Firewall Security Technical Implementation Guide - Cisco

Using dedicated paths, the OOBM backbone connects the OOBM gateway routers located at the premise of the managed networks and at the NOC. Dedicated links can be deployed using provisioned circuits (ATM, Frame Relay, SONET, T-carrier, and others or VPN technologies such as subscribing to MPLS Layer 2 and Layer 3 VPN services) or implementing a secured path with gateway-to-gateway IPsec tunnel. The tunnel mode ensures that the management traffic will be logically separated from any other traffic traversing the same path.Information Assurance OfficerEBVC-1, ECSC-1

All network devices must present a DoD approved warning banner prior to a system administrator logging on. The banner should warn any unauthorized user not to proceed. It also should provide clear and unequivocal notice to both authorized and unauthorized personnel that access to the device is subject to monitoring to detect unauthorized usage. Failure to display the required login warning banner prior to logon attempts will limit DoD’s ability to prosecute unauthorized access and also presents the potential to give rise to criminal and civil liability for systems administrators and information systems managers. In addition, DISA’s ability to monitor the device’s usage is limited unless a proper warning banner is displayed. DoD CIO has issued new, mandatory policy standardizing the wording of “notice and consent” banners and matching user agreements for all Secret and below DoD information systems, including stand-alone systems by releasing DoD CIO Memo, “Policy on Use of Department of Defense (DoD) Information Systems Standard Consent Banner and User Agreement”, dated 9 May 2008. The banner is mandatory and deviations are not permitted except as authorized in writing by the Deputy Assistant Secretary of Defense for Information and Identity Assurance. Implementation of this banner verbiage is further directed to all DoD components for all DoD assets via USCYBERCOM CTO 08-008A. Information Assurance OfficerECWM-1

Setting the timeout of the session to ten minutes or less increases the level of protection afforded critical network components.Information Assurance OfficerECSC-1

Detailed information about the network is sent across the network via SNMP. If this information is discovered by attackers it could be used to trace the network, show the networks topology, and possibly gain access to network devices.Information Assurance OfficerECSC-1

Numerous vulnerabilities exist with SNMP; therefore, without unique SNMP community names, the risk of compromise is dramatically increased. This is especially true with vendors default community names which are widely known by hackers and other networking experts. If a hacker gains access to these devices and can easily guess the name, this could result in denial of service, interception of sensitive information, or other destructive actions.Information Assurance OfficerECSC-1

The additional services that the firewall has enabled increases the risk for an attack since the firewall will listen for these services. In addition, these services provide an unsecured method for an attacker to gain access to the router.Information Assurance OfficerECSC-1

By not restricting authorized accounts to their proper privilege level, access to restricted functions may be allowed before authorized personell are trained or experienced enough to use those functions. Network disruptions or outages may occur due to mistakes made by inexperienced persons using accounts with greater privileges than necessary.Information Assurance OfficerECSC-1

A malicious user attempting to gain access to the network device may compromise an account that may be unauthorized for use. The unauthorized account may be a temporary or inactive account that is no longer needed to access the device. Denial of Service, interception of sensitive information, or other destructive actions could potentially take place if an unauthorized account is configured to access the network device.Information Assurance OfficerECSC-1, IAAC-1

Administration and management connections performed across a network are inherently dangerous because anyone with a packet sniffer and access to the right LAN segment can acquire the network device account and password information. With this intercepted information they could gain access to the router and cause denial of service attacks, intercept sensitive information, or perform other destructive actions.Information Assurance OfficerDCNR-1, ECSC-1

The additional services that the router is enabled for increases the risk for an attack since the router will listen for these services. In addition, these services provide an unsecured method for an attacker to gain access to the router. Most recent software versions support remote configuration and monitoring using the World Wide Web's HTTP protocol. In general, HTTP access is equivalent to interactive access to the router. The authentication protocol used for HTTP is equivalent to sending a clear-text password across the network, and, unfortunately, there is no effective provision in HTTP for challenge-based or one-time passwords. This makes HTTP a relatively risky choice for use across the public Internet. Any additional services that are enabled increase the risk for an attack since the router will listen for these services.Information Assurance OfficerECSC-1

A SYN-flood attack is a denial-of-service attack where the attacker send a huge amount of please-start-a-connection packets and then nothing else. This causes the device being attacked to be overloaded with the open sessions and eventually crash. A ping sweep (also known as an ICMP sweep) is a basic network scanning technique used to determine which of a range of IP addresses map to live hosts (computers). Information Assurance OfficerEBBD-1, EBBD-2, EBBD-3, ECSC-1

