Dell PowerSwitch S4128F-ON OS10 Enterprise Edition User Guide Release 10.4.1.0 - Page 595

IP fragments ACL When a packet exceeds the maximum packet size, the packet is fragmented into a number of smaller packets that contain portions of the contents of the original packet. This packet flow begins with an initial packet that contains all of the Layer 3 (L3) and Layer 4 (L4) header information contained in the original packet, and is followed by a number of packets that contain only the L3 header information. This packet flow contains all of the information from the original packet distributed through packets that are small enough to avoid the maximum packet size limit. This provides a particular problem for ACL processing. If the ACL filters based on L4 information, the non-initial packets within the fragmented packet flow will not match the L4 information, even if the original packet would have matched the filter. Because of this filtering, packets are not processed by the ACL. The examples show denying second and subsequent fragments, and permitting all packets on an interface. 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. The second example shows ACLs which permits all packets - both fragmented and non-fragmented - with destination IP 10.1.1.1. Deny second and subsequent fragments OS10(config)# ip access-list ABC OS10(conf-ipv4-acl)# deny ip any 10.1.1.1/32 fragments OS10(conf-ipv4-acl)# permit ip any 10.1.1.1/32 Permit all packets on interface OS10(config)# ip access-list ABC OS10(conf-ipv4-acl)# permit ip any 10.1.1.1/32 OS10(conf-ipv4-acl)# deny ip any 10.1.1.1/32 fragments L3 ACL rules Use ACL commands for L3 packet filtering. TCP packets from host 10.1.1.1 with the TCP destination port equal to 24 are permitted, and all others are denied. TCP packets that are first fragments or non-fragmented from host 10.1.1.1 with the TCP destination port equal to 24 are permitted, and all TCP non-first fragments from host 10.1.1.1 are permitted. All other IP packets that are non-first fragments are denied. Permit ACL with L3 information only If a packet's L3 information matches the information in the ACL, the packet's fragment offset (FO) is checked: • If a packet's FO > 0, the packet is permitted • If a packet's FO = 0, the next ACL entry processes Deny ACL with L3 information only If a packet's L3 information does not match the L3 information in the ACL, 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 processes Access Control Lists 595