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In this section we cover issues related to Cisco router configuration that affect the overall security of your internetwork This comprises Data Link (encryption) as well as Network layer (routing restriction) functions As with Application layer security, there is a trade-off between providing a user-friendly system and one that is highly secure A balance must be struck that provides reasonable ease of access to authorized users and restricts access to unauthorized persons It must be understood that users can undermine security measures if these measures do not fit with work practices Many times I have seen usernames and passwords written on pieces of paper that have been stuck to a screen Users resorted to this kind of behavior because the passwords were changed too frequently and were obscure To implement security measures fully, you must have the backing of senior management, must educate users about security issues, and must have agreed-upon procedures known to everyone in the organization
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Having said this, there are many security measures a network administrator can implement without user knowledge that will significantly improve the security of the internetwork Let's talk first about controlling access to Cisco routers Password Access to Cisco Routers First let's see what can be done to restrict unauthorized users from attaching an ASCII terminal to the console port and determining information about the internetwork by using well-known show commands There is no default password assigned to the console port; simply by attaching the appropriately configured terminal and pressing the Enter key, you get the router prompt echoed to the screen You can add a console password with the following commands: Router1(config)#line console 0 Router1(config-line)#login Router1(config-line)#password gr8scot Now each time a terminal is connected to the console port, a password prompt rather than the router prompt is presented to the user trying to get access On Cisco 2500-series routers, the auxiliary port can be used the same way as a console port to attach an ASCII terminal for displaying the router prompt It is a good idea to password-protect the auxiliary port by using the following commands: Router1(config)#line aux 0 Router1(config-line)#login Router1(config-line)#password gr8scot It is always best to make the nonprivileged-mode password a relatively easy-to-remember combination of alphanumeric characters The password shown here is an abbreviation of the statement, "Great Scott!" A similar situation is true for Telnet access, and it is a good idea to prompt users with Telnet access to the router for a nonprivileged-mode password before allowing them to see the router prompt Each Telnet session is identified by the router as a virtual terminal Many simultaneous virtual terminal accesses can be supported, but a typical configuration puts a five-session limit on Telnet access It does this by identifying terminals 0 through 4, which is implemented with the following commands: Router1(config)#line vty 0 4 Router1(config)#login Router1(config)#password you8it This discussion covers what can be done to secure access to nonprivileged mode on a Cisco router Restricting access to privileged mode is even more crucial than restricting access to nonprivileged mode Once nonprivileged mode has been gained to the router, only the security of the Enable password or secret stops an unauthorized user from getting full control of the router
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Only the network administration staff needs to know the Enable password or secret that allows a user into privileged mode, and it therefore should be obscure and changed frequently It is part of a network administrator's job to keep track of such things The Enable secret is always encrypted in the router configuration If privileged mode access to a router is given through an Enable password, it should be encrypted in the configuration as follows: Router1(config)#service password-encryption This command is followed by no arguments and encrypts all password displays Centralizing Router Access It generally is good practice to limit any type of remote access to routers on your internetwork to a limited set of network numbers It's typical for one network number to be dedicated for use by network administrative staff only It is then possible to configure a router to accept network connections that can be used for router management functions only if they originate with this particular network It is possible to restrict this type of access to just one service, such as Telnet, from a central location Ping, SNMP, and TFTP are useful utilities when managing remote devices; however, so restricting access to just one network number usually is sufficient This can be achieved by implementing a simple access list on all routers Access list 13, shown next, (defined in global configuration mode) identifies the network used by administration staff to get to the routers, which is the 200110 network Router1(config)#Access-list 13 permit 200110 000255 Once this list has been defined, it must be applied to an interface If this list is applied to the virtual terminal lines, because the only connections coming into these ports are Telnet sessions, the only Telnet sessions accepted will be those that originate from the 200110 network Applying this access list to the virtual terminal lines is done via the access-class command as shown: Router1(config)#line vty 0 4 The TACACS Security System The discussion so far has centered around defining access configurations on each individual