Network devices not running the latest tested and approved versions of software are vulnerable to network attacks. Running the most current, approved version of system and device software helps the site maintain a stable base of security fixes and patches, as well as enhancements to IP security. Viruses, denial of service attacks, system weaknesses, back doors and other potentially harmful situations could render a system vulnerable, allowing unauthorized access to DoD assets.Information Assurance OfficerECSC-1

The IDS or firewall is the first device that is under the sites control that has the possibility to alarm the local staff of an ongoing attack. An alert from either of these devices can be the first indication of an attack or system failure. Information Assurance OfficerECAT-2, ECSC-1

Authentication for administrative access to the device is required at all times. A single account can be created on the device's local database for use in an emergency such as when the authentication server is down or connectivity between the device and the authentication server is not operable. The console or emergency account logon credentials must be stored in a sealed envelope and kept in a safe.Information Assurance OfficerECSC-1

Terminating an idle session within a short time period reduces the window of opportunity for unauthorized personnel to take control of a management session enabled on the console or console port that has been left unattended. In addition quickly terminating an idle session will also free up resources committed by the managed network element. Setting the timeout of the session to 10 minutes or less increases the level of protection afforded critical network components.Information Assurance OfficerECSC-1

Enabling write access to the router via SNMP provides a mechanism that can be exploited by an attacker to set configuration variables that can disrupt network operations.Information Assurance OfficerECSC-1

Unlike GRE (a simple encapsulating header) L2TP is a full fledged communications protocol with control channel, data channels, and a robust command structure. In addition to PPP, other link layer types (called pseudowires) can be and are defined for delivery in L2TP by separate RFC documents. Further complexity is created by the capability to define vender-specific parameters beyond those defined in the L2TP specifications. The endpoint devices of an L2TP connection can be an L2TP Access Concentrator (LAC) in which case it inputs/outputs the layer 2 protocol to/from the L2TP tunnel. Otherwise it is an L2TP Network Server (LNS), in which case it inputs/outputs the layer 3 (IP) protocol to/from the L2TP tunnel. The specifications describe three reference models: LAC-LNS, LAC-LAC, and LNS-LNS, the first of which is the most common case. The LAC-LNS model allows a remote access user to reach his home network or ISP from a remote location. The remote access user either dials (or otherwise connects via layer 2) to a LAC device which tunnels his connection home to an awaiting LNS. The LAC could also be located on the remote user’s laptop which connects to an LNS at home using some generic internet connection. The other reference models may be used for more obscure scenarios. Although the L2TP protocol does not contain encryption capability, it can be operated over IPSEC which would provide authentication and confidentiality. A remote user in the LAC-LNS model would most likely obtain a dynamically assigned IP address from the home network to ultimately use through the tunnel back to the home network. Secondly, the outer IP source address used to send the L2TP tunnel packet to the home network is likely to be unknown or highly variable. Thirdly, since the LNS provides the remote user with a dynamic IP address to use, the firewall at the home network would have to be dynamically updated to accept this address in conjunction with the outer tunnel address. Finally, there is also the issue of authentication of the remote user prior to divulging an acceptable IP address. As a result of all of these complications, the strict filtering rules applied to the IP-in-IP and GRE tunneling cases will likely not be possible in the L2TP scenario. In addition to the difficulty of enforcing addresses and endpoints (as explained above), the L2TP protocol itself is a security concern if allowed through a security boundary. In particular: 1) L2TP potentially allows link layer protocols to be delivered from afar. These protocols were intended for link-local scope only, are less defended, and not as well-known 2) The L2TP tunnels can carry IP packets that are very difficult to see and filter because of the additional layer 2 overhead 3) L2TP is highly complex and variable (vender-specific variability) and therefore would be a viable target that is difficult to defend. It is better left outside of the main firewall where less damage occurs if the L2TP-processing node is compromised. 4) Filtering can not be used to detect and prevent other unintended layer 2 protocols from being tunneled. The strength of the application layer code would have to be relied on to achieve this task. 5) Regardless of whether the L2TP is handled inside or outside of the main network, a secondary layer of IP filtering is required, therefore bringing it inside doesn’t save resources. Therefore, it is not recommended to allow unencrypted L2TP packets across the security boundary into the network’s protected areas. Reference the Backbone Transport STIG for additional L2TP guidance and use. Information Assurance OfficerECSC-1

