smtp() destination options

The smtp() sends email messages using SMTP, without needing external applications. The smtp() destination has the following options:

body()

Type:string
Default:n/a

Description: The BODY field of the email. You can also use macros in the string. Use \\n to start a new line. For example:

   body("AxoSyslog received the following alert from $HOST:\n$MSG")

bcc()

Type:string
Default:n/a

Description: The BCC recipient of the email (contents of the BCC field). You can specify the email address, or the name and the email address. Set the bcc option multiple times to send the email to multiple recipients. For example: bcc("admin@example.com") or bcc("Admin" "admin@example.com") or >bcc("Admin" "admin@example.com") bcc("Admin2" "admin2@example.com")

You can also use macros to set the value of this parameter.

cc()

Type:string
Default:n/a

Description: The CC recipient of the email (contents of the CC field). You can specify the email address, or the name and the email address. Set the cc option multiple times to send the email to multiple recipients. For example: cc("admin@example.com") or cc("Admin" "admin@example.com") or cc("Admin" "admin@example.com") cc("Admin2" "admin2@example.com"

You can also use macros to set the value of this parameter.

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()

Type:number (bytes)
Default:1MiB

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()

Type:yes/no
Default:no

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()

Type:string
Default:N/A

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

flow-control-window-bytes()

Type:number (bytes)
Default:163840000

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()

Type:number(messages)
Default:10000

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()

Type:number(messages)
Default:1000

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()

Type:yes/no
Default:no

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()

Type:yes/no
Default:no

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()

Type:number((between 0 and 1))
Default:1 (do not truncate)

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:

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")
            )
        );
    };

batch-bytes()

Accepted values:number [bytes]
Default:none

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.

batch-lines()

Type:number
Default:1

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()

Type:time in milliseconds
Default:-1 (disabled)

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.

from()

Type:string
Default:n/a

Description: The sender of the email (contents of the FROM field). You can specify the email address, or the name and the email address. For example:

   from("admin@example.com")

or

   from("Admin" "admin@example.com")

If you specify the from() option multiple times, the last value will be used. Instead of the from() option, you can also use sender(), which is just an alias of the from() option.

You can also use macros to set the value of this parameter.

Type:string
Default:n/a

Description: Adds an extra header to the email with the specified name and content. The first parameter sets the name of the header, the second one its value. The value of the header can contain macros. Set the header option multiple times to add multiple headers. For example:

   header("X-Program", "$PROGRAM")

When using the header option, note the following points:

  • Do not use the header() option to set the values of headers that have dedicated options. Use it only to add extra headers.

  • If you set the same custom header multiple times, only the first will be added to the email, other occurrences will be ignored.

  • It is not possible to set the DATE, Return-Path, Original-Recipient, Content-*, MIME-*, Resent-*, Received headers.

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()

Type:string
Default:N/A

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

shutdown()

Type:string
Default:N/A

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()

Type:string
Default:N/A

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

teardown()

Type:string
Default:N/A

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 the 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")
            )
         );
    };

host()

Type:hostname or IP address
Default:n/a

Description: Hostname or IP address of the SMTP server.

log-fifo-size()

Type:number
Default:Use global setting.

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

port()

Type:number
Default:25

Description: The port number of the SMTP server.

reply-to()

Type:string
Default:n/a

Description: Replies of the recipient will be sent to this address (contents of the REPLY-TO field). You can specify the email address, or the name and the email address. Set the reply-to() option multiple times to send the email to multiple recipients. For example: reply-to("admin@example.com") or reply-to("Admin" "admin@example.com") or reply-to("Admin" "admin@example.com") reply-to("Admin2" "admin2@example.com")

You can also use macros to set the value of this parameter.

retries()

Type:number (of attempts)
Default:3

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.

subject()

Type:string
Default:n/a

Description: The SUBJECT field of the email. You can also use macros. For example:

   subject("[SYSLOG ALERT]: Critical error message received from $HOST")

If you specify the subject() option multiple times, the last value will be used.

throttle()

Type:number
Default:0

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.

time-reopen()

Accepted values:number [seconds]
Default:60

Description: The time to wait in seconds before a dead connection is reestablished.

to()

Type:string
Default:localhost

Description: The recipient of the email (contents of the TO field). You can specify the email address, or the name and the email address. Set the to() option multiple times to send the email to multiple recipients. For example: to("admin@example.com") or to("Admin" "admin@example.com") or to("Admin" "admin@example.com") to("Admin2" "admin2@example.com")

You can also use macros to set the value of this parameter.

Last modified December 4, 2023: Theme update (6377ea9)