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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


  1. Introduction
  2. NRPE Changes
  3. check_nrpe Changes
  4. Certificate Generation Example


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 ./configure time and extremely insecure) and originally allowed SSLv2. In 2004, SSLv2 and SSLv3 support was disabled.

nrpe and 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.

NRPE Changes

Running ./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 key generated.

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 script to add the new parameters to your nrpe.cfg.

The 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+). Without the +, 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 TLSv1+.

The 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, and 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 override any ssl_cipher_list entries (below) to only allow ADH.

The 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.

The ssl_cacert_file, ssl_cert_file and ssl_privatekey_file 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 protocol works.

The ssl_client_certs directive specifies whether or not a client certificate will be requested when a client tries to connect. A value of 0 means the nrpe daemon will not ask for or require a client certificate. A 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 2 to force the use of client certs. Note that the client certs must be signed by the CA cert specified in the ssl_cacert_file directive.

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, 2 to 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 Changes

The 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.

--no-adh or -d will disable the use of ADH. This option is DEPRECATED, even though it's new. It will be removed in a future version.

--ssl-version=<ver> or -S <ver> specifies minimum SSL/TLS version to use. See the ssl_version directive above for possible values.

--cipher-list=<value. or -L <value> determines which ciphers will and won't be allowed. See the ssl_cipher_list directive above.

--client-cert=<path> or -C <path> specifies an optional client certificate to use. If this value is entered, the next one below is required.

--key-file=<path> or -K <path> specifies the client certificate key file to use. This goes along with --client-cert above.

--ca-cert-file=<path> or -A <path> specifies the CA certificate to use in order to validate the nrpe daemon's public key.

--no-adh or -d is DEPRECATED

--use-adh or -d [num] is DEPRECATED, even though it is new. If you use -d or -d 0 it acts the same way as as the old -d. Otherwise, use 1 to allow ADH, and 2 to require ADH.

--ssl-logging=<num> or -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 ssl_logging directive 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 common /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) is called nag_serv; and there are two Linux machines that will run the nrpe daemon: db_server and bobs_workstation.

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 CA or 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.pem and db_server.key files to the db_server machine, and the bobs_workstation.pem and 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 nag_serv.pem, nag_serv.key and ca/ca_cert.pem files to the nag_serv machine, if you did the above on a different computer.

Put the location of each computers' three files in the nrpe.cfg file or in the check_nrpe command line. You should now have encryption and, if desired, key validation.

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