TLS/SSL (om_ssl)

The om_ssl module uses the OpenSSL library to provide an SSL/TLS transport. It behaves like the om_tcp module, except that an SSL handshake is performed at connection time and the data is received over a secure channel. Log messages transferred over plain TCP can be eavesdropped or even altered with a man-in-the-middle attack, while the om_ssl module provides a secure log message 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 directives are required for the module to start.

Host

The module connects to the IP address or hostname defined in this directive. If additional hosts are specified on new lines, the module works in a failover configuration. If a destination becomes unavailable, the module automatically fails over to the next one. If the last destination becomes unavailable, the module fails over to the first destination.

The port number can be defined by appending it at the end of the hostname or IP address using a colon as a separator (host:port). For each destination with no port number defined here, the port number specified in the Port directive is used. Port numbers defined here take precedence over any port number defined in the Port directive.

TLS/SSL directives

AllowExpired

This boolean directive specifies whether the connection should be allowed with an expired certificate. If set to TRUE, the connection will be allowed even if the remote server presents an expired certificate. The default is FALSE: the remote server must present a certificate that is not expired.

AllowHostnameValidation

This boolean directive specifies whether the certificate FQDN should be validated against the server hostname or not. If set to TRUE, the connection will only be allowed if the certificate FQDN corresponds to the server hostname. The default value is FALSE: the remote server hostname is not validated.

AllowUntrusted

This boolean directive specifies that the connection should be allowed regardless of the certificate verification results. If set to TRUE, the connection will be allowed with any unexpired certificate provided by a server. The default value is FALSE: the remote server must present a trusted certificate.

CADir

This directive specifies a path to a directory containing certificate authority (CA) certificates. These certificates will be used to verify the certificate presented by the remote server. The certificate files must be named using the OpenSSL hashed format, i.e. the hash of the certificate followed by .0, .1 etc. To find the hash of a certificate using OpenSSL:

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

For example, if the certificate hash is e2f14e4a, then the certificate filename should be e2f14e4a.0. If there is another certificate with the same hash then it should be named e2f14e4a.1 and so on.

A remote server’s self-signed certificate (which is not signed by a CA) can also be trusted by including a copy of the certificate in this directory.

The default operating system root certificate store will be used if this directive is not specified. 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.

CAFile

This specifies the path of the certificate authority (CA) certificate that will be used to verify the certificate presented by the remote server. A remote server’s self-signed certificate (which is not signed by a CA) can be trusted by specifying the remote server certificate itself. In case of certificates signed by an intermediate CA, the certificate specified must contain the complete certificate chain (certificate bundle).

CAThumbprint

This optional directive specifies the thumbprint of the certificate authority (CA) certificate that will be used to verify the certificate presented by the remote server. The hexadecimal fingerprint string can be copied from Windows Certificate Manager (certmgr.msc). Whitespaces are automatically removed. The certificate must be added to a Windows certificate store that is accessible by NXLog Agent. This directive is only supported on Windows and is mutually exclusive with the CADir and CAFile directives.

CertFile

This specifies the path of the certificate file that will be presented to the remote server during the SSL handshake.

CertKeyFile

This specifies the path of the private key file that was used to generate the certificate specified by the CertFile directive. This is used for the SSL handshake.

CertThumbprint

This optional directive specifies the thumbprint of the certificate that will be presented to the remote server during the SSL handshake. The hexadecimal fingerprint string can be copied from Windows Certificate Manager (certmgr.msc). Whitespaces are automatically removed. The certificate must be imported to the Local Computer\Personal certificate store in PFX format for NXLog Agent to find it. 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

This directive is only supported on Windows and is mutually exclusive with the CertFile and CertKeyFile directives.

The private key associated with the certificate 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.

CRLDir

This directive specifies a path to a directory containing certificate revocation list (CRL) files. These CRL files will be used to check for certificates that were revoked and should no longer be accepted. The files must be named using the OpenSSL hashed format, i.e. the hash of the issuer followed by .r0, .r1 etc. 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, then the filename should be e2f14e4a.r0. If there is another file with the same hash then it should be named e2f14e4a.r1 and so on.

CRLFile

This specifies the path of the certificate revocation list (CRL) which will be used to check for certificates that have been revoked and should no longer be accepted. Example to generate a CRL file using OpenSSL:

$ openssl ca -gencrl -out crl.pem

DHFile

This optional directive specifies a file with dh-parameters for Diffie-Hellman key exchange. These parameters can be generated with dhparam(1ssl). If this directive is not specified, default parameters will be used. See the OpenSSL Wiki for further details.

KeyPass

This directive specifies the passphrase of the private key specified by the CertKeyFile directive. A passphrase is required when the private key is encrypted. Example to generate a private key with Triple DES encryption using OpenSSL:

$ openssl genrsa -des3 -out server.key 2048

This directive is not needed for passwordless private keys.

SearchAllCertStores

This optional boolean directive, when set to TRUE, enables the loading of all available Windows certificates into NXLog Agent, for use during remote certificate verification. Any required certificates must be added to a Windows certificate store that NXLog Agent can access. This directive is mutually exclusive with the CAThumbprint, CADir, and CAFile directives.

SSLCipher

This optional directive can be used to set the permitted cipher list for TLSv1.2 and below, overriding the default. Use the format described in the ciphers(1ssl) man page. For example specify RSA:!COMPLEMENTOFALL to include all ciphers with RSA authentication but leave out ciphers without encryption.

If RSA or DSA ciphers with Diffie-Hellman key exchange are used, DHFile can be set for specifying custom dh-parameters.

SSLCiphersuites

This optional directive can be used to set the permitted cipher list for TLSv1.3. Use the same format as in the SSLCipher directive. Refer to the OpenSSL documentation for a list of valid TLS v1.3 cipher suites. The default value is:

TLS_AES_256_GCM_SHA384:TLS_CHACHA20_POLY1305_SHA256:TLS_AES_128_GCM_SHA256

SSLCompression

This boolean directive allows you to enable data compression when sending data over the network. The compression mechanism is based on the zlib compression library. If the directive is not specified, it defaults to FALSE: compression is disabled.

Some Linux packages (for example, Debian) use the OpenSSL library provided by the OS and may not support the zlib compression mechanism. The module will emit a warning on startup if the compression support is missing. The generic deb/rpm packages are bundled with a zlib-enabled libssl library.

SSLProtocol

This directive can be used to set the allowed SSL/TLS protocol(s). It takes a comma-separated list of values which can be any of the following: SSLv2, SSLv3, TLSv1, TLSv1.1, TLSv1.2, and TLSv1.3. By default, the TLSv1.2 and TLSv1.3 protocols are allowed. Note that the OpenSSL library shipped by Linux distributions may not support SSLv2 and SSLv3, and these will not work even if enabled with this directive.

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 doubles with every attempt. If the duration of the successful connection is greater than the current reconnect interval, then the reconnect interval will be 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.

SNI

This optional directive specifies the hostname used for Server Name Indication (SNI). If not specified, it defaults to the hostname in the Host directive.

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.

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 binary data to another NXLog Agent instance

This configuration reads log messages from a socket and sends them in the NXLog Agent 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>