With the parameter $PreserveFQDN on the logged-in host is not the full host(FQDN)

[Roarca]

Description

When on the client side you activate the $PreserveFQDN property on /etc/rsyslog.conf , the logs do not include the FQDN and only include the host

Configuration

My rsyslog.conf configuration (everything is default except the parameter)

$PreserveFQDN on

The /etc/hosts file

127.0.0.1 host.app.example host

And the log4j2 configuration

<?xml version="1.0" encoding="UTF-8"?>
<Configuration>
	<Appenders>
		<Console name="LogToConsole" target="SYSTEM_OUT">
			<PatternLayout pattern="%d{HH:mm:ss.SSS} [%t] %-5level %logger{36} - %msg%n" />
		</Console>
		<Syslog name="RFC5424" format="RFC5424" host="localhost" port="514" protocol="TCP" appName="app" includeMDC="true" facility="LOCAL0" enterpriseNumber="18060" newLine="true" messageId="app-ID" mdcId="mdc" id="app-MDCID" connectTimeoutMillis="1000" immediateFail="true" reconnectionDelayMillis="5000">
			<LoggerFields>
				<KeyValuePair key="thread" value="%t" />
				<KeyValuePair key="priority" value="%p" />
				<KeyValuePair key="category" value="%c" />
				<KeyValuePair key="exception" value="%ex" />
				<KeyValuePair key="message" value="%m" />
			</LoggerFields>
		</Syslog>
	</Appenders>
	<category name="org.springframework.beans">
		<priority value="info" />
	</category>
	<Loggers>
		<Logger name="foo.example.ErrorService" additivity="false">
			<AppenderRef ref="RFC5424" />
			<AppenderRef ref="LogToConsole" />
		</Logger>
	</Loggers>
</Configuration>

Version: [Log4j version]

2.17.0

Operating system: [OS and version]

SUSE Linux Enterprise Server 12 SP3

JDK: [JDK distribution and version]

OpenJDK 64-Bit Server VM (build 25.292-b10, mixed mode)

Logs

2023-08-24 14:01:49.569 host app[4539] [status] Starting...

Reproduction

Activating the $PreserveFQDN on parameter and having in /etc/hosts an FQDN longer than the normal host should be enough, but ask any questions you may have

[ppkarwasz]

@Roarca,
Thanks for the report. The bug is easily reproducible if the system call gethostname() returns an unqualified host name.

We use InetAddress#getLocalHost() to retrieve the local hostname, which calls gethostname(), performs a lookup for the fully qualified hostname and ignores the result (cf. source code).

This is why you see just host as hostname.

As a workaround, you can change the hostname of the machine:

hostname host.app.example

It is not the UNIX way, but most services should work without any problems. You can also use uts_namespaces to isolate the hostname of your app from the rest of the system.