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syslog-ng Premium Edition 6.0.18 - Administration Guide

Preface Chapter 1. Introduction to syslog-ng Chapter 2. The concepts of syslog-ng Chapter 3. Installing syslog-ng Chapter 4. The syslog-ng PE quick-start guide Chapter 5. The syslog-ng PE configuration file Chapter 6. Collecting log messages — sources and source drivers Chapter 7. Sending and storing log messages — destinations and destination drivers Chapter 8. Routing messages: log paths, reliability, and filters Chapter 9. Global options of syslog-ng PE Chapter 10. TLS-encrypted message transfer Chapter 11. FIPS-compliant syslog-ng Chapter 12.  Reliable Log Transfer Protocol™ Chapter 13. Reliability and minimizing the loss of log messages Chapter 14. Manipulating messages Chapter 15. Parsing and segmenting structured messages Chapter 16. Processing message content with a pattern database Chapter 17. Statistics and metrics of syslog-ng Chapter 18. Multithreading and scaling in syslog-ng PE Chapter 19. Troubleshooting syslog-ng Chapter 20. Best practices and examples

Using name resolution in syslog-ng

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 Procedure 20.1, “Resolving hostnames locally”.


Domain name resolution is important mainly in relay and server mode.

Procedure 20.1. Resolving hostnames locally


Resolving hostnames locally enables you to display hostnames in the log files for frequently used hosts, without having to rely on a DNS server. The known IP address – hostname pairs are stored locally in a file. In the log messages, syslog-ng will replace the IP addresses of known hosts with their hostnames. To configure local name resolution, complete the following steps:


  1. Add the hostnames and the respective IP addresses to the file used for local name resolution. On Linux and UNIX systems, this is the /etc/hosts file. Consult the documentation of your operating system for details.

  2. Instruct syslog-ng to resolve hostnames locally. Set the use-dns() option of syslog-ng to persist_only.

  3. Set the dns-cache-hosts() option to point to the file storing the hostnames.

    options {
            dns-cache-hosts(/etc/hosts); };

Configuring log rotation

The syslog-ng PE application does not rotate logs by itself. To use syslog-ng PE for log rotation, consider the following approaches:

Use logrotate together with syslog-ng PE: 

  • Ideal for workstations or when processing fewer logs.

  • It is included in most distributions by default.

  • Less scripting is required, only logrotate has to be configured correctly.

  • Requires frequent restart (syslog-ng PE must be reloaded/restarted when the files are rotated). After rotating the log files, reload syslog-ng PE using the syslog-ng-ctl reload command, or use another method to send a SIGHUP to syslog-ng PE.

  • The statistics collected by syslog-ng PE, and the correlation information gathered with Pattern Database is lost with each restart.

Separate incoming logs based on time, host or other information: 

  • Ideal for central log servers, where regular restart of syslog-ng PE is unfavorable.

  • Requires shell scripts or cron jobs to remove old logs.

  • It can be done by using macros in the destination name (in the filename, directory name, or the database table name).

Example 20.1. File destination for log rotation

This sample file destination configuration stores incoming logs in files that are named based on the current year, month and day, and places these files in directories that are named based on the hostname:

destination d_sorted { file("/var/log/remote/${HOST}/${YEAR}_${MONTH}_${DAY}.log" create-dirs(yes)); };

Example 20.2. Logstore destination for log rotation

This sample logstore destination configuration stores incoming logs in logstores that are named based on the current year, month and day, and places these logstores in directories that are named based on the hostname:

destination d_logstore { logstore("/var/log/remote/${HOST}/${YEAR}_${MONTH}_${DAY}.lgs" compress(9) create-dirs(yes)); };

Example 20.3. Command for cron for log rotation

This sample command for cron removes files older than two weeks from the /var/log/remote directory:

find /var/log/remote/ -daystart -mtime +14 -type f -exec rm {} \;

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