syslog-ng Premium Edition 7.0.12 - Administration Guide

The sender and the receiver use ALTP over the network() protocol. Since the tls() option is not configured neither on the sender nor on the receiver, the communication will be unencrypted.

Receiver configuration (syslog-ng PE server):

source s_network_altp {
        network(
            ip("127.0.0.1")
            port("5555")
            transport(altp)
            ip-protocol(4)
        );
};

Sender configuration (syslog-ng PE client):

destination d_network_altp {
        network(
            "127.0.0.1"
            port("5555")
            transport(altp)
            ip-protocol(4)
        );
};

Example: ALTP with TLS encryption

The following example configures a sender and a receiver to communicate using ALTP. Since the tls-required() option is set to optional on the receiver and yes on the sender, and the tls() option is configured, the communication will be TLS-encrypted. For the sender (syslog-ng PE client), reliable disk-buffering is enabled to prevent data loss.

Receiver configuration (syslog-ng PE server):

source s_syslog_altp {
        syslog(
            ip("127.0.0.1")
            port("4444")
            transport(altp(tls-required(optional)))
            ip-protocol(4)
            tls(
                peer-verify(required-trusted)
                ca-dir("/var/tmp/client/")
                key-file("/var/tmp/server/server_priv.key")
                cert-file("/var/tmp/server/server.crt")
            )
        );
};

Sender configuration (syslog-ng PE client):

destination d_syslog_altp {
        syslog(
            "127.0.0.1"
            port("4444")
            transport(altp(tls-required(yes)))
            ip-protocol(4)
            disk-buffer( mem-buf-size(200000) disk-buf-size(2000000) reliable(yes) )
            tls(
                peer-verify(required-trusted)
                ca-dir("/var/tmp/server/")
                key-file("/var/tmp/client/client_priv.key")
                cert-file("/var/tmp/client/client.crt")
            )
        );
};

Reliability and minimizing the loss of log messages

Introduction

Flow control, no disk buffering, no ALTP

Flow control, normal disk buffering, no ALTP

Flow control, reliable disk buffering, no ALTP

Flow control, reliable disk buffering, ALTP

Deciding which loss prevention mechanism to apply

Introduction

Reliability and minimizing the loss of log messages > Introduction

Advanced Log Transfer Protocol (ALTP) interacts with flow control and disk buffering to ensure that the loss of log messages is minimized or is prevented completely. This section explains how each loss prevention method contributes to reliability and minimizing log message loss. Flow control, disk buffering, and ALTP are explained in detail elsewhere in the document. In this section, we present a high-level overview of all of these mechanisms and highlight considerations such as:

What best practices exist in various scenarios, how to set key parameters
When is a log message considered "delivered"
Under what circumstances can log loss occur

Each of the following sections discusses a different scenario and uses figures to aid comprehension.

NOTE:

Each figure depicts a scenario in which the volume of incoming messages makes it necessary to use all buffers and control windows at maximum capacity.

Important information:

Any of the mechanisms that syslog-ng PE uses to prevent or minimize the loss of log messages only works if the hardware and operating system work normally. When there is an issue with the hardware or operating system that the application and syslog-ng PE run on, log loss may occur. Issues include operating system crash (for example, kernel panic), memory errors, disk errors, power outage, and so on.

Flow control, no disk buffering, no ALTP

Reliability and minimizing the loss of log messages > Flow control, no disk buffering, no ALTP

How it works:

log-iw-size() sets a control window that tracks how many messages syslog-ng PE can accept. Every source has its own control window. If the window gets full, syslog-ng PE stops reading messages from the sources until some messages are successfully sent to the destination(s).

Figure 33: Flow control, no disk buffering, no ALTP

How to set key parameters:

Set flags(flow-control) in the log path.

The output buffer must be large enough to store the incoming messages of every connected source:

log-fifo-size() > sum of log-iw-size() of sources connected to this destination

Benefits:

This configuration minimizes the loss of log messages in the following situations:

Unreachable destination server(s): Only as many incoming log messages are read as can be "delivered". When flow control is used, those messages are considered delivered that have been written to the output buffer. When the output buffer is full, syslog-ng PE stops reading messages from the connected sources. This means that no log messages get lost.

NOTE:

In case the application is sending its log messages through a blocking I/O socket, then it is the application that stops sending new log messages and waits until the previous batch has been delivered. If the application is not sending logs through a blocking I/O socket, then it will keep sending messages (regardless of whether or not the previous batch has been delivered), and this can result in the loss of log messages. For example, it is not possible to apply flow control in the case of a UDP source.

Drawbacks:

While this configuration gives you the fastest processing time, it has some limitations. It does not provide protection against the loss of log messages in the following situations:

TCP error: In the case of a TCP connection, when messages are sent from the destination drivers to the destination servers, messages are written to the TCP socket. The TCP socket sends an acknowledgement to the destination drivers once it has successfully processed messages. A message is considered "delivered" when no error occurs during the process of writing the data to the socket, and the acknowledgement is received. Note, however, that if something goes wrong after messages have been successfully written to the TCP socket, log messages can still get lost. Also note that TCP errors can occur on both the source and the destination side, and both can cause the loss of log messages.
Message loss outside of syslog-ng PE: Because syslog-ng PE stores only a small number of log messages in the memory, it is possible to lose messages outside of syslog-ng. For example, if the output buffer is full because the server is not reachable, syslog-ng PE will not read the source, meaning that the external application that generates the logs can drop the logs. If you want to minimize the risk, use disk buffering. For details, see Flow control, normal disk buffering, no ALTP and Flow control, reliable disk buffering, no ALTP.
Message loss when syslog-ng PE is stopped or restarted: When syslog-ng is stopped or restarted, the contents of the output buffers are lost. If you want to minimize the risk, use disk buffering. For details, see Flow control, normal disk buffering, no ALTP and Flow control, reliable disk buffering, no ALTP.
When syslog-ng PE is not able to operate normally (for example, when syslog-ng PE crashes due to some unforeseen event): Log messages that were in the output buffer when the issue occurred get lost because those messages are stored in the memory.