ZyXEL 2WG Guía Del Usuario
Chapter 12 Firewall Screens
ZyWALL 2WG User’s Guide
265
By putting your LAN and Gateway A in different subnets, all returning network traffic must
pass through the ZyWALL to your LAN. The following steps describe such a scenario.
pass through the ZyWALL to your LAN. The following steps describe such a scenario.
1 A computer on the LAN initiates a connection by sending a SYN packet to a receiving
server on the WAN.
2 The ZyWALL reroutes the packet to Gateway A, which is in Subnet 2.
3 The reply from the WAN goes to the ZyWALL.
4 The ZyWALL then sends it to the computer on the LAN in Subnet 1.
3 The reply from the WAN goes to the ZyWALL.
4 The ZyWALL then sends it to the computer on the LAN in Subnet 1.
Figure 168 Using IP Alias to Solve the Triangle Route Problem
DoS Thresholds
For TCP, half-open means that the session has not reached the established state-the TCP three-
way handshake has not yet been completed. Under normal circumstances, the application that
initiates a session sends a SYN (synchronize) packet to the receiving server. The receiver
sends back an ACK (acknowledgment) packet and its own SYN, and then the initiator
responds with an ACK (acknowledgment). After this handshake, a connection is established.
way handshake has not yet been completed. Under normal circumstances, the application that
initiates a session sends a SYN (synchronize) packet to the receiving server. The receiver
sends back an ACK (acknowledgment) packet and its own SYN, and then the initiator
responds with an ACK (acknowledgment). After this handshake, a connection is established.
Figure 169 Three-Way Handshake
For UDP, half-open means that the firewall has detected no return traffic. An unusually high
number (or arrival rate) of half-open sessions could indicate a DOS attack.
number (or arrival rate) of half-open sessions could indicate a DOS attack.