NRPE With SSL/TLS
This document covers the different methods of SSL transport that NRPE allows for.
If there was a TL;DR here, it is these:
Don't use NRPE without encryption
Use Public Key Encryption
NRPE has had basic support for SSL/TLS for some time now, but it was
severely lacking. It only allowed anonymous Diffie Hellman (ADH) key
exchange, it used a fixed 512-bit key (generated at
time and extremely insecure) and originally allowed SSLv2. In 2004,
SSLv2 and SSLv3 support was disabled.
check_nrpe have been updated to offer much more secure
encryption and more options. And the updates are done in a backward-
compatible way, allowing you to migrate to the newer versions
without having to do it all at once, and possibly miss updating some
machines, causing lost reporting.
./configure will now create a 2048-bit DH key instead
of the old 512-bit key. The most current versions of openSSL will
still not allow it. In my testing, openSSL 1.0.1e allowed DH keys
of 512 bits, and 1.0.1k would not allow 2048 bit keys. In addition
we now call
SSL_CTX_set_options(ctx, SSL_OP_SINGLE_DH_USE) so a
new key is generated on each connection, based on the 2048-bit
The NRPE configuration file has added new SSL/TLS options. The defaults currently will allow old check_nrpe plugins to continue to connect to the nrpe daemon, but can report on "old style" connections, or enforce more secure communication as your migration progresses. The new options are in the "SSL/TLS OPTIONS" section of nrpe.cfg, about two-thirds of the way down.
If you are upgrading NRPE from a prior version, you can run the
update-cfg.pl script to add the new parameters to your nrpe.cfg.
ssl_version directive lets you set which versions of SSL/TLS
you want to allow. SSLv2, SSLv3, TLSv1, TLSv1.1 and TLSv1.2 are
allowed, or those litereals with a
+ after them (as in TLSv1.1+).
+, that version only will be used. With the
that version or above will be used. openSSL will always negotiate
the highest available allowed version available on both ends. This
directive currently defaults to
ssl_use_adh directive is DEPRECATED, even though it is new.
Possible values are
0 to not allow ADH at all,
1 to allow ADH,
2 to require ADH. The
2 should never be required, but it's
there just in case it's needed, for whatever reason.
1 is currently
the default, which allows older
check_nrpe plugins to connect using
ADH. When all the plugins are migrated to the newer version, it
should be set to
0. In an upcoming version of NRPE, ADH will no
longer be allowed at all. Note that if you use a
2 here, NRPE will
ssl_cipher_list entries (below) to only allow ADH.
ssl_cipher_list directive lets you specify which ciphers you
want to allow. It currently defaults to
ALL:!MD5:@STRENGTH but can
take any value allowed by openSSL. In an upcoming version of NRPE, it
will be changed to something more secure, something like
ALL:!aNULL:!eNULL:!SSLv2:!LOW:!EXP:!RC4:!MD5:@STRENGTH. Note that
if you have
ssl_use_adh=2, this string will be overridden with
ADH which only only allow ADH.
directives are used to specify which *.pem files are to be used for
Public-Key Encryption (PKE). Setting these will allow clients to use
PKE to communicate with the server, similar to how the HTTPS
ssl_client_certs directive specifies whether or not a client
certificate will be requested when a client tries to connect. A value
0 means the nrpe daemon will not ask for or require a client
1 will cause it to ask for a client certificate, but
not require one. A
2 will require the client to present a valid
certificate. This currently defaults to
0. If you want to use
client certificates and are upgrading the clients over time, you can
set this to
1 once many have been upgraded, then set to
force the use of client certs. Note that the client certs must be
signed by the CA cert specified in the
ssl_logging directive allows you to log some additional data
to syslog. OR (or add) values together to have more than one option
enabled. Values are
0 for no additional logging (the default),
1 to log startup SSL/TLS parameters from the nrpe.cfg file,
log the SSL/TLS version of connections,
4 to log which cipher is
being used for the connection,
8 to log if the plugin has a cert, and
16 to log details of plugin's certificate.
-1 will enable all.
This can be especially helpful during plugin migration, so you can
tell which plugins have certificates, what SSL/TLS version is being
used, and which ciphers are being used.
check_nrpe plugin has also been updated to provide more secure
encryption and allow the use of client certificates. The command line
has several new options, which are outlined below. Both the long and
short arguments are presented.
-d will disable the use of ADH. This option is
DEPRECATED, even though it's new. It will be removed in a
-S <ver> specifies minimum SSL/TLS version
to use. See the
ssl_version directive above for possible values.
-L <value> determines which ciphers will
and won't be allowed. See the
ssl_cipher_list directive above.
-C <path> specifies an optional client
certificate to use. If this value is entered, the next one below is
-K <path> specifies the client certificate
key file to use. This goes along with
-A <path> specifies the CA certificate
to use in order to validate the nrpe daemon's public key.
