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Dell PowerEdge FN I/O Module Configuration Guide 9.10(0.0)

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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

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)

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