Configuring Kerberos

Kerberos is a secret-key network authentication protocol, developed at the Massachusetts Institute of Technology (MIT), that uses the Data Encryption Standard (DES) cryptographic algorithm for encryption and authentication. Kerberos was designed to authenticate requests for network resources. Kerberos, like other secret-key systems, is based on the concept of a trusted third party that performs secure verification of users and services. In the Kerberos protocol, this trusted third party is called the key distribution center (KDC).

Finding Feature Information

Your software release may not support all the features documented in this module. For the latest caveats and feature information, see Bug Search Tool and the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the feature information table.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.

Prerequisites for Configuring Kerberos

  • For hosts and the KDC in your Kerberos realm to communicate and mutually authenticate, you must identify them to each other. To do this, you add entries for the hosts to the Kerberos database on the KDC and add SRVTAB files generated by the KDC to all hosts in the Kerberos realm. You also make entries for users in the KDC database.

  • The Kerberos administrative programs (known as the KDC) must be installed on a UNIX host and initialized on the database. A Kerberos realm name and password must also be configured. For instructions about completing these tasks, refer to the Kerberos software instructions.


Note

Write down the host name or IP address of the KDC, the port number you want the KDC to monitor for queries, and the name of the Kerberos realm it will serve. This information is required to configure the router.

Information About Configuring Kerberos

The primary use of Kerberos is to verify that users and the network services they use are really who and what they claim to be. To accomplish this, a trusted Kerberos server issues tickets to users. These tickets, which have a limited lifespan, are stored in a user’s credential cache and can be used in place of the standard username-and-password authentication mechanism.

The Kerberos credential scheme embodies a concept called “single logon.” This process requires authenticating a user once, and then allows secure authentication (without encrypting another password) wherever that user’s credential is accepted.

Starting with Cisco IOS Release 11.2, Cisco IOS software includes Kerberos 5 support, which allows organizations already deploying Kerberos 5 to use the same Kerberos authentication database on their routers that they are already using on their other network hosts (such as UNIX servers and PCs).

The following network services are supported by the Kerberos authentication capabilities in Cisco IOS software:

  • Telnet

  • rlogin

  • rsh

  • rcp


Note

Cisco Systems’ implementation of Kerberos client support is based on code developed by CyberSafe, which was derived from the MIT code. As a result, the Cisco Kerberos implementation has successfully undergone full compatibility testing with the CyberSafe Challenger commercial Kerberos server and MIT’s server code, which is freely distributed.

The table below lists common Kerberos-related terms and their definitions.

Table 1. Kerberos Terminology

Term

Definition

authentication

A process by which a user or service identifies itself to another service. For example, a client can authenticate to a router or a router can authenticate to another router.

authorization

A means by which the router determines what privileges you have in a network or on the router and what actions you can perform.

credential

A general term that refers to authentication tickets, such as ticket granting tickets (TGTs) and service credentials. Kerberos credentials verify the identity of a user or service. If a network service decides to trust the Kerberos server that issued a ticket, it can be used in place of retyping in a username and password. Credentials have a default lifespan of eight hours.

instance

An authorization level label for Kerberos principals. Most Kerberos principals are of the form user@REALM (for example, smith@EXAMPLE.COM). A Kerberos principal with a Kerberos instance has the form user/instance@REALM (for example, smith/admin@EXAMPLE.COM). The Kerberos instance can be used to specify the authorization level for the user if authentication is successful. It is up to the server of each network service to implement and enforce the authorization mappings of Kerberos instances. Note that the Kerberos realm name must be in uppercase characters.

Kerberized

Applications and services that have been modified to support the Kerberos credential infrastructure.

Kerberos realm

A domain consisting of users, hosts, and network services that are registered to a Kerberos server. The Kerberos server is trusted to verify the identity of a user or network service to another user or network service. Kerberos realms must always be in uppercase characters.

Kerberos server

A daemon running on a network host. Users and network services register their identity with the Kerberos server. Network services query the Kerberos server to authenticate to other network services.

key distribution center (KDC)

A Kerberos server and database program running on a network host.

principal

Also known as a Kerberos identity, this is who you are or what a service is according to the Kerberos server.

service credential

A credential for a network service. When issued from the KDC, this credential is encrypted with the password shared by the network service and the KDC, and with the user’s TGT.

SRVTAB

A password that a network service shares with the KDC. The network service authenticates an encrypted service credential by using the SRVTAB (also known as a KEYTAB) to decrypt it.

ticket granting ticket (TGT)

A credential that the key distribution center (KDC) issues to authenticated users. When users receive a TGT, they can authenticate to network services within the Kerberos realm represented by the KDC.

Kerberos Client Support Operation

The Kerberos security system works with a router functioning as the security server. Although (for convenience or technical reasons) you can customize Kerberos in a number of ways, remote users attempting to access network services must pass through three layers of security before they can access network services.

