TLS options

The syslog-ng OSE application can encrypt incoming and outgoing syslog message flows using TLS if you use the network() or syslog() drivers.

NOTE: The format of the TLS connections used by syslog-ng OSE is similar to using syslog-ng OSE and stunnel, but the source IP information is not lost.

To encrypt connections, use the transport("tls") and tls() options in the source and destination statements.

The tls() option can include the following settings:

allow-compress()

Accepted values:	yes \| no
Default:	no

Description: Enable on-the-wire compression in TLS communication. Note that this option must be enabled both on the server and the client to have any effect. Enabling compression can significantly reduce the bandwidth required to transport the messages, but can slightly decrease the performance of syslog-ng OSE, reducing the number of transferred messages during a given period.

Available in version 3.19 and later.

ca-dir()

Type:	Directory name
Default:	none

Description: The name of a directory that contains a set of trusted CA certificates in PEM format. The CA certificate files have to be named after the 32-bit hash of the subject's name. This naming can be created using the c_rehash utility in openssl. For an example, see Configuring TLS on the syslog-ng OSE clients. The syslog-ng OSE application uses the CA certificates in this directory to validate the certificate of the peer.

This option can be used together with the optional ca-file() option.

NOTE: During a TLS handshake, syslog-ng OSE automatically sets the certificate_authorities field of the certificate request based on the ca-file() and ca-dir() options.

ca-file()

Type:	File name
Default:	empty

Description: Optional. The name of a file that contains a set of trusted CA certificates in PEM format. The syslog-ng OSE application uses the CA certificates in this file to validate the certificate of the peer.

Example format in configuration:

ca-file("/etc/pki/tls/certs/ca-bundle.crt")

NOTE: The ca-file() option can be used together with the ca-dir() option, and it is relevant when peer-verify() is set to other than no or optional-untrusted.

NOTE: During a TLS handshake, syslog-ng OSE automatically sets the certificate_authorities field of the certificate request based on the ca-file() and ca-dir() options.

cert-file()

Accepted values:	Filename
Default:	none

Description: Name of a file, that contains an X.509 certificate (or a certificate chain) in PEM format, suitable as a TLS certificate, matching the private key set in the key-file() option. The syslog-ng OSE application uses this certificate to authenticate the syslog-ng OSE client on the destination server. If the file contains a certificate chain, the file must begin with the certificate of the host, followed by the CA certificate that signed the certificate of the host, and any other signing CAs in order.

cipher-suite()

Accepted values:	Name of a cipher, or a colon-separated list
Default:	Depends on the OpenSSL version that syslog-ng OSE uses

Description: Specifies the cipher, hash, and key-exchange algorithms used for the encryption, for example, ECDHE-ECDSA-AES256-SHA384. The list of available algorithms depends on the version of OpenSSL used to compile syslog-ng OSE. To specify multiple ciphers, separate the cipher names with a colon, and enclose the list between double-quotes, for example:

cipher-suite("ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES256-GCM-SHA384")

For a list of available algorithms, execute the openssl ciphers -v command. The first column of the output contains the name of the algorithms to use in the cipher-suite() option, the second column specifies which encryption protocol uses the algorithm (for example, TLSv1.2). That way, the cipher-suite() also determines the encryption protocol used in the connection: to disable SSLv3, use an algorithm that is available only in TLSv1.2, and that both the client and the server supports. You can also specify the encryption protocols using ssl-options().

