Administrator Guide

Layer 4 ACL Rules Examples
The following examples show the ACL commands for Layer 4 packet filtering.
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.
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.
Example of Layer 4 ACL Rules
Example of TCP Packets
In this first example, fragments or non-fragmented TCP packets from 10.1.1.1 with TCP destination port equal
to 24 are permitted. All other fragments are denied.
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, 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.
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)
Configure a Standard IP ACL
To configure an ACL, use commands in IP ACCESS LIST mode and INTERFACE mode.
For a complete list of all the commands related to IP ACLs, refer to the Dell Networking OS Command Line
Interface Reference Guide. To set up extended ACLs, refer to Configure an Extended IP ACL.
Access Control Lists (ACLs) 131