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Issuing subordinate CA certificates from FreeIPA

FreeIPA, since version 4.4, has supported creating subordinate CAs within the deployment’s Dogtag CA instance. This feature is called lightweight sub-CAs. But what about when you need to issue a subordinate CA certificate to an external entity? One use case would be chaining a FreeIPA deployment up to some existing FreeIPA deployment. This is similar to what many customers do with Active Directory. In this post I’ll show how you can issue subordinate CA certificates from FreeIPA.

Scenario description §

The existing FreeIPA deployment has the realm IPA.LOCAL and domain ipa.local. Its CA’s Subject Distinguished Name (Subject DN) is CN=Certificate Authority,O=IPA.LOCAL 201808022359. The master’s hostname is f28-0.ipa.local. I will refer to this deployment as the existing or primary deployment.

I will install a new FreeIPA deployment on the host f28-1.ipa.local, with realm SUB.IPA.LOCAL and domain sub.ipa.local. This will be called the secondary deployment. Its CA will be signed by the CA of the primary deployment.

Choice of subject principal and Subject DN §

All certificate issuance via FreeIPA (with some limited exceptions) requires a nominated subject principal. Subject names in the CSR (Subject DN and Subject Alternative Names) are validated against the subject principal. We must create a subject principal in the primary deployment to represent the CA of the secondary deployment.

When validating CSRs, the Common Name (CN) of the Subject DN is checked against the subject principal, in the following ways:

This validation regime imposes a restriction on what the CN of the subordinate CA can be. In particular:

For this scenario, I chose to create a host principal for the domain of the secondary deployment:

[f28-0]% ipa host-add --force sub.ipa.local
Added host "sub.ipa.local"
  Host name: sub.ipa.local
  Principal name: host/sub.ipa.local@IPA.LOCAL
  Principal alias: host/sub.ipa.local@IPA.LOCAL
  Password: False
  Keytab: False
  Managed by: sub.ipa.local

Creating a certificate profile for sub-CAs §

We will tweak the caIPAserviceCert profile configuration to create a new profile for subordinate CAs. Export the profile configuration:

[f28-0]% ipa certprofile-show caIPAserviceCert --out SubCA.cfg
Profile configuration stored in file 'SubCA.cfg'
  Profile ID: caIPAserviceCert
  Profile description: Standard profile for network services
  Store issued certificates: TRUE

Perform the following edits to SubCA.cfg:

  1. Replace profileId=caIPAserviceCert with profileId=SubCA.

  2. Replace the subjectNameDefaultImpl component with the userSubjectNameDefaultImpl component. This will use the Subject DN from the CSR as is, without restriction:

    policyset.serverCertSet.1.constraint.name=No Constraint
    policyset.serverCertSet.1.default.name=Subject Name Default
  3. Edit the keyUsageExtDefaultImpl and keyUsageExtConstraintImpl configurations. They should have the following settings:

    • keyUsageCrlSign=true
    • keyUsageDataEncipherment=false
    • keyUsageDecipherOnly=false
    • keyUsageDigitalSignature=true
    • keyUsageEncipherOnly=false
    • keyUsageKeyAgreement=false
    • keyUsageKeyCertSign=true
    • keyUsageKeyEncipherment=false
    • keyUsageNonRepudiation=true
  4. Add the Basic Constraints extension configuration:

    policyset.serverCertSet.15.constraint.name=Basic Constraint Extension Constraint
    policyset.serverCertSet.15.default.name=Basic Constraints Extension Default

    Add the new components’ index to the component list, to ensure they get processed:

  5. Remove the commonNameToSANDefaultImpl and Extended Key Usage related components. This can be accomplished by removing the relevant indices (in my case, 7 and 12) from the component list:

  6. (Optional) edit the validity period in the validityDefaultImpl and validityConstraintImpl components. The default is 731 days. I did not change it.

For the avoidance of doubt, the diff between the caIPAserviceCert profile configuration and SubCA is:

--- caIPAserviceCert.cfg        2018-08-21 12:44:01.748884778 +1000
+++ SubCA.cfg   2018-08-21 14:05:53.484698688 +1000
@@ -13,5 +13,3 @@
-policyset.serverCertSet.1.constraint.name=Subject Name Constraint
+policyset.serverCertSet.1.constraint.name=No Constraint
@@ -19 +16,0 @@
-policyset.serverCertSet.1.default.params.name=CN=$request.req_subject_name.cn$, o=IPA.LOCAL 201808022359
@@ -66,2 +63,2 @@
@@ -72,2 +69,2 @@
@@ -78,2 +75,2 @@
@@ -84,2 +81,2 @@
@@ -111,2 +108,13 @@
+policyset.serverCertSet.15.constraint.name=Basic Constraint Extension Constraint
+policyset.serverCertSet.15.default.name=Basic Constraints Extension Default

Now import the profile:

[f28-0]% ipa certprofile-import SubCA \
            --desc "Subordinate CA" \
            --file SubCA.cfg \
Imported profile "SubCA"
  Profile ID: SubCA
  Profile description: Subordinate CA
  Store issued certificates: TRUE

