Infrastructure Router Security Technical Implementation Guide Cisco

If the keys used for routing protocol authentication are guessed, the malicious user could create havoc within the network by advertising incorrect routes and redirecting traffic. Changing the keys frequently reduces the risk of them eventually being guessed. When configuring authentication for routing protocols that provide key chains, configure two rotating keys with overlapping expiration dates, both with 180-day or less expirations.

Since NTP is used to ensure accurate log file time stamp 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 loses connectivity to its 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 is not used to authenticate NTP clients because NTP servers do not care about the authenticity of their clients, as they never accept any time from them.

Using a loopback address as the source address offers a multitude of uses for security, access, management, and scalability of routers. It is easier to construct appropriate ingress filters for router management plane traffic destined to the network management subnet since the source addresses will be from the range used for loopback interfaces instead of a larger range of addresses used for physical interfaces. Log information recorded by authentication and syslog servers will record the router’s loopback address instead of the numerous physical interface addresses. Syslog messages sent to management servers should use the loopback address as the source address.

Using a loopback address as the source address offers a multitude of uses for security, access, management, and scalability of routers. It is easier to construct appropriate ingress filters for router management plane traffic destined to the network management subnet since the source addresses will be from the range used for loopback interfaces instead of a larger range of addresses used for physical interfaces. Log information recorded by authentication and syslog servers will record the router’s loopback address instead of the numerous physical interface addresses. SNMP messages sent to management servers should use the loopback address as the source address.

Using a loopback address as the source address offers a multitude of uses for security, access, management, and scalability of network devices. It is easier to construct appropriate ingress filters for management plane traffic destined to the network management subnet since the source addresses will be from the range used for loopback interfaces instead of a larger range of addresses used for physical interfaces. Log information recorded by authentication and syslog servers will record the router’s loopback address instead of the numerous physical interface addresses. TFTP and FTP messages sent to management servers should use the loopback address as the source address.

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.

Unicast Reverse Path Forwarding (uRPF) provides a mechanism for IP address spoof protection. When uRPF is enabled on an interface, the router examines all packets received as input on that interface to make sure that the source address and source interface appear in the routing table and match the interface on which the packet was received. If the packet was received from one of the best reverse path routes, the packet is forwarded as normal. If there is no reverse path route on the same interface from which the packet was received, it might mean that the source address was modified. If Unicast RPF does not find a reverse path for the packet, the packet is dropped.

If internal nodes automatically configure an address based on a prefix from a bogus Router Advertisement a dangerous situation may exist. An internal host may contact an internal server which responds with a packet that could be routed outside of the network via default routing (because the routers do not recognize the destination address as an internal address).

To prevent this, filtering should be applied to network interfaces between internal host LANs and internal server LANs to insure that source addresses have valid prefixes.

ISATAP is an automatic tunnel mechanism that does not provide authentication such as IPSec. As a result of this limitation, ISATAP is thought of as a tool that is used inside the enclave among trusted hosts, which would limit it to internal attacks. ISATAP is a service versus a product, and is readily available to most users. If a user knows the ISATAP router IP address, they can essentially get onto the IPv6 intranet. To control the vulnerability of this tunnel mechanism, it is critical to control the use of protocol 41 and use IPv4 filters to control what IPv4 nodes can send protocol 41 packets to an ISATAP router interface. Although the ISATAP tunneling mechanism is similar to other automatic tunneling mechanisms, such as IPv6 6to4 tunneling, ISATAP is designed for transporting IPv6 packets between sites within an enclave, not between enclaves.

The emergency administration account is to be configured as a local account on the network devices. It is to be used only when the authentication server is offline or not reachable via the network. The emergency account must be set to an appropriate authorization level to perform necessary administrative functions during this time.

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.

If the gateway router is not a dedicated device for the OOBM network, several safeguards must be implemented for containment of management and production traffic boundaries. Since the managed network and the management network are separate routing domains, separate IGP routing instances must be configured on the router—one for the managed network and one for the OOBM network. In addition, the routes from the two domains must not be redistributed to each other.

If the gateway router is not a dedicated device for the OOBM network, several safeguards must be implemented for containment of management and production traffic boundaries such as using interface ACLs or filters at the boundaries between the two networks.

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.

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, both data plane and control plane, does not leak into the managed network and production traffic does not leak into the management network.

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.

When network congestion occurs, all traffic has an equal chance of being dropped.
Prioritization of network management traffic must be implemented to ensure that even during periods of severe network congestion, the network can be managed and monitored. Quality of Service (QoS) provisioning categorizes network traffic, prioritizes it according to its relative importance, and provides priority treatment through congestion avoidance techniques. Implementing QoS within the network makes network performance more predictable and bandwidth utilization more effective. Most important, since the same bandwidth is being used to manage the network, it provides some assurance that there will be bandwidth available to troubleshoot outages and restore availability when needed.