You can also use the following command to automatically list only ciphers permitted in a specific encryption protocol, for example, TLSv1.2:

echo "cipher-suite(\"$(openssl ciphers -v | grep TLSv1.2 | awk '{print $1}' | xargs echo -n | sed 's/ /:/g' | sed -e 's/:$//')\")"

Note that starting with version 3.10, when syslog-ng OSE receives TLS-encrypted connections, the order of ciphers set on the syslog-ng OSE server takes precedence over the client settings.

crl-dir()

Accepted values:	Directory name
Default:	none

Description: Name of a directory that contains the Certificate Revocation Lists for trusted CAs. Similarly to ca-dir() files, use the 32-bit hash of the name of the issuing CAs as filenames. The extension of the files must be .r0.

dhparam-file()

Accepted values:	string (filename)
Default:	none

Description: Specifies a file containing Diffie-Hellman parameters, generated using the openssl dhparam utility. Note that syslog-ng OSE supports only DH parameter files in the PEM format. If you do not set this parameter, syslog-ng OSE uses the 2048-bit MODP Group, as described in RFC-3526.

ecdh-curve-list()

Accepted values:	colon-separated list
Default:	none

Description: A colon-separated list that specifies the curves that are permitted in the connection when using Elliptic Curve Cryptography (ECC).

This option is only available when syslog-ng OSE is compiled with OpenSSL version 1.0.2 or later. In the case of older versions, prime256v1 (NIST P-256) is used.

The following example curves work for all versions of OpenSSL that are equal to or later than version 1.0.2:

ecdh-curve-list("prime256v1:secp384r1")

tls-options-sigalgs

Accepted values:	colon-separated list
Default:	none

Description: A colon-separated list that specifies the supported signature algorithms associated with client authentication for TLSv1.2 and higher, for example, RSA-PSS+SHA256:ed25519.

For servers, the value is used in the

signature_algorithms

field of a

CertificateRequest

message.

For clients, it is used to determine which signature algorithm to use with the client certificate.

key-file()

Accepted values:	Filename
Default:	none

Description: The name of a file that contains an unencrypted private key in PEM format, suitable as a TLS key. If properly configured, the syslog-ng OSE application uses this private key and the matching certificate (set in the cert-file() option) to authenticate the syslog-ng OSE client on the destination server.

keylog-file()

Accepted values:	Filename
Default:	N/A

Description: This option enables saving TLS secrets (decryption keys) for a given source or destination, which can be used to decrypt data with, for example, Wireshark. The given path and name of a file will be used to save these secrets.

This option is only available with the following drivers:

network
syslog
tcp
tcp6

CAUTION: Using keylog-file() makes TLS connections less secure by writing secret key materials into the given file. This option should only be enabled for debugging purposes and should be disabled after that. It is also recommended to delete the file after the debugging session is over.

ocsp-stapling-verify()

Accepted values:	yes, no
Default:	no

This option is available in syslog-ng OSE 4.0 and later versions.

Description: In case the OCSP stapling verification is enabled, syslog-ng OSE requests the server to return its OCSP status. This status response is verified by syslog-ng OSE using the trust store configured by the ca-file(), ca-dir(), or the pkcs12-file() options.

NOTE: RFC-6961 multi-stapling and TLS 1.3-provided multiple responses are currently not validated, only the peer certificate is verified.

Example: OCSP stapling verification

destination {

    network("example.com" transport(tls)
        tls(
            pkcs12-file("/path/to/test.p12")
            peer-verify(yes)
            ocsp-stapling-verify(yes)
        )
    );
};

openssl-conf-cmds()

This option is available in syslog-ng OSE 4.0 and later versions.

CAUTION: openssl-conf-cmds() always has the highest priority. It overrides any other option found in the tls() section.

OpenSSL offers an alternative and software-independent configuration mechanism through the SSL_CONF_cmd interface for configuring the various SSL_CTX and SSL options.

The order of operations within openssl-conf-cmds() affects the order of execution. The commands are executed in top-down order. If the same option occurs multiple times, the last executed option takes effect. This is also true for options that can be set multiple ways (for example, cipher suites or protocols).

