Citrix ADC 101 – Fundamentals

The following is a collection of basic information about Citrix ADC. From licensing, to the most important commands, to the update procedures that can be performed.

General Information

Here is some basic information about Citrix ADC.

Operating System and Architecture

Citrix ADC is based on the open source operating system FreeBSD. Unlike the very similar Linux, FreeBSD has a modular kernel and Citrix has taken advantage of this to modify FreeBSD’s Bash shell by removing the networking subsystem and replacing it with its own. The modifications were placed in a custom kernel module called NetScaler Core Packet Processing Engine (PPE).

Citrix ADC FreeBSD
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SAML Authentication between Citrix & Microsoft with Azure MFA

As a result of increasing projects, here is a little how to with the summary of my previous articles. The main points are:

  • Azure AD Seamless Single Sign-On (PTA / PHS)
  • SAML Authentication (Azure AD as IdP & Citrix Gateway as SP)
  • Citrix Federated Authentication Service (FAS)
  • Microsoft Azure Multi-Factor-Authentication with Conditional Access

Requirements

  • Fully working Citrix Virtual Apps and Desktop Environment (StoreFront & DDC Minimum Version 7.9)
  • Citrix ADC with successful base configuration & activated Enterprise or Platinum license (Minimum Version 12.1 Build 50+ for native workspace app, for browser Minimum Version 11.1)
  • Configured Unified Gateway vServer
  • Internal and external DNS entries for Unified Gateway vServer (e.g. citrix.deyda.net)
  • Certificates for DNS entries (wildcard certificates are the easiest)
  • Existing Azure Tenant with Azure-AD base configuration (Domain, AAD Sync) & activated Azure AD Premium license
  • AD Connect version installed and configured (Minimum Version 1.1.644.0)
  • Firewall release for *.msappproxy.net on port 443
  • Domain administrator credentials for the domains that connected to Azure AD via AD Connect
  • Installed Authenticator App on Test User Mobile Phone
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ADV190023 – Enable LDAPS in Windows DC and Citrix ADC

Important Info:
The scheduled update (ADV190023), regarding LDAP Signing and Channel Binding for new and existing domain controllers, scheduled for March 10, 2020, has been postponed to the second half of calendar year 2020. The March 2020 update will only provide additional auditing capabilities to identify and configure LDAP systems before they become inaccessible with the later update.

The later update results in no more connections to the domain controller, via unsigned / Clear Text LDAP on port 389. Then it is only possible to use either LDAPS via port 636 or Signed LDAP (StartTLS) on port 389.

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Checklist for Citrix ADC CVE-2019-19781

Citrix has released a critical vulnerability warning (CVE-2019-19781) in all Citrix ADC & Gateway systems one week before Christmas. Several working exploits have been released since Jan. 10, 2020 and are available to everyone.

Important ! The fix from Citrix with the Responder Policy does not work on systems with version 12.1.51.16/51.19, 50.31 and older. If this version is in use, please update to the latest 12.1 version.

The exploits allow remote code to be executed anonymously, allowing unauthenticated attackers to take over the various machines with root privileges.

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SAML Authentication with Azure AD as IdP and Citrix as SP

Since Citrix XenApp / XenDesktop 7.9 the Federated Authentication Service (FAS) is available. Via Citrix FAS it is possible to authenticate a user via SAML and thus connect Citrix as a service provider to existing identity providers, such as Azure-AD.

Sequence of SAML authentication

  1. The user browse the FQDN (e.g. citrix.deyda.net) of the Citrix Gateway vServer (Service Provider) to start his VA / VD resources
  2. The Citrix Gateway vServer directs the unauthenticated user directly to the Identity Provider (Azure-AD) to authenticate itself (saml: authnRequest)
  3. The Identity Provider points to its SingleSignOnService URL (e.g. login.microsoftonline.com) and the user must authenticate
  4. The user enters his AD credentials and these are checked by the Identity Provider against the user database
  5. Upon successful verification in the user database, the IdP is informed
  6. The IdP issues a token (SAML assertion) and sends it to the Citrix Gateway (saml: response)
  7. Citrix Gateway checks the token (assertion signature) and extracts the UPN from the assertion token. This allows access via SSO to the VA / VD farm via FAS (The SP does not have access to the user’s credentials)
SAML Auth Azure AD & Citrix Gateway with FAS
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