Authenticating to the Boundary Router

The first step in the Kerberos authentication process is for remote users to authenticate themselves to the boundary router when they attempt to access a network. The following process describes how users authenticate to a boundary router:

  1. The remote user opens a PPP connection to the corporate site router.

  2. The router prompts the user for a username and password.

  3. The router requests a TGT from the KDC for this particular user.

  4. The KDC sends an encrypted TGT to the router that includes (among other things) the user’s identity.

  5. The router attempts to decrypt the TGT using the password the user entered. If the decryption is successful, the remote user is authenticated to the router.

A remote user who successfully initiates a PPP session and authenticates to the boundary router is inside the firewall but still must authenticate to the KDC directly before being allowed to access network services. This is because the TGT issued by the KDC is stored on the router and is not useful for additional authentication unless the user physically logs on to the router.

Obtaining a TGT from a KDC

This section describes how remote users who are authenticated to the boundary router authenticate themselves to a KDC.

When a remote user authenticates to a boundary router, that user technically becomes part of the network; that is, the network is extended to include the remote user and the user’s machine or network. To gain access to network services, however, the remote user must obtain a TGT from the KDC. The following process describes how remote users authenticate to the KDC:

  1. The remote user, at a workstation on a remote site, launches the KINIT program (part of the client software provided with the Kerberos protocol).

  2. The KINIT program finds the user’s identity and requests a TGT from the KDC.

  3. The KDC creates a TGT, which contains the identity of the user, the identity of the KDC, and the expiration time of the TGT.

  4. Using the user’s password as a key, the KDC encrypts the TGT and sends the TGT to the workstation.

  5. When the KINIT program receives the encrypted TGT, it prompts the user for a password (this is the password that is defined for the user in the KDC).

  6. If the KINIT program can decrypt the TGT with the password the user enters, the user is authenticated to the KDC, and the KINIT program stores the TGT in the user’s credential cache.

At this point, the user has a TGT and can communicate securely with the KDC. In turn, the TGT allows the user to authenticate to other network services.

Authenticating to Network Services

The following process describes how a remote user with a TGT authenticates to network services within a given Kerberos realm. Assume the user is on a remote workstation (Host A) and wants to log in to Host B.

  1. The user on Host A initiates a Kerberized application (such as Telnet) to Host B.

  2. The Kerberized application builds a service credential request and sends it to the KDC. The service credential request includes (among other things) the user’s identity and the identity of the desired network service. The TGT is used to encrypt the service credential request.

  3. The KDC tries to decrypt the service credential request with the TGT it issued to the user on Host A. If the KDC can decrypt the packet, it is assured that the authenticated user on Host A sent the request.

  4. The KDC notes the network service identity in the service credential request.

  5. The KDC builds a service credential for the appropriate network service on Host B on behalf of the user on Host A. The service credential contains the client’s identity and the desired network service’s identity.

  6. The KDC then encrypts the service credential twice. It first encrypts the credential with the SRVTAB that it shares with the network service identified in the credential. It then encrypts the resulting packet with the TGT of the user (who, in this case, is on Host A).

  7. The KDC sends the twice-encrypted credential to Host A.

  8. Host A attempts to decrypt the service credential with the user’s TGT. If Host A can decrypt the service credential, it is assured the credential came from the real KDC.

  9. Host A sends the service credential to the desired network service. Note that the credential is still encrypted with the SRVTAB shared by the KDC and the network service.

  10. The network service attempts to decrypt the service credential using its SRVTAB.

  11. If the network service can decrypt the credential, it is assured the credential was in fact issued from the KDC. Note that the network service trusts anything it can decrypt from the KDC, even if it receives it indirectly from a user. This is because the user first authenticated with the KDC.

At this point, the user is authenticated to the network service on Host B. This process is repeated each time a user wants to access a network service in the Kerberos realm.

How to Configure Kerberos

Configuring the KDC Using Kerberos Commands

After a host is configured to function as the KDC in the Kerberos realm, entries must be made to the KDC database (and to modify existing database information) for all principals in the realm. Principals can be network services on routers and hosts or principals can be users.


Note

All Kerberos command examples are based on Kerberos 5 Beta 5 of the original MIT implementation. Later versions use a slightly different interface.

Adding Users to the KDC Database

Follow these steps to add users to the KDC and create privileged instances for those users:

SUMMARY STEPS

  1. Use the su command to become root on the host running the KDC.
  2. Use the kdb5_edit program to configure the commands in the next steps.
  3. Use the ank (add new key) command in privileged EXEC mode to add a user to the KDC. This command prompts for a password that the user must enter to authenticate the router. For example:
  4. Use the ank command to add a privileged instance of a user. For example:

DETAILED STEPS

Step 1

Use the su command to become root on the host running the KDC.

Step 2

Use the kdb5_edit program to configure the commands in the next steps.

Note 

The Kerberos realm name in the following steps must be in uppercase characters.

