PKINIT configuration

PKINIT is a preauthentication mechanism for Kerberos 5 which uses X.509 certificates to authenticate the KDC to clients and vice versa. PKINIT can also be used to enable anonymity support, allowing clients to communicate securely with the KDC or with application servers without authenticating as a particular client principal.

Creating certificates

PKINIT requires an X.509 certificate for the KDC and one for each client principal which will authenticate using PKINIT. For anonymous PKINIT, a KDC certificate is required, but client certificates are not. A commercially issued server certificate can be used for the KDC certificate, but generally cannot be used for client certificates.

The instruction in this section describe how to establish a certificate authority and create standard PKINIT certificates. Skip this section if you are using a commercially issued server certificate as the KDC certificate for anonymous PKINIT, or if you are configuring a client to use an Active Directory KDC.

Generating a certificate authority certificate

You can establish a new certificate authority (CA) for use with a PKINIT deployment with the commands:

openssl genrsa -out cakey.pem 2048
openssl req -key cakey.pem -new -x509 -out cacert.pem -days 3650

The second command will ask for the values of several certificate fields. These fields can be set to any values. You can adjust the expiration time of the CA certificate by changing the number after -days. Since the CA certificate must be deployed to client machines each time it changes, it should normally have an expiration time far in the future; however, expiration times after 2037 may cause interoperability issues in rare circumstances.

The result of these commands will be two files, cakey.pem and cacert.pem. cakey.pem will contain a 2048-bit RSA private key, which must be carefully protected. cacert.pem will contain the CA certificate, which must be placed in the filesytems of the KDC and each client host. cakey.pem will be required to create KDC and client certificates.

Generating a KDC certificate

A KDC certificate for use with PKINIT is required to have some unusual fields, which makes generating them with OpenSSL somewhat complicated. First, you will need a file containing the following:

[kdc_cert]
basicConstraints=CA:FALSE
keyUsage=nonRepudiation,digitalSignature,keyEncipherment,keyAgreement
extendedKeyUsage=1.3.6.1.5.2.3.5
subjectKeyIdentifier=hash
authorityKeyIdentifier=keyid,issuer
issuerAltName=issuer:copy
subjectAltName=otherName:1.3.6.1.5.2.2;SEQUENCE:kdc_princ_name

[kdc_princ_name]
realm=EXP:0,GeneralString:${ENV::REALM}
principal_name=EXP:1,SEQUENCE:kdc_principal_seq

[kdc_principal_seq]
name_type=EXP:0,INTEGER:1
name_string=EXP:1,SEQUENCE:kdc_principals

[kdc_principals]
princ1=GeneralString:krbtgt
princ2=GeneralString:${ENV::REALM}

If the above contents are placed in extensions.kdc, you can generate and sign a KDC certificate with the following commands:

openssl genrsa -out kdckey.pem 2048
openssl req -new -out kdc.req -key kdckey.pem
env REALM=YOUR_REALMNAME openssl x509 -req -in kdc.req \
    -CAkey cakey.pem -CA cacert.pem -out kdc.pem -days 365 \
    -extfile extensions.kdc -extensions kdc_cert -CAcreateserial
rm kdc.req

The second command will ask for the values of certificate fields, which can be set to any values. In the third command, substitute your KDC’s realm name for YOUR_REALMNAME. You can adjust the certificate’s expiration date by changing the number after -days. Remember to create a new KDC certificate before the old one expires.

The result of this operation will be in two files, kdckey.pem and kdc.pem. Both files must be placed in the KDC’s filesystem. kdckey.pem, which contains the KDC’s private key, must be carefully protected.

If you examine the KDC certificate with openssl x509 -in kdc.pem -text -noout, OpenSSL will not know how to display the KDC principal name in the Subject Alternative Name extension, so it will appear as othername:<unsupported>. This is normal and does not mean anything is wrong with the KDC certificate.

