3Com CoreBuilder 3500, Руководство по внедрению, Страница: 252 / 592

252 C
HAPTER
10: P
ACKET
F
ILTERING
4 Enter executable instruction #3:
ge
# Compare if the value of the socket is greater than
# or equal to the lower bound.
5 Enter executable instruction #4:
pushLiteral.w 0x0898
# Put the highest socket value on
# top of the stack.
6 Enter executable instruction #5:
pushField.w 30
# Put the value of the socket from the
# packet on top of the stack.
7 Enter executable instruction #6:
lt
# Compare if the value of the socket is less than the
# upper bound.
8 Enter executable instruction #7:
and
# “and” together with “ge” and “lt” test to determine
# if the socket value is “within” the range. If it is,
# place a non-zero value on the stack.
Combining a Subset of the Filters
The next filter places a non-zero value on the stack for IP packets with a
socket range of 0x76c (1900) and 0x898 (2200). The filter combines
packet filters one and two, modifying them for IP. These steps show how
to create this filter.
1 Name the filter:
name “Only IP pkts w/in socket range”
2 Perform steps 6 through 8 as described earlier in “Packet Filter One”
except give the pushLiteral instruction (in step 7) a value of 0x0800 for IP.
3 Perform steps 2 through 8 as described earlier in “Packet Filter Two”
except the socket value for IP (in steps 3 and 6) is located 24 bytes into
the packet (instead of 30 as for XNS).