slack: Send alerts and notifications to a Slack channel

• 1: Slack destination options

   destination d_slack {
      slack(
        hook-url("https://hooks.slack.com/services/T00000000/B00000000/XXXXXXXXXXXXXXXXXXXXXXXX")
      );
    };

   @include "scl.conf"

   @include "scl.conf"
    
    destination d_slack1 {
      slack(
        hook-url("https://hooks.slack.com/services/T00000000/B00000000/XXXXXXXXXXXXXXXXXXXXXXXX")
        colors("#000000,#222222,#444444,#666666,#888888,#AAAAAA,#CCCCCC,#EEEEEE")
        color-chooser(7)
        author-name("Example BOT")
        author-link("https://www.syslog-ng.com/products/open-source-log-management")
        author-icon("https://raw.githubusercontent.com/MrAnno/vscode-syslog-ng/master/images/syslog-ng-icon.png")
      );
    };

1 - Slack destination options

The slack destination of AxoSyslog can directly post log messages and notifications to Slack channels. The slack destination has the following options.

author-name()

Description: The sender of the message as displayed in Slack. For details, see the author_name option in the Slack documentation.

author-link()

Description: A hyperlink for the sender of the message as displayed in Slack. For details, see the author_link option in the Slack documentation.

author-icon()

Description: A hyperlink for icon of the author to be displayed in Slack. For details, see the author_icon option in the Slack documentation.

batch-bytes()

Description: Sets the maximum size of payload in a batch. If the size of the messages reaches this value, AxoSyslog sends the batch to the destination even if the number of messages is less than the value of the batch-lines() option.

Note that if the batch-timeout() option is enabled and the queue becomes empty, AxoSyslog flushes the messages only if batch-timeout() expires, or the batch reaches the limit set in batch-bytes().

Available in AxoSyslog version 3.19 and later.

For details on how this option influences batch mode, see http: Posting messages over HTTP without Java

batch-lines()

Description: Specifies how many lines are flushed to a destination in one batch. The AxoSyslog application waits for this number of lines to accumulate and sends them off in a single batch. Increasing this number increases throughput as more messages are sent in a single batch, but also increases message latency.

For example, if you set batch-lines() to 100, AxoSyslog waits for 100 messages.

If the batch-timeout() option is disabled, the AxoSyslog application flushes the messages if it has sent batch-lines() number of messages, or the queue became empty. If you stop or reload AxoSyslog or in case of network sources, the connection with the client is closed, AxoSyslog automatically sends the unsent messages to the destination.

Note that if the batch-timeout() option is enabled and the queue becomes empty, AxoSyslog flushes the messages only if batch-timeout() expires, or the batch reaches the limit set in batch-lines().

For optimal performance, make sure that the AxoSyslog source that feeds messages to this destination is configured properly: the value of the log-iw-size() option of the source must be higher than the batch-lines()*workers() of the destination. Otherwise, the size of the batches cannot reach the batch-lines() limit.

batch-timeout()

Description: Specifies the time AxoSyslog waits for lines to accumulate in the output buffer. The AxoSyslog application sends batches to the destinations evenly. The timer starts when the first message arrives to the buffer, so if only few messages arrive, AxoSyslog sends messages to the destination at most once every batch-timeout() milliseconds.

For details on how this option influences batch mode, see http: Posting messages over HTTP without Java

ca-dir()

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 AxoSyslog clients. The AxoSyslog 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.

An alternative way to specify this option is to put it into a tls() block, together with any other TLS options. This allows you to separate these options and ensure better readability.

Make sure that you specify TLS options either using their own dedicated option (ca-dir(), ca-file(), cert-file(), cipher-suite(), key-file(), peer-verify(), and ssl-version()), or using the tls() block and inserting the relevant options within tls(). Avoid mixing the two methods. In case you do specify TLS options in both ways, the one that comes later in the configuration file will take effect.

ca-file()

Description: Name of a file that contains an X.509 CA certificate (or a certificate chain) in PEM format. The AxoSyslog application uses this certificate to validate the certificate of the HTTPS 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.