Generating client certificates

PKINIT client certificates also must have some unusual certificate fields. To generate a client certificate with OpenSSL for a single-component principal name, you will need an extensions file (different from the KDC extensions file above) containing:

[client_cert]
basicConstraints=CA:FALSE
keyUsage=digitalSignature,keyEncipherment,keyAgreement
extendedKeyUsage=1.3.6.1.5.2.3.4
subjectKeyIdentifier=hash
authorityKeyIdentifier=keyid,issuer
issuerAltName=issuer:copy
subjectAltName=otherName:1.3.6.1.5.2.2;SEQUENCE:princ_name

[princ_name]
realm=EXP:0,GeneralString:${ENV::REALM}
principal_name=EXP:1,SEQUENCE:principal_seq

[principal_seq]
name_type=EXP:0,INTEGER:1
name_string=EXP:1,SEQUENCE:principals

[principals]
princ1=GeneralString:${ENV::CLIENT}

If the above contents are placed in extensions.client, you can generate and sign a client certificate with the following commands:

openssl genrsa -out clientkey.pem 2048
openssl req -new -key clientkey.pem -out client.req
env REALM=YOUR_REALMNAME CLIENT=YOUR_PRINCNAME openssl x509 \
    -CAkey cakey.pem -CA cacert.pem -req -in client.req \
    -extensions client_cert -extfile extensions.client \
    -days 365 -out client.pem
rm client.req

Normally, the first two commands should be run on the client host, and the resulting client.req file transferred to the certificate authority host for the third command. As in the previous steps, the second command will ask for the values of certificate fields, which can be set to any values. In the third command, substitute your realm’s name for YOUR_REALMNAME and the client’s principal name (without realm) for YOUR_PRINCNAME. You can adjust the certificate’s expiration date by changing the number after -days.

The result of this operation will be two files, clientkey.pem and client.pem. Both files must be present on the client’s host; clientkey.pem, which contains the client’s private key, must be protected from access by others.

As in the KDC certificate, OpenSSL will display the client principal name as othername:<unsupported> in the Subject Alternative Name extension of a PKINIT client certificate.

If the client principal name contains more than one component (e.g. host/example.com@REALM), the [principals] section of extensions.client must be altered to contain multiple entries. (Simply setting CLIENT to host/example.com would generate a certificate for host\/example.com@REALM which would not match the multi-component principal name.) For a two-component principal, the section should read:

[principals]
princ1=GeneralString:${ENV::CLIENT1}
princ2=GeneralString:${ENV::CLIENT2}

The environment variables CLIENT1 and CLIENT2 must then be set to the first and second components when running openssl x509.

Configuring the KDC

The KDC must have filesystem access to the KDC certificate (kdc.pem) and the KDC private key (kdckey.pem). Configure the following relation in the KDC’s kdc.conf file, either in the [kdcdefaults] section or in a [realms] subsection (with appropriate pathnames):

pkinit_identity = FILE:/var/lib/krb5kdc/kdc.pem,/var/lib/krb5kdc/kdckey.pem

If any clients will authenticate using regular (as opposed to anonymous) PKINIT, the KDC must also have filesystem access to the CA certificate (cacert.pem), and the following configuration (with the appropriate pathname):

pkinit_anchors = FILE:/var/lib/krb5kdc/cacert.pem

Because of the larger size of requests and responses using PKINIT, you may also need to allow TCP access to the KDC:

kdc_tcp_listen = 88

Restart the krb5kdc daemon to pick up the configuration changes.

The principal entry for each PKINIT-using client must be configured to require preauthentication. Ensure this with the command:

kadmin -q 'modprinc +requires_preauth YOUR_PRINCNAME'

Starting with release 1.12, it is possible to remove the long-term keys of a principal entry, which can save some space in the database and help to clarify some PKINIT-related error conditions by not asking for a password:

kadmin -q 'purgekeys -all YOUR_PRINCNAME'

These principal options can also be specified at principal creation time as follows:

kadmin -q 'add_principal +requires_preauth -nokey YOUR_PRINCNAME'

By default, the KDC requires PKINIT client certificates to have the standard Extended Key Usage and Subject Alternative Name attributes for PKINIT. Starting in release 1.16, it is possible to authorize client certificates based on the subject or other criteria instead of the standard PKINIT Subject Alternative Name, by setting the pkinit_cert_match string attribute on each client principal entry. For example:

kadmin set_string user@REALM pkinit_cert_match "<SUBJECT>CN=user@REALM$"

