TLS/SSL (om_ssl)

This module uses the OpenSSL library to send data over a secure TLS/SSL connection. It works like the TCP output module, but performs a TLS/SSL handshake at connection time and encrypts data in transit. Use this module instead of the TCP output module when you need to protect data from eavesdropping or tampering in transit.

To examine the supported platforms, see the list of installation packages.

Configuration

The om_ssl module accepts the following directives in addition to the common module directives.

Required directives

The following directive is required for the module to start.

Host

Set this directive to the IP address or hostname of the remote host to connect to. If you specify a hostname, the module resolves it to an IP address on each new connection.

The module uses port 514 by default. To connect to a different port, add the port number after the IP address or hostname, separated by a colon (host:port).

Enclose IPv6 addresses in square brackets ([addr]:port). For example, [2001:0db8:85a3:0:0:8a2e:0370:7334]:514.

You can set this directive more than once to allow failover between different hosts or ports. List IPv4 and IPv6 addresses separately if needed. If you use a DNS name, make sure it resolves to fewer than 16 addresses or CNAMEs to prevent problems with DNS response size limits.

If you configure more than one Host directive, or a DNS lookup returns multiple addresses, the module connects to the first reachable address and stays connected until it stops or the connection drops. It does not detect DNS changes on its own.

If a connection attempt fails, the module tries the next address in the current Host directive, then moves to the next Host directive, if any. Once it checks all Host directives, it starts over from the first and keeps trying until it connects.

TLS/SSL directives

The following directives configure secure data transfer via TLS/SSL.

AllowExpired

Set this directive to TRUE to allow connections with remote hosts presenting an expired certificate. The default is FALSE; the module rejects connections with expired certificates. This directive only takes effect when RequireCert is set to TRUE.

AllowHostnameValidation

Set this directive to TRUE to require that the certificate FQDN matches the server hostname before allowing a connection. The default is FALSE; the module does not validate the remote server hostname.

AllowUntrusted

Set this directive to TRUE to allow connections with remote hosts regardless of certificate verification results. The default is FALSE; the module only accepts connections with trusted certificates.

CADir

Set this directive to the path of a directory containing Certificate Authority (CA) certificates. The module uses these certificates to verify the certificate presented by the remote host. Name the certificate files using the OpenSSL hashed format: the hash of the certificate followed by .0, .1, and so on. To find the hash of a certificate using OpenSSL:

$ openssl x509 -hash -noout -in ca.crt

For example, if the certificate hash is e2f14e4a, name the certificate file e2f14e4a.0. If there is another certificate with the same hash, name it e2f14e4a.1, and so on.

To trust a remote host’s self-signed certificate, include a copy of it in this directory.

If this directive isn’t specified, NXLog Agent falls back to the default operating system root certificate store. Unix-like operating systems commonly store root certificates in /etc/ssl/certs. Windows operating systems use the Windows Certificate Store, while macOS uses the Keychain Access Application as the default certificate store. See Certificate Authority (CA) certificates in the NXLog Platform User Guide for more information on using this directive.

In addition, Microsoft’s PKI repository contains root certificates for Microsoft services.

This directive is mutually exclusive with the CAThumbprint, CAPattern, and SearchAllCertStores directives.

CAFile

Set this directive to the path of the Certificate Authority (CA) certificate used to verify the certificate presented by the remote host. To trust a remote host’s self-signed certificate, specify the remote host’s certificate itself. For certificates signed by an intermediate CA, the specified certificate must contain the complete certificate chain (certificate bundle).

This directive is mutually exclusive with the CAThumbprint, CAPattern, and SearchAllCertStores directives.

CAPattern

Set this directive to a PCRE2-compliant regular expression to locate a suitable Certificate Authority (CA) certificate. This directive is supported on Windows and macOS.

On Windows, the certificate must be in the Windows Certificate Store. On macOS, the certificate must be in the system keychain.

NXLog Agent matches the pattern against a string built from the certificate’s fields in this order:

SUBJECT=<value>, CN=<value>; DN=CN=<value>, O=<value>, OU=<value>, L=<value>, ST=<value>, C=<value>; SAN=<value>

The pattern doesn’t need to match every field. Include only the fields you want to match, and use .*? to skip over the rest.

