Where is the log file for pptpd on Ubuntu?

I was running a working pptp server months ago. After tried and failed l2tp server, I wanted to go back to pptp. But for some reason it won't work any more. I cannot even find the data log for the pptpd.

I am using Ubuntu 15.04. I setup iptables to accept tcp 1723+ gre ACCEPT tcp -- anywhere anywhere tcp dpt:ssh ACCEPT tcp -- anywhere anywhere tcp dpt:1723 also has sysctl net.ipv4.ip_forward=1 net.ipv4.conf.all.accept_redirects = 0 net.ipv4.conf.all.send_redirects = 0

When I tried to connect to the pptp server using my android phone, I got unsuccessful after a few seconds.

And I remember there was a data log could be checked even with failed connection, but I can not find it anymore. I tried /var/log/auth.log and ./messages

I think if I can find the datalog for pptpd, it might help me debug..

my pptpd.conf option /etc/ppp/pptpd-options logwtmp localip 192.168.0.111 remoteip 192.168.0.10-15

also, I am using wlan0 as single interface

After everytime I tried to connect to the server, I see the iptables -L -v shows the number of bytes of tcp 1723 increase by about 60

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