Step 3

Use the ank (add new key) command in privileged EXEC mode to add a user to the KDC. This command prompts for a password that the user must enter to authenticate the router. For example:

Example:

Router# ank 
username@REALM
Step 4

Use the ank command to add a privileged instance of a user. For example: 


Router# ank  username/instance@REALM
Example

The following example adds the user loki to the Kerberos realm COMPANY.COM: 


ank loki@COMPANY.COM

Privileged instances can be created to allow network administrators to connect to the router at the enable level so that a clear text password is not used to avoid compromising security and to enter enabled modes. See the Enabling Kerberos Instance Mapping for more information on mapping Kerberos instances to various Cisco IOS privilege levels.

Creating and Extracting a SRVTAB on the KDC

All routers authenticated through Kerberos must have a SRVTAB that contains the password or randomly generated key for the service principal key that was entered into the KDC database. A service principal key must be shared with the host running that service. To do this, the SRVTAB entry must be saved (extracted) to a file and copied to the router and all hosts in the Kerberos realm.

Follow these steps to make a SRVTAB entry and extract this SRVTAB to a file on the KDC in privileged EXEC mode:

SUMMARY STEPS

  1. Use the ark (add random key) command to add a network service supported by a host or router to the KDC. For example:
  2. Use the kdb5_edit command xst to write an SRVTAB entry to a file. For example:
  3. Use the quit command to exit the kdb5_edit program.

DETAILED STEPS

Step 1

Use the ark (add random key) command to add a network service supported by a host or router to the KDC. For example:

Example:

Router# ark 
SERVICE/HOSTNAME@REALM
Step 2

Use the kdb5_edit command xst to write an SRVTAB entry to a file. For example:

Example:

Router# xst
 router-name host
Step 3

Use the quit command to exit the kdb5_edit program.

Example

The following example shows how to add a Kerberized authentication service for a router called router1 to the Kerberos realm COMPANY.COM: 


ark host/router1.company.com@COMPANY.COM

The following example shows how to write an entry for all network services on all Kerberized hosts that use this KDC for authentication to a file: 


xst router1.company.com@COMPANY.COM host

Configuring the Router to Use the Kerberos Protocol

Defining a Kerberos Realm

For a router to authenticate a user defined in the Kerberos database, it must know the host name or IP address of the host running the KDC, the name of the Kerberos realm and, optionally, be able to map the host name or Domain Name System (DNS) domain to the Kerberos realm.

To configure the router to authenticate to a specified KDC in a specified Kerberos realm, use the following commands in global configuration mode. Note that DNS domain names must begin with a leading dot (.):

SUMMARY STEPS

  1. Router(config)# kerberos local-realm kerberos-realm
  2. Router(config)# kerberos server kerberos-realm {hostname | ip-address } [port-number ]
  3. Router(config)# kerberos realm {dns-domain | host } kerberos-realm

DETAILED STEPS

  Command or Action Purpose
Step 1

Router(config)# kerberos local-realm kerberos-realm

Defines the default realm for the router.

Step 2

Router(config)# kerberos server kerberos-realm {hostname | ip-address } [port-number ]

Specifies to the router which KDC to use in a given Kerberos realm and, optionally, the port number that the KDC is monitoring. (The default is 88.)

Step 3

Router(config)# kerberos realm {dns-domain | host } kerberos-realm

(Optional) Maps a host name or DNS domain to a Kerberos realm.

What to do next

Note

Because the machine running the KDC and all Kerberized hosts must interact within a 5-minute window or authentication fails, all Kerberized machines, and especially the KDC, should be running the Network Time Protocol (NTP).

The kerberos local-realm , kerberos realm , and kerberos server commands are equivalent to the UNIX krb.conf file. The table below identifies mappings from the Cisco IOS configuration commands to a Kerberos 5 configuration file (krb5.conf).
Table 2. Kerberos 5 Configuration File and Commands

krb5.conf File

Cisco IOS Configuration Command

[libdefaults] 


default_realm = DOMAIN.COM

(in configuration mode) 


kerberos local-realm
 DOMAIN.COM

[domain_realm] 


.domain.com = DOMAIN.COM


domain.com = DOMAIN.COM

(in configuration mode) 


kerberos realm
.domain.com
 DOMAIN.COM
kerberos realm
 domain.com DOMAIN.COM

[realms] 


kdc = DOMAIN.PIL.COM:750


admin_server = DOMAIN.PIL.COM


default_domain = DOMAIN.COM

(in configuration mode) 


kerberos server
 DOMAIN.COM 172.65.44.2
(172.65.44.2
 is the example IP address for DOMAIN.PIL.COM
)
See Defining a Kerberos Realm Examples for a Kerberos realm configuration example.

Copying SRVTAB Files

To make it possible for remote users to authenticate to the router using Kerberos credentials, the router must share a secret key with the KDC. To do this, you must give the router a copy of the SRVTAB you extracted on the KDC.

The most secure method to copy an SRVTAB file to the hosts in your Kerberos realm is to copy it onto physical media and go to each host in turn and manually copy the files onto the system. To copy an SRVTAB file to the router, which does not have a physical media drive, it must be transfered over the network using TFTP.

