3Com 4007, Руководство по внедрению, Страница: 342 / 684

342 C
HAPTER
15: P
ACKET
F
ILTERING
Packet Filter Solution The solution described here is to create a highly sophisticated packet filter
that prevents only the broadcast packets from the market data servers
from being forwarded onto the segments that are not part of an active
trading floor.
Before you write the packet filter, it is important to understand the
functions that the filter must provide. The broadcast packets that are
transmitted by the servers are based on either TCP/IP or XNS protocol. In
both cases, the broadcast packets have socket values that are greater
than 0x076c and less than 0x0898. The socket value is located 24 bytes
into the packet in IP datagrams and 30 bytes into the packet in XNS
datagrams.
You can use this information to create pseudocode that simplifies the
process of writing the actual filter. It helps to first write the pseudocode in
outline form, as shown here:
1 Determine if the packet has a broadcast address.
2 Determine if the packet is an XNS datagram.
3 Examine socket values and discard the packet if:
■
The socket value is greater than or equal to 0x76c
AND
■
The socket value is less than 0x898
4 Determine if the packet is an IP datagram.
5 If so, then examine socket values and discard the packet if:
■
The socket value is greater than or equal to 0x76c
AND
■
The socket value is less than 0x898
6 End the filter.
The pseudocode translates into the following complex packet filter: