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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 and ### Use Public Key Encryption Contents -------- 1. [Introduction](#introduction) 2. [NRPE Changes](#nrpe-changes) 3. [check_nrpe Changes](#check_nrpe-changes) 4. [Certificate Generation Example](#certificate-generation-example) Introduction ------------ 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 `update-cfg.pl` 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.