syslog-ng Premium Edition 6.0.21 - Administration Guide

This driver writes messages to the terminal of a logged-in user.

The usertty() driver has a single required argument, specifying a username who should receive a copy of matching messages. Use the asterisk * to specify every user currently logged in to the system.

Declaration: 

usertty(username);

The usertty() does not have any further options nor does it support templates.

destination d_usertty { usertty("root"); };

This section describes the internal message-processing model of syslog-ng, as well as the flow-control feature that can prevent message losses. To use flow-control, the flow-control flag must be enabled for the particular log path.

The syslog-ng application monitors (polls) the sources defined in its configuration file, periodically checking each source for messages. When a log message is found in one of the sources, syslog-ng polls every source and reads the available messages. These messages are processed and put into the output buffer of syslog-ng (also called fifo). From the output buffer, the operating system sends the messages to the appropriate destinations.

In large-traffic environments many messages can arrive during a single poll loop, therefore syslog-ng reads only a fixed number of messages from each source. The log-fetch-limit() option specifies the number of messages read during a poll loop from a single source.

Figure 8.3. Managing log messages in syslog-ng

Every destination has its own output buffer. The output buffer is needed because the destination might not be able to accept all messages immediately. The log-fifo-size() parameter sets the size of the output buffer. The output buffer must be larger than the log-fetch-limit() of the sources, to ensure that every message read during the poll loop fits into the output buffer. If the log path sends messages to a destination from multiple sources, the output buffer must be large enough to store the incoming messages of every source.

TCP and unix-stream sources can receive the logs from several incoming connections (for example many different clients or applications). For such sources, syslog-ng reads messages from every connection, thus the log-fetch-limit() parameter applies individually to every connection of the source.

Figure 8.4. Managing log messages of TCP sources in syslog-ng

The flow-control of syslog-ng introduces a control window to the source that tracks how many messages can syslog-ng accept from the source. Every message that syslog-ng reads from the source lowers the window size by one, every message that syslog-ng successfully sends from the output buffer increases the window size by one. If the window is full (that is, its size decreases to zero), syslog-ng stops reading messages from the source. The initial size of the control window is by default 1000: the log-fifo-size() must be larger than this value in order for flow-control to have any effect. If a source accepts messages from multiple connections, all messages use the same control window.

When flow-control is used, every source has its own control window. As a worst-case situation, the output buffer of the destination must be set to accommodate all messages of every control window, that is, the log-fifo-size() of the destination must be greater than number_of_sources*log-iw-size(). This applies to every source that sends logs to the particular destination. Thus if two sources having several connections and heavy traffic send logs to the same destination, the control window of both sources must fit into the output buffer of the destination. Otherwise, syslog-ng does not activate the flow-control, and messages may be lost.

The syslog-ng application handles outgoing messages the following way:

Figure 8.5. Handling outgoing messages in syslog-ng PE

There are two types of flow-control: Hard flow-control and soft flow-control.

source s_localhost {
            network(ip(127.0.0.1) port(1999) max-connections(300)); };
destination d_tcp {
            network("10.1.2.3" port(1999); localport(999)); log-fifo-size(30000); };
log { source(s_localhost); destination(d_tcp); flags(flow-control); };

source s_localhost {
            network(ip(127.0.0.1) port(1999) max-connections(300)); };
source s_tcp {
            network(ip(192.168.1.5) port(1999) max-connections(100)); };
destination d_tcp {
            network("10.1.2.3" port(1999); localport(999)); log-fifo-size(40000); };
log { source(s_localhost); destination(d_tcp); flags(flow-control); };

The syslog-ng Premium Edition application can store messages on the local hard disk if the destination (for example, the central log server) or the network connection to the destination becomes unavailable. The syslog-ng PE application automatically sends the stored messages to the destination when the connection is reestablished. The disk buffer is used as a queue: when the connection to the destination is reestablished, syslog-ng PE sends the messages to the destination in the order they were received.

The following destination drivers can use disk-based buffering: elasticsearch(), elasticsearch2(), hdfs(), kafka(), mongodb(), program(), smtp(), snmp(), sql(), unix-dgram(), and unix-stream(). The network(), syslog(), tcp(), and tcp6() destination drivers can also use disk-based buffering, except when using the udp transport method. (The other destinations or protocols do not provide the necessary feedback mechanisms required for disk-based buffering.)

Every such destination uses a separate disk buffer (similarly to the output buffers controlled by log-fifo-size()). The hard disk space is not pre-allocated, so ensure that there is always enough free space to store the disk buffers even when the disk buffers are full.

If syslog-ng PE is restarted (using the /etc/init.d/syslog-ng restart command, or another appropriate command on your platform), it automatically saves any unsent messages from the disk buffer and the output queue. After the restart, syslog-ng PE sends the saved messages to the destination. In other words, the disk buffer is persistent. The disk buffer is also resistant to syslog-ng PE crashes.

The syslog-ng PE application supports two types of disk buffering: reliable and normal. For details, see the section called “Enabling reliable disk-based buffering” and the section called “Enabling normal disk-based buffering”, respectively.

Message handling and normal disk-based buffering. When you use disk-based buffering, and the reliable() option is set to no, syslog-ng PE handles outgoing messages the following way:

Figure 8.6. Handling outgoing messages in syslog-ng PE

Message handling and reliable disk-based buffering. When you use disk-based buffering, and the reliable() option is set to yes, syslog-ng PE handles outgoing messages the following way.

The mem-buf-size() option determines when flow-control is triggered. All messages arriving to the log path that includes the destination using the disk-buffer are written into the disk-buffer, until the size of the disk-buffer reaches (disk-buf-size() minus mem-buf-size()). Above that size, messages are written into both the disk-buffer and the memory-buffer, indicating that flow-control needs to slow down the message source. These messages are not taken out from the control window (governed by log-iw-size()), causing the control window to fill up. If the control window is full, the flow-control completely stops reading incoming messages from the source. (As a result, mem-buf-size() must be at least as large as log-iw-size().)

destination d_BSD {
    network(
            "127.0.0.1"
            port(3333)
            disk-buffer(
                mem-buf-size(10000)
                disk-buf-size(2000000)
                reliable(yes)
            )
        );
}; 

destination d_BSD {
    network(
            "127.0.0.1"
            port(3333)
            disk-buffer(
                mem-buf-length(10000)
                disk-buf-size(2000000)
                reliable(no)
            )
        );
        }; 

destination d_BSD {
    network(
            "127.0.0.1"
            port(3333)
            log-disk-fifo-size(2000000)
            log-fifo-size(10000)
        );
}; 

destination d_BSD {
    network(
            "127.0.0.1"
            port(3333)
            log-fifo-size(10000)
        );
};