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Source Interface Selection for Outgoing Traffic with Certificate Authority (CA)
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Source Interface Selection for Outgoing Traffic with Certificate Authority

Table Of Contents

Source Interface Selection for Outgoing Traffic with Certificate Authority

Finding Feature Information

Contents

Information About Source Interface Selection for Outgoing Traffic with Certificate Authority

Certificates That Identify an Entity

Source Interface for Outgoing TCP Connections Associated with a Trustpoint

How to Configure Source Interface Selection for Outgoing Traffic with Certificate Authority

Configuring the Interface for All Outgoing TCP Connections Associated with a Trustpoint

Troubleshooting Tips

Configuration Examples for Source Interface Selection for Outgoing Traffic with Certificate Authority

Source Interface Selection for Outgoing Traffic with Certificate Authority Example

Additional References

Related Documents

MIBs

Technical Assistance

Feature Information for Source Interface Selection for Outgoing Traffic with Certificate Authority

Glossary


Source Interface Selection for Outgoing Traffic with Certificate Authority


First Published: March 17, 2003
Last Updated: March 31, 2011

The Source Interface Selection for Outgoing Traffic with Certificate Authority feature allows the IP address of an interface to be specified and used as the source address for all outgoing TCP connections associated with that trustpoint when a designated trustpoint has been configured.

Finding Feature Information

Your software release may not support all the features documented in this module. For the latest feature information and caveats, see 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 for Source Interface Selection for Outgoing Traffic with Certificate Authority" section.

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

Contents

Information About Source Interface Selection for Outgoing Traffic with Certificate Authority

How to Configure Source Interface Selection for Outgoing Traffic with Certificate Authority

Configuration Examples for Source Interface Selection for Outgoing Traffic with Certificate Authority

Additional References

Command Reference, page 7

Glossary

Information About Source Interface Selection for Outgoing Traffic with Certificate Authority

To configure the Source Interface Selection for Outgoing Traffic with Certificate Authority feature, you must understand the following concepts:

Certificates That Identify an Entity

Source Interface for Outgoing TCP Connections Associated with a Trustpoint

Certificates That Identify an Entity

Certificates can be used to identify an entity. A trusted server, known as the certification authority (CA), issues the certificate to the entity after determining the identity of the entity. A router that is running Cisco IOS software obtains its certificate by making a network connection to the CA. Using the Simple Certificate Enrollment Protocol (SCEP), the router transmits its certificate request to the CA and receives the granted certificate. The router obtains the certificate of the CA in the same manner using SCEP. When validating a certificate from a remote device, the router may again contact the CA or a Lightweight Directory Access Protocol (LDAP) or HTTP server to determine whether the certificate of the remote device has been revoked. (This process is known as checking the certificate revocation list [CRL].)

In some configurations, the router may make the outgoing TCP connection using an interface that does not have a valid or IP address that can be routed. The user must specify that the address of a different interface be used as the source IP address for the outgoing connection. Cable modems are a specific example of this requirement because the outgoing cable interface (the RF interface) usually does not have an IP address that can be routed. However, the user interface (usually Ethernet) does have a valid IP address.

Source Interface for Outgoing TCP Connections Associated with a Trustpoint

The crypto ca trustpoint command is used to specify a trustpoint. The source interface command is used along with the crypto ca trustpoint command to specify the address of the interface that is to be used as the source address for all outgoing TCP connections associated with that trustpoint.


Note If the interface address is not specified using the source interface command, the address of the outgoing interface is used.


How to Configure Source Interface Selection for Outgoing Traffic with Certificate Authority

This section includes the following procedure:

Configuring the Interface for All Outgoing TCP Connections Associated with a Trustpoint

Configuring the Interface for All Outgoing TCP Connections Associated with a Trustpoint

Perform this task to configure the interface that you want to use as the source address for all outgoing TCP connections associated with a trustpoint.

SUMMARY STEPS

1. enable

2. configure terminal

3. crypto ca trustpoint name

4. enrollment url url

5. source interface interface-address

6. interface type slot/port

7. description string

8. ip address ip-address mask

9. interface type slot/port

10. description string

11. ip address ip-address mask

12. crypto map map-name

DETAILED STEPS

 
Command or Action
Purpose

Step 1 

enable

Example:

Router> enable

Enables privileged EXEC mode.

Enter your password if prompted.

Step 2 

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3 

crypto ca trustpoint name

Example:

Router (config)# crypto ca trustpoint ms-ca

Declares the Certificate Authority (CA) that your router should use and enters ca-trustpoint configuration mode.

Step 4 

enrollment url url

Example:

Router (ca-trustpoint)# enrollment url http://yourname:80/certsrv/mscep/mscep.dll

Specifies the enrollment parameters of your CA.

Step 5 

source interface interface-address

Example:

Router (ca-trustpoint)# interface ethernet 0

Interface to be used as the source address for all outgoing TCP connections associated with that trustpoint.

Step 6 

interface type slot/port

Example:

Router (ca-trustpoint)# interface ethernet 1

Configures an interface type and enters interface configuration mode.

Step 7 

description string

Example:

Router (config-if)# description inside interface

Adds a description to an interface configuration.

Step 8 

ip address ip-address mask

Example:

Router (config-if)# ip address 10.1.1.1 255.255.255.0

Sets a primary or secondary IP address for an interface.

Step 9 

interface type slot/port

Example:

Router (config-if)# interface ethernet1/0

Configures an interface type.

