Reference Guide

Example of Denying Second and Subsequent Fragments
Dell(conf)#ip access-list extended ABC
Dell(conf-ext-nacl)#deny ip any 10.1.1.1/32 fragments
Dell(conf-ext-nacl)#permit ip any 10.1.1.1/32
Dell(conf-ext-nacl)
Layer 4 ACL Rules Examples
The following examples show the ACL commands for Layer 4 packet filtering.
Permit an ACL line with L3 information only, and the fragments keyword is present:
If a packet’s L3 information matches the L3 information in the ACL line, the packet's FO is checked.
● If a packet's FO > 0, the packet is permitted.
● If a packet's FO = 0, the next ACL entry is processed.
Deny ACL line with L3 information only, and the fragments keyword is present:
If a packet's L3 information does match the L3 information in the ACL line, the packet's FO is checked.
● If a packet's FO > 0, the packet is denied.
● If a packet's FO = 0, the next ACL line is processed.
In this first example, TCP packets from host 10.1.1.1 with TCP destination port equal to 24 are permitted. All others are denied.
Example of Permitting All Packets from a Specified Host
Dell(conf)#ip access-list extended ABC
Dell(conf-ext-nacl)#permit tcp host 10.1.1.1 any eq 24
Dell(conf-ext-nacl)#deny ip any any fragment
Dell(conf-ext-nacl)
In the following example, the TCP packets that are first fragments or non-fragmented from host 10.1.1.1 with TCP destination
port equal to 24 are permitted. Additionally, all TCP non-first fragments from host 10.1.1.1 are permitted. All other IP packets
that are non-first fragments are denied.
Example of Permitting Only First Fragments and Non-Fragmented Packets from a Specified Host
Dell(conf)#ip access-list extended ABC
Dell(conf-ext-nacl)#permit tcp host 10.1.1.1 any eq 24
Dell(conf-ext-nacl)#permit tcp host 10.1.1.1 any fragment
Dell(conf-ext-nacl)#deny ip any any fragment
Dell(conf-ext-nacl)
To log all the packets denied and to override the implicit deny rule and the implicit permit rule for TCP/ UDP fragments, use a
configuration similar to the following.
Example of Logging Denied Packets
Dell(conf)#ip access-list extended ABC
Dell(conf-ext-nacl)#permit tcp any any fragment
Dell(conf-ext-nacl)#permit udp any any fragment
Dell(conf-ext-nacl)#deny ip any any log
Dell(conf-ext-nacl)
When configuring ACLs with the fragments keyword, be aware of the following.
When an ACL filters packets, it looks at the fragment offset (FO) to determine whether it is a fragment.
● FO = 0 means it is either the first fragment or the packet is a non-fragment.
● FO > 0 means it is dealing with the fragments of the original packet.
Access Control Lists (ACLs)
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