packet filtering in The Network Encyclopedia
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`packet filtering
`
`Definition of packet filtering in The Network Encyclopedia.
`
`packet filtering
`
`The process of controlling the flow of packets based on packet attributes such as source
`address, destination address, type, length, and port number.
`How It Works
`
`Many routers and proxy sewers use some form of packet filtering that provides firewall
`capabil tim for protecting the network from unauthorized traffic. Administrators can create
`rules for filtering out unwanted packets and can arrange these rules in the mofi efficient order.
`A packet that passes all the rules is allowed through, while a packet that violates any rule is
`dropped.
`
`Packet filtering can be implemented on routers and firewall devices in two ways: static filtering
`and dynamic filtering.
`
`Stat c packet filtering prov d$ limited security by configuring selected ports as either
`permanently open or permanently closed. For example, to deny outside pad<ets access to a
`company intranet server on port 80 (the standard port number for the Hypertext Tramfer
`Protocol, or HTTP) you could configure the router or firewall to block all incoming packets
`directed toward port 80.
`
`Dynamic packet filtering provides enhanced securty by allowing selected ports to be opened at
`the start of a legitimate session and then closed at the end of the sess on to secure the port
`against attempts at unauthorized access. This is part culariy useful for protocols that allocate
`ports dynamically—for example, with the File Transfer Protocol (FTP). If you want to grant
`outside users secure access to an FTP server behind the firewall (within the corporate
`network), you need to cons der the following:
`
`0 Port 21 (the FTP control port) needs to be left pemianenfly open so that the FTP server
`ran ‘listen" for connection attempts from outside clients. A static filtering rule can
`accomplish this.
`0 Port 20 (the FTP data port) needs to be opened only when data will be uploaded to or
`downloaded from the FTP server. with statc filtering this port would have to be
`configured as permanently open, wh ch could provide a door for hacking attempts.
`Dynamic filtering allows this port to be opened at the start of an FI'P smsion and then
`closed at the end of the sesson.
`o In order to stablish an FI'P connection with the client, the FI'P server randomly assigns
`two port numbers in the range 1024 through 65,535 to the client, one for the control
`connection and one to transfer data. Because these ports are assigned randomly, there
`is no way to predict which ports above 1024 must be able to be opened by the firewall.
`w th static filtering, you would therefore have to leave all ports above 1024 permanently
`open if you wanted to allow FTP access through the firewall, which would be a real
`security risk. W th dynam c filtering, however, you (an configure rules on the firewall
`that will read the packets issued by the server, dynam cally open the two randomly
`assigned ports to allow a session to be opened, monitor the flow of packets to ensure
`that no attempt is made to hijack the session by an unauthorized user, and close the
`randomly assigned ports when the FTP session ends.
`
`Static filtering
`
`Firewall [\I l N
`
`Randme assigned
`
`/% port numbers
`
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`http://www.thenetworkencyclopedia.com/entry/packet-filtering/
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`4/25/201 7
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`PACKET INTELLIGENCE LLC 2003 - 00001
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`packet filtering in The Network Encyclopedia
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`1024—65535
`Fons 20. 21.
`and 1024—65535
`permanently open
`
`mm“ "Rank"
`
`port numbers
` ' External
`
`Randomly assigned
`
`' Chem
`
`Part 21 pennanently open
`Packets from FTP server cause port
`20. and two randomly assigned port
`numbers. to be temporarily opened.
`Graphic P-2. Packet filtering.
`TIP
`Microsoft Proxy Server includes a number of predefined filters that you can use to configure
`
`aceptions for common protocols. You can use these to quickly configure Proxy Sewer for
`securing your network from the Internet.
`
`Packet filtering on a typical router can cause a perfomiance hit of about 30 percent on the
`router’s ability to handle network traff c. This suggests that instead of using a packet-filtering
`router for a firewall, you should consider installing proper firewall soltware such as Microsofl:
`Proxy Server on a dedicated server. Proxy Server includes dynamic packet filtering among its
`security features. If packet filtering is enabled, all incoming and outgoing packets are rejected
`unless an except on is expl c tly created that allows them to pass. Packet filters can be enabled
`on Proxy Server only if the machine has an extemal network interface, such as one connected
`to a distrusted network (the Internet, for example).
`NOTE
`
`Some routers and firewalls can actually ping the source address of each packet to ensure that
`addresses local to the company network are coming from inside the network and are not being
`spoofed by a hacker outside the network.
`
`Proxy Server also supports domain filters for allowing or denying access to World Wide Web
`(W) or FI'P services based on the source IP addrms or Domain Name System (DNS)
`domain name. Proxy Server can issue alerts to inform you when packets are rejected or illegal
`packets are detected. It will also keep a log of alerts that ocwr for analysis and record
`keeping.
`
`
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`PACKET INTELLIGENCE LLC 2003 - 00002
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