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 subsequent sections.
Do not base the separation of log messages into 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 the section called “Using name resolution in syslog-ng”.
Procedure 20.2. Collecting logs from chroot
Purpose:
To collect logs from a chroot using a syslog-ng client running on the host, complete the following steps:
Steps:
Create a /dev
directory within the chroot. The applications running in the chroot send their log messages here.
Create a local source in the configuration file of the syslog-ng application running outside the chroot. This source should point to the /dev/log
file within the chroot (for example to the /chroot/dev/log
directory).
Include the source in a log statement.
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 the section called “Using name resolution in syslog-ng”.
Enable flow-control for the TCP sources. For details, see the section called “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 the section called “Regular expressions”.
Increase the value of the flush-lines()
parameter. Increasing flush-lines()
from 0
to 100
can increase the performance of syslog-ng PE 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 Procedure 20.1, “Resolving hostnames locally”.
|
NOTE:
Domain name resolution is important mainly in relay and server mode. |
Procedure 20.1. Resolving hostnames locally
Purpose:
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:
Steps:
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.
Instruct syslog-ng to resolve hostnames locally. Set the use-dns()
option of syslog-ng to persist_only
.
Set the dns-cache-hosts()
option to point to the file storing the hostnames.
options { use-dns(persist_only); dns-cache-hosts(/etc/hosts); };
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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