An alternative way to specify this option is to put it into a tls() block, together with any other TLS options. This allows you to separate these options and ensure better readability.

Make sure that you specify TLS options either using their own dedicated option (ca-dir(), ca-file(), cert-file(), cipher-suite(), key-file(), peer-verify(), and ssl-version()), or using the tls() block and inserting the relevant options within tls(). Avoid mixing the two methods. In case you do specify TLS options in both ways, the one that comes later in the configuration file will take effect.

cipher-suite()

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 AxoSyslog. 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 AxoSyslog receives TLS-encrypted connections, the order of ciphers set on the AxoSyslog server takes precedence over the client settings.

An alternative way to specify this option is to put it into a tls() block, together with any other TLS options. This allows you to separate these options and ensure better readability.

Make sure that you specify TLS options either using their own dedicated option (ca-dir(), ca-file(), cert-file(), cipher-suite(), key-file(), peer-verify(), and ssl-version()), or using the tls() block and inserting the relevant options within tls(). Avoid mixing the two methods. In case you do specify TLS options in both ways, the one that comes later in the configuration file will take effect.

Declaration:

   destination d_http {
        http(
            url("http://127.0.0.1:8080")
            tls(
                ca-dir("dir")
                ca-file("ca")
                cert-file("cert")
                cipher-suite("cipher")
                key-file("key")
                peer-verify(yes|no)
                ssl-version(<the permitted SSL/TLS version>)
            )
        );
    };

colors()

Description: The colors to be assigned to the messages of different importance levels.

color-chooser()

Description: An integer that assigns a color to the message from the list of colors set in the colors() option.

disk-buffer()

Description: This option enables putting outgoing messages into the disk buffer of the destination to avoid message loss in case of a system failure on the destination side. It has the following options:

capacity-bytes()

Description: This is a required option. The maximum size of the disk-buffer in bytes. The minimum value is 1048576 bytes. If you set a smaller value, the minimum value will be used automatically. It replaces the old log-disk-fifo-size() option.

In AxoSyslog version 4.2 and earlier, this option was called disk-buf-size().

compaction()

Description: If set to yes, AxoSyslog prunes the unused space in the LogMessage representation, making the disk queue size smaller at the cost of some CPU time. Setting the compaction() argument to yes is recommended when numerous name-value pairs are unset during processing, or when the same names are set multiple times.

dir()

Description: Defines the folder where the disk-buffer files are stored.

flow-control-window-bytes()

Description: Use this option if the option reliable() is set to yes. This option contains the size of the messages in bytes that is used in the memory part of the disk buffer. It replaces the old log-fifo-size() option. It does not inherit the value of the global log-fifo-size() option, even if it is provided. Note that this option will be ignored if the option reliable() is set to no.

In AxoSyslog version 4.2 and earlier, this option was called mem-buf-size().

flow-control-window-size()

Description: Use this option if the option reliable() is set to no. This option contains the number of messages stored in overflow queue. It replaces the old log-fifo-size() option. It inherits the value of the global log-fifo-size() option if provided. If it is not provided, the default value is 10000 messages. Note that this option will be ignored if the option reliable() is set to yes.

In AxoSyslog version 4.2 and earlier, this option was called mem-buf-length().

front-cache-size()

Description: The number of messages stored in the output buffer of the destination. Note that if you change the value of this option and the disk-buffer already exists, the change will take effect when the disk-buffer becomes empty.

Options reliable() and capacity-bytes() are required options.

In AxoSyslog version 4.2 and earlier, this option was called qout-size().

prealloc()

Description:

By default, AxoSyslog doesn’t reserve the disk space for the disk-buffer file, since in a properly configured and sized environment the disk-buffer is practically empty, so a large preallocated disk-buffer file is just a waste of disk space. But a preallocated buffer can prevent other data from using the intended buffer space (and elicit a warning from the OS if disk space is low), preventing message loss if the buffer is actually needed. To avoid this problem, when using AxoSyslog 4.0 or later, you can preallocate the space for your disk-buffer files by setting prealloc(yes).

