13.7. Fragmentation Settings
IP is able to transport up to 65536 bytes of data. However, most media, such as Ethernet, cannot
carry such huge packets. To compensate, the IP stack fragments the data to be sent into separate
packets, each one given their own IP header and information that will help the recipient reassemble
the original packet correctly.
Many IP stacks, however, are unable to handle incorrectly fragmented packets, a fact that can be
exploited by intruders to crash such systems. NetDefendOS provides protection against
fragmentation attacks in a number of ways.
Pseudo Reass Max Concurrent
Maximum number of concurrent fragment reassemblies. To drop all fragmented packets, set
PseudoReass_MaxConcurrent to 0.
Default: 1024
Illegal Fragments
Determines how NetDefendOS will handle incorrectly constructed fragments. The term "incorrectly
constructed" refers to overlapping fragments, duplicate fragments with different data, incorrect
fragment sizes, etc. Possible settings include:
•
Drop – Discards the illegal fragment without logging it. Also remembers that the packet that is
being reassembled is "suspect", which can be used for logging further down the track.
•
DropLog – Discards and logs the illegal fragment. Also remembers that the packet that is being
reassembled is "suspect", which can be used for logging further down the track.
•
DropPacket – Discards the illegal fragment and all previously stored fragments. Will not allow
further fragments of this packet to pass through during ReassIllegalLinger seconds.
•
DropLogPacket – As DropPacket, but also logs the event.
•
DropLogAll – As DropLogPacket, but also logs further fragments belonging to this packet that
arrive during ReassIllegalLinger seconds.
The choice of whether to discard individual fragments or disallow the entire packet is governed by
two factors:
•
It is safer to discard the whole packet.
•
If, as the result of receiving an illegal fragment, it is chosen to discard the whole packet,
attackers will be able to disrupt communication by sending illegal fragments during a
reassembly, and in this way block almost all communication.
Default: DropLog – discards individual fragments and remembers that the reassembly attempt is
"suspect".
Duplicated Fragment Data
If the same fragment arrives more than once, this can mean either that it has been duplicated at some
point on its journey to the recipient or that an attacker is trying to disrupt the reassembly of the
packet. In order to determine which is more likely, NetDefendOS compares the data components of
the fragment. The comparison can be made in 2 to 512 random locations in the fragment, four bytes
of each location being sampled. If the comparison is made in a larger number of samples, it is more
likely to find mismatching duplicates. However, more comparisons result in higher CPU load.
13.7. Fragmentation Settings
Chapter 13. Advanced Settings
527
Summary of Contents for NetDefend DFL-260E
Page 27: ...1 3 NetDefendOS State Engine Packet Flow Chapter 1 NetDefendOS Overview 27...
Page 79: ...2 7 3 Restore to Factory Defaults Chapter 2 Management and Maintenance 79...
Page 146: ...3 9 DNS Chapter 3 Fundamentals 146...
Page 227: ...4 7 5 Advanced Settings for Transparent Mode Chapter 4 Routing 227...
Page 241: ...5 4 IP Pools Chapter 5 DHCP Services 241...
Page 339: ...6 7 Blacklisting Hosts and Networks Chapter 6 Security Mechanisms 339...
Page 360: ...7 4 7 SAT and FwdFast Rules Chapter 7 Address Translation 360...
Page 382: ...8 3 Customizing HTML Pages Chapter 8 User Authentication 382...
Page 386: ...The TLS ALG 9 1 5 The TLS Alternative for VPN Chapter 9 VPN 386...
Page 439: ...Figure 9 3 PPTP Client Usage 9 5 4 PPTP L2TP Clients Chapter 9 VPN 439...
Page 450: ...9 7 6 Specific Symptoms Chapter 9 VPN 450...
Page 488: ...10 4 6 Setting Up SLB_SAT Rules Chapter 10 Traffic Management 488...
Page 503: ...11 6 HA Advanced Settings Chapter 11 High Availability 503...
Page 510: ...12 3 5 Limitations Chapter 12 ZoneDefense 510...
Page 533: ...13 9 Miscellaneous Settings Chapter 13 Advanced Settings 533...
