I have a problem with pptpd. My log :

Feb 7 20:34:09 Debian pptpd[20888]: MGR: Launching /usr/sbin/pptpctrl to handle client
Feb 7 20:34:09 Debian pptpd[20888]: CTRL: local address = 192.168.1.100
Feb 7 20:34:09 Debian pptpd[20888]: CTRL: remote address = 192.168.1.101
Feb 7 20:34:09 Debian pptpd[20888]: CTRL: pppd options file = /etc/ppp/pptpd-options
Feb 7 20:34:09 Debian pptpd[20888]: CTRL: Client 81.105.140.150 control connection started
Feb 7 20:34:09 Debian pptpd[20888]: CTRL: Received PPTP Control Message (type: 1)
Feb 7 20:34:09 Debian pptpd[20888]: CTRL: Made a START CTRL CONN RPLY packet
Feb 7 20:34:09 Debian pptpd[20888]: CTRL: I wrote 156 bytes to the client.
Feb 7 20:34:09 Debian pptpd[20888]: CTRL: Sent packet to client
Feb 7 20:34:09 Debian pptpd[20888]: CTRL: Received PPTP Control Message (type: 7)
Feb 7 20:34:09 Debian pptpd[20888]: CTRL: Set parameters to 100000000 maxbps, 64 window size
Feb 7 20:34:09 Debian pptpd[20888]: CTRL: Made a OUT CALL RPLY packet
Feb 7 20:34:09 Debian pptpd[20888]: CTRL: Starting call (launching pppd, opening GRE)
Feb 7 20:34:09 Debian pptpd[20888]: CTRL: pty_fd = 6
Feb 7 20:34:09 Debian pptpd[20888]: CTRL: tty_fd = 7
Feb 7 20:34:09 Debian pptpd[20889]: CTRL (PPPD Launcher): program binary = /usr/sbin/pppd
Feb 7 20:34:09 Debian pptpd[20889]: CTRL (PPPD Launcher): local address = 192.168.4.100
Feb 7 20:34:09 Debian pptpd[20889]: CTRL (PPPD Launcher): remote address = 192.168.4.101
Feb 7 20:34:09 Debian pptpd[20888]: CTRL: I wrote 32 bytes to the client.
Feb 7 20:34:09 Debian pptpd[20888]: CTRL: Sent packet to client
Feb 7 20:34:09 Debian pptpd[20888]: CTRL: Received PPTP Control Message (type: 15)
Feb 7 20:34:09 Debian pptpd[20888]: CTRL: Got a SET LINK INFO packet with standard ACCMs
Feb 7 20:34:39 Debian pptpd[20888]: GRE: read(fd=6,buffer=80507e0,len=8196) from PTY failed: status = -1 error = Input/output error, usually caused by unexpected termination of pppd, check option syntax and pppd logs
Feb 7 20:34:39 Debian pptpd[20888]: CTRL: PTY read or GRE write failed (pty,gre)=(6,7)
Feb 7 20:34:39 Debian pptpd[20888]: CTRL: Reaping child PPP[20889]
Feb 7 20:34:39 Debian pptpd[20888]: CTRL: Client 81.105.140.150 control connection finished
Feb 7 20:34:39 Debian pptpd[20888]: CTRL: Exiting now
Feb 7 20:34:39 Debian pptpd[20784]: MGR: Reaped child 20888

It works on the LAN, doesn't work from Internet. My iptables rules :

iptables -A INPUT -i lo -j ACCEPT
iptables -A OUTPUT -o lo -j ACCEPT

iptables -A INPUT -p tcp --dport 1723 -i eth1 -j ACCEPT
iptables -A OUTPUT -p tcp --sport 1723 -o eth1 -j ACCEPT

iptables -A INPUT -p 47 -j ACCEPT

iptables -A INPUT -p GRE -j ACCEPT
iptables -A OUTPUT -p GRE -j ACCEPT
iptables -A FORWARD -p GRE -j ACCEPT

iptables -F FORWARD
iptables -A FORWARD -j ACCEPT

iptables -A POSTROUTING -t nat -o eth1 -j MASQUERADE

echo 1 > /proc/sys/net/ipv4/ip_forward

Please help !