1) If RRAS based VPN server is behind a firewall (i.e. a firewall is placed between Internet and RRAS server), then following ports need to be opened (bidirectional) on this firewall to allow VPN traffic to pass through: –

• For PPTP:
  • IP Protocol=TCP, TCP Port number=1723   <- Used by PPTP control path
  • IP Protocol=GRE (value 47)   <- Used by PPTP data path
• For L2TP:
  • IP Protocol Type=UDP, UDP Port Number=500    <- Used by IKEv1 (IPSec control path)
  • IP Protocol Type=UDP, UDP Port Number=4500   <- Used by IKEv1 (IPSec control path)
  • IP Protocol Type=ESP (value 50)   <- Used by IPSec data path
• For SSTP:
  • IP Protocol=TCP, TCP Port number=443   <- Used by SSTP control and data path
• For IKEv2:
  • IP Protocol Type=UDP, UDP Port Number=500    <- Used by IKEv2 (IPSec control path)
  • IP Protocol Type=UDP, UDP Port Number=4500   <- Used by IKEv2 (IPSec control path)
  • IP Protocol Type=ESP (value 50)   <- Used by IPSec data path

2) If RRAS server is directly connected to Internet, then you need to protect RRAS server from the Internet side (i.e. only allow access to the services on the public interface that isaccessible from the Internet side). This can be done using RRAS static filters or running Windows Firewall on the public interface (or the interface towards the Internet side). In this scenario following ports need to be opened (bidirectional) on RRAS box to allow VPN traffic to pass through

• For PPTP:
  • IP Protocol=TCP, TCP Port number=1723  <- Used by PPTP control path
  • IP Protocol=GRE (value 47)  <- Used by PPTP data path
• For L2TP:
  • IP Protocol Type=UDP, UDP Port Number=500   <- Used by IKEv1 (IPSec control path)
  • IP Protocol Type=UDP, UDP Port Number=4500 <- Used by IKEv1 (IPSec control path)
  • IP Protocol Type=UDP, UDP Port Number=1701  <- Used by L2TP control/data path
  • IP Protocol Type=50  <- Used by data path (ESP)

• For SSTP:
• IP Protocol=TCP, TCP Port number=443   <- Used by SSTP control and data path

• For IKEv2:
• IP Protocol Type=UDP, UDP Port Number=500   <- Used by IKEv2 (IPSec control path)
• IP Protocol Type=UDP, UDP Port Number=4500 <- Used by IKEv2 (IPSec control path)
• IP Protocol Type=UDP, UDP Port Number=1701  <- Used by L2TP control/data path
• IP Protocol Type=50 <- Used by data path (ESP)

Note: Please DO NOT configure RRAS static filters if you are running on the same server RRAS based NAT router functionality. This is because RRAS static filters are stateless and NAT translation requires a stateful edge firewall like ISA firewall.

Do not forget: If you enable Windows firewall or RRAS static filters on the public interface and only enable VPN traffic to pass-through, then all the other traffic may be dropped. For example, if the same server is running as a mail server facing internet or a DNS server or a reverse web proxy server, then you need to enable the ports used by those services explicitly. For further details, refer to this article: http://blogs.technet.com/rrasblog/archive/2006/07/06/enabling-rras-drops-all-other-traffic-except-vpn-traffic.aspx

Ref: https://blogs.technet.microsoft.com/rrasblog/2006/06/14/which-ports-to-unblock-for-vpn-traffic-to-pass-through/

另外如果L2TP服务器是在防火墙背后的话，Mac可以连，但是Win连不了。这个时候需要改Win的注册表：

Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\services\PolicyAgent]
"AssumeUDPEncapsulationContextOnSendRule"=dword:00000002

Ref : http://support.microsoft.com/kb/926179