Cabletron Systems SmartSwitch 8-slot, Руководство пользователя

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Chapter 9: Security Configuration Guide
9 - 10 SSR User Reference Manual
For IPX ACLs, the following fields can be specified:
• Source network address
• Destination network address
• Source IPX socket
• Destination IPX socket
When defining an ACL rule, each field in the rule is position sensitive. For example,
for TCP, the source address must be followed by the destination address, followed by
the source socket and the destination socket and so on. For example, the following
describes the syntax of a TCP ACL:
acl name permit tcp source-addr dest-addr source-port dest-port tos
Not all the fields are required. If a field is not specified, it is treated as don't care.
However, since each field is position sensitive, it may be necessary to “skip” some
fields in order to specify a value for another field. To skip a field, the keyword any is
used. For example, the following rule denies SMTP traffic between any two hosts:
acl nosmtp deny tcp any any smtp smtp
Note that in the above example, the tos field (Type of Service) is not specified and is
treated as don't care. The keyword any is needed only to skip a don't care field in order
to explicitly specify another field that is further down in the rule. If there are no other
fields to specify, the keyword any is not really needed. For example, the following
ACL permits all IP traffic to go through:
acl yesip permit ip
The Ordering of ACL rules
For an ACL with multiple rules, the ordering of the rules is very important. When the
router looks at an ACL to determine whether a packet should be forwarded or not, it
goes through each rule in the ACL sequentially. When the router finds a rule that
matches the packet, all subsequent rules are ignored. That is, a first match algorithm is
used. The action defined by this ACL, to permit or deny, is used to forward or drop the
packet. There are no hidden or implied ordering of these rules. Nor is there precedence
attached to each field. The router simply goes down the list, one rule at a time until
there is a match. Consequently, rules that are more specific (i.e. with more details)
should always be listed ahead of rules that are less specific. For example, the following
ACL permits all TCP traffic except those from subnet 10.2.0.0/16:
acl 101 deny tcp 10.2.0.0/16 any any any
acl 101 permit tcp any any any any