If the host that a firewall engine is operating on is not secured, the firewall itself is exposed to greater risk.Information Assurance OfficerDCCS-1, DCCS-2, ECSC-1

Remote administration is inherently dangerous because anyone with a sniffer and access to the right LAN segment, could acquire the device account and password information. With this intercepted information they could gain access to the infrastructure and cause denial of service attacks, intercept sensitive information, or perform other destructive actions.Information Assurance OfficerECSC-1

An attacker may attempt to connect to the device using SSH by guessing the authentication method, encryption algorithm, and keys. Limiting the amount of time allowed for authenticating and negotiating the SSH session reduces the window of opportunity for the malicious user attempting to make a connection to the network element.Information Assurance OfficerECSC-1

An attacker may attempt to connect to the device using SSH by guessing the authentication method and authentication key or shared secret. Setting the authentication retry to 3 or less strengthens against a Brute Force attack.Information Assurance OfficerECSC-1

A TCP connection consists of a three-way handshake message sequence. A connection request is transmitted by the originator, an acknowledgement is returned from the receiver, and then an acceptance of that acknowledgement is sent by the originator. An attacker’s goal in this scenario is to cause a denial of service to the network or device by initiating a high volume of TCP packets, then never sending an acknowledgement, leaving connections in a half-opened state. Without the device having a connection or time threshold for these half-opened sessions, the device risks being a victim of a denial of service attack. Setting a TCP timeout threshold will instruct the device to shut down any incomplete connections. Services such as SSH, BGP, SNMP, LDP, etc. are some services that may be prone to these types of denial of service attacks. If the router does not have any BGP connections with BGP neighbors across WAN links, values could be set to even tighter constraints.Information Assurance OfficerECSC-1

Vulnerability assessments must be reviewed by the SA and protocols must be approved by the IA staff before entering the enclave. Access Control Lists (ACLs) are the first line of defense in a layered security approach. They permit authorized packets and deny unauthorized packets based on port or service type. They enhance the posture of the network by not allowing packets to even reach a potential target within the security domain. The list provided are highly susceptible ports and services that should be blocked or limited as much as possible without adversely affecting customer requirements. Auditing packets attempting to penetrate the network but are stopped by an ACL will allow network administrators to broaden their protective ring and more tightly define the scope of operation. If the perimeter is in a Deny-by-Default posture and what is allowed through the filter is IAW DoD Instruction 8551.1, and if the permit rule is explicitly defined with explicit ports and protocols allowed, then all requirements related to PPS being blocked would be satisfied. Information Assurance OfficerECSC-1

Many of the known attacks in stateless autoconfiguration are defined in RFC 3756 were present in IPv4 ARP attacks. IPSec AH was originally suggested as mitigation for the link local attacks, but has since been found to have bootstrapping problems and to be very administrative intensive. Due to first requiring an IP address in order to set up the IPSec security association creates the chicken-before-the-egg dilemma. There are solutions being developed (Secure Neighbor Discovery and Cryptographic Generated Addressing) to secure these threats but are not currently available at the time of this writing. To mitigate these vulnerabilities, links that have no hosts connected such as the interface connecting to external gateways will be configured to suppress router advertisements. Disable (or do not configure) all IPv6 Neighbor Discovery functions across tunnels including the Neighbor Unreachability Detection (NUD) function. Note: this is applicable only when the inner IP layer is IPv6 since IPv4 does not have the Neighbor Discovery functionality. Information Assurance OfficerDCBP-1, ECSC-1

Creating a filter to allow a port or service through the firewall without a proxy or content inspection, protocol inspection, and flow control creates a direct connection between the host in the private network and a host on the outside; thereby, bypassing additional security measures that could be provided. This places the internal host at a greater risk of exploitation that could make the entire network vulnerable to an attack. Information Assurance OfficerDCCS-2, ECSC-1

The loopback address is used by an Inter-Processor Control (IPC) mechanism that enables the client and server portion of an application running on the same machine to communicate, and so it is trusted. It should never be used as the source IP address of an inbound or outbound transmission.Information Assurance OfficerECSC-1

By immediately displaying an alarm message, identifying the potential security violation and making it accessible with the audit record contents associated with the event(s) that generated the alarm provides the administration staff prompt alert messages 7 x 24 regardless of if they are logged on.Information Assurance OfficerECAR-1, ECAR-2, ECAR-3, ECSC-1

