ISA Server’s external adapter can’t get an address from a DHCP server

If you are trying to get your external NIC on the ISA Server obtain an IP address from a DHCP server and can’t, check out this KB article 841141 from Microsoft. This solution applies to both ISA Server 2004/2006.

The external network adapter on your ISA Server 2006 or ISA Server 2004 computer cannot obtain an IP address from a DHCP server

SYMPTOMS
When you try to configure the external network adapter on your Microsoft Internet Security and Acceleration (ISA) Server 2006 computer or on your ISA Server 2004 computer to obtain its Internet Protocol (IP) address from a Dynamic Host Configuration Protocol (DHCP) server, the external network adapter does not receive a valid IP address.

CAUSE
This behavior occurs because the default ISA Server system policy does not permit DHCP replies from external DHCP servers to the ISA Server computer.

RESOLUTION
To resolve this behavior, follow these steps:
1.    Click Start, point to Programs, point to Microsoft ISA Server, and then click ISA Server Management.
2.    In the console tree, click Firewall Policy.
3.    In the right pane, click the Tasks tab, and then click Show System Policy Rules.
4.    Click Allow DHCP replies from DHCP servers to ISA Server.
5.    In the details pane, click Edit System Policy.
6.    Click the From tab.
7.    Click Add.
8.    If you know the IP address of the external DHCP server, follow these steps:
a.     In the New list, click Computer.
b.     In the New Computer Rule Element dialog box, type a name for the DHCP computer rule element in the Name box, type the IP address of the DHCP server in the Computer IP Address box, and then click OK.
c.     Expand Computers, click the DHCP computer rule element that you just created, click Add, and then click Close.
To add the external network instead of the specific DHCP server, expand Networks, click External, click Add, and then click Close.

Note Microsoft recommends that you add the specific DHCP server instead of the external network to make the ISA Server computer less susceptible to external attacks.
9.    Click OK, and then click Apply to save the changes and update the configuration.
Note This procedure is for renewals only. If you do not have an IP address, you may want to allow DHCP traffic from any network until an address is leased. If you do not already have a lease, the “specific DHCP server” setting in step 8 will not work because Windows will be forced into DHCP Discover mode. This mode is strictly for broadcast traffic.