where are l2tp vpn logs in CentOS?

I am setting up an l2tp vpn using openswan(libreswan) and xl2tpd. Unfortunately, the vpn does not work. So I want to debug the problem. First, I should know where the l2tp vpn logs are. Searching the internet for where ipsec log is, I got the answer: /var/log/syslog and /var/log/audit.log. But I cannot find the log files in there. In fact, these are log files for ubuntu and my OS is CentOS 6. Another answer is /var/log/pluto.log. It sounds likely since the ipsec process is pluto. But I still cannot find the log file. Where on earth is the pluto log file? In fact, pluto writes the logs in /var/log/secure. And xl2tpd logs are in /var/log/messages.

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