For example, the following pattern matches only the DN and SAN fields:

CAPattern    'DN=CN=SERVER01\.example\.com.*?SAN=DNS:SERVER01\.example\.com'

This produces the following output in the NXLog Agent log file:

matching pattern [DN=CN=SERVER01\.example\.com;.*?SAN=DNS:SERVER01\.example\.com] to certificate [SUBJECT=US, ClientState, ClientCity, ClientCompany, ClientUnit, SERVER01.example.com, CN=SERVER01.example.com; DN=CN=SERVER01.example.com, O=ClientCompany, OU=ClientUnit, L=ClientCity, ST=ClientState, C=US; SAN=DNS:SERVER01.example.com; DNS:www.SERVER01.example.com; IP:127.0.0.3; ]

If the pattern matches multiple certificates, NXLog Agent uses the first match. If the certificate changes, restart NXLog Agent to apply the new certificate.

This directive is mutually exclusive with the CAThumbprint, CADir, CAFile, and SearchAllCertStores directives.

On macOS, keychain behavior and certificate requirements depend on your operating system version. Newer versions enforce stricter rules on certificate algorithms and key sizes. A certificate and private key may be imported into the keychain, yet remain unusable if they do not comply with the current macOS requirements. After importing, you may also need to give NXLog Agent permission to use the certificate in the keychain.

Check your operating system documentation to find out which certificates are trusted and valid. See also Apple’s certificate requirements for macOS.

CAThumbprint

Set this directive to the thumbprint of the Certificate Authority (CA) certificate used to verify the certificate presented by the remote host. This directive is supported on Windows and macOS.

On Windows, copy the hexadecimal fingerprint string from Windows Certificate Manager (certmgr.msc). The certificate must be in a Windows certificate store accessible by NXLog Agent.

On macOS, obtain the hexadecimal fingerprint string from the Keychain Access application. The certificate must be accessible in the system keychain.

Whitespaces and colons are automatically removed.

This directive is mutually exclusive with the CADir, CAFile, CAPattern, and SearchAllCertStores directives.

Check your operating system documentation to find out which certificates are trusted and valid. See also Apple’s certificate requirements for macOS.

CertFile

Set this directive to the path of the certificate file to present to the remote host during the TLS/SSL handshake.

CertKeyFile

Set this directive to the path of the private key file corresponding to the certificate specified by CertFile.

CertPattern

Set this directive to a PCRE2-compliant pattern to identify the certificate and its corresponding private key. This directive is supported on Windows and macOS.

NXLog Agent matches the pattern against a string built from the certificate’s fields in this order:

SUBJECT=<value>, CN=<value>; DN=CN=<value>, O=<value>, OU=<value>, L=<value>, ST=<value>, C=<value>; SAN=<value>

The pattern doesn’t need to match every field. Include only the fields you want to match, and use .*? to skip over the rest.

On Windows, import the certificate in PFX format into the Local Computer\Personal certificate store.

On macOS, ensure the certificate and its private key are present in the System Keychain (/Library/Keychains/System.keychain).

If the pattern matches multiple certificates, NXLog Agent uses the first match. If the certificate changes, restart NXLog Agent to apply the new certificate.

This directive is mutually exclusive with the CertThumbprint, CertFile and CertKeyFile directives.

For example:

CertPattern    $hostname + 'Cert'

CertPattern    DN=CN=Client\.example\.com;.*?SAN=DNS:Client\.example\.com

This produces the following output in the NXLog Agent log file:

matching pattern [DN=CN=Client\.example\.com;.*?SAN=DNS:Client\.example\.com] to certificate [SUBJECT=US, ClientState, ClientCity, ClientCompany, ClientUnit, Client.example.com, CN=Client.example.com; DN=CN=Client.example.com, O=ClientCompany, OU=ClientUnit, L=ClientCity, ST=ClientState, C=US; SAN=DNS:Client.example.com; DNS:www.Client.example.com; IP:127.0.0.3; ]

Check your operating system documentation to find out which certificates are trusted and valid. See also Apple’s certificate requirements for macOS.

CertThumbprint

Set this directive to the thumbprint of the certificate to present to the remote server during the TLS/SSL handshake. Whitespaces and colons are automatically removed.

