Customers who leverage the Smart Install feature for more than zero-touch deployment (configuration and image management). The algorithm is not designed to protect configuration files against serious analysis by even slightly sophisticated attackers and must not be used for this purpose. By default, IGPs are dynamic and discover additional routers that communicate with the particular IGP in use. The presence of IP options within a packet might indicate an attempt to subvert security controls in the network or otherwise alter the transit characteristics of a packet. If password recovery is not required, then an administrator can remove the ability to perform the password recovery procedure using the no service password-recovery global configuration command; however, once the no service password-recovery command has been enabled, an administrator can no longer perform password recovery on a device. These packets, which transit the devices deployed throughout the network, can impact CPU operations of a device. Refer to Flexible Packet Matching, located on the Cisco IOS Flexible Packet Matching homepage, for more information about the feature. In particular, these privileges allow an administrator to perform the password recovery procedure. Without PVLANs, all devices on a Layer 2 VLAN can communicate freely. ICMP redirects are disabled with the interface configuration no ip redirects command , as shown in the example configuration: IP Directed Broadcasts make it possible to send an IP broadcast packet to a remote IP subnet. When you configure this feature with the neighbor maximum-prefix BGP router configuration command, one argument is required: the maximum number of prefixes that are accepted before a peer is shutdown. While the network troubleshooting tools ping and traceroute use ICMP, external ICMP connectivity is rarely needed for the proper operation of a network. This feature, added in Cisco IOS Software Release 12.3(11)T, allows a device to reclaim space in order to create new crashinfo files when the device crashes. This example ACL filters packets with TTL values less than six. If SSH is enabled, it is recommended to disable SSHv1 by using the ip ssh version 2 command. An authorized user who is configured with privilege level 15 cannot be locked out with this feature. An AUX port can be disabled with these commands: Interactive management sessions in Cisco IOS software use a tty or virtual tty (vty). This example instructs the Cisco IOS device to store archived configurations as files named archived-config-N on the disk0: file system, to maintain a maximum of 14 backups, and to archive once per day (1440 minutes) and when an administrator issues the write memory EXEC command. This feature is not available in all Cisco IOS software releases. Dynamic Address Resolution Protocol (ARP) Inspection (DAI) mitigates attack vectors that use ARP poisoning on local segments. SSHv1 and SSHv2 are not compatible. As a result, many networks are vulnerable because they have well-known holes in their security armor that should have been fixed but weren’t. Refer to Reserve Memory for Console Access for more information about this feature. In Cisco IOS Software Release 12.4(4)T and later, Control Plane Protection (CPPr) can be used in order to restrict or police control plane traffic by the CPU of a Cisco IOS device. IPSec can also be used in order to validate and secure routing protocols, but these examples do not detail its use. This is an example of NetFlow output from the CLI. In addition, CPPr includes these control plane protection features: Refer to Control Plane Protection and Understanding Control Plane Protection (CPPr) for more information on the configuration and use of the CPPr feature. NetFlow can provide visibility into all traffic on the network. Refer to ACL Support for Filtering on TTL Value for more information about this functionality. This feature focuses on memory allocations that are dynamic. It can also be pushed via the director when switches are first deployed. This information about Cisco IOS software features and configurations can help ensure the resilience of the control plane. The presence of IP options within a packet can also indicate an attempt to subvert security controls in the network or otherwise alter the transit characteristics of a packet. This configuration builds upon previous examples that include configuration of the TACACS servers. Method lists enable you to designate one or more security protocols to be used for authentication, and thus ensure a backup system for authentication in case the initial method fails. By adding MD5 hash capabilities to the authentication process, routing updates no longer contain cleartext passwords, and the entire contents of the routing update is more resistant to tampering. IP source routing, which is enabled by default in all Cisco IOS Software Releases, is disabled via the no ip source-route global configuration command. Each device that an IP packet traverses decrements this value by one. This configuration example shows the use of these commands: Refer to Cisco IOS Network Management Command Reference for more information about global configuration commands. This configuration example illustrates the use of this command: ICMP redirects are used in order to inform a network device of a better path to an IP destination. If you use IPSec, it also adds additional CPU overhead to the device. An iACL should contain a policy that denies unauthorized SNMP packets on UDP port 161. This allows for a locally defined user to be created for one or more network administrators. Although the configuration archive functionality can