nagios_nrpe

NRPE With SSL/TLS

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.

The changes to the NRPE daemon are outlined below first, followed by the changes to the check_nrpe client.

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####CHANGES IN THE CURRENT VERSION OF NRPE

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.

The NRPE configuration file has added new SSL/TLS options. The defaults currently will allow old check_nrpe clients 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.

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 clients to connect using ADH. When all the clients 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 client has a cert, and 16 to log details of client's certificate. -1 will enable all. This can be especially helpful during client migration, so you can tell which clients have certificates, what SSL/TLS version is being used, and which ciphers are being used.

The ssl_adh_key directive is DEPRECATED, even though it's new. It should be used (if at all) during client migration. This will allow you to change the ADH key without having to recompile the NRPE daemon. It can either contain the path to a key file, or it can contain a base-64 encoded key. See the examples in the nrpe.cfg file.

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####CHANGES IN THE CURRENT VERSION OF CHECK_NRPE

The check_nrpe client 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.