HP 6120XG HP ProCurve Series 6120 Blade Switches Access Security Guide - Page 340

Example of How an ACL Filters Packets, deny any

Page 340 highlights

IPv4 Access Control Lists (ACLs) ACL Operation 2. Deny only the inbound Telnet traffic sent from IP address 11.11.11.101. 3. Permit only inbound Telnet traffic sent from IP address 11.11.11.33. 4. Deny all other inbound traffic on port 12. The following ACL model, when assigned to inbound filtering on port 12, supports the above case: 1 2 3 4 5 1. Permits IP traffic inbound from source address 11.11.11.42. Packets matching this criterion are permitted and will not be compared to any later ACE in the list. Packets not matching this criterion will be compared to the next entry in the list. 4. Permits Telnet traffic from source address 11.11.11.33. Packets matching this criterion are permitted and are not compared to any later criteria in the list. Packets not matching this criterion are compared to the next entry in the list. 2. Denies Telnet traffic from source address 11.11.11.101. Packets matching this criterion are dropped and are not compared to later criteria in the list. Packets not matching this criterion are compared to the next entry in the list. 5. This entry does not appear in an actual ACL, but is implicit as the last entry in every ACL. Any inbound packets on port 12 that do not match any of the criteria in the ACL's preceding entries will be denied (dropped). 3. Permits any IP traffic from source address 11.11.11.101. Any packets matching this criterion will be permitted and will not be compared to any later criteria in the list. Because this entry comes after the entry blocking Telnet traffic from this same address, there will not be any Telnet packets to compare with this entry; they have already been dropped as a result of matching the preceding entry. Figure 9-4. Example of How an ACL Filters Packets It is important to remember that this ACL (and all ACLs) include an implicit deny any. That is, inbound IP packets (including switched packets having the switch as the destination IP address) that the ACL does not explicitly permit or deny will be implicitly denied, and therefore dropped. You can preempt the implicit deny by inserting a "permit IP any" at the end of an ACL, but this solution does not apply in the preceding example, where the intention is for the switch to allow only explicitly permitted packets inbound on port 12. 9-16

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IPv4 Access Control Lists (ACLs)
ACL Operation
2.
Deny
only
the inbound Telnet traffic sent from IP address 11.11.11.101.
3.
Permit
only
inbound Telnet traffic sent from IP address 11.11.11.33.
4.
Deny
all other
inbound traffic on port 12.
The following ACL model, when assigned to inbound filtering on port 12,
supports the above case:
4
1
2
3
5
1.
Permits
IP traffic inbound from source address 11.11.11.42.
4.
Permits
Telnet traffic from source address 11.11.11.33. Packets
Packets matching this criterion are permitted and will not be
matching this criterion are permitted and are not compared to
compared to any later ACE in the list. Packets not matching this
any later criteria in the list.
Packets not matching this criterion
criterion will be compared to the next entry in the list.
are compared to the next entry in the list.
2.
Denies
Telnet traffic from source address 11.11.11.101. Packets
5. This entry does not appear in an actual ACL, but is implicit as
matching this criterion are dropped and are not compared to
the last entry in every ACL. Any inbound packets on port 12 that
later criteria in the list. Packets not matching this criterion are
do not match any of the criteria in the ACL’s preceding entries
compared to the next entry in the list.
will be denied (dropped).
3.
Permits
any IP traffic from source address 11.11.11.101. Any
packets matching this criterion will be permitted and will not be
compared to any later criteria in the list. Because this entry
comes after the entry blocking Telnet traffic from this same
address, there will not be any Telnet packets to compare with
this entry; they have already been dropped as a result of
matching the preceding entry.
Figure 9-4. Example of How an ACL Filters Packets
It is important to remember that this ACL (and all ACLs) include an implicit
deny any
. That is, inbound IP packets (including switched packets having the
switch as the destination IP address) that the ACL does not
explicitly
permit
or deny will be
implicitly
denied, and therefore dropped. You can preempt
the implicit deny by inserting a “permit IP any” at the end of an ACL, but this
solution does not apply in the preceding example, where the intention is for
the switch to allow only explicitly permitted packets inbound on port 12.
9-16