Security
7-13
Parts of a filter
A filter consists of criteria based on packet attributes. A typical filter
can match a packet on any one of the following attributes:
■
The source IP address (where the packet was sent from)
■
The destination IP address (where the packet is going)
■
The type of higher-layer Internet protocol the packet is carr ying,
such as TCP or UDP
Port numbers
A filter can also match a packet’s por t number attributes, but only if
the filter’s protocol type is set to TCP or UDP, since only those
protocols use por t numbers. The filter can be configured to match
the following:
■
The source por t number (the por t on the sending host that
originated the packet)
■
The destination por t number (the por t on the receiving host
that the packet is destined for)
By matching on a por t number, a filter can be applied to selected
TCP or UDP ser vices, such as Telnet, FTP, and World Wide Web. The
tables below show a few common ser vices and their associated
por t numbers.
Internet service
TCP port
Internet service
UDP port
FTP
20/21
Who Is
43
Telnet
23
World Wide Web
80
SMTP (mail)
25
SNMP
161
Gopher
70
TFTP
69
Finger
79
AppleTalk Routing
Maintenance (at-r tmp)
201
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