syslog-ng Open Source Edition 3.33 - Administration Guide

This chapter discusses some special examples and recommendations.

This section provides general tips and recommendations on using syslog-ng. Some of the recommendations are detailed in the sections below:

• Do not base the separation of log messages in different files on the facility parameter. As several applications and processes can use the same facility, the facility does not identify the application that sent the message. By default, the facility parameter is not even included in the log message itself. In general, sorting the log messages into several different files can make finding specific log messages difficult. If you must create separate log files, use the application name.

• Standard log messages include the local time of the sending host, without any time zone information. It is recommended to replace this timestamp with an ISODATE timestamp, because the ISODATE format includes the year and timezone as well. To convert all timestamps to the ISODATE format, include the following line in the syslog-ng configuration file:

  options {ts-format(iso) ; };

• Resolving the IP addresses of the clients to domain names can decrease the performance of syslog-ng. For details, see Using name resolution in syslog-ng.

This section provides tips on optimizing the performance of syslog-ng. Optimizing the performance is important for syslog-ng hosts that handle large traffic.

• Disable DNS resolution, or resolve hostnames locally. For details, see Using name resolution in syslog-ng.

• Enable flow-control for the TCP sources. For details, see Managing incoming and outgoing messages with flow-control.

• Do not use the usertty() destination driver. Under heavy load, the users are not be able to read the messages from the console, and it slows down syslog-ng.

• Do not use regular expressions in our filters. Evaluating general regular expressions puts a high load on the CPU. Use simple filter functions and logical operators instead. For details, see Regular expressions.

• Caution: When receiving messages using the UDP protocol, increase the size of the UDP receive buffer on the receiver host (that is, the syslog-ng OSE server or relay receiving the messages). Note that on certain platforms, for example, on Red Hat Enterprise Linux 5, even low message load (~200 messages per second) can result in message loss, unless the so-rcvbuf() option of the source is increased. In this cases, you will need to increase the net.core.rmem_max parameter of the host (for example, to 1024000), but do not modify net.core.rmem_default parameter. As a general rule, increase the so-rcvbuf() so that the buffer size in kilobytes is higher than the rate of incoming messages per second. For example, to receive 2000 messages per second, set the so-rcvbuf() at least to 2 097 152 bytes.

• Increase the value of the flush-lines() parameter. Increasing flush-lines() from 0 to 100 can increase the performance of syslog-ng OSE by 100%.

The syslog-ng application can resolve the hostnames of the clients and include them in the log messages. However, the performance of syslog-ng is severely degraded if the domain name server is unaccessible or slow. Therefore, it is not recommended to resolve hostnames in syslog-ng. If you must use name resolution from syslog-ng, consider the following:

• Use DNS caching. Verify that the DNS cache is large enough to store all important hostnames. (By default, the syslog-ng DNS cache stores 1007 entries.)

  options { dns-cache-size(2000); };

• If the IP addresses of the clients change only rarely, set the expiry of the DNS cache large.

  options { dns-cache-expire(87600); };

• If possible, resolve the hostnames locally. For details, see Resolving hostnames locally.