White Papers

IP Fragment Handling
The Dell Networking OS supports a configurable option to explicitly deny IP fragmented packets, specifically second and
subsequent packets.
It extends the existing ACL command syntax with the fragments keyword for all Layer 3 rules applicable to all Layer protocols
(permit/deny ip/tcp/udp/icmp).
● Both standard and extended ACLs support IP fragments.
● Second and subsequent fragments are allowed because a Layer 4 rule cannot be applied to these fragments. If the packet is
denied eventually, the first fragment is denied and the packet as a whole cannot be reassembled.
● Implementing the required rules uses a significant number of CAM entries per TCP/UDP entry.
● For IP ACL, Dell Networking OS always applies implicit deny. You do not have to configure it.
● For IP ACL, Dell Networking OS applies implicit permit for second and subsequent fragment just prior to the implicit deny.
● If you configure an explicit deny, the second and subsequent fragments do not hit the implicit permit rule for fragments.
● Loopback interfaces do not support ACLs using the IP fragment option. If you configure an ACL with the fragments
option and apply it to a Loopback interface, the command is accepted but the ACL entries are not actually installed the
offending rule in CAM.
IP Fragments ACL Examples
The following examples show how you can use ACL commands with the fragment keyword to filter fragmented packets.
The following configuration permits all packets (both fragmented and non-fragmented) with destination IP 10.1.1.1. The second
rule does not get hit at all.
Example of Permitting All Packets on an Interface
Dell(conf)#ip access-list extended ABC
Dell(conf-ext-nacl)#permit ip any 10.1.1.1/32
Dell(conf-ext-nacl)#deny ip any 10.1.1.1./32 fragments
Dell(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
Dell(conf)#ip access-list extended ABC
Dell(conf-ext-nacl)#deny ip any 10.1.1.1/32 fragments
Dell(conf-ext-nacl)#permit ip any 10.1.1.1/32
Dell(conf-ext-nacl)
Layer 4 ACL Rules Examples
The following examples show the ACL commands for Layer 4 packet filtering.
NOTE: The following rules apply when configuring ACLs with the fragments keyword.
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.
Access Control Lists (ACLs)
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