Configuring certificate-based authentication

You can configure certificate-based authentication for FortiGate administrators, SSL VPN users, and IPsec VPN users.

In Microsoft Windows 7, you can use the certificate manager to keep track of all the different certificates on your local computer. To access certificate manager, in Windows 7 press the Windows key, enter “certmgr.msc” at the search prompt, and select the displayed match. Remember that in addition to these system certificates, many applications require you to register certificates with them directly.

To see FortiClient certificates, open the FortiClient Console, and select VPN. The VPN menu has options for My Certificates (local or client) and CA Certificates (root or intermediary certificate authorities). Use Import on those screens to import certificate files from other sources.

Authenticating administrators with security certificates

You can install a certificate on the management computer to support strong authentication for administrators. When a personal certificate is installed on the management computer, the FortiGate unit processes the certificate after the administrator supplies a username and password.

To enable strong administrative authentication:

• Obtain a signed personal certificate for the administrator from a CA and load the signed personal certificate into the web browser on the management computer according to the browser documentation.
• Install the root certificate and the CRL from the issuing CA on the FortiGate unit (see Installing a CA root certificate and CRL to authenticate remote clients on page 529 ).
• Create a PKI user account for the administrator.
• Add the PKI user account to a firewall user group dedicated to PKI-authenticated administrators.
• In the administrator account configuration, select PKI as the account Type and select the User Group to which the administrator belongs.

Authenticating SSL VPN users with security certificates

While the default self-signed certificates can be used for HTTPS connections, it is preferable to use the X.509 server certificate to avoid the redirection as it can be misinterpreted as possible session hijacking. However, the server certificate method is more complex than self-signed security certificates. Also the warning message is typically displayed for the initial connection, and future connections will not generate these messages.

X.509 certificates can be used to authenticate IPsec VPN peers or clients, or SSL VPN clients. When configured to authenticate a VPN peer or client, the FortiGate unit prompts the VPN peer or client to authenticate itself using the X.509 certificate. The certificate supplied by the VPN peer or client must be verifiable using the root CA certificate installed on the FortiGate unit in order for a VPN tunnel to be established.

To enable certificate authentication for an SSL VPN user group:

1. Install a signed server certificate on the FortiGate unit and install the corresponding root certificate (and CRL) from the issuing CA on the remote peer or client.

2. Obtain a signed group certificate from a CA and load the signed group certificate into the web browser used by each user. Follow the browser documentation to load the certificates.

3. Install the root certificate and the CRL from the issuing CA on the FortiGate unit (see Installing a CA root certificate and CRL to authenticate remote clients on page 529).

4. Create a PKI user for each SSL VPN user. For each user, specify the text string that appears in the Subject field of the user’s certificate and then select the corresponding CA certificate.

5. Use the config user peergrp CLI command to create a peer user group. Add to this group all of the SSL VPN users who are authenticated by certificate.

6. Go to Policy & Objects > Policy > IPv4.

7. Edit the SSL-VPN security policy.

8. Select the user group created earlier in the Source User(s) field.

9. Select OK.

Authenticating IPsec VPN users with security certificates

To require VPN peers to authenticate by means of a certificate, the FortiGate unit must offer a certificate to authenticate itself to the peer.

To enable the FortiGate unit to authenticate itself with a certificate:

1. Install a signed server certificate on the FortiGate unit.

See To install or import the signed server certificate – web-based manager on page 529.

2. Install the corresponding CA root certificate on the remote peer or client. If the remote peer is a FortiGate unit, see To install a CA root certificate on page 529.

3. Install the certificate revocation list (CRL) from the issuing CA on the remote peer or client. If the remote peer is a

FortiGate unit, see To import a certificate revocation list on page 529.

4. In the VPN phase 1 configuration, set Authentication Method to Signature and from the Certificate Name list select the certificate that you installed in Step 1.

To authenticate a VPN peer using a certificate, you must install a signed server certificate on the peer. Then, on the FortiGate unit, the configuration depends on whether there is only one VPN peer or if this is a dialup VPN that can be multiple peers.

To configure certificate authentication of a single peer

1. Install the CA root certificate and CRL.

2. Create a PKI user to represent the peer. Specify the text string that appears in the Subject field of the user’s certificate and then select the corresponding CA certificate.

3. In the VPN phase 1 Peer Options, select peer certificate for Accept Types field and select the PKI user that you created in the Peer certificate field.

To configure certificate authentication of multiple peers (dialup VPN)

1. Install the corresponding CA root certificate and CRL.

2. Create a PKI user for each remote VPN peer. For each user, specify the text string that appears in the Subject field of the user’s certificate and then select the corresponding CA certificate.

3. Use the config user peergrp CLI command to create a peer user group. Add to this group all of the PKI

users who will use the IPsec VPN.

In the VPN phase 1 Peer Options, select peer certificate group for Accept Types field and select the PKI

user group that you created in the Peer certificate group field.

Online updates to certificates and CRLs

If you obtained your local or CA certificate using SCEP, you can configure online renewal of the certificate before it expires. Similarly, you can receive online updates to CRLs.

