forward

The out_forward Buffered Output plugin forwards events to other fluentd nodes. This plugin supports load-balancing and automatic fail-over (a.k.a. active-active backup). For replication, please use the out_copy plugin.

The out_forward plugin detects server faults using a "φ accrual failure detector" algorithm. You can customize the parameters of the algorithm. When a server fault recovers, the plugin makes the server available automatically after a few seconds.

The out_forward plugin supports at-most-once and at-least-once semantics. The default is at-most-once.

Example Configuration

out_forward is included in Fluentd's core. No additional installation process is required.

<match pattern>
@type forward
send_timeout 60s
recover_wait 10s
hard_timeout 60s
<server>
name myserver1
host 192.168.1.3
port 24224
weight 60
</server>
<server>
name myserver2
host 192.168.1.4
port 24224
weight 60
</server>
...
<secondary>
@type file
path /var/log/fluent/forward-failed
</secondary>
</match>

Please see the Config File article for the basic structure and syntax of the configuration file.

Supported modes

  • Synchronous

  • Asynchronous

See Output Plugin Overview for more details.

Plugin helpers

Parameters

Common Parameters

@type

The value must be forward.

<server> (at least one is required)

required

multi

version

true

true

0.14.5

The destination servers. Each server has following parameters.

  • host

  • name

  • port

  • shared_key

  • username

  • password

  • standby

  • weight

host

type

default

version

string

required parameter

0.14.5

The IP address or host name of the server.

name

type

default

version

string

nil

0.14.5

The name of the server. Used for logging and certificate verification in TLS transport (when host is address).

port

type

default

version

integer

24224

0.14.5

The port number of the host. Note that both TCP packets (event stream) and UDP packets (heartbeat message) are sent to this port.

shared_key

type

default

version

string

nil

0.14.5

The shared key per server.

username

type

default

version

string

"" (empty string)

0.14.5

The username for authentication.

password

type

default

version

string

"" (empty string)

0.14.5

The password for authentication.

standby

type

default

version

bool

false

0.14.5

Marks a node as the standby node for an Active-Standby model between Fluentd nodes. When an active node goes down, the standby node is promoted to an active node. The standby node is not used by the out_forward plugin until then.

<match pattern>
@type forward
...
<server>
name myserver1
host 192.168.1.3
weight 60
</server>
<server> # forward doesn't use myserver2 until myserver1 goes down
name myserver2
host 192.168.1.4
weight 60
standby
</server>
...
</match>

weight

type

default

version

integer

60

0.14.5

The load balancing weight. If the weight of one server is 20 and the weight of the other server is 30, events are sent in a 2:3 ratio. The default weight is 60.

require_ack_response

type

default

version

bool

false

0.14.0

Change the protocol to at-least-once. The plugin waits the ack from destination's in_forward plugin.

ack_response_timeout

type

default

version

time

190

0.14.0

This option is used when require_ack_response is true. This default value is based on popular tcp_syn_retries.

If set 0, this plugin doesn't wait the ack response.

send_timeout

type

default

version

time

60

0.14.0

The timeout time when sending event logs.

connect_timeout

type

default

version

time

nil(no timeout)

1.6.0

The timeout time for socket connect. When the connection timed out during establishment, Errno::ETIMEDOUT is raised.

recover_wait

type

default

version

time

10

0.14.0

The wait time before accepting a server fault recovery.

heartbeat_type

type

default

available

version

enum

transport

transport, tcp, udp, none

0.14.12

The transport protocol to use for heartbeats. Set "none" to disable heartbeat.

heartbeat_interval

type

default

version

time

1

0.14.0

The interval of the heartbeat packer.

phi_failure_detector

type

default

version

bool

true

0.14.0

Use the "Phi accrual failure detector" to detect server failure.

phi_threshold

type

default

version

integer

16

0.14.0

The threshold parameter used to detect server faults.

`phi_threshold` is deeply related to `heartbeat_interval`. If you are using longer `heartbeat_interval`, please use the larger `phi_threshold`. Otherwise you will see frequent detachments of destination servers. The default value 16 is tuned for `heartbeat_interval` 1s.

hard_timeout

type

default

version

time

60

0.14.0

The hard timeout used to detect server failure. The default value is equal to the send_timeout parameter.

expire_dns_cache

type

default

version

time

nil (persistent cache)

0.14.0

Set TTL to expire DNS cache in seconds. Set 0 not to use DNS Cache.

dns_round_robin

type

default

version

bool

false

0.14.0

Enable client-side DNS round robin. Uniform randomly pick an IP address to send data when a hostname has several IP addresses.

