Proxicast LAN-Cell 2 User's Guide download pdf (Page 206)

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Chapter 9 Firewall Screens

LAN-Cell 2 User’s Guide

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Asymmetrical Routes

If an alternate gateway on the LAN has an IP address in the same subnet as the LAN-Cell’s

LAN IP address, return traffic may not go through the LAN-Cell. This is called an

asymmetrical or “triangle” route. This causes the LAN-Cell to reset the connection, as the

connection has not been acknowledged.

You can have the LAN-Cell permit the use of asymmetrical route topology on the network

(not reset the connection).

Allowing asymmetrical routes may let traffic from the WAN go directly to the LAN without

passing through the LAN-Cell. A better solution is to use IP alias to put the LAN-Cell and the

backup gateway on separate subnets.

Asymmetrical Routes and IP Alias

You can use IP Alias instead of allowing asymmetrical routes. IP Alias allow you to partition

your network into logical sections over the same interface.

By putting your LAN and Gateway A in different subnets, all returning network traffic must

pass through the LAN-Cell 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 LAN-Cell reroutes the packet to Gateway A, which is in Subnet 2.

3 The reply from the WAN goes to the LAN-Cell.

4 The LAN-Cell then sends it to the computer on the LAN in Subnet 1.

Figure 121 Using IP Alias to Solve the Triangle Route Problem

DoS Firewall 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.

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