By immediately displaying an alarm message, identifying the potential security violation and making it accessible with the audit record contents associated with the auditable event(s) that generated the alarm provides the administration staff prompt alert messages at their work areas.Information Assurance OfficerECAR-1, ECAR-2, ECAR-3, ECSC-1

Since NTP is used to ensure accurate log file timestamp information, NTP could pose a security risk if a malicious user were able to falsify NTP information. To launch an attack on the NTP infrastructure, a hacker could inject time that would be accepted by NTP clients by spoofing the IP address of a valid NTP server. To mitigate this risk, the time messages must be authenticated by the client before accepting them as a time source. Two NTP-enabled devices can communicate in either client-server mode or peer-to-peer mode (aka “symmetric mode”). The peering mode is configured manually on the device and indicated in the outgoing NTP packets. The fundamental difference is the synchronization behavior: an NTP server can synchronize to a peer with better stratum, whereas it will never synchronize to its client regardless of the client’s stratum. From a protocol perspective, NTP clients are no different from the NTP servers. The NTP client can synchronize to multiple NTP servers, select the best server and synchronize with it, or synchronize to the averaged value returned by the servers. A hierarchical model can be used to improve scalability. With this implementation, an NTP client can also become an NTP server providing time to downstream clients at a higher stratum level and of decreasing accuracy than that of its upstream server. To increase availability, NTP peering can be used between NTP servers. In the event the device looses connectivity to it upstream NTP server, it will be able to choose time from one of its peers. The NTP authentication model is opposite of the typical client-server authentication model. NTP authentication enables an NTP client or peer to authenticate time received from their servers and peers. It’s not used to authenticate NTP clients because NTP servers don’t care about the authenticity of their clients, as they never accept any time from them. Information Assurance OfficerECSC-1

As currently defined, site local addresses are ambiguous and can be present in multiple sites. The address itself does not contain any indication of the site to which it belongs. The use of site-local addresses has the potential to adversely affect network security through leaks, ambiguity and potential misrouting, as documented in section 2 of RFC3879. RFC3879 formally deprecates the IPv6 site-local unicast prefix defined in RFC3513, i.e., 1111111011 binary or FEC0::/10.Information Assurance OfficerECSC-1

SSH Version 1 is a protocol that has never been defined in a standard. Since SSH-1 has inherent design flaws which make it vulnerable to attacks, e.g., man-in-the-middle attacks, it is now generally considered obsolete and should be avoided by explicitly disabling fallback to SSH-1. Information Assurance OfficerECSC-1

Network Address Translation with Protocol Translation (NAT-PT), defined in [RFC2766], is a service that can be used to translate data sent between IP-heterogeneous nodes. NAT-PT translates a IPv4 datagram into a semantically equivalent IPv6 datagram or vice versa. For this service to work it has to be located in the connection point between the IPv4 network and the IPv6 network. The PT-part of the NAT-PT handles the interpretation and translation of the semantically equivalent IP header, either from IPv4 to IPv6 or from IPv6 to IPv4. Like NAT, NATPT also uses a pool of addresses which it dynamically assigns to the translated datagrams. The NAT-PT architecture is not one of the preferred DoD IPv6 transition paradigms due to the deprecation of NAT-PT within the DoD community. However, as described in the "DoD IPv6 Guidance for Information Assurance (IA) Milestone Objective 3 (MO3) Requirements, some services/agencies may chose to implement this transition mechanism within an enclave. The following sub-sections provide guidelines for the use of NAT-PT within a controlled enclave. In addition to the single point of failure, the reduced performance of an application level gateway, coupled with limitations on the kinds of applications that work, decreases the overall value and utility of the network. NAT-PT also inhibits the ability to deploy security at the IP layer. Information Assurance OfficerECSC-1

The use of Authentication, Authorization, and Accounting (AAA) affords the best methods for controlling user access, authorization levels, and activity logging. By enabling AAA on the routers in conjunction with an authentication server such as TACACS+ or RADIUS, the administrators can easily add or remove user accounts, add or remove command authorizations, and maintain a log of user activity. The use of an authentication server provides the capability to assign router administrators to tiered groups that contain their privilege level that is used for authorization of specific commands. For example, user mode would be authorized for all authenticated administrators while configuration or edit mode should only be granted to those administrators that are permitted to implement router configuration changes. Information Assurance OfficerIAIA-1

The Out-of-Band Management (OOBM) network is an IP network used exclusively for the transport of OAM&P data from the network being managed to the OSS components located at the NOC. Its design provides connectivity to each managed network element enabling network management traffic to flow between the managed NEs and the NOC. This allows the use of paths separate from those used by the network being managed. Traffic from the managed network to the management network and vice-versa must be secured via IPSec encapsulation. Information Assurance OfficerECNK-1

