Source Interface Selection for Outgoing Traffic with Certificate Authority
Source Interface Selection for Outgoing Traffic with Certificate Authority
Last Updated: July 18, 2012
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.
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Information About Source Interface Selection for Outgoing Traffic with Certificate Authority
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Directory Access Protocol (LDAP) or HTTP server to determine whether the certificate of the remote device has been revoked.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 interfacecommand is used along with the crypto ca trustpointcommand 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.
How to Configure Source Interface Selection for Outgoing Traffic with Certificate Authority
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.
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 ipcommand 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
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
Feature Information for Source Interface Selection for Outgoing Traffic with Certificate Authority
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.
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.
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Any Internet Protocol (IP) addresses and phone numbers used in this document are not intended to be actual addresses and phone numbers. Any examples, command display output, network topology diagrams, and other figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses or phone numbers in illustrative content is unintentional and coincidental.
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