Dell PowerSwitch S4820T Configuration Guide for the S4820T System 9.14.1.0 - Page 117

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. Example of Permitting All Packets from a Specified Host 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. 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) Example of Permitting Only First Fragments and Non-Fragmented Packets from a Specified Host 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. 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) Example of Logging Denied Packets 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. 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. • FO > 0 means it is dealing with the fragments of the original packet. 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 EMC Networking OS Command Line Interface Reference Guide. To set up extended ACLs, refer to Configure an Extended IP ACL. Access Control Lists (ACLs) 117