To remotely copy an SRVTAB file to the router from the KDC, use the kerberos srvtab remote command in global configuration mode:

Router(config)# kerberos srvtab remote {hostname | ip-address } {filename }

When you copy the SRVTAB file from the router to the KDC, the kerberos srvtab remote command parses the information in this file and stores it in the router’s running configuration in the kerberos srvtab entry format. To ensure that the SRVTAB is available (does not need to be acquired from the KDC) when you reboot the router, use the write memory configuration command to write your running configuration (which contains the parsed SRVTAB file) to NVRAM. See Copying a SRVTAB File Example for an example.

Specifying Kerberos Authentication

See the Configuring Authentication feature module for more information on configuring authentication on the router aaa authentication command is used to specify Kerberos as the authentication method.

Enabling Credentials Forwarding

With Kerberos configured thus far, a user authenticated to a Kerberized router has a TGT and can use it to authenticate to a host on the network. However, if the user tries to list credentials after authenticating to a host, the output will show no Kerberos credentials present.

You can optionally configure the router to forward users’ TGTs with them as they authenticate from the router to Kerberized remote hosts on the network when using Kerberized Telnet, rcp, rsh, and rlogin (with the appropriate flags).

To force all clients to forward users’ credentials as they connect to other hosts in the Kerberos realm, use the following command in global configuration mode:

Command

Purpose 


Router(config)# kerberos credentials forward

Forces all clients to forward user credentials upon successful Kerberos authentication.

With credentials forwarding enabled, users’ TGTs are automatically forwarded to the next host they authenticate to. In this way, users can connect to multiple hosts in the Kerberos realm without running the KINIT program each time to get a new TGT.

Opening a Telnet Session to the Router

To use Kerberos to authenticate users opening a Telnet session to the router from within the network, use the following command in global configuration mode:

Command

Purpose 


Router(config)# aaa authentication login  {default  | list-name
} krb5_telnet

Sets login authentication to use the Kerberos 5 Telnet authentication protocol when using Telnet to connect to the router.

Although Telnet sessions to the router are authenticated, users must still enter a clear text password if they want to enter enable mode. The kerberos instance map command, discussed in a later section, allows them to authenticate to the router at a predefined privilege level.

Establishing an Encrypted Kerberized Telnet Session

Another way for users to open a secure Telnet session is to use Encrypted Kerberized Telnet. With Encrypted Kerberized Telnet, users are authenticated by their Kerberos credentials before a Telnet session is established. The Telnet session is encrypted using 56-bit Data Encryption Standard (DES) encryption with 64-bit Cipher Feedback (CFB). Because data sent or received is encrypted, not clear text, the integrity of the dialed router or access server can be more easily controlled.


Note

This feature is available only if you have the 56-bit encryption image. 56-bit DES encryption is subject to U.S. Government export control regulations.

To establish an encrypted Kerberized Telnet session from a router to a remote host, use either of the following commands in EXEC command mode:

Command

Purpose 


Router(config)# connect  host
 [port
] /encrypt kerberos
or 

Router(config)# telnet  host
 [port
] /encrypt kerberos

Establishes an encrypted Telnet session.

When a user opens a Telnet session from a router to a remote host, the router and remote host negotiate to authenticate the user using Kerberos credentials. If this authentication is successful, the router and remote host then negotiate whether or not to use encryption. If this negotiation is successful, both inbound and outbound traffic is encrypted using 56-bit DES encryption with 64-bit CFB.

When a user dials in from a remote host to a router configured for Kerberos authentication, the host and router will attempt to negotiate whether or not to use encryption for the Telnet session. If this negotiation is successful, the router will encrypt all outbound data during the Telnet session.

If encryption is not successfully negotiated, the session will be terminated and the user will receive a message stating that the encrypted Telnet session was not successfully established.

For information about enabling bidirectional encryption from a remote host, refer to the documentation specific to the remote host device.

For an example of using encrypted Kerberized Telnet to open a secure Telnet session, see the section “Encrypting a Telnet Session Example” later in this chapter.

Enabling Mandatory Kerberos Authentication

As an added layer of security, you can optionally configure the router so that, after remote users authenticate to it, these users can authenticate to other services on the network only with Kerberized Telnet, rlogin, rsh, and rcp. If you do not make Kerberos authentication mandatory and Kerberos authentication fails, the application attempts to authenticate users using the default method of authentication for that network service; for example, Telnet and rlogin prompt for a password, and rsh attempts to authenticate using the local rhost file.

To make Kerberos authentication mandatory, use the following command in global configuration mode:

Command

Purpose 


Router(config)# kerberos clients mandatory

Sets Telnet, rlogin, rsh, and rcp to fail if they cannot negotiate the Kerberos protocol with the remote server.

Enabling Kerberos Instance Mapping

As mentioned in the section “Creating and Extracting a SRVTAB on the KDC,” you can create administrative instances of users in the KDC database. The kerberos instance map command allows you to map those instances to Cisco IOS privilege levels so that users can open secure Telnet sessions to the router at a predefined privilege level, obviating the need to enter a clear text password to enter enable mode.