Step 10 

description string

Example:

Router (config-if)# description outside interface 10.1.1.205 255.255.255.0

Adds a description to an interface configuration.

Step 11 

ip address ip-address mask

Example:

Router (config-if)# ip address 10.2.2.205 255.255.255.0

Sets a primary or secondary IP address for an interface.

Step 12 

crypto map map-name

Example:

Router (config-if)# crypto map mymap

Applies a previously defined crypto map set to an interface.

Troubleshooting Tips

Ensure that the interface specified in the command has a valid address. Attempt to ping the router using the address of the specified interface from another device (possibly the HTTP or LDAP server that is serving the CRL). You can do the same thing by using a traceroute to the router from the external device.

You can also test connectivity between the router and the CA or LDAP server by using Cisco IOS command-line interface (CLI). Enter the ping ip command and respond to the prompts. If you answer "yes" to the "Extended commands [n]:" prompt, you can specify the source address or interface.

In addition, you can use Cisco IOS CLI to input a traceroute command. If you enter the traceroute ip command (in EXEC mode), you are prompted for the destination and source address. You should specify the CA or LDAP server as the destination and the address of the interface that you specified in the "source interface" as the source address.

Configuration Examples for Source Interface Selection for Outgoing Traffic with Certificate Authority

This section includes the following example:

Source Interface Selection for Outgoing Traffic with Certificate Authority Example

Source Interface Selection for Outgoing Traffic with Certificate Authority Example

In the following example, the router is located in a branch office. The router uses IP Security (IPSec) to communicate with the main office. Ethernet 1 is the "outside" interface that connects to the Internet Service Provider (ISP). Ethernet 0 is the interface connected to the LAN of the branch office. To access the CA server located in the main office, the router must send its IP datagrams out interface Ethernet 1 (address 10.2.2.205) using the IPSec tunnel. Address 10.2.2.205 is assigned by the ISP. Address 10.2.2.205 is not a part of the branch office or main office.

The CA cannot access any address outside the company because of a firewall. The CA sees a message coming from 10.2.2.205 and cannot respond (that is, the CA does not know that the router is located in a branch office at address 10.1.1.1, which it is able to reach).

Adding the source interface command tells the router to use address 10.1.1.1 as the source address of the IP datagram that it sends to the CA. The CA is able to respond to 10.1.1.1.

This scenario is configured using the source interface command and the interface addresses as described above.

crypto ca trustpoint ms-ca
 enrollment url http://ms-ca:80/certsrv/mscep/mscep.dll
 source interface ethernet0
!
interface ethernet 0
 description inside interface
 ip address 10.1.1.1 255.255.255.0
!
interface ethernet 1
 description outside interface
 ip address 10.2.2.205 255.255.255.0
 crypto map main-office

Additional References

Related Documents

Related Topic
Document Title

Cisco IOS commands

Cisco IOS Master Commands List, All Releases

Configuring IPSec and certification authority

Security for VPNs with IPsec

IPSec and certification authority commands

Cisco IOS Security Command Reference


MIBs

MIBs
MIBs Link

None.

To locate and download MIBs for selected platforms, Cisco software releases, and feature sets, use Cisco MIB Locator found at the following URL:

http://www.cisco.com/go/mibs


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 Source Interface Selection for Outgoing Traffic with Certificate Authority

Table 1 lists the release history for this feature.

Use Cisco Feature Navigator to find information about platform support and software image support. Cisco Feature Navigator enables you to determine which software images support a specific software release, feature set, or platform. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.


Note Table 1 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.


Table 1 Feature Information for Source Interface Selection for Outgoing Traffic with Certificate Authority

Feature Name
Releases
Feature Information

Source Interface Selection for Outgoing Traffic with Certificate Authority

12.2(15)T

This feature allows the IP address of an interface to be specified and used as the source address for all outgoing TCP connections associated with that trustpoint when a designated trustpoint has been configured.

This feature was introduced in Cisco IOS Release 12.2(15)T.

The following command was introduced or modified: source interface.


Glossary

authenticate—To prove the identity of an entity using the certificate of an identity and a secret that the identity poses (usually the private key corresponding to the public key in the certificate).

CA—Certificate Authority. A CA is an entity that issues digital certificates (especially X.509 certificates) and vouches for the binding between the data items in a certificate.

CA authentication—The user manually approves a certificate from a root CA. Usually a fingerprint of the certificate is presented to the user, and the user is asked to accept the certificate based on the fingerprint. The certificate of a root CA is signed by itself (self-signed) so that it cannot be automatically authenticated using the normal certificate verification process.

CRL—certificate revocation list. A CRL is a data structure that enumerates digital certificates that have been invalidated by their issuer prior to when they were scheduled to expire.

enrollment—A router receives its certificate through the enrollment process. The router generates a request for a certificate in a specific format (known as PKCS #10). The request is transmitted to a CA, which grants the request and generates a certificate encoded in the same format as the request. The router receives the granted certificate and stores it in an internal database for use during normal operations.

certificate—A data structure defined in International Organization for Standardization (ISO) standard X.509 to associate an entity (machine or human) with the public key of that entity. The certificate contains specific fields, including the name of the entity. The certificate is normally issued by a CA on behalf of the entity. In this case the router acts as its own CA. Common fields within a certificate include the distinguished name (DN) of the entity, the DN of the authority issuing the certificate, and the public key of the entity.

LDAP—Lightweight Directory Access Protocol. A LDAP is a protocol that provides access for management and browser applications that provide read-and-write interactive access to the X.500 directory.