7.4. 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(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. See Section 7.4.1, “Resolving hostnames locally ” for details.

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

7.4.1. 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:

Procedure 7.1. Resolving hostnames locally

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); };
```

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The syslog-ng 3.0 Administrator Guide

Document version: Tenth
Release date: January 8, 2010
Latest version available at the BalaBit Documentation Page