qr code c# tutorial Be very familiar with the general syntax of an extended ACL in Objective-C

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statement: you might have to con gure an extended ACL on the exam
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ACL Types
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TCP and UDP
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Use the following syntax to configure an extended ACL for TCP or UDP:
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Router(config)# access-list 100-199|2000-2699 permit|deny tcp|udp source_address source_wildcard_mask [operator source_port_#] destination_address destination_wildcard_mask [operator destination_port_#] [established] [log]
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After specifying the action (permit or deny), you configure the IP protocol: tcp or udp
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Operators With TCP and UDP, you can specify the source, destination, or both source and destination port numbers or names To specify how to perform the match, you must configure an operator The operator tells the router how to match on the port number(s) or names Table 22-5 lists the valid operators for TCP and UDP ACL entries Note that these operators apply only to TCP and UDP connections Other IP protocols do not use them If you omit the port number or name, the ACL looks for a match on all TCP or UDP connections Ports Numbers and Names For TCP and UDP connections, you can list either the name or the number of the port For example, if you wanted to match on telnet traffic, you could use either the keyword telnet or the number 23 Table 22-6 lists some of the most common port names and numbers for TCP connections
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TABLE 22-5
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lt gt neq eq range
Explanation
Less than Greater than Not equal to Equal to Range of port numbers
TCP and UDP Operators
22: Access Control Lists
TABLE 22-6
Port Name
FTP Data FTP Control Telnet SMTP WWW POP3
Command Parameter
ftp-data ftp telnet smtp www pop3
Port Number
20 21 23 25 80 110
Common TCP Port Names and Numbers
Table 22-7 shows some of the common UDP port names and numbers
One common problem that occurs when setting up an ACL is that the administrator speci es the wrong protocol for the application, such as TCP for TFTP, RIP, or DNS queries, instead of UDPThis is also true of port numbers or names
You need to be familiar with the TCP/IP protocols and their ports when setting up ltering policies Remember the TCP and UDP application names and numbers in Tables 22-6 and 22-7
established Keyword The established keyword is used only for TCP connections The assumption behind the use of this keyword is that you are originating TCP traffic on the inside of the network and filtering the returning traffic as it comes back into your network In this situation, this keyword allows (or denies) any TCP traffic that has a certain flag or flag bits set in the TCP segment header, indicating that this is returning traffic back into your network Refer to 2 for an explanation of connection-oriented transport protocols
TABLE 22-7
Port Name
DNS Query TFTP SNMP IP RIP
Command Parameter
dns tftp snmp rip
Port Number
53 69 161 520
Common UDP Port Names and Numbers
ACL Types
Understand the use of the established keyword with TCP ACL statements
and 9 for the mechanics of TCP My book with Cisco Press, Cisco Router Firewall Security 2007 , discusses this topic in much more depth, comparing and contrasting the use of the established keyword with how stateful firewalls, such as the Cisco ASA, work
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2203The CD contains a multimedia demonstration of configuring an extended numbered ACL to allow telnet traffic through a router
ICMP
The following command shows the syntax of filtering ICMP traffic:
Router(config)# access-list 100-199|2000-2699 permit|deny icmp source_address source_wildcard_mask destination_address destination_wildcard_mask [icmp_message] [log]
Unlike TCP and UDP, ICMP doesn t use ports Instead, ICMP uses message types And where TCP and UDP extended ACLs allow you to specify both source and destination ports, ICMP allows you to enter an ICMP message Table 22-8 shows some of the common ICMP messages and brief descriptions You can enter the ICMP message by either its name or its number If you omit the ICMP message type, all message types are included
TABLE 22-8
Message Type
administrativelyprohibited echo echo-reply host-unreachable net-unreachable traceroute
Message Description
Message that says that someone filtered a packet Used by ping to check a destination Is a response to an echo message created by ping The subnet is reachable, but the host is not responding The network/subnet is not reachable Filters on traceroute information when ICMP is used
Common ICMP Messages
22: Access Control Lists
If you execute the no access-list command, followed by the ACL number, the entire ACL and its referenced commands are deleted What most administrators don t realize, or forget, is that if you preface any numbered ACL statement with the no parameter, it has exactly the same effect: the entire ACL is deleted For example, executing the no access-list 100 permit tcp any any command causes the router basically to ignore everything after the 100 parameter, causing the router to execute the command as if it were no access-list 100! 2204The CD contains a multimedia demonstration of configuring an extended numbered ACL to permit ICMP traffic through a router
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