`heartbeat_type udp` is not available with `dns_round_robin true`. Use `heartbeat_type tcp` or `heartbeat_type none`.

ignore_network_errors_at_startup

type

default

version

bool

false

0.14.12

Ignore DNS resolution and errors at startup time.

tls_version

type

default

available

version

enum

TLSv1_2

TLSv1_1, TLSv1_2

0.14.12

The default version of TLS transport.

tls_ciphers

type

default

version

string

ALL:!aNULL:!eNULL:!SSLv2 (OpenSSL > 1.0.0 default)

0.14.12

The cipher configuration of TLS transport.

tls_insecure_mode

type

default

version

bool

false

0.14.12

Skip all verification of certificates or not.

tls_allow_self_signed_cert

type

default

version

bool

false

0.14.12

Allow self signed certificates or not.

tls_verify_hostname

type

default

version

bool

true

0.14.12

Verify hostname of servers and certificates or not in TLS transport.

tls_cert_path

type

default

version

array of string

nil

0.14.12

The additional CA certificate path for TLS.

tls_client_cert_path

type

default

version

string

nil

1.3.2

The client certificate path for TLS

tls_client_private_key_path

type

default

version

string

nil

1.3.2

The client private key path for TLS.

tls_client_private_key_passphrase

type

default

version

string

nil

1.3.2

The client private key passphrase for TLS.

tls_cert_thumbprint

type

default

version

string

nil

1.7.1

The certificate thumbprint for searching from Windows system certstore This parameter is for Windows only.

tls_cert_logical_store_name,

type

default

version

string

nil

1.7.1

The certificate logical store name on Windows system certstore. This parameter is for Windows only.

tls_cert_use_enterprise_store

type

default

version

string

true

1.7.1

Enable to use certificate enterprise store on Windows system certstore. This parameter is for Windows only.

keepalive

type

default

version

bool

fales

1.5.0

Enable keepalive connection.

keepalive_timeout

type

default

version

time

nil

1.5.0

Expired time of keepalive. Default value is nil, which means to keep connection as long as possible.

<security> section

required

multi

version

false

false

0.14.5

This section contains parameters related to authentication.

  • self_hostname

  • shared_key

self_hostname

type

default

version

string

required parameter

0.14.5

The hostname.

shared_key

type

default

version

string

required parameter

0.14.5

Shared key for authentication. If you want to specify shared_key for specific server, use <server> section.

<secondary>

required

multi

version

false

false

0.14.0

The backup destination that is used when all servers are unavailable.

For more details, see Secondary Output.

verify_connection_at_startup

type

default

version

bool

false

1.3.1

Verify that a connection can be made with one of out_forward nodes at the time of startup.

Tips & Tricks

How to connect to a TLS/SSL enabled server

If you've set up TLS/SSL encryption in the receiving server, you need to tell the output forwarder to use encryption by setting the transport parameter:

<match debug.**>
@type forward
transport tls
<server>
host 192.168.1.2
port 24224
</server>
</match>

If you're using a self-singed certificate, copy the certificate file to the forwarding server, then add the following settings:

<match debug.**>
@type forward
transport tls
tls_cert_path /path/to/fluentd.crt # Set the path to the certificate file.
tls_verify_hostname true # Set false to ignore cert hostname.
<server>
host 192.168.1.2
port 24224
</server>
</match>

After updating the settings, please confirm that the forwarded data is being received by the destination node properly.

How to connect to a TLS/SSL enabled server with Windows Certstore Certificate

If you've set up TLS/SSL encryption in the receiving server, you need to tell the output forwarder to use encryption by setting the transport parameter.

Valid logical store name are:

  • MY

  • CA

  • ROOT

  • AUTHROOT

  • DISALLOWED

  • SPC

  • TRUST

  • TRUSTEDPEOPLE

  • TRUSTEDPUBLISHER

  • CLIENTAUTHISSUER

  • TRUSTEDDEVICES

  • SMARTCARDROOT

  • WEBHOSTING

  • REMOTE DESKTOP

Logical store name is case-insensitive. Note that this section configurations work on only Windows.