Creating the CA ACL §

Before issuing a certificate, CA ACLs are checked to determine if the combination of CA, profile and subject principal is acceptable. We must create a CA ACL that permits use of the SubCA profile to issue certificate to our subject principal:

[f28-0]% ipa caacl-add SubCA
Added CA ACL "SubCA"
  ACL name: SubCA
  Enabled: TRUE

[f28-0]% ipa caacl-add-profile SubCA --certprofile SubCA
  ACL name: SubCA
  Enabled: TRUE
  Profiles: SubCA
Number of members added 1

[f28-0]% ipa caacl-add-ca SubCA --ca ipa
  ACL name: SubCA
  Enabled: TRUE
  CAs: ipa
  Profiles: SubCA
Number of members added 1

[f28-0]% ipa caacl-add-host SubCA --hosts sub.ipa.local
  ACL name: SubCA
  Enabled: TRUE
  CAs: ipa
  Profiles: SubCA
  Hosts: sub.ipa.local
Number of members added 1

Installing the secondary FreeIPA deployment §

We are finally ready to run ipa-server-install to set up the secondary deployment. We need to use the --ca-subject option to override the default Subject DN that will be included in the CSR, providing a valid DN according to the rules discussed above.

[root@f28-1]# ipa-server-install \
    --realm SUB.IPA.LOCAL \
    --domain sub.ipa.local \
    --external-ca \
    --ca-subject 'CN=SUB.IPA.LOCAL,O=Red Hat'


The IPA Master Server will be configured with:
Hostname:       f28-1.ipa.local
IP address(es):
Domain name:    sub.ipa.local
Realm name:     SUB.IPA.LOCAL

The CA will be configured with:
Subject DN:   CN=SUB.IPA.LOCAL,O=Red Hat
Subject base: O=SUB.IPA.LOCAL
Chaining:     externally signed (two-step installation)

Continue to configure the system with these values? [no]: yes


Configuring certificate server (pki-tomcatd). Estimated time: 3 minutes
  [1/8]: configuring certificate server instance

The next step is to get /root/ipa.csr signed by your CA and re-run
/usr/sbin/ipa-server-install as:
The ipa-server-install command was successful

Let’s inspect /root/ipa.csr:

[root@f28-1]# openssl req -text < /root/ipa.csr |grep Subject:
        Subject: O = Red Hat, CN = SUB.IPA.LOCAL

The desired Subject DN appears in the CSR (note that openssl shows DN components in the opposite order from FreeIPA). After copying the CSR to f28-0.ipa.local we can request the certificate:

[f28-0]% ipa cert-request ~/ipa.csr \
            --principal host/sub.ipa.local \
            --profile SubCA \
            --certificate-out ipa.pem
  Issuing CA: ipa
  Certificate: MIIEAzCCAuugAwIBAgIBFTANBgkqhkiG9w0BAQsF...
  Subject: CN=SUB.IPA.LOCAL,O=Red Hat
  Issuer: CN=Certificate Authority,O=IPA.LOCAL 201808022359
  Not Before: Tue Aug 21 04:16:24 2018 UTC
  Not After: Fri Aug 21 04:16:24 2020 UTC
  Serial number: 21
  Serial number (hex): 0x15

The certificate was saved in the file ipa.pem. We can see from the command output that the Subject DN in the certificate is exactly what was in the CSR. Further inspecting the certificate, observe that the Basic Constraints extension is present and the Key Usage extension contains the appropriate assertions:

[f28-0]% openssl x509 -text < ipa.pem
      X509v3 extensions:
          X509v3 Key Usage: critical
              Digital Signature, Non Repudiation, Certificate Sign, CRL Sign
          X509v3 Basic Constraints: critical
              CA:TRUE, pathlen:0

Now, after copying the just-issued subordinate CA certificate and the primary CA certificate (/etc/ipa/ca.crt) over to f28-1.ipa.local, we can continue the installation:

[root@f28-1]# ipa-server-install \
                --external-cert-file ca.crt \
                --external-cert-file ipa.pem

The log file for this installation can be found in /var/log/ipaserver-install.log
Directory Manager password: XXXXXXXX


Adding [ f28-1.ipa.local] to your /etc/hosts file
Configuring ipa-custodia
  [1/5]: Making sure custodia container exists
The ipa-server-install command was successful

And we’re done.

Discussion §

I’ve shown how to create a profile for issuing subordinate CA certificates in FreeIPA. Because of the way FreeIPA validates certificate requests—always against a subject principal—there are restrictions on the what the subject DN of the subordinate CA can be. The Subject DN must contain a CN attribute matching either the hostname of a host or service principal, or the UID of a user principal.

If you want to avoid these Subject DN restrictions, right now there is no choice but to use the Dogtag CA directly, instead of via the FreeIPA commands. If such a requirement emerges it might make sense to implement some “special handling” for issuing sub-CA certificates (similar to what we currently do for the KDC certificate). But the certificate request logic is already complicated; I am hesitant to complicate it even more.

Currently there is no sub-CA profile included in FreeIPA by default. It might make sense to include it, or at least to produce an official solution document describing the procedure outlined in this post.

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