Example: OpenSSL command configuration

tls(
        ca-dir("/etc/ca.d")
        key-file("/etc/cert.d/serverkey.pem")
        cert-file("/etc/cert.d/servercert.pem")
        peer-verify(yes)

        openssl-conf-cmds(
            # For system wide available cipher suites use: /usr/bin/openssl ciphers -v
            # For formatting rules see: https://www.openssl.org/docs/man1.1.1/man3/SSL_CONF_cmd.html
            "CipherString" => "ECDHE-RSA-AES128-SHA",                                   # TLSv1.2 and bellow
            "CipherSuites" => "TLS_CHACHA20_POLY1305_SHA256:TLS_AES_256_GCM_SHA384",    # TLSv1.3+ (OpenSSl 1.1.1+)

            "Options" => "PrioritizeChaCha",
            "Protocol" => "-ALL,TLSv1.3",
        )
    )

peer-verify()

Accepted values:	optional-trusted \| optional-untrusted \| required-trusted \| required-untrusted \| yes \| no
Default:	required-trusted

Description: Verification method of the peer, the four possible values is a combination of two properties of validation:

Whether the peer is required to provide a certificate (required or optional prefix).
Whether the certificate provided needs to be valid or not.

The following table summarizes the possible options and their results depending on the certificate of the peer.

The remote peer has:

		no certificate	invalid certificate	valid certificate
Local peer-verify() setting	optional-untrusted	TLS-encryption	TLS-encryption	TLS-encryption
	optional-trusted	TLS-encryption	rejected connection	TLS-encryption
	required-untrusted	rejected connection	TLS-encryption	TLS-encryption
	required-trusted	rejected connection	rejected connection	TLS-encryption

For untrusted certificates only the existence of the certificate is checked, but it does not have to be valid — syslog-ng OSE accepts the certificate even if it is expired, signed by an unknown CA, or its CN and the name of the machine mismatches.

CAUTION: When validating a certificate, the entire certificate chain must be valid, including the CA certificate. If any certificate of the chain is invalid, syslog-ng OSE will reject the connection.

Starting with syslog-ng OSE version 3.10, you can also use a simplified configuration method for the peer-verify option, simply setting it to yes or no. The following table summarizes the possible options and their results depending on the certificate of the peer.

The remote peer has:

		no certificate	invalid certificate	valid certificate
Local peer-verify() setting	no (optional-untrusted)	TLS-encryption	TLS-encryption	TLS-encryption
	yes (required-trusted)	rejected connection	rejected connection	TLS-encryption

pkcs12-file()

Accepted values:	Filename
Default:	none

Description: The name of a PKCS #12 file that contains an unencrypted private key, an X.509 certificate, and an optional set of trusted CA certificates.

If this option is used in the configuration, the value of key-file() and cert-file() will be omitted.

You can use the ca-dir() option together with pkcs12-file(). However, this is optional because the PKCS #12 file may contain CA certificates as well.

Passphrase is currently not supported.

Example: Using pkcs12-file()

In the following example, the first command creates a single PKCS #12 file from the private key, X.509 certificate, and CA certificate files. Then, the second half of the example uses the same PKCS #12 file in the syslog-ng OSE configuration.

openssl pkcs12 -export -inkey server.key -in server.crt -certfile ca.crt -out server.p12

Example configuration

source s_tls {
    syslog(
        transport(tls)
        tls(
            pkcs12-file("/path/to/server.p12")
            ca-dir("/path/to/cadir") # optional
            peer-verify(yes)
        )
    );
};

sni()

Accepted values:	yes \| no
Default:	no

Description: When set to yes in a destination that uses TLS encryption, this option enables Server Name Indication (also called Server Name Identification, SNI). The syslog-ng OSE sends the hostname or the IP address set in the destination to the server during the TLS handshake.

Available in syslog-ng OSE 3.24 and newer.