<match debug.**>
@type forward
transport tls
# Set valid logical store name.
tls_cert_logical_store_name Trust
tls_verify_hostname true # Set false to ignore cert hostname.
<server>
host 192.168.1.2
port 24224
</server>
</match>

If you're using a self-signed certificate, export the certificate file from Windows Certstore and copy to the forwarding server, then add the following settings:

<match debug.**>
@type forward
transport tls
# Set certificate SHA1 hash which is usually called as thumbprint.
tls_cert_thumbprint <YOUR CERTIFICATE THUMBPRINT>
tls_cert_logical_store_name Trust
tls_verify_hostname true # Set false to ignore cert hostname.
tls_cert_use_enterprise_store true # Set false to use non-enterprise certificate.
<server>
host 192.168.1.2
port 24224
</server>
</match>

Note that these configuration works for root certificate which is put in Windows Certstore. Currently, chained certificate is not supported.

Certificate thumbprint is able to obtain with certutil command. Here is an example which uses a certificate that is generated by fluent-ca-generate command.

PS> certutil -store -enterprise <YOUR CERTIFICATE STORE NAME>
trust "Enterprise Trust"
================ Certificate 0 ================
Serial Number: 01
Issuer: CN=Fluentd Forward CA, L=Mountain View, S=CA, C=US
NotBefore: 1/1/1970 12:00 AM
NotAfter: 8/21/2024 7:43 AM
Subject: CN=Fluentd Forward CA, L=Mountain View, S=CA, C=US
Signature matches Public Key
Root Certificate: Subject matches Issuer
Cert Hash(sha1): <YOUR CERTIFICATE THUMBPRINT>
No key provider information
Cannot find the certificate and private key for decryption.
CertUtil: -store command completed successfully.

Cert hash(sha1) is called as thumbprint in this section.

Note that -enterprise flag represents to use enterprise certstore. Please pay attension to using whether enterprise certificates store or not.

How to Enable Password Authentication

If you want to connect to a server that requires password authentication, you need to set your credentials in the configuration file.

<match debug.**>
@type forward
<server>
host 192.168.1.2
port 24224
</server>
<security>
self_hostname HOSTNAME
shared_key secret
</security>
</match>

Note that, as to the option self_hostname, you need to set the name of the server on which your out_forward instance is running. In the current implementation, it is considered invalid if your in_forward and out_forward shares the same hostname.

How to enable gzip compression

Since v0.14.7, Fluentd supports transparent data compression. You can use this feature to reduce the transferred payload size.

To enable this feature, set the compress option as follows:

<match debug.**>
@type forward
compress gzip
<server>
host 192.168.1.2
port 24224
</server>
</match>

You don't need any configuration in the receiving server. Data compression is auto-detected and handled transparently by the destination node.

What is a Phi accrual failure detector?

Fluentd implements an adaptive failure detection mechanism called "Phi accrual failure detector". Here is how it works:

  1. Each in_forward node sends heartbeat packets to its out_foward

    server at a regular interval.

  2. The out_forward server records the arrival time of heartbeat

    packets sent by each node.

  3. If the server does not receive a heartbeat from one of its nodes for

    "a long time", it assumes the node is down.

But how long should the server wait before detaching a node? The phi accrual failure detector answers this question by computing the probability of a node being down based on the assumption that heartbeat intervals follow normal distribution. Internally it represent the confidence of a node being down by a continuous function φ(t) which grows as the time from the last packet increases.

For example, suppose that the historical average interval is 1 seconds and the standard deviation is 1, it's not likely that the node is still being active when its heartbeat does not arrive for the last 10 seconds.

For details, please read the original paper: Hayashibara, Naohiro, et al. "The φ accrual failure detector." IEEE, 2004.

Troubleshooting

"no nodes are available"

Please make sure that you can communicate with port 24224 using not only TCP, but also UDP. These commands will be useful for checking the network configuration.

$ telnet host 24224
$ nmap -p 24224 -sU host

Please note that there is one known issue where VMware will occasionally lose small UDP packets used for heartbeat.

If this article is incorrect or outdated, or omits critical information, please let us know. Fluentd is a open source project under Cloud Native Computing Foundation (CNCF). All components are available under the Apache 2 License.