The IPSec tunnel end points may be configured on the OOBM gateway routers connecting the managed network and the NOC. They may also be configured on a firewall or VPN concentrator located behind the gateway router. In either case, the crypto access-list used to identify the traffic to be protected must be a mirror (both IP source and destination address) of the crypto access list configured at the remote VPN peer.System AdministratorInformation Assurance OfficerECSC-1

The OOBM access switch will connect to the management interface of the managed network elements. The management interface of the managed network element will be directly connected to the OOBM network. An OOBM interface does not forward transit traffic; thereby, providing complete separation of production and management traffic. Since all management traffic is immediately forwarded into the management network, it is not exposed to possible tampering. The separation also ensures that congestion or failures in the managed network do not affect the management of the device. If the OOBM interface does not have an IP address from the managed network address space, it will not have reachability from the NOC using scalable and normal control plane and forwarding mechanisms.System AdministratorInformation Assurance OfficerECSC-1

The OOBM access switch will connect to the management interface of the managed network elements. The management interface can be a true OOBM interface or a standard interface functioning as the management interface. In either case, the management interface of the managed network element will be directly connected to the OOBM network. An OOBM interface does not forward transit traffic; thereby, providing complete separation of production and management traffic. Since all management traffic is immediately forwarded into the management network, it is not exposed to possible tampering. The separation also ensures congestion or failures in the managed network do not affect the management of the device. If the device does not have an OOBM port, the interface functioning as the management interface must be configured so management traffic does not leak into the managed network and production traffic does not leak into the management network. System AdministratorInformation Assurance OfficerECSC-1

The management network must still have its own subnet in order to enforce control and access boundaries provided by Layer 3 network nodes such as routers and firewalls. Management traffic between the managed network elements and the management network is routed via the same links and nodes as that used for production or operational traffic. Safeguards must be implemented to ensure that the management traffic does not leak past the managed network’s premise equipment. It there is a firewall located behind the premise router, then all management traffic should be blocked at that point—with the exception of management traffic destined to premise equipment.System AdministratorInformation Assurance OfficerECSC-1

Similar to the OOBM model, when the production network is managed in-band, the management network could also be housed at a NOC that is located locally or remotely at a single or multiple interconnected sites. NOC interconnectivity as well as connectivity between the NOC and the managed networks’ premise routers would be enabled using either provisioned circuits or VPN technologies such as IPSec tunnels or MPLS VPN services. System AdministratorInformation Assurance OfficerECSC-1

Protecting data sitting in a server VLAN is necessary and can be accomplished using access control lists on VLANs provisioned for servers. Without proper access control of traffic entering or leaving the server VLAN, potential threats such as a denial of service, data corruption, or theft could occur, resulting in the inability to complete mission requirements by authorized users.Information Assurance OfficerECND-1, ECSC-1

ACLs on VLAN interfaces do not protect against compromised servers. The Server farm vlans need to protect the servers located on one subnet from servers located on another subnet. Protecting a client’s data from other clients is necessary and can be accomplished using VLAN provisioning, layer 3 filtering and content filtering at the Server Farm entry point. Restricting protocol, source and destination traffic via filters is an option; however additional security practices such as content filtering are required. The Server farm private vlans need to protect the servers located on one subnet from servers located on another subnet.Information Assurance OfficerECND-1

Most current applications are deployed as a multi-tier architecture. The multi-tier model uses separate server machines to provide the different functions of presentation, business logic, and database. Multi-tier server farms provide added security because a compromised web server does not provide direct access to the application itself or to the database. The multi-tier separation is accomplished in several architectures, by a layer 2 switch, by a layer3 switch/router or by a firewall located at the server farm. Using the firewall implementation is the most secure method and is the only approved DoD architecture. Firewalls get packets from VLAN-supporting switches complete with 802.1Q tags in their headers. What the VLAN-aware firewall can do is extract the tags and use the information within the tags to make policy-based security decisions. Information Assurance OfficerEBBD-1

“6-to-4” is a tunneling IPv6 transition mechanism [RFC 3056]. The guidance is the default case, which assumes that 6-to-4 is not being used as an IPv6 transition mechanism. If 6-to-4 is implemented, reference addition 6-to-4 guidance defined in the STIG. Drop all inbound IPv6 packets containing a source address of type 2002::/16. This assumes the 6-to-4 transition mechanism is not being used. Drop all inbound IPv6 packets containing a destination address of type 2002::/16. This assumes the 6-to-4 transition mechanism is not being used.Information Assurance OfficerECSC-1