Example: Using Server Name Indication

The following destination sends the hostname of its destination during the TLS handshake.

destination demo_tls_destination_with_sni {
    network(
          "logserver.example.com" port(6514)
        transport("tls")
        tls(
            ca_dir("/etc/syslog-ng/ca.d")
            key-file("/etc/syslog-ng/cert.d/clientkey.pem")
            cert-file("/etc/syslog-ng/cert.d/clientcert.pem")
            sni(yes)
        )
    );
};

ssl-options()

Accepted values:	comma-separated list of the following options: `no-sslv2`, `no-sslv3`, `no-tlsv1`, `no-tlsv11`, `no-tlsv12`, `no-tlsv13`, `none`, `ignore-hostname-mismatch`, `ignore-validity-period`
Default:	`no-sslv2`

This option is available in syslog-ng OSE 3.7 and newer.

Description: Sets the specified options of the SSL/TLS protocols. Currently, you can use it to disable specific protocol versions. Note that disabling a newer protocol version (for example, TLSv1.1) does not automatically disable older versions of the same protocol (for example, TLSv1.0). For example, use the following option to permit using only TLSv1.1 or newer:

ssl-options(no-sslv2, no-sslv3, no-tlsv1)

Using ssl-options(none) means that syslog-ng OSE does not specify any restrictions on the protocol used. However, in this case, the underlying OpenSSL library can restrict the available protocols, for example, certain OpenSSL versions automatically disable SSLv2.

By specifying ignore-hostname-mismatch, the subject name of a certificate can be ignored during the validation process. This means that syslog-ng OSE checks only if the certificate itself is trusted by the current set of trust anchors (for example trusted CAs), and ignores the mismatch between the targeted hostname and the certificate subject. ignore-hostname-mismatch is available in syslog-ng OSE 4.4 and newer versions.

By specifying ignore-validity-period, the validity periods of a certificate can be ignored during the certificate validation process. ignore-validity-period is available in syslog-ng OSE 4.5 and newer.

Example: Using ssl-options

The following destination explicitly disables SSL and TLSv1.0

destination demo_tls_destination {
    network(
          "172.16.177.147" port(6514)
        transport("tls")
        tls(
            ca_dir("/etc/syslog-ng/ca.d")
            key-file("/etc/syslog-ng/cert.d/clientkey.pem")
            cert-file("/etc/syslog-ng/cert.d/clientcert.pem")
            ssl-options(no-sslv2, no-sslv3, no-tlsv1)
        )
    );
};

ssl-version()

Type:	string
Default:	None, uses the libcurl default

Available in syslog-ng OSE 4.5 and later versions.

Description: This option specifies the allowed SSL/TLS version. The available values are the following: sslv2, sslv3, tlsv1, tlsv1_0, tlsv1_1, tlsv1_2, tlsv1_3.

trusted-dn()

Accepted values:	list of accepted distinguished names
Default:	none

Description: To accept connections only from hosts using certain certificates signed by the trusted CAs, list the distinguished names of the accepted certificates in this parameter. For example, using trusted-dn("*, O=Example Inc, ST=Some-State, C=*") will accept only certificates issued for the Example Inc organization in Some-State state.

trusted-keys()

Accepted values:	list of accepted SHA-1 fingerprints
Default:	none

Description: To accept connections only from hosts using certain certificates having specific SHA-1 fingerprints, list the fingerprints of the accepted certificates in this parameter.
For example,

trusted-keys(\"SHA1:00:EF:ED:A4:CE:00:D1:14:A4:AB:43:00:EF:00:91:85:FF:89:28:8F\",
\"SHA1:0C:42:00:3E:B2:60:36:64:00:E2:83:F0:80:46:AD:00:A8:9D:00:15\").

To find the fingerprint of a certificate, you can use the following command: openssl x509 -in <certificate-filename> -sha1 -noout -fingerprint

NOTE: When using the trusted-keys() and trusted-dn() parameters, note the following:

First, the trusted-keys() parameter is checked. If the fingerprint
of the peer is listed, the certificate validation is performed.

If the fingerprint of the peer is not listed in the trusted-keys()
parameter, the trusted-dn() parameter is checked. If the DN of the
peer is not listed in the trusted-dn() parameter, the authentication
of the peer fails and the connection is closed.