Reference Guide

120 | Access Control Lists (ACLs)
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IP Fragment Handling
Dell Networking OS supports a configurable option to explicitly deny IP fragmented packets, particularly
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 to be denied eventually, the first fragment would be denied and hence the
packet as a whole cannot be reassembled.
• Implementing the required rules will use 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 an explicit deny is configured, the second and subsequent fragments will 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 configuration permits all packets (both fragmented & non-fragmented) with destination IP
10.1.1.1. The second rule does not get hit at all (Figure 7-3).
Figure 7-3. Permit All Packets
To deny second/subsequent fragments, use the same rules in a different order. These ACLs deny all second
& subsequent fragments with destination IP 10.1.1.1 but permit the first fragment & non fragmented
packets with destination IP 10.1.1.1 (Figure 7-4).
Figure 7-4. Deny Second Packets
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)
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)