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TLS

Fluent Bit provides integrated support for Transport Layer Security (TLS) and its predecessor Secure Sockets Layer (SSL). This section refers only to TLS for both implementations.

Fluent Bit 4.1.0 introduced the replacement of Next Protocol Negotiation (NPN) with Application Layer Protocol Negotiation (ALPN) as its implementation for TLS. Both NPN and ALPN are used when client and server are establishing SSL/TLS connections. ALPN avoids an additional round trip because the client list the application layer protocols supported by the client in the client hello message.

Both input and output plugins that perform Network I/O can optionally enable TLS and configure the behavior. The following table describes the properties available:

Property
Description
Default

tls

Enable or disable TLS support.

off

tls.debug

Set TLS debug verbosity level. Accepted values: 0 (No debug), 1 (Error), 2 (State change), 3 (Informational) and 4 (Verbose).

1

tls.ca_file

Absolute path to CA certificate file.

none

tls.ca_path

Absolute path to scan for certificate files.

none

tls.ciphers

Specify TLS ciphers up to TLSv1.2.

none

tls.crt_file

Absolute path to Certificate file.

none

tls.key_file

Absolute path to private Key file.

none

tls.key_passwd

Optional password for tls.key_file file.

none

tls.max_version

Specify the maximum version of TLS.

none

tls.min_version

Specify the minimum version of TLS.

none

tls.verify

Force certificate validation.

on

tls.vhost

Hostname to be used for TLS SNI extension.

none

tls.verify_hostname

Force TLS verification of host names.

off

To use TLS on input plugins, you must provide both a certificate and a private key.

The listed properties can be enabled in the configuration file, specifically in each output plugin section or directly through the command line.

The following output plugins can take advantage of the TLS feature:

The following input plugins can take advantage of the TLS feature:

In addition, other plugins implement a subset of TLS support, with restricted configuration:

Example: enable TLS on HTTP input

By default, the HTTP input plugin uses plain TCP. Run the following command to enable TLS:

fluent-bit -i http \
           -p port=9999 \
           -p tls=on \
           -p tls.verify=off \
           -p tls.crt_file=self_signed.crt \
           -p tls.key_file=self_signed.key \
           -o stdout \
           -m '*'

See the Tips and Tricks section for details on generating self_signed.crt and self_signed.key files shown in these examples.

In the previous command, the two properties tls and tls.verify are set for demonstration purposes. Always enable verification in production environments.

The same behavior can be accomplished using a configuration file:

pipeline:
  inputs:
    - name: http
      port: 9999
      tls: on
      tls.verify: off
      tls.cert_file: self_signed.crt
      tls.key_file: self_signed.key

  outputs:
    - name: stdout
      match: '*'

Example: enable TLS on HTTP output

By default, the HTTP output plugin uses plain TCP. Run the following command to enable TLS:

fluent-bit -i cpu -t cpu -o http://192.168.2.3:80/something \
           -p tls=on         \
           -p tls.verify=off \
           -m '*'

In the previous command, the properties tls and tls.verify are enabled for demonstration purposes. Always enable verification in production environments.

The same behavior can be accomplished using a configuration file:

pipeline:
  inputs:
    - name: cpu
      tag: cpu

  outputs:
    - name: http
      match: '*'
      host: 192.168.2.3
      port: 80
      uri: /something
      tls: on
      tls.verify: off

Tips and tricks

Generate a self-signed certificates for testing purposes

The following command generates a 4096-bit RSA key pair and a certificate that's signed using SHA-256 with the expiration date set to 30 days in the future. In this example, test.host.net is set as the common name. This example opts out of DES, so the private key is stored in plain text.

openssl req -x509 \
            -newkey rsa:4096 \
            -sha256 \
            -nodes \
            -keyout self_signed.key \
            -out self_signed.crt \
            -subj "/CN=test.host.net"

Connect to virtual servers using TLS

Fluent Bit supports TLS server name indication. If you are serving multiple host names on a single IP address (for example, using virtual hosting), you can make use of tls.vhost to connect to a specific hostname.

pipeline:
  inputs:
    - name: cpu
      tag: cpu

  outputs:
    - name: forward
      match: '*'
      host: 192.168.10.100
      port: 24224
      tls: on
      tls.verify: off
      tls.ca_file: '/etc/certs/fluent.crt'
      tls.vhost: 'fluent.example.com'

Verify subjectAltName

By default, TLS verification of host names isn't done automatically. As an example, you can extract the X509v3 Subject Alternative Name from a certificate:

X509v3 Subject Alternative Name:
    DNS:my.fluent-aggregator.net

This certificate covers only my.fluent-aggregator.net so if you use a different hostname it should fail.

To fully verify the alternative name and demonstrate the failure, enable tls.verify_hostname:

pipeline:
  inputs:
    - name: cpu
      tag: cpu

  outputs:
    - name: forward
      match: '*'
      host: other.fluent-aggregator.net
      port: 24224
      tls: on
      tls.verify: on
      tls.verify_hostname: on
      tls.ca_file: '/path/to/fluent-x509v3-alt-name.crt'

This outgoing connect will fail and disconnect:

[2024/06/17 16:51:31] [error] [tls] error: unexpected EOF with reason: certificate verify failed
[2024/06/17 16:51:31] [debug] [upstream] connection #50 failed to other.fluent-aggregator.net:24224
[2024/06/17 16:51:31] [error] [output:forward:forward.0] no upstream connections available

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