When management traffic must traverse several nodes to reach the management network, management traffic should be classified and marked at the nearest upstream MLS or router. In addition, all core routers within the managed network must be configured to provide preferred treatment based on the QoS markings. This will ensure that management traffic receives preferred treatment (per-hop behavior) at each forwarding device along the path to the management network. traffic.

Routing in the network containing the tunnel entry point must be configured to direct the intended traffic into the tunnel. Depending on the router products used this may be done by creating routes to a tunnel by name, by address, or by interface.

If multiple tunnels are defined or IPv6 interfaces, you must be selective with static routes, policy based routing, or even let the interior gateway protocol (IGP) make the decision since a ipv4 or ipv6 address has been configured on the tunnel. The key is the administrator should carefully plan and configure or let the IGP determine what goes into each tunnel.

A scope zone is an instance of a connected region of a given scope. Zones of the same scope cannot overlap while zones of a smaller scope will fit completely within a zone of a larger scope. For example, Admin-local scope is smaller than Site-local scope, so the administratively configured boundary fits within the bounds of a site. According to RFC 4007 IPv6 Scoped Address Architecture (section 5), scope zones are also required to be "convex from a routing perspective"-that is, packets routed within a zone must not pass through any links that are outside of the zone. This requirement forces each zone to be one contiguous island rather than a series of separate islands. As stated in the DoD IPv6 IA Guidance for MO3, "One should be able to identify all interfaces of a zone by drawing a closed loop on their network diagram, engulfing some routers and passing through some routers to include only some of their interfaces."

Administrative scoped multicast addresses are locally assigned and are to be used exclusively by the enterprise network or enclave. Hence, administrative scoped multicast traffic must not cross the perimeter of the enclave in either direction. Admin-local scope could be used to contain multicast traffic to a portion of an enclave or within a site. This can make it more difficult for a malicious user to access sensitive traffic if the traffic is restricted to links that the user does not have access to. Admin-local scope is encouraged for any multicast traffic within a network that is intended for network management as well as control plane traffic that must reach beyond link-local destinations.

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.

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.

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 logon 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.

The susceptibility of IP addresses to spoofing translates to DNS host name and IP address mapping vulnerabilities. For example, suppose a source host wishes to establish a connection with a destination host and queries a DNS server for the IP address of the destination host name. If the response to this query is the IP address of a host operated by an attacker, the source host will establish a connection with the attackers host, rather than the intended target. The user on the source host might then provide logon, authentication, and other sensitive data.

A rogue router could send a fictitious routing update to convince a site’s premise router to send traffic to an incorrect or even a rogue destination. This diverted traffic could be analyzed to learn confidential information of the site’s network, or merely used to disrupt the network’s ability to effectively communicate with other networks.

Group accounts configured for use on a network device do not allow for accountability or repudiation of individuals using the shared account. If group accounts are not changed when someone leaves the group, that person could possibly gain control of the network device. Having group accounts does not allow for proper auditing of who is accessing or changing the network.

A scope zone is an instance of a connected region of a given scope. Zones of the same scope cannot overlap while zones of a smaller scope will fit completely within a zone of a larger scope. For example, Admin-local scope is smaller than Site-local scope, so the administratively configured boundary fits within the bounds of a site. According to RFC 4007 IPv6 Scoped Address Architecture (section 5), scope zones are also required to be "convex from a routing perspective"—that is, packets routed within a zone must not pass through any links that are outside of the zone. This requirement forces each zone to be one contiguous island rather than a series of separate islands. As stated in the DoD IPv6 IA Guidance for MO3, "One should be able to identify all interfaces of a zone by drawing a closed loop on their network diagram, engulfing some routers and passing through some routers to include only some of their interfaces." Hence, it is imperative that the network has documented their multicast topology and thereby knows which interfaces are enabled for multicast. Once, this is done, the zones can be scoped as required.

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.

Many attacks information systems and network elements are launched from within the network. Hence, it is imperative that all passwords are encrypted so they cannot be intercepted by viewing the console or printout of the configuration.

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.

If the running and startup router configurations are not synchronized properly and a router malfunctions, it will not restart with all of the recent changes incorporated. If the recent changes were security related, then the routers would be vulnerable to attack.

L2TPv3 sessions can be used to transport layer-2 protocols across an IP backbone. These protocols were intended for link-local scope only and are therefore less defended and not as well-known. As stated in DoD IPv6 IA Guidance for MO3 (S4-C7-1), the L2TP tunnels can also carry IP packets that are very difficult to filter because of the additional encapsulation. Hence, it is imperative that L2TP sessions are authenticated prior to transporting traffic.

