificate database configuring CERTPATH and CERTIDENT in upsmon.conf in the same way as upsd.

CERTPATH /usr/local/ups/etc/cert_db
CERTIDENT "certificate name" "database password"

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 files in the certificate directory. NUT reads the keys and certificates after forking and dropping privileges.

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.


SSL support should be considered stable but purposely under-documented 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) should avoid the above licensing issues.

10.7. 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.


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 file descriptors into the bit bucket to make sure nothing else grabs fds 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.


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 (or equivalent on your system) 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 "golden" copy and push it into the chroot path after making changes.

The upsdrvctl itself does not chroot, so the ups.conf still needs to be in the usual confpath.


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.

The upsd chroots before opening any config files, so there is no way to add support for that in upsd.conf at the present time.