Virtual Fragmentation Reassembly
Virtual Fragmentation Reassembly
Last Updated: March 22, 2012
Currently, the Cisco IOS Firewall--specifically context-based access control (CBAC) and the intrusion detection system (IDS)--cannot identify the contents of the IP fragments nor can it gather port information from the fragment. These inabilities allow the fragments to pass through the network without being examined or without dynamic access control list (ACL) creation.
Virtual fragmentation reassembly (VFR) enables the Cisco IOS Firewall to create the appropriate dynamic ACLs, thereby, protecting the network from various fragmentation attacks.
Feature History for Virtual Fragmentation Reassembly
Finding Support Information for Platforms and Cisco IOS Software Images
Use Cisco Feature Navigator to find information about platform support and Cisco IOS software image support. Access Cisco Feature Navigator at http://www.cisco.com/go/fn . You must have an account on Cisco.com. If you do not have an account or have forgotten your username or password, click Cancel at the login dialog box and follow the instructions that appear.
Restrictions for Virtual Fragmentation Reassembly
VFR will cause a performance impact on the basis of functions such as packet copying, fragment validation, and fragment reorder. This performance impact will vary depending on the number of concurrent IP datagram that are being reassembled.
VFR Configuration Restriction
VFR should not be enabled on a router that is placed on an asymmetric path. The reassembly process requires all of the fragments within an IP datagram. Routers placed in the asymmetric path may not receive all of the fragments, so the fragment reassembly will fail.
Information About Virtual Fragmentation Reassembly
To use fragmentation support for Cisco IOS Firewall, you should understand the following concept:
Detected Fragment Attacks
VFR is responsible for detecting and preventing the following types of fragment attacks:
VFR drops all tiny fragments, and an alert message such as follows is logged to the syslog server: "VFR-3-TINY_FRAGMENTS."
VFR drops all fragments within a fragment chain if an overlap fragment is detected, and an alert message such as follows is logged to the syslog server: "VFR-3-OVERLAP_FRAGMENT."
To avoid buffer overflow and control memory usage, configure a maximum threshold for the number of IP datagrams that are being reassembled and the number of fragments per datagram. (Both of these parameters can be specified via the ip virtual-reassembly command.)
When the maximum number of datagrams that can be reassembled at any given time is reached, all subsequent fragments are dropped, and an alert message such as the following is logged to the syslog server: "VFR-4_FRAG_TABLE_OVERFLOW."
When the maximum number of fragments per datagram is reached, subsequent fragments will be dropped, and an alert message such as the following is logged to the syslog server: "VFR-4_TOO_MANY_FRAGMENTS."
In addition to configuring the maximum threshold values, each IP datagram is associated with a managed timer. If the IP datagram does not receive all of the fragments within the specified time, the timer will expire and the IP datagram (and all of its fragments) will be dropped.
Automatically Enabling or Disabling VFR
VFR is designed to work with any feature that requires fragment reassembly (such as Cisco IOS Firewall and NAT). Currently, NAT enables and disables VFR internally; that is, when NAT is enabled on an interface, VFR is automatically enabled on that interface.
If more than one feature attempts to automatically enable VFR on an interface, VFR will maintain a reference count to keep track of the number of features that have enabled VFR. When the reference count is reduced to zero, VFR is automatically disabled.
How to Use Virtual Fragmentation Reassembly
Use this task to enable VFR on an interface, specify maximum threshold values to combat buffer overflow and control memory usage, and verify any VFR configurations.
4. ip virtual-reassembly [max-reassemblies number] [max-fragments number] [timeout seconds] [drop-fragments]
7. show ip virtual-reassembly [interface type]
The following commands are introduced or modified in the feature or features documented in this module. For information about these commands, see the Cisco IOS Security Command Reference. For information about all Cisco IOS commands, go to the Command Lookup Tool at http://tools.cisco.com/Support/CLILookup or to the Cisco IOS Master Commands List.
fragment --Part of an IP datagram that is fragmented into multiple pieces. Each piece is called a fragment or an IP fragment.
fragmentation --Process of breaking down an IP datagram into smaller packets (fragments) that are transmitted over different types of network media.
initial fragment -- First fragment within a fragment set. This fragment should have a Layer 4 header and should have an offset of zero.
noninitial fragment --All fragments within a fragment set, except the initial fragment.
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