This directive is supported on Windows and macOS. On Windows, copy the hexadecimal fingerprint string from Windows Certificate Manager (certmgr.msc). Import the certificate to the Local Computer\Personal certificate store in PFX format. To create a PFX file from the certificate and private key using OpenSSL:

$ openssl pkcs12 -export -out server.pfx -inkey server.key -in server.pem

When the global directive UseCNGCertificates is set to FALSE, the private key must be exportable.

If you generate the certificate request using Windows Certificate Manager, enable the Make private key exportable option from the certificate properties.
If you import the certificate with the Windows Certificate Import Wizard, make sure that the Mark this key as exportable option is enabled.
If you migrate the certificate and associated private key from one Windows machine to another, select Yes, export the private key when exporting from the source machine.

When UseCNGCertificates is set to TRUE, the private key does not need to be exportable. For example, private keys stored on a TPM are always non-exportable.

On macOS, obtain the hexadecimal fingerprint string from the Keychain Access application. The certificate and its associated private key must be present in the System Keychain (/Library/Keychains/System.keychain).

For example:

CertThumbprint    7c2cc5a5fb59d4f46082a510e74df17da95e2152

This directive is mutually exclusive with the CertFile and CertKeyFile directives.

Check your operating system documentation to find out which certificates are trusted and valid. See also Apple’s certificate requirements for macOS.

CRLDir

Set this directive to the path of a directory containing certificate revocation list (CRL) files. The module uses these files to reject connections from hosts presenting a revoked certificate. Name the files using the OpenSSL hashed format: the hash of the issuer followed by .r0, .r1, and so on. To find the hash of the issuer of a CRL file using OpenSSL:

$ openssl crl -hash -noout -in crl.pem

For example, if the hash is e2f14e4a, name the file e2f14e4a.r0. If there is another file with the same hash, name it e2f14e4a.r1, and so on.

CRLFile

Set this directive to the path of the certificate revocation list (CRL) file used to reject connections from hosts presenting a revoked certificate. To generate a CRL file using OpenSSL:

$ openssl ca -gencrl -out crl.pem

DHFile

Set this directive to the path of a file containing Diffie-Hellman parameters for key exchange. Generate the parameters using dhparam(1ssl). If this directive is not set, the module uses default parameters. See the OpenSSL Wiki for further details.

KeyPass

Set this directive to the passphrase of the private key specified by CertKeyFile. This directive applies only to encrypted private keys.

To generate a private key with Triple DES encryption using OpenSSL:

$ openssl genrsa -des3 -out server.key 2048

LoadCertificateChains

Set this directive to TRUE to load the full certificate chain from the PEM file referenced by CertFile, which may contain a single certificate or the complete chain. The default is FALSE; the module loads certificates from the operating system certificate store instead.

This directive is only supported on Windows.

RequireCert

Set this directive to TRUE to require the remote host to present a certificate during the TLS/SSL handshake. The module refuses the connection if no certificate is presented. The default is TRUE.

SearchAllCertStores

Set this directive to TRUE to load all available Windows certificates into NXLog Agent for use during remote certificate verification. Any required certificates must be in a Windows certificate store accessible by NXLog Agent. This directive is only supported on Windows and is mutually exclusive with the CAThumbprint, CAPattern, CADir, and CAFile directives.

Sigalgs

Set this directive to the signature algorithm parameter to pass to the Windows SSL library. Accepted values depend on the available encryption providers. This directive is only supported on Windows.

SNI

Set this directive to the hostname to use for Server Name Indication (SNI). If not set, the module uses the hostname specified in Host.

SSLCipher

Set this directive to override the permitted cipher list for TLSv1.2 and below. Use the format described in the ciphers(1ssl) man page. For example, specify RSA:!COMPLEMENTOFALL to include all ciphers with RSA authentication but exclude ciphers without encryption.

If RSA or DSA ciphers with Diffie-Hellman key exchange are used, set DHFile to specify custom Diffie-Hellman parameters.

SSLCiphersuites

Set this directive to override the permitted cipher list for TLSv1.3. Use the same format as SSLCipher. Refer to the OpenSSL documentation for a list of valid TLS v1.3 cipher suites. The default is:

TLS_AES_256_GCM_SHA384:TLS_CHACHA20_POLY1305_SHA256:TLS_AES_128_GCM_SHA256

SSLCompression

Set this directive to TRUE to compress data using zlib when sending over the network. The default is FALSE; the module sends data without compression.

