Writing log data to syslog using log4j

Add the following lines to rsyslog.conf file

$ModLoad imudp
$UDPServerRun 514

It worked for me.

Need to restart the rsyslog after modfications.

The answer from @Sandeep above is the correct one, but it's from 2012 so I wanted to expand a little bit for folks who are using more recent setups. For instance, on Ubuntu 18.04 the /etc/rsyslog.conf file now has data near the top of the file that looks like this:

#################
#### MODULES ####
#################

module(load="imuxsock") # provides support for local system logging
#module(load="immark")  # provides --MARK-- message capability

# provides UDP syslog reception
#module(load="imudp")
#input(type="imudp" port="514")

# provides TCP syslog reception
#module(load="imtcp")
#input(type="imtcp" port="514")

Uncommenting the two UDP lines and then running sudo service rsyslog restart worked for me. The Java Log4J Syslog appender (https://logging.apache.org/log4j/1.2/apidocs/org/apache/log4j/net/SyslogAppender.html) expects syslog to be listening on UDP port 514 on localhost.

As a potential further security improvement, you may also consider binding to the loopback address so port 514 isn't visible external to the host if you don't need it to be:

input(type="imudp" port="514" address="127.0.0.1")

It's also possible to make this update without having to touch the existing /etc/rsyslog.conf file; instead you can add a new conf file under the /etc/rsyslog.d/ directory, e.g. /etc/rsyslog.d/10-open-upd-port.conf, that only contains these lines:

module(load="imudp")
input(type="imudp" port="514" address="127.0.0.1")

And then restart the rsyslog daemon as described above.

To see whether or not the rsyslog daemon is actively listening on the UDP port 514, I found this command useful as well: sudo lsof -iUDP:514 -nP -c rsyslogd -a (show listeners on port UDP 514 whose command is "rsyslogd").