The finger service supports the UNIX finger protocol, which is used for querying a host about the users that are logged on. This service is not necessary for generic users. If an attacker were to find out who is using the network, they may use social engineering practices to try to elicit classified DoD information.

Source routing is a feature of IP, whereby individual packets can specify routes. This feature is used in several different network attacks by bypassing perimeter and internal defense mechanisms.

The additional services 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. The HTTPS server may be enabled for administrative access.

As specified in RFC 793, TCP utilizes sequence checking to ensure proper ordering of received packets. RFC 793 also specifies that RST (reset) control flags should be processed immediately, without waiting for out of sequence packets to arrive. RFC 793 also requires that sequence numbers are checked against the window size before accepting data or control flags as valid. A router receiving an RST segment will close the TCP session with the BGP peer that is being spoofed; thereby, purging all routes learned from that BGP neighbor. A RST segment is valid as long as the sequence number is within the window. The TCP reset attack is made possible due to the requirements that Reset flags should be processed immediately and that a TCP endpoint must accept out of order packets that are within the range of a window size. This reduces the number of sequence number guesses the attack must make by a factor equivalent to the active window size. Each sequence number guess made by the attacker can be simply incremented by the receiving connections window size. The BGP peering session can protect itself against such an attack by authenticating each TCP segment. The TCP header options include an MD5 signature in every packet and are checked prior to the acceptance and processing of any TCP packet—including RST flags.

One way to create havoc in a network is to advertise bogus routes to a network. A rogue router could send a fictitious routing update to convince a BGP router to send traffic to an incorrect or rogue destination. This diverted traffic could be analyzed to learn confidential information of the site’s network, or merely used to disrupt the network’s ability to effectively communicate with other networks. An autonomous system can advertise incorrect information by sending BGP updates messages to routers in a neighboring AS. A malicious AS can advertise a prefix originated from another AS and claim that it is the originator (prefix hijacking). Neighboring autonomous systems receiving this announcement will believe that the malicious AS is the prefix owner and route packets to it.

Network devices that are 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.

SNMP Versions 1 and 2 are not considered secure. Without the strong authentication and privacy that is provided by the SNMP Version 3 User-based Security Model (USM), an unauthorized user can gain access to network management information used to launch an attack against the network.

Authentication for administrative access to the device is required at all times. A single account of last resort 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 local account of last resort logon credentials must be stored in a sealed envelope and kept in a safe.

Enabling write access to the device via SNMP provides a mechanism that can be exploited by an attacker to set configuration variables that can disrupt network operations.

Logging is a critical part of router security. Maintaining an audit trail of system activity logs (syslog) can help identify configuration errors, understand past intrusions, troubleshoot service disruptions, and react to probes and scans of the network. Syslog levels 0-6 are the levels required to collect the necessary information to help in the recovery process.

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.

Packet Assembler Disassembler (PAD) is an X.25 component seldom used. It collects the data transmissions from the terminals and gathers them into a X.25 data stream and vice versa. PAD acts like a multiplexer for the terminals. If enabled, it can render the device open to attacks. Some voice vendors use PAD on internal routers.

Identification support allows one to query a TCP port for identification. This feature enables an unsecured protocol to report the identity of a client initiating a TCP connection and a host responding to the connection. Identification support can connect a TCP port on a host, issue a simple text string to request information, and receive a simple text-string reply. This is another mechanism to learn the router vendor, model number, and software version being run.

The Cisco Express Forwarding (CEF) switching mode replaces the traditional Cisco routing cache with a data structure that mirrors the entire system routing table. Because there is no need to build cache entries when traffic starts arriving for new destinations, CEF behaves more predictably when presented with large volumes of traffic addressed to many destinations such as a SYN flood attacks. Because many SYN flood attacks use randomized source addresses to which the hosts under attack will reply to, there can be a substantial amount of traffic for a large number of destinations that the router will have to handle. Consequently, routers configured for CEF will perform better under SYN floods directed at hosts inside the network than routers using the traditional cache.

Only Interior Gateway Protocols (IGPs) use key chains. When configuring authentication for routing protocols that provide key chains, configure two rotating keys with overlapping expiration dates--both with a 180-day or less lifetime. A third key must also be defined with an infinite lifetime. Both of these steps ensure there will always be a key that can be placed into service by all peers. If a time period occurs during which no key is activated, authentication cannot occur; hence, route updates will not occur. The lifetime key should be changed 7 days after successful key rotation and synchronization has occurred with all peers.