In addition to making sure that the required disk space is available when needed, preallocated disk-buffer files provide radically better (3-4x) performance as well: in case of an outage the amount of messages stored in the disk-buffer is continuously growing, and using large continuous files is faster, than constantly waiting on a file to change its size.

If you are running AxoSyslog on a dedicated host (always recommended for any high-volume settings), use prealloc(yes).

Available in AxoSyslog 4.0 and later.

reliable()

Description: If set to yes, AxoSyslog cannot lose logs in case of reload/restart, unreachable destination or AxoSyslog crash. This solution provides a slower, but reliable disk-buffer option. It is created and initialized at startup and gradually grows as new messages arrive. If set to no, the normal disk-buffer will be used. This provides a faster, but less reliable disk-buffer option.

truncate-size-ratio()

Description: Limits the truncation of the disk-buffer file. Truncating the disk-buffer file can slow down the disk IO operations, but it saves disk space. By default, AxoSyslog version 4.0 and later doesn’t truncate disk-buffer files by default (truncate-size-ratio(1)). Earlier versions freed the disk-space when at least 10% of the disk-buffer file could be freed (truncate-size-ratio(0.1)).

AxoSyslog only truncates the file if the possible disk gain is more than truncate-size-ratio() times capacity-bytes().

• Smaller values free disk space quicker.
• Larger ratios result in better performance.

If you want to avoid performance fluctuations:

• use truncate-size-ratio(1) (never truncate), or
• use prealloc(yes) to reserve the entire size of the disk-buffer on disk.

Example: Examples for using disk-buffer()

In the following case reliable disk-buffer() is used.

destination d_demo {
    network(
        "127.0.0.1"
        port(3333)
        disk-buffer(
            flow-control-window-bytes(10000)
            capacity-bytes(2000000)
            reliable(yes)
            dir("/tmp/disk-buffer")
        )
    );
};

In the following case normal disk-buffer() is used.

destination d_demo {
    network(
        "127.0.0.1"
        port(3333)
            disk-buffer(
            flow-control-window-size(10000)
            capacity-bytes(2000000)
            reliable(no)
            dir("/tmp/disk-buffer")
        )
    );
};

fallback()

Description: The plain-text summary of the Slack attachment. For details, see the fallback option in the Slack documentation.

footer()

Description: The footer of the message. For details, see the footer option in the Slack documentation.

footer-icon()

Description: A hyperlink for an image. For details, see the footer_icon option in the Slack documentation.

hook-commands()

Description: This option makes it possible to execute external programs when the relevant driver is initialized or torn down. The hook-commands() can be used with all source and destination drivers with the exception of the usertty() and internal() drivers.

Using hook-commands() when AxoSyslog starts or stops

To execute an external program when AxoSyslog starts or stops, use the following options:

startup()

Description: Defines the external program that is executed as AxoSyslog starts.

shutdown()

Description: Defines the external program that is executed as AxoSyslog stops.

Using the hook-commands() when AxoSyslog reloads

To execute an external program when the AxoSyslog configuration is initiated or torn down, for example, on startup/shutdown or during a AxoSyslog reload, use the following options:

setup()

Description: Defines an external program that is executed when the AxoSyslog configuration is initiated, for example, on startup or during a AxoSyslog reload.

teardown()

Description: Defines an external program that is executed when the AxoSyslog configuration is stopped or torn down, for example, on shutdown or during a AxoSyslog reload.

Example: Using hook-commands() with a network source

In the following example, the hook-commands() is used with the network() driver and it opens an iptables port automatically as AxoSyslog is started/stopped.

