Best practice for authenticating DMZ against AD in LAN
If you're using PAM for your authentication stack, you can use pam_krb5 to provide kerberos authentication for your services. Kerberos was designed out-of-the-box to deal with hostile environments, handles authentication-by-proxy, and is already a part of the AD spec. Why struggle with LDAP when you can get Kerberos to do the heavy lifting for you, and get on with life? Yeah, you'll have to do some reading, and yeah, it'll take a bit of time, but I've used Kerb-to-AD authentication for years and have found it to be the easiest, quickest way to get SSO working out of the box when you have Active Directory as the authentication backend.
The main thing you'll run into is that Microsoft decided to be very specific about the default encryption types (they basically made their own), so you'll need to set up your Kerberos clients to have the correct matching encryption types, or the AD servers will continue to reject it. This is thankfully an easy procedure and shouldn't require more than a few edits to krb5.conf.
And now, some links for you to consider...
Microsoft's View of Kerberos
- Kerberos Explained
Meshing Kerberos and Active Directory
- Step-by-Step Guide to Kerberos 5 (krb5 1.0) Interoperability
ssh and Kerberos authentication via PAM
- A snippet from O'Reilly's book on SSH
- IBM has a few words on OpenSSH and Kerberos
Apache and Kerberos
- mod_auth_kerb via SourceForge
- Providing Active Directory authentication via Kerberos protocol in Apache
ProFTP and Kerberos
- ProFTPD module mod_gss also via SourceForge
RFCs of Microsoft's Activities with Kerberos (which you really don't want to read about):
- RFC3244 Microsoft Windows 2000 Kerberos Change Password and Set Password Protocols
- RFC4757 The RC4-HMAC Kerberos Encryption Types Used by Microsoft Windows