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Skyhigh Security

Configuring Kerberos - Simplified Guide

Introduction

This guide is a trimmed-down version of the Ultimate Kerberos Guide, it includes only the basics for setting up Kerberos. No background or historical information will be found here. My comments will also be short and sweet.

IMPORTANT: By the end of this guide you should only have one user and keytab per domain for all of your Secure Web Gateways!

Use the Three-Headed Dog

A Kerberos setup tool has been created to make the setup process much easier -- it will provide you with the commands to give to your Active Directory team. You can find the tool here: Secure Web Gateway: Three Headed Dog v1.0.3 (A Kerberos Setup Tool).

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Create User in Active Directory

You know how to do this, this account will be treated as a service account so adjust accordingly.

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Generate Keytab in Active Directory

When generating the keytab syntax is essential! The commands are case-sensitive. The syntax below is for Windows Server 2008+, start a command prompt as administrator.

ktpass -princ HTTP/[fqdn-of-appliance_lowercase]@[DOMAIN_UPPERCASE] -mapuser[DOMAIN]\[USERNAME] -pass [PASSWORD] -ptype KRB5_NT_PRINCIPAL -crypto All -out[OUTPUT-FILENAME].keytab

# Example:

ktpass -princ HTTP/proxy.domain.local@DOMAIN.LOCAL -mapuser vegas\mwg-kerb-user -passpassword -ptype KRB5_NT_PRINCIPAL -crypto All -out proxy.domain.local.keytab

 

Enable AES support in Active Directory (optional)

If you wish to enable AES support, do it *after* generating a keytab. If you adjust the AES support generate the keytab again. In my testing if I enabled AES support before generating a keytab, authentication would fail; only deleting the account and starting again would get it to work.

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Set additional SPNs in Active Directory

Additional SPNs are necessary if you have multiple Secure Web Gateways, they are behind a load balancer, or there are multiple DNS names.

setspn -a HTTP/[fqdn-of-appliance] mwg-kerb-user

# Example:

setspn -a HTTP/load-balancer.domain.local mwg-kerb-user

setspn -a HTTP/mwg-alias.domain.local mwg-kerb-user

# SOCKS Example:

setspn -a RCMD/proxy.domain.local mwg-kerb-user

setspn -a RCMD/load-balancer.domain.local mwg-kerb-user

setspn -a RCMD/mwg-alias.domain.local mwg-kerb-user

 

Upload keytab into the Web Gateway

Configuration > [Select your appliance] > Kerberos Administration. Upload the single keytab to each appliance.

 

Import Authentication Rules into Web Gateway

Use the ruleset from the Ultimate Kerberos Guide. Download ruleset here. The screenshot below shows Direct Proxy Authentication rules with NTLM Fallback, rules would be different for Authentication Server. We also assume you have already joined the Secure Web Gateway to the domain () forgetting groups.

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Common Issues

Proxy Settings

You must have the proxy settings set to use the FQDN (used in the keytab creation process). Do not use the IP.

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Duplicate SPN

You probably created multiple user accounts after generating keytabs and forgot to delete them. To check for it run the command below on the Active Directory server. Replace "SPN-SEARCH-STRING" with the actual search string (e.g. proxy.domain.local)...

ldifde -f c:\dump.txt -l dn,sAMAccountName,msds-keyversionnumber,serviceprincipalname,userprincipalname -p subtree -r "(serviceprincipalname=*SPN-SEARCH-STRING*)"

User account / keytab version mismatch

You probably re-created the keytab or updated the account password, and now theversions are off.

Run the ldifde command again:

ldifde -f c:\dump.txt -l dn,sAMAccountName,msds-keyversionnumber,serviceprincipalname,userprincipalname -p subtree -r "(serviceprincipalname=*SPN-SEARCH-STRING*)"

Example output (showing version 6):

> ldifde -f c:\dump.txt -l dn,sAMAccountName,msds-keyversionnumber,serviceprincipalname,userprincipalname -p subtree -r "(serviceprincipalname=*proxy.domain.local*)"

c:\dump.txt:

dn: CN=mwg-kerb-user,CN=Users,DC=domain,DC=local

changetype: add

sAMAccountName: mwg-kerb-user

userPrincipalName: HTTP/proxy.domain.local@domain.local

servicePrincipalName: HTTP/proxy.domain.local

servicePrincipalName: HTTP/mwg-alias.domain.local

msDS-KeyVersionNumber: 6

 

Compare the version listed in the ldifde output with the version in the keytab:

yum install krb5-workstation

klist -tek /etc/krb5.mwg.keytab

 

Example output (showing version 5):

[root@proxy ~]# klist -tek /etc/krb5.mwg.keytab

Keytab name: FILE:/etc/krb5.mwg.keytab

KVNO Timestamp Principal

---- ----------------- --------------------------------------------------------

5 12/31/69 18:00:00 HTTP/proxy.domain.local@DOMAIN.LOCAL (des-cbc-crc)

5 12/31/69 18:00:00 HTTP/proxy.domain.local@DOMAIN.LOCAL (des-cbc-md5)

5 12/31/69 18:00:00 HTTP/proxy.domain.local@DOMAIN.LOCAL (arcfour-hmac)

5 12/31/69 18:00:00 HTTP/proxy.domain.local@DOMAIN.LOCAL (aes256-cts-hmac-sha1-96)

5 12/31/69 18:00:00 HTTP/proxy.domain.local@DOMAIN.LOCAL (aes128-cts-hmac-sha1-96)

[root@proxy ~]#

Troubleshooting

If you have problems gather the following... (if you don't then we can't pinpoint your issue)

  • Flush DNS and purge Kerberos tickets:

ipconfig /flushdns

klist purge

  • Screenshot of proxy settings (if applicable)
  • ldifde output from Active directory server
  • klist output from Secure Web Gateway
  • Client-side Wireshark capture while reproducing whatever problem you are having.

Conclusion

By the end of all of this you should have one user and one keytab created (per domain) for all of your Secure Web Gateways. Authentication rules should be imported into Secure Web Gateway with NTLM Fallback in place.

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