Some Linux distributions, including Debian, provide an OpenSSL library without zlib support. The module emits a warning on startup if compression support is missing. The generic deb/rpm packages are bundled with a zlib-enabled libssl library.

SSLProtocol

Set this directive to restrict the TLS/SSL protocols the module accepts. Specify a comma-separated list of the following values: SSLv2, SSLv3, TLSv1, TLSv1.1, TLSv1.2, and TLSv1.3. The default is TLSv1.2 and TLSv1.3. The OpenSSL library shipped by some Linux distributions does not support SSLv2 and SSLv3; these protocols will not work even if specified here.

TLSConnectLog

Set this directive to TRUE to log the TLS protocol version and cipher suite on each successful TLS/SSL handshake. This is useful for debugging, security audits, and verifying compliance with encryption standards.

TLSCertLog

Set this directive to TRUE to log the remote host’s certificate details during the TLS/SSL handshake. When available, the module logs the Subject, Issuer, NotBefore, and NotAfter fields to the NXLog Agent log file. The default is FALSE.

This directive depends on the values of the AllowUntrusted and RequireCert directives:

AllowUntrusted RequireCert Outcome

FALSE

Output depends on TLSCertLog value

FALSE

TRUE

Output depends on TLSCertLog value

TRUE

FALSE

No output regardless of TLSCertLog value

TRUE

Output depends on TLSCertLog value

UseCNGCertificates

Set this directive to TRUE to use the Windows Cryptography API: Next Generation (CNG) to access the private keys of certificates identified by a thumbprint. This directive is only supported on Windows.

Optional directives

LocalPort

Set this directive to the local port number to use for the connection, for example to satisfy a firewall rule that expects traffic from a known port. If you do not set this directive, the module uses a random high port number, which a firewall may block.

Attempts to bind to LocalPort may fail because of the required TIME-WAIT delay in closing connections. When this happens, the module writes Address already in use to the NXLog Agent log file. If it persists, it can degrade network performance.

OutputType

Set this directive to specify how the module formats output records. See the common OutputType directive for the available values, including support for data converters.

The default is LineBased_LF, terminating each record with LF.

Reconnect

Set this directive to the number of seconds to wait between reconnection attempts. If you do not set this directive, the module starts with a 1 second interval and doubles it after each failed attempt, resetting to 1 second once it reconnects successfully.

Setting this directive on multiple systems can cause them to send reconnection requests to the same destination at the same time, potentially overloading it. This can also cause NXLog Agent to consume unusually high system resources or become unresponsive.

ReconnectOnData

Set this directive to TRUE to have the module reconnect to the remote host only when it has data to send, instead of keeping the connection open at all times. The default is FALSE; the module always keeps a connection open with the remote host.

TCPNoDelay

Set this directive to TRUE to deactivate Nagle’s algorithm and send packets immediately instead of buffering small writes into larger frames. Nagle’s algorithm normally merges small outgoing packets and waits for an acknowledgment before sending the next one, which reduces network overhead but adds latency. The default is FALSE; the module does not set the TCP_NODELAY socket option, so Nagle’s algorithm remains active.

TcpSendStallDetect

Set this directive to TRUE to monitor for pipeline backpressure by logging a warning whenever the module cannot send data to the destination. The module writes the following message to the NXLog Agent log file:

TCP send buffer full for <instance_name> - write blocked

The default is FALSE; the module does not check for send stalls.

Procedures

The following procedures are exported by om_ssl.

reconnect();

Forces a reconnection to the server. For example, you can call this procedure from a Schedule block to reconnect periodically.

Avoid calling this procedure for every record the module processes. Doing so forces a reconnection each time, potentially overloading the destination.

Examples

Example 1. Basic om_ssl configuration with failover

The following is a basic configuration that sends data over a secure connection. If the connection to the first Host fails, the module fails over to the next one.

nxlog.conf

<Output ssl>
    Module         om_ssl
    Host           192.168.1.2:1514
    Host           192.168.1.3:1514
    Host           example.com:1234
    CAFile         %CERTDIR%/ca.pem
    CertFile       %CERTDIR%/agent-cert.pem
    CertKeyFile    %CERTDIR%/agent-key.pem
</Output>