To map a Kerberos instance to a Cisco IOS privilege level, use the following command in global configuration mode:

Command

Purpose 


Router(config)# kerberos instance map  instance
 privilege-level

Maps a Kerberos instance to a Cisco IOS privilege level.

If there is a Kerberos instance for user loki in the KDC database (for example, loki/admin ), user loki can now open a Telnet session to the router as loki/admin and authenticate automatically at privilege level 15, assuming instance “admin” is mapped to privilege level 15. (See the section “Adding Users to the KDC Database” earlier in this chapter.)

Cisco IOS commands can be set to various privilege levels using the privilege level command.

After you map a Kerberos instance to a Cisco IOS privilege level, you must configure the router to check for Kerberos instances each time a user logs in. To run authorization to determine if a user is allowed to run an EXEC shell based on a mapped Kerberos instance, use the aaa authorization command with the krb5-instance keyword. For more information, refer to the chapter “Configuring Authorization.”

Monitoring and Maintaining Kerberos

To display or remove a current user’s credentials, use the following commands in EXEC mode:

SUMMARY STEPS

  1. Router# show kerberos creds
  2. Router# clear kerberos creds

DETAILED STEPS

  Command or Action Purpose
Step 1

Router# show kerberos creds

Lists the credentials in a current user’s credentials cache.

Step 2

Router# clear kerberos creds

Destroys all credentials in a current user’s credentials cache, including those forwarded.

Configuration Examples for Kerberos

Defining a Kerberos Realm Examples

To define CISCO.COM as the default Kerberos realm, use the following command: 


kerberos local-realm CISCO.COM

To tell the router that the CISCO.COM KDC is running on host 10.2.3.4 at port number 170, use the following Kerberos command: 


kerberos server CISCO.COM 10.2.3.4 170

To map the DNS domain cisco.com to the Kerberos realm CISCO.COM, use the following command: 


kerberos realm.cisco.com CISCO.COM

Copying a SRVTAB File Example

To copy over the SRVTAB file on a host named host123.cisco.com for a router named router1.cisco.com, the command would look like this: 


kerberos srvtab remote host123.cisco.com router1.cisco.com-new-srvtab

Configuring Kerberos Examples

This section provides a typical non-Kerberos router configuration and shows output for this configuration from the write term command, then builds on this configuration by adding optional Kerberos functionality. Output for each configuration is presented for comparison against the previous configuration.

This example shows how to use the kdb5_edit program to perform the following configuration tasks:

  • Adding user chet to the Kerberos database

  • Adding a privileged Kerberos instance of user chet (chet/admin) to the Kerberos database

  • Adding a restricted instance of chet (chet/restricted) to the Kerberos database

  • Adding workstation chet-ss20.cisco.com

  • Adding router chet-2500.cisco.com to the Kerberos database

  • Adding workstation chet-ss20.cisco.com to the Kerberos database

  • Extracting SRVTABs for the router and workstations

  • Listing the contents of the KDC database (with the ldb command)


Note

In this sample configuration, host chet-ss20 is also the KDC: 


chet-ss20# sbin/kdb5_edit
kdb5_edit:  ank chet
Enter password:
Re-enter password for verification:
kdb5_edit:  ank chet/admin
Enter password:
Re-enter password for verification:
kdb5_edit:  ank chet/restricted
Enter password:
Re-enter password for verification:
kdb5_edit:  ark host/chet-ss20.cisco.com
kdb5_edit:  ark host/chet-2500.cisco.com
kdb5_edit:  xst chet-ss20.cisco.com host
'host/chet-ss20.cisco.com@CISCO.COM' added to keytab 'WRFILE:chet-ss20.cisco.com-new-srvtab'
kdb5_edit:  xst chet-2500.cisco.com host
'host/chet-2500.cisco.com@CISCO.COM' added to keytab 'WRFILE:chet-2500.cisco.com-new-srvtab'
kdb5_edit:  ldb
entry: host/chet-2500.cisco.com@CISCO.COM
entry: chet/restricted@CISCO.COM
entry: chet@CISCO.COM
entry: K/M@CISCO.COM
entry: host/chet-ss20.cisco.com@CISCO.COM
entry: krbtgt/CISCO.COM@CISCO.COM
entry: chet/admin@CISCO.COM
kdb5_edit:  q
chet-ss20#

The following example shows output from a write term command, which displays the configuration of router chet-2500. This is a typical configuration with no Kerberos authentication. 