store up to 14 backup configurations, you are advised to consider the space requirements before you use the maximum command. LLDP is similar to CDP. The syntax for PACLs creation, which takes precedence over VLAN maps and router ACLs, is the same as router ACLs. Current versions of Cisco IOS software have this functionality disabled by default; however, it can be enabled via the ip directed-broadcast interface configuration command. The command is supported in Cisco IOS Software Release 12.2(18)SXD (for Sup 720) and Cisco IOS Software Releases 12.2(33)SRA or later. Spoofed packets could enter the network through a Unicast RPF-enabled interface if an appropriate return route to the source IP address exists. In addition, ACLs and null routing are often deployed as a manual means of spoofing prevention. The only reliable transport that is defined for SSH is TCP. By default, the Cisco IOS software sends a redirect if it receives a packet that must be routed through the interface it was received. However, the algorithm is subject to dictionary attacks. The security configuration framework is designed to help simplify security configuration while still allowing enough flexibility to allow you to balance security, productivity, and user experience. OSPF does not utilize Key Chains. Unicast RPF relies on you to enable Cisco Express Forwarding on each device and is configured on a per-interface basis. This requires a level of CPU effort that is not required for typical packets that traverse the network. This kind of communication can allow an attacker to pose as an FHRP-speaking device to assume the default gateway role on the network. SSHv1 is considered to be insecure and can have adverse effects on the system. EIGRP and RIPv2 utilize Key Chains as part of the configuration. This functionality is enabled with the logging enable configuration change logger configuration mode command. Control Plane Protection (CPPr) builds on the functionality of Control Plane Policing in order to restrict and police control plane traffic that is destined to the route processor of the IOS device. Only special and production keys can be revoked in the event of a key compromise. IGPs also discover routes that can be used during a network link failure. Information leaks, or the introduction of false information into an IGP, can be mitigated through use of the passive-interface command that assists in controlling the advertisement of routing information. SNMPv3 consists of three primary configuration options: An authoritative engine ID must exist in order to use the SNMPv3 security mechanisms - authentication or authentication and encryption - to handle SNMP packets; by default, the engine ID is generated locally. In Cisco IOS Software Release 15.1(1)T and later, Key Replacement for Digitally Signed Cisco Software was introduced. The primary purpose of routers and switches is to forward packets and frames through the device onward to final destinations. For server authentication, the Cisco IOS SSH client must assign a host key for each server. Protocols that leverage virtual MAC addresses such as HSRP do not function when the maximum number is set to one. After the Configuration Change Notification and Logging feature has been enabled, the privileged EXEC command show archive log config all can be used in order to view the configuration log. These hardware rate limiters are referred to as special-case rate limiters because they cover a specific predefined set of IPv4, IPv6, unicast, and multicast DoS scenarios. Where possible and appropriate, this document contains recommendations that, if implemented, help secure a network. Users are the weakest link in any network security scenario. This interface command has to be applied on the ingress interface and it instructs the forwarding engine to not inspect the IP header. Logging at level 7 produces an elevated CPU load on the device that can lead to device and network instability. The Network Time Protocol (NTP) is not an especially dangerous service, but any unneeded service can represent an attack vector. This checklist is a collection of all the hardening steps that are presented in this guide. A vty and tty should be configured in order to accept only encrypted and secure remote access management connections to the device or through the device if it is used as a console server. In the previous CPPr policy, the access control list entries that match packets with the permit action result in these packets being discarded by the policy-map drop function, while packets that match the deny action (not shown) are not affected by the policy-map drop function. Subsequent methods are only attempted in cases where earlier methods fail due to server unavailability or incorrect configuration. However, if outgoing connections are allowed, then an encrypted and secure remote access method for the connection should be enforced through the use of transport output ssh. The SSH server computes a hash over the public key provided by the user. Cisco IOS software provides Unicast RPF and IP Source Guard (IPSG) in order to deter attacks that rely on source IP address spoofing. Note that ttys can be used for connections to console ports of other devices. In a dictionary attack, an attacker tries every word in a dictionary or other list of candidate passwords in order to find a match. In manual mode, the administrator uses the configure terminal lock command in order to lock the configuration when it enters configuration mode. In order to encrypt a user password with MD5 hashing, issue the username secret global configuration command. 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