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Security 13-131
Proto: The protocol to match. This can be entered as a number (see the table below) or as TCP or UDP if those
protocols are used.
Src. Port: The source port to match. This is the port on the sending host that originated the packet.
D. Port: The destination port to match. This is the port on the receiving host for which the packet is intended.
On?: Displays Yes when the filter is in effect or No when it is not.
Fwd: Shows whether the filter forwards (Yes) a packet or discards (No) it when there’s a match.
Filtering example #1
Returning to our filtering rule example from above (see page 13-128), look at how a rule is translated into a
filter. Start with the rule, then fill in the filter’s attributes:
1. The rule you want to implement as a filter is:
Block all Telnet attempts that originate from the remote host 199.211.211.17.
2. The host 199.211.211.17 is the source of the Telnet packets you want to block, while the destination
address is any IP address. How these IP addresses are masked determines what the final match will be,
although the mask is not displayed in the table that displays the filter sets (you set it when you create the
filter). In fact, since the mask for the destination IP address is 0.0.0.0, the address for Dest IP Addr could
have been anything. The mask for Source IP Addr must be 255.255.255.255 since an exact match is
desired.
■ Source IP Addr = 199.211.211.17
■ Source IP address mask = 255.255.255.255
■ Dest IP Addr = 0.0.0.0
■ Destination IP address mask = 0.0.0.0
Note: To learn about IP addresses and masks, see Appendix B, “Understanding IP Addressing.”
3. Using the tables on page 13-129, find the destination port and protocol numbers (the local Telnet port):
■ Proto = TCP (or 6)
■ D. Port = 23
Protocol Number to use Full name
N/A 0 Ignores protocol type
ICMP 1 Internet Control Message Protocol
TCP 6 Transmission Control Protocol
UDP 17 User Datagram Protocol