syslog-ng Premium Edition 7.0.33 - Administration Guide

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 PE 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 PE by 100%.

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 inaccessible 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.

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

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

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); };

Collecting logs from chroot

The following describes how to collect logs from a chroot using a syslog-ng client running on the host.

Figure 47: Collecting logs from chroot

To collect logs from a chroot using a syslog-ng client running on the host

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.

NOTE: You need to set up timezone information within your chroot as well. This usually means creating a symlink to /etc/localtime.

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syslog-ng Premium Edition 7.0.33 - Administration Guide

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