The pkinit_cert_match string attribute follows the syntax used by the krb5.conf pkinit_cert_match relation. To allow the use of non-PKINIT client certificates, it will also be necessary to disable key usage checking using the pkinit_eku_checking relation; for example:

[kdcdefaults]
    pkinit_eku_checking = none

Configuring the clients

Client hosts must be configured to trust the issuing authority for the KDC certificate. For a newly established certificate authority, the client host must have filesystem access to the CA certificate (cacert.pem) and the following relation in krb5.conf in the appropriate [realms] subsection (with appropriate pathnames):

pkinit_anchors = FILE:/etc/krb5/cacert.pem

If the KDC certificate is a commercially issued server certificate, the issuing certificate is most likely included in a system directory. You can specify it by filename as above, or specify the whole directory like so:

pkinit_anchors = DIR:/etc/ssl/certs

A commercially issued server certificate will usually not have the standard PKINIT principal name or Extended Key Usage extensions, so the following additional configuration is required:

pkinit_eku_checking = kpServerAuth
pkinit_kdc_hostname = hostname.of.kdc.certificate

Multiple pkinit_kdc_hostname relations can be configured to recognize multiple KDC certificates. If the KDC is an Active Directory domain controller, setting pkinit_kdc_hostname is necessary, but it should not be necessary to set pkinit_eku_checking.

To perform regular (as opposed to anonymous) PKINIT authentication, a client host must have filesystem access to a client certificate (client.pem), and the corresponding private key (clientkey.pem). Configure the following relations in the client host’s krb5.conf file in the appropriate [realms] subsection (with appropriate pathnames):

pkinit_identities = FILE:/etc/krb5/client.pem,/etc/krb5/clientkey.pem

If the KDC and client are properly configured, it should now be possible to run kinit username without entering a password.

Anonymous PKINIT

Anonymity support in Kerberos allows a client to obtain a ticket without authenticating as any particular principal. Such a ticket can be used as a FAST armor ticket, or to securely communicate with an application server anonymously.

To configure anonymity support, you must generate or otherwise procure a KDC certificate and configure the KDC host, but you do not need to generate any client certificates. On the KDC, you must set the pkinit_identity variable to provide the KDC certificate, but do not need to set the pkinit_anchors variable or store the issuing certificate if you won’t have any client certificates to verify. On client hosts, you must set the pkinit_anchors variable (and possibly pkinit_kdc_hostname and pkinit_eku_checking) in order to trust the issuing authority for the KDC certificate, but do not need to set the pkinit_identities variable.

Anonymity support is not enabled by default. To enable it, you must create the principal WELLKNOWN/ANONYMOUS using the command:

kadmin -q 'addprinc -randkey WELLKNOWN/ANONYMOUS'

Some Kerberos deployments include application servers which lack proper access control, and grant some level of access to any user who can authenticate. In such an environment, enabling anonymity support on the KDC would present a security issue. If you need to enable anonymity support for TGTs (for use as FAST armor tickets) without enabling anonymous authentication to application servers, you can set the variable restrict_anonymous_to_tgt to true in the appropriate [realms] subsection of the KDC’s kdc.conf file.

To obtain anonymous credentials on a client, run kinit -n, or kinit -n @REALMNAME to specify a realm. The resulting tickets will have the client name WELLKNOWN/ANONYMOUS@WELLKNOWN:ANONYMOUS.

Freshness tokens

Freshness tokens can ensure that the client has recently had access to its certificate private key. If freshness tokens are not required by the KDC, a client program with temporary possession of the private key can compose requests for future timestamps and use them later.

In release 1.17 and later, freshness tokens are supported by the client and are sent by the KDC when the client indicates support for them. Because not all clients support freshness tokens yet, they are not required by default. To check if freshness tokens are supported by a realm’s clients, look in the KDC logs for the lines:

PKINIT: freshness token received from <client principal>
PKINIT: no freshness token received from <client principal>

To require freshness tokens for all clients in a realm (except for clients authenticating anonymously), set the pkinit_require_freshness variable to true in the appropriate [realms] subsection of the KDC’s kdc.conf file. To test that this option is in effect, run kinit -X disable_freshness and verify that authentication is unsuccessful.