chet-2500# write term
Building configuration...
Current configuration:
!
! Last configuration 
change at 14:03:55 PDT Mon May 13 1996
!
version 11.2
service udp-small-servers
service tcp-small-servers
!
hostname chet-2500
!
clock timezone PST -8
clock summer-time PDT recurring
aaa new-model
aaa authentication login console none
aaa authentication ppp local local
enable password sMudgKin
!
username chet-2500 password 7 sMudgkin
username chet-3000 password 7 sMudgkin
username chetin password 7 sMudgkin
!
interface Ethernet0
 ip address 172.16.0.0 255.255.255.0
!
interface Serial0
 no ip address
 shutdown
 no fair-queue
!
interface Serial1
 no ip address
 shutdown
 no fair-queue
!
interface Async2
 ip unnumbered Ethernet0
 encapsulation ppp
 shutdown
 async dynamic routing
 async mode dedicated
 no cdp enable
 ppp authentication pap local
 no tarp propagate
!
interface Async3
 ip unnumbered Ethernet0
 encapsulation ppp
 shutdown
 async dynamic address
 async dynamic routing
 async mode dedicated
 no cdp enable
 ppp authentication pap local
 no tarp propagate
!
router eigrp 109
 network 172.17.0.0
 no auto-summary
!
ip default-gateway 172.30.55.64
ip domain-name cisco.com
ip name-server 192.168.0.0
ip classless
!
!
line con 0
 exec-timeout 0 0
 login authentication console
line 1 16
 transport input all
line aux 0
 transport input all
line vty 0 4
 password sMudgKin
!
ntp clock-period 17179703
ntp peer 172.19.10.0
ntp peer 172.19.0.0
end

The following example shows how to enable user authentication on the router via the Kerberos database. To enable user authentication via the Kerberos database, you would perform the following tasks:

  • Entering configuration mode

  • Defining the Kerberos local realm

  • Identifying the machine hosting the KDC

  • Enabling credentials forwarding

  • Specifying Kerberos as the method of authentication for login

  • Exiting configuration mode (CTL-Z)

  • Writing the new configuration to the terminal 


chet-2500# configure term
Enter configuration commands, one per line. End with CNTL/Z.
chet-2500(config)# kerberos local-realm CISCO.COM
chet-2500(config)# kerberos server CISCO.COM chet-ss20
Translating "chet-ss20"...domain server (192.168.0.0) [OK]
chet-2500(config)# kerberos credentials forward
 
chet-2500(config)# aaa authentication login default krb5
chet-2500(config)#
chet-2500#
%SYS-5-CONFIG_I: Configured from console by console
chet-2500# write term

Compare the following configuration with the previous one. In particular, look at the lines beginning with the words “aaa,” “username,” and “kerberos” (lines 10 through 20) in this new configuration. 


Building configuration...
Current configuration:
!
! Last configuration change at 14:05:54 PDT Mon May 13 1996
!
version 11.2
service udp-small-servers
service tcp-small-servers
!
hostname chet-2500
!
clock timezone PST -8
clock summer-time PDT recurring
aaa new-model
aaa authentication login default krb5
aaa authentication login console none
aaa authentication ppp local local
enable password sMudgKin
!
username chet-2500 password 7 sMudgkin
username chet-3000 password 7 sMudgkin
username chetin password 7 sMudgkin
kerberos local-realm CISCO.COM
kerberos server CISCO.COM 172.71.54.14
kerberos credentials forward
!
interface Ethernet0
 ip address 172.16.0.0 255.255.255.0
!
interface Serial0
 no ip address
 shutdown
 no fair-queue
!
interface Serial1
 no ip address
 shutdown
 no fair-queue
!
interface Async2
 ip unnumbered Ethernet0
 encapsulation ppp
 shutdown
 async dynamic routing
 async mode dedicated
 no cdp enable
 ppp authentication pap local
 no tarp propagate
!
interface Async3
 ip unnumbered Ethernet0
 encapsulation ppp
 shutdown
 async dynamic address
 async dynamic routing
 async mode dedicated
 no cdp enable
 ppp authentication pap local
 no tarp propagate
!
router eigrp 109
 network 172.17.0.0
 no auto-summary
!
ip default-gateway 172.30.55.64
ip domain-name cisco.com
ip name-server 192.168.0.0
ip classless
!
!
line con 0
 exec-timeout 0 0
 login authentication console
line 1 16
 transport input all
line aux 0
 transport input all
line vty 0 4
 password sMudgKin
!
ntp clock-period 17179703
ntp peer 172.19.10.0
ntp peer 172.19.0.0
end

With the router configured thus far, user chet can log in to the router with a username and password and automatically obtain a TGT, as illustrated in the next example. With possession of a credential, user chet successfully authenticates to host chet-ss20 without entering a username/password. 


chet-ss20% telnet chet-2500
Trying 172.16.0.0 ...
Connected to chet-2500.cisco.com.
Escape character is '^]'.
User Access Verification
Username: chet
Password: 
chet-2500> show kerberos creds
 
Default Principal:  chet@CISCO.COM
Valid Starting          Expires                 Service Principal
13-May-1996 14:05:39    13-May-1996 22:06:40    krbtgt/CISCO.COM@CISCO.COM
chet-2500> telnet chet-ss20
Trying chet-ss20.cisco.com (172.71.54.14)... Open
Kerberos:       Successfully forwarded credentials
SunOS UNIX (chet-ss20) (pts/7)
Last login: Mon May 13 13:47:35 from chet-ss20.cisco.c
Sun Microsystems Inc.   SunOS 5.4       Generic July 1994
unknown mode: new
chet-ss20%