Local certificates

In the config vpn certificate local command, you can specify automatic certificate renewal. The relevant fields are:

scep-url <URL_str>             The URL of the SCEP server. This can be HTTP or HTTPS. The following options appear after you add the <URL_str>.

scep-password <password_str>  The password for the SCEP server.

auto-regenerate-days <days_

int>

How many days before expiry the FortiGate unit requests an updated local certificate. The default is 0, no auto-update.

auto-regenerate-days-warning

<days_int>

How many days before local certificate expiry the FortiGate gen- erates a warning message. The default is 0, no warning.

In this example, an updated certificate is requested three days before it expires.

config vpn certificate local edit mycert

set scep-url http://scep.example.com/scep set scep-server-password my_pass_123

set auto-regenerate-days 3

set auto-regenerate-days-warning 2 end

CA certificates

In the config vpn certificate ca command, you can specify automatic certificate renewal. The relevant fields are:

Variable                                                    Description

scep-url <URL_str>             The URL of the SCEP server. This can be HTTP or HTTPS.

Variable                                                    Description

auto-update-days <days_int>   How many days before expiry the FortiGate unit requests an updated CA certificate. The default is 0, no auto-update.

auto-update-days-warning

<days_int>

How many days before CA certificate expiry the FortiGate gen- erates a warning message. The default is 0,no warning.

In this example, an updated certificate is requested three days before it expires.

config vpn certificate ca edit mycert

set scep-url http://scep.example.com/scep set auto-update-days 3

set auto-update-days-warning 2 end

Certificate Revocation Lists

If you obtained your CRL using SCEP, you can configure online updates to the CRL using the config vpn certificate crl command. The relevant fields are:

Variable                                                    Description

http-url <http_url>            URL of the server used for automatic CRL certificate updates.

This can be HTTP or HTTPS.

scep-cert <scep_certificate>  Local certificate used for SCEP communication for CRL auto- update.

scep-url <scep_url>            URL of the SCEP CA server used for automatic CRL certificate updates. This can be HTTP or HTTPS.

update-interval <seconds>

How frequently, in seconds, the FortiGate unit checks for an updated CRL. Enter 0 to update the CRL only when it expires. Not available for http URLs.

update-vdom <update_vdom>      VDOM used to communicate with remote SCEP server for CRL

auto-update.

In this example, an updated CRL is requested only when it expires.

config vpn certificate crl edit cert_crl

set http-url http://scep.example.com/scep set scep-cert my-scep-cert

set scep-url http://scep.ca.example.com/scep set update-interval 0

set update-vdom root end

Troubleshooting certificates

There are times when there are problems with certificates — a certificate is seen as expired when its not, or it can’t be found. Often the problem is with a third party web site, and not FortiOS. However, some problems can be traced back to FortiOS such as DNS or routing issues.

Certificate is reported as expired when it is not

Certificates often are issued for a set period of time such as a day or a month, depending on their intended use. This ensures everyone is using up-to-date certificates. It is also more difficult for hackers to steal and use old certificates.

Reasons a certificate may be reported as expired include:

• It really has expired based on the “best before” date in the certificate
• The FortiGate unit clock is not properly set. If the FortiGate clock is fast, it will see a certificate as expired before the expiry date is really here.
• The requesting server clock is not properly set. A valid example is if your certificate is 2 hours from expiring, a server more than two time zones away would see the certificate as expired. Otherwise, if the server’s clock is set wrongly it will also have the same effect.
• The certificate was revoked by the issuer before the expiry date. This may happen if the issuer believes a certificate was either stolen or misused. Its possible it is due to reasons on the issuer’s side, such as a system change or such. In either case it is best to contact the certificate issuer to determine what is happening and why.

A secure connection cannot be completed (Certificate cannot be found)

Everyone who uses a browser has encountered a message such as This connection is untrusted. Normally when you try to connect securely to a web site, that web site will present its valid certificate to prove their identity is valid. When the web site’s certificate cannot be verified as valid, the message appears stating This connection is untrusted or something similar. If you usually connect to this web site without problems, this error could mean that someone is trying to impersonate or hijack the web site, and best practices dictates you not continue.

Reasons a web site’s certificate cannot be validated include:

• The web site uses an unrecognized self-signed certificate. These are not secure because anyone can sign them. If you accept self-signed certificates you do so at your own risk. Best practices dictate that you must confirm the ID of the web site using some other method before you accept the certificate.
• The certificate is valid for a different domain. A certificate is valid for a specific location, domain, or sub-section of a domain such as one certificate for support.example.com that is not valid for marketing.example.com. If you encounter this problem, contact the webmaster for the web site to inform them of the problem.
• There is a DNS or routing problem. If the web site’s certificate cannot be verified, it will not be accepted. Generally to be verified, your system checks with the third party certificate signing authority to verify the certificate is valid. If you cannot reach that third party due to some DNS or routing error, the certificate will not be verified.
• Firewall is blocking required ports. Ensure that any firewalls between the requesting computer and the web site allow the secure traffic through the firewall. Otherwise a hole must be opened to allow it through. This includes ports such as 443 (HTTPS) and 22 (SSH).

