Administrator Guide

Example of Permitting All Packets on an Interface
DellEMC(conf)#ip access-list extended ABC
DellEMC(conf-ext-nacl)#permit ip any 10.1.1.1/32
DellEMC(conf-ext-nacl)#deny ip any 10.1.1.1/32 fragments
DellEMC(conf-ext-nacl)
To deny the second/subsequent fragments, use the same rules in a different order. These ACLs deny all second and subsequent
fragments with destination IP 10.1.1.1 but permit the first fragment and non-fragmented packets with destination IP 10.1.1.1.
Example of Denying Second and Subsequent Fragments
DellEMC(conf)#ip access-list extended ABC
DellEMC(conf-ext-nacl)#deny ip any 10.1.1.1/32 fragments
DellEMC(conf-ext-nacl)#permit ip any 10.1.1.1/32
DellEMC(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
DellEMC(conf)#ip access-list extended ABC
DellEMC(conf-ext-nacl)#permit tcp host 10.1.1.1 any eq 24
DellEMC(conf-ext-nacl)#deny ip any any fragment
DellEMC(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
DellEMC(conf)#ip access-list extended ABC
DellEMC(conf-ext-nacl)#permit tcp host 10.1.1.1 any eq 24
DellEMC(conf-ext-nacl)#permit tcp host 10.1.1.1 any fragment
DellEMC(conf-ext-nacl)#deny ip any any fragment
DellEMC(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
DellEMC(conf)#ip access-list extended ABC
DellEMC(conf-ext-nacl)#permit tcp any any fragment
DellEMC(conf-ext-nacl)#permit udp any any fragment
DellEMC(conf-ext-nacl)#deny ip any any log
DellEMC(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.
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Access Control Lists (ACLs)