The IPv6 Jumbo Payload allows IP packets to be larger than 65,535 bytes. This feature is only useful on very specialized high performance systems (e.g. super computers). Common place link layer technologies do not support these payload sizes and special link layer designs would be necessary. This header should be dropped unless the system is specifically designed to use very large payloads, since it only serves as an opportunity to break implementations. Information Assurance OfficerECSC-1

Call home services or features will routinely send data such as configuration and diagnostic information to the vendor for routine or emergency analysis and troubleshooting. The risk that transmission of sensitive data sent to unauthorized persons could result in data loss or downtime due to an attack.Information Assurance OfficerNetwork Security OfficerECSC-1

1) Drop IPv6 Undetectable protocol/port (May be an intrinsic FW feature.) - IPv6 allows an unlimited number of extension headers to be applied to a packet. A FW may not be able to locate the layer 4 protocol and port values if too many extension headers exhaust its resources. As a minimum, a FW must be able to drop any packet for which it cannot identify the layer 4 protocol and ports (if applicable). The security policy would be subverted if these packets were allowed to pass through a FW. If the FW cannot traverse through extension headers at all, it must drop packets using any extension header. This measure will disable a large amount of IPv6’s functionality and should only be used if the Primary guidance cannot be implemented. 2) Drop IPv6 Type 0 Routing Header - The IPv6 Type 0 Routing Header (extension header) is functionally equivalent to the IPv4 loose source routing header option, which is typically blocked for security reasons. The Type 0 Routing Header is dangerous because it allows attackers to spoof source addresses and get traffic in response (rather than to the real owner of the address). Secondly, a packet with an allowed destination address could be sent through a FW only to bounce to a different (disallowed) node once inside using the Routing Header functionality. If the Type 0 Routing Header must be used, it must be used in conjunction with either the IPSec AH or the IPSec Encapsulation Security Payload (ESP) headers. If the FW cannot distinguish the type field of a routing header, it should be configured to drop all routing headers. Note that Mobile IP is disabled without the Type 2 Routing Header. Although deprecated by a recent RFC, there may be existing implementations that still recognize this header. 3) Drop Undefined IPv6 Header Extensions/Protocol Values (May be an intrinsic FW feature.) - Undefined IPv6 header extensions means that the Next Header type is not registered with Internet Assigned Numbers Authority (IANA). The header extension is the same as the protocol value, and should be dropped. Drop all undefined extension headers/protocol values. 4) Drop at least one fragment of any inbound fragmented packet for which the complete data set for filtering to include protocol/port values cannot be determined.(May be an intrinsic FW feature) - A FW must be able to properly enforce its filtering policy upon fragmented packets. This requires that the FW be able to find the complete set of header data including extension headers and the upper layer protocol/port values. It also requires that the packet not be susceptible to fragment overlap attacks. Fragment overlaps are a more serious problem in IPv6 than in IPv4 because the presence of extension headers can push the upper layer protocol/port information outward (toward packet boundaries) making it much harder to protect. How a FW achieves these requirements is not important as long as both aspects are met. The wording “drop at least one fragment” used in the actions below is a statement of the bare minimum action to secure a packet, and is chosen to allow FW venders flexibility in achieving it. Refer to Firewall Design Considerations for IPv6 section 3.6 for extensive detail on this topic. https://www.us.army.mil/suite/doc/10209656 5) Drop all inbound IPv6 packets containing more than one Fragmentation Header within an IP header chain. (May be an intrinsic FW feature) - Nested fragmentation is an unnecessary and unwanted IPv6 condition that is not forbidden by the specifications. It occurs when an IP header chain contains more than one Fragmentation Header implying that a fragment has been fragmented. In the specification, the phrase “IP header chain” rather than “packet” is used, because a tunneled packet has more than one IP header chain and each chain can have a Fragment Header (this case is not nested fragmentation). Nested fragmentation is a new phenomenon with IPv6. It is not possible in IPv4, because the fragmentation fields are part of the main header and are modified in the event of a secondary fragmentation event. Nested fragmentation in IPv6 should be dropped by FWs since internal nodes that process the fragmentation may or may not be equipped to handle this unexpected case. These nodes may crash or behave in some unpredictable manner.Information Assurance OfficerDCDS-1, EBBD-1