-d is DEPRECATED
-d [num] is DEPRECATED, even though it is new.
If you use
-d 0 it acts the same way as as the old
1 to allow ADH, and
2 to require ADH.
-s <num> allows you to log some additional
data to syslog. OR (or add) values together to have more than one
option enabled. See the description of the
from NRPE above.
Certificate Generation Example
Note The following example does not follow best practice for creating and running a CA or creating certificates. It is for testing or possibly for use in a small environment. Sloppy security is as bad as no security.
In this example, we are going to put everything in the
/usr/local/nagios/etc/ssl directory. You may want to use the more
/etc/ssl directory, or somewhere else entirely.
We are going to assume your company name is Foo Widgets, LLC; the
server running the nagios process (and thus the check_nrpe program)
nag_serv; and there are two Linux machines that will
run the nrpe daemon:
Set up the directories
As root, do the following:
mkdir -p -m 750 /usr/local/nagios/etc/ssl chown root:nagios /usr/local/nagios/etc/ssl cd /usr/local/nagios/etc/ssl mkdir -m 750 ca chown root:root ca mkdir -m 750 server_certs chown root:nagios server_certs mkdir -m 750 client_certs chown root:nagios client_certs
Create Certificate Authority
If you want to validate client or server certificates, you will need to create a Certificate Authority (CA) that will sign all client and server certificates. If your organization already has a CA, you can use that.
As root, do the following:
cd /usr/local/nagios/etc/ssl/ca openssl req -x509 -newkey rsa:4096 -keyout ca_key.pem \ -out ca_cert.pem -utf8 -days 3650
When asked, enter a passphrase. Then follow the prompts. You will
probably want to include
Certificate Authority in for
Organizational Unit Name and
Common Name. For example:
Organization Name (eg, company) :Foo Widgets LLC Organizational Unit Name (eg, section) :Foo Certificate Authority Common Name (e.g. server FQDN or YOUR name) :Foo Nagios CA
Create NRPE Server Certificate Requests
For each of the hosts that will be running the nrpe daemon, you will need a server certificate. You can create a key, and the CSR (Certificate Signing Request) separately, but the following commands will do both with one command. As root, do the following:
cd /usr/local/nagios/etc/ssl/server_certs openssl req -new -newkey rsa:2048 -keyout db_server.key \ -out db_server.csr -nodes openssl req -new -newkey rsa:2048 -keyout bobs_workstation.key \ -out bobs_workstation.csr -nodes
Follow the prompts. The
-nodes at the end of the lines tells
openssl to generate the key without a passphrase. Leave it off if you
want someone to enter a passphrase whenever the machine boots.
Now you need to sign the CSRs with your CA key.
If you have the default
/etc/openssl.cnf, either change it, or as root, do:
cd /usr/local/nagios/etc/ssl mkdir demoCA mkdir demoCA/newcerts touch demoCA/index.txt echo "01" > demoCA/serial chown -R root:root demoCA chmod 700 demoCA chmod 700 demoCA/newcerts chmod 600 demoCA/serial chmod 600 demoCA/index.txt
Now, sign the CSRs. As root, do the following:
cd /usr/local/nagios/etc/ssl openssl ca -days 365 -notext -md sha256 \ -keyfile ca/ca_key.pem -cert ca/ca_cert.pem \ -in server_certs/db_server.csr \ -out server_certs/db_server.pem chown root:nagios server_certs/db_server.pem chmod 440 server_certs/db_server.pem openssl ca -days 365 -notext -md sha256 \ -keyfile ca/ca_key.pem -cert ca/ca_cert.pem \ -in server_certs/bobs_workstation.csr \ -out server_certs/bobs_workstation.pem chown root:nagios server_certs/bobs_workstation.pem chmod 440 server_certs/bobs_workstation.pem
Now, copy the
db_server.key files to the
db_server machine, and the
bobs_workstation.key files to bobs_workstation. Copy the
ca/ca_cert.pem file to both machines.
Create NRPE Client Certificate Requests
Now you need to do the same thing for the machine that will be running the check_nrpe program.
cd /usr/local/nagios/etc/ssl/client_certs openssl req -new -newkey rsa:2048 -keyout nag_serv.key \ -out nag_serv.csr -nodes cd /usr/local/nagios/etc/ssl openssl ca -extensions usr_cert -days 365 -notext -md sha256 \ -keyfile ca/ca_key.pem -cert ca/ca_cert.pem \ -in client_certs/nag_serv.csr \ -out client_certs/nag_serv.pem chown root:nagios client_certs/nag_serv.pem chmod 440 client_certs/nag_serv.pem
Now, copy the
files to the nag_serv machine, if you did the above on a different
Put the location of each computers' three files in the
file or in the check_nrpe command line. You should now have
encryption and, if desired, key validation.