router It is possible to centralize password administration for a large number of remote routers by using the TACACS system TACACS stands for the Terminal Access Controller Access Control System Though TACACS usually is deployed to centralize management of CHAP usernames and passwords, which are defined on a per-interface basis, it also can be used to authenticate users seeking Telnet (and hence Enable) access to a router TACACS provides the freedom to authenticate individual users and log their activity, whereas an Enable password defined on a router is a global configuration and its use cannot be traced to a specific user To configure this type of access checking, you need to set up the TACACS daemon on a Unix machine, configure all routers to reference that Unix machine for TACACS authorization, and configure the virtual terminals to use TACACS to check login requests Assuming that a Unix machine is appropriately configured for TACACS, with the address 210111, the configuration for each remote router to will be as follows:
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tacacs-server host 200111 tacacs-server last-resort password ! line vty 0 4 login tacacs The first line identifies as a global configuration the IP address of the TACACS host machine The next entry configures the router to prompt the user trying to get access to use the standard login password defined with the Enable password command This command comes into play if the router cannot gain access to the TACACS server defined in the first configuration entry The entry login tacacs refers all requests for connections coming in over the virtual terminals to the TACACS server for authentication With this configuration, access to the nonprivileged mode is authenticated by the TACACS server Access to Enable mode can be similarly checked by TACACS if the following configuration commands are added: ! tacacs-server extended enable use-tacacs tacacs-server authenticate enable enable last-resort password ! Here's what these commands do: Command tacacs-server extended initializes the router to use extended TACACS mode Command enable use-tacacs tells the router to use TACACS to decide whether a user should be allowed to enter privileged mode Command tacacs-server authenticate enable is necessary, and if it is not in the configuration, you will be locked out of the router In this example, it may appear redundant, as this command defines the Enable form of authentication, but it can be used to authenticate protocol connections and more sophisticated options using access lists Command enable last-resort password allows use of the Enable password in the router's configuration if the TACACS server is unavailable The TACACS server authenticates users against those listed in its configuration Usernames and passwords are simply listed in the TACACS server as shown here: username user1 password aaa12
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username user2 password bbb34 Extensions to the Cisco-supplied TACACS system allow for the use of a token card that is synchronized with the TACACS server software, which changes the password for users every three minutes To successfully log in to such a system, the user must carry a token card that displays the new password every three minutes This makes a system very secure; however, the user must keep the token card in a safe place, separate from the computer where the login takes place Leaving the token card next to the computer being used for login is as ineffective in terms of providing the intended level of security as posting the password on the screen Securing Intercomputer Communication In the previous section we looked at using passwords and TACACS to restrict access to privileged and nonprivileged mode on the router Here we will look at using CHAP and TACACS for the authenticating, authorizing, and accounting of computers attempting to make connections to the internetwork and participate in the routing of packets CHAP is preferred over PAP as a method for authenticating users because it is not susceptible to the modem playback issues discussed in Chap 6 CHAP is available only on a point-to-point link; in Cisco router terms, this means serial interfaces, async interfaces, or ISDN interfaces You cannot implement CHAP on a LAN interface The basic idea behind the operation of CHAP is that the router receiving a request for a connection will have a list of usernames and associated passwords The computer wanting to connect will have to supply one of these valid username and password pairs in order to gain access Implementing CHAP on serial interfaces connecting routers together uses the same configuration as defined for the ISDN connections using CHAP as illustrated in Chap 6 Here we will discuss how TACACS, can enhance the security features of CHAP Many of the configuration commands for using TACACS to provide security on network connections begin with the letters AAA, which stand for Authentication, Authorization, and Accounting Authentication is used to identify valid users and allow them access, and to disallow access for intruders Authorization determines what services on the internetwork the user can access Accounting tracks which user did what and when, which can be used for audit-trail purposes We now will examine the commands that you put into a router or access server to enable TACACS when using AAA security on network connections Let's list a typical configuration that would be input on a router to configure it for centralized TACACS+ management, and then discuss each command in turn Figure 7-10 shows the configuration; command explanations follow
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