The following example shows how to authenticate to the router using Kerberos credentials. To authenticate using Kerberos credentials, you would perform the following tasks:

  • Entering configuration mode

  • Remotely copying over the SRVTAB file from the KDC

  • Setting authentication at login to use the Kerberos 5 Telnet authentication protocol when using Telnet to connect to the router

  • Writing the configuration to the terminal

Note that the new configuration contains a kerberos srvtab entry line. This line is created by the kerberos srvtab remote command. 


chet-2500# configure term
Enter configuration commands, one per line.  End with CNTL/Z.
chet-2500(config)# kerberos srvtab remote earth chet/chet-2500.cisco.com-new-srvtab
Translating "earth"...domain server (192.168.0.0) [OK]
Loading chet/chet-2500.cisco.com-new-srvtab from 172.68.1.123 (via Ethernet0): !
[OK - 66/1000 bytes]
chet-2500(config)# aaa authentication login default krb5-telnet krb5 
chet-2500(config)#
chet-2500#
%SYS-5-CONFIG_I: Configured from console by console
chet-2500# write term
Building configuration...
Current configuration:
!
! Last configuration change at 14:08:32 PDT Mon May 13 1996
!
version 11.2
service udp-small-servers
service tcp-small-servers
!
hostname chet-2500
!
clock timezone PST -8
clock summer-time PDT recurring
aaa new-model
aaa authentication login default krb5-telnet krb5
aaa authentication login console none
aaa authentication ppp local local
enable password sMudgKin
!
username chet-2500 password 7 sMudgkin
username chet-3000 password 7 sMudgkin
username chetin password 7 sMudgkin
kerberos local-realm CISCO.COM
kerberos srvtab entry host/chet-2500.cisco.com@CISCO.COM 0 832015393 1 1 8 7 sMudgkin
kerberos server CISCO.COM 172.71.54.14
kerberos credentials forward
!
interface Ethernet0
 ip address 172.16.0.0 255.255.255.0
!
interface Serial0
 no ip address
 shutdown
 no fair-queue
!
interface Serial1
 no ip address
 shutdown
 no fair-queue
!
interface Async2
 ip unnumbered Ethernet0
 encapsulation ppp
 shutdown
 async dynamic routing
 async mode dedicated
 no cdp enable
 ppp authentication pap local
 no tarp propagate
!
interface Async3
 ip unnumbered Ethernet0
 encapsulation ppp
 shutdown
 async dynamic address
 async dynamic routing
 async mode dedicated
 no cdp enable
 ppp authentication pap local
 no tarp propagate
!
router eigrp 109
 network 172.17.0.0
 no auto-summary
!
ip default-gateway 172.30.55.64
ip domain-name cisco.com
ip name-server 192.168.0.0
ip classless
!
!
line con 0
 exec-timeout 0 0
 login authentication console
line 1 16
 transport input all
line aux 0
 transport input all
line vty 0 4
 password sMudgKin
!
ntp clock-period 17179703
ntp peer 172.19.10.0
ntp peer 172.19.0.0
end
chet-2500#

With this configuration, the user can Telnet in to the router using Kerberos credentials, as illustrated in the next example: 


chet-ss20% bin/telnet -a -F chet-2500
Trying 172.16.0.0...
Connected to chet-2500.cisco.com.
Escape character is '^]'.
[ Kerberos V5 accepts you as "chet@CISCO.COM" ]
User Access Verification
chet-2500>[ Kerberos V5 accepted forwarded credentials ]
chet-2500> show kerberos creds
Default Principal:  chet@CISCO.COM
Valid Starting          Expires                 Service Principal
13-May-1996 15:06:25    14-May-1996 00:08:29    krbtgt/CISCO.COM@CISCO.COM
chet-2500>q
Connection closed by foreign host.
chet-ss20%

The following example shows how to map Kerberos instances to Cisco’s privilege levels. To map Kerberos instances to privilege levels, you would perform the following tasks:

  • Entering configuration mode

  • Mapping the Kerberos instance admin to privilege level 15

  • Mapping the Kerberos instance restricted to privilege level 3

  • Specifying that the instance defined by the kerberos instance map command be used for AAA Authorization

