TLS/SSL (om_ssl)

The om_ssl module uses the OpenSSL library to provide TLS/SSL transport. It is like the om_tcp module, except that a TLS/SSL handshake is performed at connection time and the data is sent over a secure channel. Data transferred over plain TCP can be eavesdropped or even altered in a man-in-the-middle attack, while the om_ssl module provides secure transport.

Configuration

The om_ssl module accepts the following directives in addition to the common module directives. The Host directive is required.

Required directives

The following directive is required for the module to start.

Host

The module connects to this IP address or hostname. If using a hostname, the module resolves the hostname to an IP address on each new connection.

You can define the port number by appending it to the IP address or hostname using a colon as a separator (host:port). The default port is 514.

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

You can define this directive multiple times to connect to multiple hosts or ports in failover mode. Specify IPv4 and IPv6 addresses separately as needed. If you use a DNS name, you should keep the addresses or CNAMEs below 16 to avoid issues caused by DNS response size limits.

If the module is configured with multiple Host directives or a resolver, such as DNS, returns multiple addresses, the module will connect to the first reachable address. The module will remain connected to that address until it is stopped or the connection is severed. DNS changes are therefore not picked up by the module without intervention.

If the connection to an address fails, the module will attempt to connect to the next address until it reaches the end of the set of addresses for the same Host directive. Then, it performs a lookup on the next Host directive, if configured. Once all options are exhausted, the module will start over from the first address of the first Host directive until the connection can be re-established.

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.

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).

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.

The pattern must be in the format "SUBJECT=, CN=; DN=CN=, O=, OU=, L=, ST=, C=; SAN=;". For example:

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, and CAFile 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 and CAPattern 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. The pattern must use the format "SUBJECT=, CN=, DN=, SAN=" where DN is "CN=, O=, OU=, L=, ST=, C=". This directive is supported on Windows and macOS.

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, 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

This optional directive specifies the local port number of the connection. If this is not specified, a random high port number will be used, which is not always ideal in firewalled network environments.

Due to the required TIME-WAIT delay in closing connections, attempts to bind to LocalPort may fail. In such cases, the message Address already in use will be written to nxlog.log. If the situation persists, it could impede network performance.

OutputType

See the OutputType directive in the list of common module directives. The default is LineBased_LF.

Reconnect

This optional directive sets the reconnect interval in seconds. If it is set, the module attempts to reconnect in every defined second. If it is not set, the reconnect interval will start at 1 second and double with every attempt. In the latter case, when the system decides that the reconnection is successful, the reconnect interval is immediately reset to 1 sec.

The Reconnect directive must be used with caution. If it is used on multiple systems, it can send reconnect requests simultaneously to the same destination, potentially overloading the destination system. It may also cause NXLog Agent to use unusually high system resources or cause NXLog Agent to become unresponsive.

ReconnectOnData

This optional directive defines the behavior when the connection with the remote host is lost. When set to TRUE, the module only attempts to reconnect when it has data to send. The default value is FALSE; it will always keep a connection open with the remote host.

TCPNoDelay

This boolean directive is used to turn off the network optimization performed by Nagle’s algorithm. Nagle’s algorithm is a network optimization tweak that tries to reduce the number of small packets sent out to the network, by merging them into bigger frames, and by not sending them to the other side of the session before receiving the ACK. If this directive is unset, the TCP_NODELAY socket option will not be set.

TcpSendStallDetect

Use this directive to log a warning when the module cannot send data to the destination, allowing you to monitor pipeline backpressure. When set to TRUE, the module logs the following message to the NXLog Agent log file: TCP send buffer full for <instance_name> - write blocked. The default is FALSE.

Procedures

The following procedures are exported by om_ssl.

reconnect();

Force a reconnection. This can be used from a Schedule block to periodically reconnect to the server.

The reconnect() procedure must be used with caution. If configured, it can attempt to reconnect after every event sent, potentially overloading the destination system.

Examples

Example 1. Sending logs to another NXLog Agent instance

This configuration reads log messages from a socket and sends them in the NXLog Binary format to another NXLog Agent instance.

nxlog.conf

<Input uds>
    Module          im_uds
    UDS             tmp/socket
</Input>

<Output ssl>
    Module          om_ssl
    Host            example.com:23456
    CAFile          %CERTDIR%/ca.pem
    CertFile        %CERTDIR%/client-cert.pem
    CertKeyFile     %CERTDIR%/client-key.pem
    KeyPass         secret
    AllowUntrusted  TRUE
    OutputType      Binary
</Output>

Example 2. Sending logs to another NXLog Agent instance with failover

This configuration sends logs to another NXLog Agent instance in a failover configuration (multiple Hosts defined).

nxlog.conf

<Output ssl>
    Module  om_ssl
    Host    192.168.1.2:23456
    Host    192.168.1.3:23456
    Host    example.com:1514
    CAFile  %CERTDIR%/ca.pem
</Output>