The assumption in this example is that the LOGCHAIN chain is part of a larger ruleset that routes traffic to it. Whenever the AxoSyslog created rule is there, packets can flow, otherwise the port is closed.

source {
    network(transport(udp)
    hook-commands(
          startup("iptables -I LOGCHAIN 1 -p udp --dport 514 -j ACCEPT")
          shutdown("iptables -D LOGCHAIN 1")
        )
     );
};

hook-url()

Description: The Webhook URL for the Incoming Webhook of your Slack app. This URL must also include the authentication token that AxoSyslog uses to authenticate to Slack. For example: https://hooks.slack.com/services/T00000000/B00000000/XXXXXXXXXXXXXXXXXXXXXXXX

For details, see the Slack documentation about Incoming Webhooks.

image-url()

Description: A hyperlink for an image. For details, see the image_url option in the Slack documentation.

log-fifo-size()

Description: The number of messages that the output queue can store.

persist-name()

Description: If you receive the following error message during AxoSyslog startup, set the persist-name() option of the duplicate drivers:

Error checking the uniqueness of the persist names, please override it with persist-name option. Shutting down.

This error happens if you use identical drivers in multiple sources, for example, if you configure two file sources to read from the same file. In this case, set the persist-name() of the drivers to a custom string, for example, persist-name("example-persist-name1").

pretext()

Description: The text that appears above the attachment block. For details, see the pretext option in the Slack documentation.

retries()

Description: If AxoSyslog cannot send a message, it will try again until the number of attempts reaches retries().

If the number of attempts reaches retries(), AxoSyslog will wait for time-reopen() time, then tries sending the message again.

To handle HTTP error responses, if the HTTP server returns 5xx codes, AxoSyslog will attempt to resend messages until the number of attempts reaches retries. If the HTTP server returns 4xx codes, AxoSyslog will drop the messages.

ssl-version()

Description: Specifies the permitted SSL/TLS version. Possible values: sslv2, sslv3, tlsv1, tlsv1_0, tlsv1_1, tlsv1_2, tlsv1_3.

An alternative way to specify this option is to put it into a tls() block, together with any other TLS options. This allows you to separate these options and ensure better readability.

Make sure that you specify TLS options either using their own dedicated option (ca-dir(), ca-file(), cert-file(), cipher-suite(), key-file(), peer-verify(), and ssl-version()), or using the tls() block and inserting the relevant options within tls(). Avoid mixing the two methods. In case you do specify TLS options in both ways, the one that comes later in the configuration file will take effect.

template()

Description: Specifies a template defining the logformat to be used in the destination. Macros are described in Macros of AxoSyslog. Please note that for network destinations it might not be appropriate to change the template as it changes the on-wire format of the syslog protocol which might not be tolerated by stock syslog receivers (like syslogd or syslog-ng itself). For network destinations make sure the receiver can cope with the custom format defined.

throttle()

Description: Sets the maximum number of messages sent to the destination per second. Use this output-rate-limiting functionality only when using disk-buffer as well to avoid the risk of losing messages. Specifying 0 or a lower value sets the output limit to unlimited.

By default, the throttle() option is set to 1, because Slack has a 1 message/second limit on Webhooks. It can allow more message in short bursts, so you can set it to 0, if you only expect messages in a short period of time. For details, see the Web API rate limiting in the Slack documentation.

thumb-url()

Description: A hyperlink for a thumbnail image. For details, see the thumb_url option in the Slack documentation.

timeout()

Description: The value (in seconds) to wait for an operation to complete, and attempt to reconnect the server if exceeded. By default, the timeout value is 0, meaning that there is no timeout. Available in version 3.11 and later.

title()

Description: The message title in Slack. For details, see the title option in the Slack documentation.

title-link()

Description: A hyperlink for the message title in Slack. For details, see the title_link option in the Slack documentation.

user-agent()

Description: The value of the USER-AGENT header in the messages sent to the server.

use-system-cert-store()

Description: Use the certificate store of the system for verifying HTTPS certificates. For details, see the curl documentation.

workers()

Description: Specifies the number of worker threads (at least 1) that AxoSyslog uses to send messages to the server. Increasing the number of worker threads can drastically improve the performance of the destination.

If you are using load-balancing (that is, you have configured multiple servers in the url() option), increase the number of worker threads at least to the number of servers. For example, if you have set three URLs (url("site1", "site2", "site3")), set the workers() option to 3 or more.