Filter
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7-10 Reference Guide
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 carrying,
such as TCP or UDP
Port numbers
A filter can also match a packet’s port number attributes, but only if
the filter’s protocol type is set to TCP or UDP, since only those
protocols use port numbers. The filter can be configured to match
the following:
■ The source port number (the port on the sending host that
originated the packet)
■ The destination port number (the port on the receiving host
that the packet is destined for)
By matching on a port number, a filter can be applied to selected
TCP or UDP services, such as Telnet, FTP, and World Wide Web. The
tables below show a few common services and their associated
port numbers..
Internet service TCP port Internet service TCP port
FTP 20/21 Finger 79
Telnet 23 World Wide Web 80
SMTP (mail) 25 News 144
Gopher 70 rlogin 513