APM Cookbook: Single Sign On (SSO) using Kerberos

To get the APM Cookbook series moving along, I’ve decided to help out by documenting the common APM solutions I help customers and partners with on a regular basis.

Kerberos SSO is nothing new, but seems to stump people who have never used Kerberos before. Getting Kerberos SSO to work with APM is straight forward once you have the Active Directory components configured.

Overview

I have a pre-configured web service (IIS 7.5/Sharepoint 2010) that is configured for Windows Authentication, which will send a “Negotiate” in the header of the “401 Request for Authorization”. Make sure the web service is configured to send the correct header before starting the APM configuration by accessing the website directly and viewing the headers using browser tools.

In my example, I used the Sharepoint 2010/2013 iApp to build the LTM configuration. I’m using a single pool member, sp1.f5.demo (10.10.30.2) listening on HTTP and the Virtual Server listening on HTTPS performing SSL offload.

Step 1 - Create a delegation account on your domain

1.1 Open Active Directory Users and Computers administrative tool and create a new user account.

User logon name: host/apm-kcd.f5.demo
User logon name (pre-Windows 2000): apm-kcd
Set the password and not expire

1.2 Alter the account and set the servicePrincipcalName. Run setspn from the command line:

setspn –A host/apm-kcd.f5.demo apm-kcd

A delegation tab will now be available for this user.

Step 2 - Configure the SPN

2.1 Open Active Directory Users and Computers administrative tool and select the user account created in the previous step.

Edit the Properties for this user
Select the Delegation tab
Select: Trust this user for delegation to specified services only
Select: Use any authentication protocol
Select Add, to add services.
Select Users or Computers…
Enter the host name, in my example I will be adding HTTP service for sp1.f5.demo (SP1).

Select Check Names and OK

Select the http Service Type and OK

2.2 Make sure there are no duplicate SPNs and run setspn –x from the command line.

Step 3 - Check Forward and Reverse DNS

DNS is critical and a missing PTR is common error I find when troubleshooting Kerberos SSO problems.

From the BIG-IP command line test forward and reverse records exist for the web service using dig:

# dig sp1.f5.demo

;; QUESTION SECTION:

;sp1.f5.demo. IN A

;; ANSWER SECTION:

sp1.f5.demo. 1200 IN A 10.10.30.2

# dig -x 10.10.30.2

;; QUESTION SECTION:

;2.30.10.10.in-addr.arpa. IN PTR

;; ANSWER SECTION:

2.30.10.10.in-addr.arpa. 1200 IN PTR sp1.f5.demo.

Step 4 - Create the APM Configuration

In this example I will use a Logon Page to capture the user credentials that will be authenticated against Active Directory and mapped to the SSO variables for the Kerberos SSO.

4.1 Configure AAA Server for Authentication

Access Policy >> AAA Servers >> Active Directory >> “Create”

Supply the following:

Name: f5.demo_ad_aaa
Domain Name: f5.demo
Domain Controller: (Optional – BIG-IP will use DNS to discover if left blank)
Admin Name and Password

Select “Finished" to save.

4.2 Configure Kerberos SSO

Access Policy >> SSO Configurations >> Kerberos >> “Create”

Supply the following:

Name: f5.demo_kerberos_sso
Username Source: session.sso.token.last.username
User Realm Source: session.ad.last.actualdomain
Kerberos Realm: F5.DEMO
Account Name: apm-kcd (from Step 1)
Account Password & Confirm Account Password (from Step1)

Select “Finished” to save.

4.3 Create an Access Profile and Policy

We can now bring it all together using the Visual Policy Editor (VPE).

Access Policy >> Access Profiles >> Access Profile List >> “Create”

Supply the following:

Name: intranet.f5.demo_sso_ap
SSO Configuration: f5.demo_kerberos_sso
Languages: English (en)
Use the default settings for all other settings.

Select “Finished” to save.

4.4 Edit the Access Policy in the VPE

Access Policy >> Access Profiles >> Access Profile List >> “Edit” (intranet.f5.demo_sso_ap)

On the fallback branch after the Start object, add a Logon Page object.

Leave the defaults and “Save”.

On the fallback branch after the Logon Page object, add an AD Auth object.

Select the Server

Select “Save” when your done.

On the Successful branch after the AD Auth object, add a SSO Credential Mapping object.

Leave the defaults and “Save”.

On the fallback branch after the SSO Credential Mapping, change Deny ending to Allow. The finished policy should look similar to this:

Don't forget to “Apply Access Policy”.

Step 5 – Attach the APM Policy to the Virtual Server and Test

5.1 Edit the Virtual Server

Local Traffic >> Virtual Servers >> Virtual Server List >> intranet.f5.demo_vs

Scroll down to the Access Policy section and select the Access Profile.

Select “Update” to save.

5.2 Test

Open a browser, access the Virtual Server URL (https://intranet.f5.demo in my example), authenticate and verify the client is automatically logged on (SSO) to the web service. To verify Kerberos SSO has worked correctly, check /var/log/apm on APM by turning on debug. You should see log events similar to the ones below when the BIG-IP has fetched a Kerberos Ticket.

