This article describes how to configure a firewall for domains and trusts.

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To establish a domain trust or a security channel across a firewall, the following ports must be opened. Be aware that there may be hosts functioning with both client and server roles on both sides of the firewall. Therefore, ports rules may have to be mirrored.

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Windows NT

In this environment, one side of the trust is a Windows NT 4.0 trust, or the trust was created by using the NetBIOS names.

Client Port(s)	Server Port	Service
137/UDP	137/UDP	NetBIOS Name
138/UDP	138/UDP	NetBIOS Netlogon and Browsing
1024-65535/TCP	139/TCP	NetBIOS Session
1024-65535/TCP	42/TCP	WINS Replication

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Windows Server 2003 and Windows 2000 Server

For a mixed-mode domain that uses either Windows NT domain controllers or legacy clients, trust relationships between Windows Server 2003-based domain controllers and Windows 2000 Server-based domain controllers may necessitate that all the ports for Windows NT that are listed in the previous table be opened in addition to the following ports.

Note The two domain controllers are both in the same forest, or the two domain controllers are both in a separate forest. Also, the trusts in the forest are Windows 2003 trusts or later version trusts.

Client Port(s)	Server Port	Service
1024-65535/TCP	135/TCP	RPC
1024-65535/TCP	1024-65535/TCP	LSA RPC Services (*)
1024-65535/TCP/UDP	389/TCP/UDP	LDAP
1024-65535/TCP	636/TCP	LDAP SSL
1024-65535/TCP	3268/TCP	LDAP GC
1024-65535/TCP	3269/TCP	LDAP GC SSL
53,1024-65535/TCP/UDP	53/TCP/UDP	DNS
1024-65535/TCP/UDP	88/TCP/UDP	Kerberos
1024-65535/TCP	445/TCP	SMB

(*) To define RPC server ports that are used by the LSA RPC services, see the "Domain controllers and Active Directory" section in the following Microsoft Knowledge Base article:For Active Directory to function correctly through a firewall, the Internet Control Message Protocol (ICMP) protocol must be allowed through the firewall from the clients to the domain controllers so that the clients can receive Group Policy information.

ICMP is used to determine whether the link is a slow link or a fast link. ICMP is a legitimate protocol that Active Directory uses for Group Policy detection and for Maximum Transfer Unit (MTU) detection. The Windows Redirector also uses ICMP to verify that a server IP is resolved by the DNS service before a connection is made.

If you want to minimize ICMP traffic, you can use the following sample firewall rule:
Unlike the TCP protocol layer and the UDP protocol layer, ICMP does not have a port number. This is because ICMP is directly hosted by the IP layer.

By default, Windows Server 2003 and Windows 2000 Server DNS servers use ephemeral client-side ports when they query other DNS servers. However, this behavior may be modified with a specific registry setting that is described in the following article in the Microsoft Knowledge Base:
For more information about Active Directory and firewall configuration, view the "Active Directory in Networks Segmented by Firewalls" Microsoft White Paper. To do this, visit the following Web site: Alternatively, you can establish a trust through the Point-to-Point Tunneling Protocol (PPTP) compulsory tunnel, and this will limit the number of ports that the firewall will need to open. For PPTP, the following ports must be enabled.

Client Ports	Server Port	Protocol
1024-65535/TCP	1723/TCP	PPTP

In addition, you would have to enable IP PROTOCOL 47 (GRE).

Note When you add permissions to a resource on a trusting domain for users in a trusted domain, there are some differences between the Windows 2000 and Windows NT 4.0 behavior. If the computer cannotdisplay a list of the remote domain's users:

•	Windows NT 4.0 tries to resolve manually-typed names by contacting the PDC for the remote user's domain (UDP 138). If that communication fails, a Windows NT 4.0-based computer contacts its own PDC, and then asks for resolution of the name.
•	Windows 2000 and Windows Server 2003 also try to contact the remote user's PDC for resolution over UDP 138, but they do not rely on using their own PDC. Make sure that all Windows 2000-based member servers and Windows Server 2003-based member servers that will be granting access to resources have UDP 138 connectivity to the remote PDC.

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