  • Writing the configuration to the terminal 


chet-2500# configure term
Enter configuration commands, one per line.  End with CNTL/Z.
chet-2500(config)# kerberos instance map admin 15
chet-2500(config)# kerberos instance map restricted 3
chet-2500(config)# aaa authorization exec default krb5-instance
chet-2500(config)#
chet-2500#
%SYS-5-CONFIG_I: Configured from console by console
chet-2500# write term
Building configuration...
Current configuration:
!
! Last configuration change at 14:59:05 PDT Mon May 13 1996
!
version 11.2
service udp-small-servers
service tcp-small-servers
!
hostname chet-2500
!
aaa new-model
aaa authentication login default krb5-telnet krb5
aaa authentication login console none
aaa authentication ppp default krb5 local
aaa authorization exec default krb5-instance
enable password sMudgKin
!
username chet-2500 password 7 sMudgkin
username chet-3000 password 7 sMudgkin
username chetin password 7 sMudgkin
ip domain-name cisco.com
ip name-server 192.168.0.0
kerberos local-realm CISCO.COM
kerberos srvtab entry host/chet-2500.cisco.com@CISCO.COM 0 832015393 1 1 8 7 sMudgkin
kerberos server CISCO.COM 172.71.54.14
kerberos instance map admin 15
kerberos instance map restricted 3
kerberos credentials forward
clock timezone PST -8
clock summer-time PDT recurring
!
interface Ethernet0
 ip address 172.16.0.0 255.255.255.0
!
interface Serial0
 no ip address
 shutdown
 no fair-queue
!
interface Serial1
 no ip address
 shutdown
 no fair-queue
!
interface Async2
 ip unnumbered Ethernet0
 encapsulation ppp
 shutdown
 async dynamic routing
 async mode dedicated
 no cdp enable
 ppp authentication pap local
 no tarp propagate
!
interface Async3
 ip unnumbered Ethernet0
 encapsulation ppp
 shutdown
 async dynamic address
 async dynamic routing
 async mode dedicated
 no cdp enable
 ppp authentication pap local
 no tarp propagate
!
router eigrp 109
 network 172.17.0.0
no auto-summary
!
ip default-gateway 172.30.55.64
ip classless
!
!
line con 0
 exec-timeout 0 0
 login authentication console
line 1 16
 transport input all
line aux 0
 transport input all
line vty 0 4
 password sMudgKin
!
ntp clock-period 17179703
ntp peer 172.19.10.0
ntp peer 172.19.0.0
end
chet-2500#

The following example shows output from the three types of sessions now possible for user chet with Kerberos instances turned on: 


chet-ss20% telnet chet-2500
Trying 172.16.0.0 ...
Connected to chet-2500.cisco.com.
Escape character is '^]'.
User Access Verification
Username: chet
Password: 
chet-2500> show kerberos creds
Default Principal:  chet@CISCO.COM
Valid Starting          Expires                 Service Principal
13-May-1996 14:58:28    13-May-1996 22:59:29    krbtgt/CISCO.COM@CISCO.COM
chet-2500> show privilege
Current privilege level is 1
chet-2500> q
Connection closed by foreign host.
chet-ss20% telnet chet-2500
Trying 172.16.0.0 ...
Connected to chet-2500.cisco.com.
Escape character is '^]'.
User Access Verification
Username: chet/admin
Password: 
chet-2500# show kerberos creds
Default Principal:  chet/admin@CISCO.COM
Valid Starting          Expires                 Service Principal
13-May-1996 14:59:44    13-May-1996 23:00:45    krbtgt/CISCO.COM@CISCO.COM
chet-2500# show privilege
Current privilege level is 15
chet-2500# q
Connection closed by foreign host.
chet-ss20% telnet chet-2500
Trying 172.16.0.0 ...
Connected to chet-2500.cisco.com.
Escape character is '^]'.
User Access Verification
Username: chet/restricted
Password: 
chet-2500# show kerberos creds
Default Principal:  chet/restricted@CISCO.COM
Valid Starting          Expires                 Service Principal
13-May-1996 15:00:32    13-May-1996 23:01:33    krbtgt/CISCO.COM@CISCO.COM
chet-2500# show privilege
Current privilege level is 3
chet-2500# q
Connection closed by foreign host.
chet-ss20%

Encrypting a Telnet Session Example

The following example shows how to establish an encrypted Telnet session from a router to a remote host named “host1”: 


Router>
 telnet host1 /encrypt kerberos

Additional References

Related Documents

Related Topic

Document Title

User Authentication

Cisco IOS Security Guide: Securing User Services

RFCs

RFC

Title

RFC 2942

Telnet Authentication: Kerberos Version 5

Technical Assistance

Description

Link

The Cisco Support and Documentation website provides online resources to download documentation, software, and tools. Use these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products and technologies. Access to most tools on the Cisco Support and Documentation website requires a Cisco.com user ID and password.

http://www.cisco.com/cisco/web/support/index.html

Feature Information for Configuring Kerberos

The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
Table 3. Feature Information for Configuring Kerberos

Feature Name

Releases

Feature Information

Configuring Kerberos

11.1

Kerberos is a secret-key network authentication protocol, developed at the Massachusetts Institute of Technology (MIT), that uses the Data Encryption Standard (DES) cryptographic algorithm for encryption and authentication. Kerberos was designed to authenticate requests for network resources. Kerberos, like other secret-key systems, is based on the concept of a trusted third party that performs secure verification of users and services. In the Kerberos protocol, this trusted third party is called the key distribution center (KDC).

In Cisco IOS Release 11.1, this feature was introduced.