info websso.1[9041]: 014d0011:6: 33186a8c: Websso Kerberos authentication for user 'test.user' using config '/Common/f5.demo_kerberos_sso'
debug websso.1[9041]: 014d0018:7: sid:33186a8c ctx:0x917e4a0 server address = ::ffff:10.10.30.2
debug websso.1[9041]: 014d0021:7: sid:33186a8c ctx:0x917e4a0 SPN = HTTP/sp1.f5.demo@F5.DEMO
debug websso.1[9041]: 014d0023:7: S4U ======> ctx: 33186a8c, sid: 0x917e4a0, user: test.user@F5.DEMO, SPN: HTTP/sp1.f5.demo@F5.DEMO
debug websso.1[9041]: 014d0001:7: Getting UCC:test.user@F5.DEMO@F5.DEMO, lifetime:36000
debug websso.1[9041]: 014d0001:7: fetched new TGT, total active TGTs:1
debug websso.1[9041]: 014d0001:7: TGT: client=apm-kcd@F5.DEMO server=krbtgt/F5.DEMO@F5.DEMO expiration=Tue Apr 29 08:33:42 2014 flags=40600000
debug websso.1[9041]: 014d0001:7: TGT expires:1398724422 CC count:0
debug websso.1[9041]: 014d0001:7: Initialized UCC:test.user@F5.DEMO@F5.DEMO, lifetime:36000 kcc:0x92601e8
debug websso.1[9041]: 014d0001:7: UCCmap.size = 1, UCClist.size = 1
debug websso.1[9041]: 014d0001:7: S4U ======> - NO cached S4U2Proxy ticket for user: test.user@F5.DEMO server: HTTP/sp1.f5.demo@F5.DEMO - trying to fetch
debug websso.1[9041]: 014d0001:7: S4U ======> - NO cached S4U2Self ticket for user: test.user@F5.DEMO - trying to fetch
debug websso.1[9041]: 014d0001:7: S4U ======> - fetched S4U2Self ticket for user: test.user@F5.DEMO
debug websso.1[9041]: 014d0001:7: S4U ======> trying to fetch S4U2Proxy ticket for user: test.user@F5.DEMO server: HTTP/sp1.f5.demo@F5.DEMO
debug websso.1[9041]: 014d0001:7: S4U ======> fetched S4U2Proxy ticket for user: test.user@F5.DEMO server: HTTP/sp1.f5.demo@F5.DEMO
debug websso.1[9041]: 014d0001:7: S4U ======> OK!

Conclusion

Like I said in the beginning, once you know how Kerberos SSO works with APM, it’s a piece of cake!

Published Apr 28, 2014

Version 1.0

BIG-IP Access Policy Manager (APM)

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28 Comments

Martin_Kylian_1
Nimbostratus
Oct 03, 2018
Hi, this works perfectly. What if i need a client to do the kerberos auth and the APM do the krb SSO. Can you describe how to configure it correctly Brett?
Smithy
Cirrostratus
Oct 03, 2018
Hi Martin,

Best place to start is here: https://support.f5.com/kb/en-us/products/big-ip_apm/manuals/product/apm-authentication-single-sign-on-12-1-0/9.html

You need to configure Kerberos Auth on the Client side and KCD SSO (this article) for server side.

Sounds like I need to write another article with a Kerberos Auth example when I get time. :)
Martin_Kylian_1
Nimbostratus
Oct 17, 2018
Hi Brett, thanks, I think I've done the AAA krb auth right.

Ending up with this

Oct 17 15:54:52 sok5-f5 err websso.3[23639]: 014d0026:3: /Common/pptest2:Common:61e5157b: Could not find SSO username, check SSO credential mapping agent setting Oct 17 15:54:52 sok5-f5 debug websso.3[23639]: 014d0041:7: /Common/pptest2:Common:61e5157b: Could not find SSO domain, using default realm defined for Kerberos SSO object Oct 17 15:54:52 sok5-f5 err websso.3[23639]: 014d0043:3: /Common/pptest2:Common:61e5157b: SSO username is empty - SSO is disabled

I can see the authenticated session. Variable session.logon.last.username by AAA is something like this user@KRB-REALM.ORG. Is the right thought that this should be mapped to be used by SSO (backend krb delegation) to defined variables (session.sso.token.last.username and session.ad.last.actualdomain in this scenario) by SSO credentail mapping? Can you advise me me how to achieve this?
Martin_Kylian_1
Nimbostratus
Oct 18, 2018
Hi, i've solved the trouble. I'll post the solution here.
Smithy
Cirrostratus
Oct 21, 2018
Hi Martin,

You can split the user@KRB-REALM.ORG (session.logon.last.username) into seperate variables using TCL or add an AD Query object to the VPE and set the "SearchFilter" to "userPrincipalName=%{session.logon.last.username}". It will populate "session.ad.last.attr.sAMAccountName" and "session.ad.last.actualdomain". Then change the "SSO Credential Mapping" object to use "session.ad.last.attr.sAMAccountName" instead of "session.logon.last.username".

Cheers,

Brett
Martin_Kylian_1
Nimbostratus
Nov 28, 2018
I have chosen the TCL way. AD query consumes time :)

Variable assign contains:

session.sso.token.last.username = expr { [regsub {(.+\|^)} [mcget {session.logon.last.username}] "" x; set username $x; regsub {(\@.+)} $username "" y; set username $y]}

session.ad.last.actualdomain = expr { [regsub {(\.+)} [mcget {session.logon.last.username}] "" x; set username $x; regsub {(.+\@)} $username "" y; set username $y]}

This splits the krb username@realm into two variables
awan_m
Cirrus
Jul 25, 2023
Martin_Kylian_1
Hi - i am having the same issue - F5 logging this Message
Could not find SSO domain, using default realm defined for Kerberos SSO object
How did you resolve this issue ?
Thanks
SV2022
Cirrus
Feb 25, 2024
Hi . i am new to APM.
why is that we are not using HTTP 401 response code in VPE instead i could see Logon .
In this article it has been stated that windows server has been configured to respond with http 4001 response code, but when the traffic comes to the F5 the VPE will be executed and first it will go to the logon page and then AD AUTH. is that not mandatory to use http 401 instead of Logon ?