Installing a CA root certificate and CRL to authenticate remote clients

When you apply for a signed personal or group certificate to install on remote clients, you can obtain the corresponding root certificate and CRL from the issuing CA. When you receive the signed personal or group certificate, install the signed certificate on the remote client(s) according to the browser documentation. Install the corresponding root certificate (and CRL) from the issuing CA on the FortiGate unit according to the procedures given below.

To install a CA root certificate

1. After you download the root certificate of the CA, save the certificate on the management computer. Or, you can use online SCEP to retrieve the certificate.

2. On the FortiGate unit, go to System > Certificates > Import > CA Certificates.

3. Do one of the following:

• To import using SCEP, select SCEP. Enter the URL of the SCEP server from which to retrieve the CA

certificate. Optionally, enter identifying information of the CA, such as the filename.

• To import from a file, select Local PC, then select Browse and find the location on the management computer where the certificate has been saved. Select the certificate, and then select Open.

5. Select OK, and then select Return.

The system assigns a unique name to each CA certificate. The names are numbered consecutively (CA_Cert_1, CA_Cert_2, CA_Cert_3, and so on).

To import a certificate revocation list

A Certificate Revocation List (CRL) is a list of the CA certificate subscribers paired with certificate status information. The list contains the revoked certificates and the reason(s) for revocation. It also records the certificate issue dates and the CAs that issued them.

When configured to support SSL VPNs, the FortiGate unit uses the CRL to ensure that the certificates belonging to the CA and remote peers or clients are valid. The CRL has an “effective date” and a “next update” date. The interval is typically 7 days (for Microsoft CA). FortiOS will update the CRL automatically. Also, there is a CLI command to specify an “update-interval” in seconds. Recommendation should be 24 hours (86400 seconds) but depends on company security policy.

1. After you download the CRL from the CA web site, save the CRL on the management computer.

2. Go to System > Certificates > Import > CRL.

3. Do one of the following:

• To import using an HTTP server, select HTTP and enter the URL of the HTTP server.
• To import using an LDAP server see this KB article.
• To import using an SCEP server, select SCEP and select the Local Certificate from the list. Enter the URL of the SCEP server from which the CRL can be retrieved.
• To import from a file, select Local PC, then select Browse and find the location on the management computer where the CRL has been saved. Select the CRL and then select Open.

5. Select OK, and then select Return.

Obtaining and installing a signed server certificate from an external CA

To obtain a signed server certificate for a FortiGate unit, you must send a request to a CA that provides digital certificates that adhere to the X.509 standard. The FortiGate unit provides a way for you to generate the request.

To submit the certificate signing request (file-based enrollment):

1. Using the web browser on the management computer, browse to the CA web site.

2. Follow the CA instructions for a base-64 encoded PKCS#10 certificate request and upload your certificate request.

3. Follow the CA instructions to download their root certificate and CRL.

When you receive the signed server certificate from the CA, install the certificate on the FortiGate unit.

To install or import the signed server certificate – web-based manager

1. On the FortiGate unit, go to System > Certificates > Import > Local Certificates.

2. From Type, select Local Certificate.

3. Select Browse, browse to the location on the management computer where the certificate was saved, select the certificate, and then select Open.

4. Select OK, and then select Return.

Generating certificates with CA software

CA software allows you to generate unmanaged certificates and CA certificates for managing other certificates locally without using an external CA service. Examples of CA software include ssl-ca from OpenSSL (available for Linux, Windows, and Mac) or gensslcert from SuSE, MS Windows Server 2000 and 2003 come with a CA as part of their certificate services, and in MS Windows 2008 CA software can be installed as part of the Active Directory installation. See Example — Generate and Import CA certificate with private key pair on OpenSSL on page 537.

The general steps for generating certificates with CA software are

1. Install the CA software as a stand-alone root CA.

2. Provide identifying information for your self-administered CA.

While following these steps, the methods vary slightly when generating server certificates, CA certificates, and

PKI certificates.

Server certificate

1. Generate a Certificate Signing Request (CSR) on the FortiGate unit.

2. Copy the CSR base-64 encoded text (PKCS10 or PKCS7) into the CA software and generate the certificate.

PKCS10 is the format used to send the certificate request to the signing authority. PKCS7 is the format the signing authority can use for the newly signed certificate.

3. Export the certificate as a X.509 DER encoded binary file with .CER extension

4. Upload the certificate file to the FortiGate unit Local Certificates page (type is Certificate).

CA certificate

1. Retrieve the CA Certificate from the CA software as a DER encoded file.

2. Import the CA certificate file to the FortiGate unit at System > Certificates > Import > CA Certificates.

PKI certificate

1. Generate a Certificate Signing Request (CSR) on the FortiGate unit.

2. Copy the CSR base-64 encoded text (PKCS#10 or PKCS#7) into the CA software and generate the certificate.

PKCS10 is the format used to send the certificate request to the signing authority. PKCS7 is the format the signing authority can use for the newly signed certificate.

3. Export the certificate as a X.509 DER encoded binary file with .CER extension.

4. Install the certificate in the user’s web browser or IPsec VPN client as needed.