By default, the Forefront UAG DirectAccess Configuration Wizard creates Group Policy objects for DirectAccess clients and servers when:
- ICMP traffic, for both IPv4 and IPv6, is
exempted from IPsec protection.
- Teredo discovery traffic does not travel
within the IPsec tunnels between DirectAccess clients and Forefront
UAG DirectAccess servers.
These default settings allow Teredo-based DirectAccess clients to perform Teredo discovery of intranet resources, but they also allow the following security risks:
- Any computer with a Teredo or 6to4 client can
send ICMPv6 traffic to intranet locations through the Forefront UAG
DirectAccess server to probe for valid intranet destination IPv6
addresses. The volume of this traffic is limited by the Denial of
Service Protection (DoSP) component of the Forefront UAG
DirectAccess server.
- A malicious user on the same subnet as a
Teredo-based DirectAccess client can determine the IPv6 addresses
of intranet servers by capturing ICMPv6 Echo Request and Echo Reply
message exchanges.
To prevent these possible security issues, you can modify some default configuration settings, as follows:
- Configure the global IPsec settings for the Group Policy object
for DirectAccess clients to not exempt ICMP traffic from IPsec
protection (on the IPsec Settings tab for the properties of
the Windows Firewall with Advanced Security snap-in).
- Configure the global IPsec settings for the Group Policy object
for the Forefront UAG DirectAccess server to not exempt ICMP
traffic from IPsec protection (on the IPsec Settings tab for
the properties of the Windows Firewall with Advanced Security
snap-in).
- For the Group Policy object for the Forefront UAG DirectAccess
server, create a new connection security rule that exempts ICMPv6
traffic when it is tunneled from the Forefront UAG DirectAccess
server.
- For the Group Policy object for DirectAccess clients, create a
new connection security rule that exempts ICMPv6 traffic when it is
tunneled to the Forefront UAG DirectAccess server.
With these modifications:
- All ICMPv6 traffic sent through the Forefront
UAG DirectAccess server must be sent using a tunnel. Only
DirectAccess clients can send ICMPv6 traffic to intranet
locations.
- Malicious users on the same subnet as the
DirectAccess client will only be able to determine the IPv6
addresses of the DirectAccess client and the Forefront UAG
DirectAccess server. Intranet IPv6 addresses will be tunneled and
protected with IPsec encryption.
Although these modifications address the security issues of the default configuration, Teredo discovery messages cannot pass through the Forefront UAG DirectAccess server, and DirectAccess clients cannot use Teredo as a connectivity method. If you make these changes, you must also do the following:
- Disable Teredo client functionality on your DirectAccess
clients.
From the Group Policy object for DirectAccess clients, set Computer Configuration\Administrative Templates\Networking\TCPIP Settings\IPv6 Transition Technologies\Teredo State to Disabled.
- Disable Teredo server and relay functionality on your Forefront
UAG DirectAccess server.
Type the netsh interface teredo set state state=disable command from an administrator-level command prompt on your Forefront UAG DirectAccess server.
- Configure your Internet firewall to block UDP port 3544 traffic
to and from the Forefront UAG DirectAccess server. If you
previously added a port exemption for Teredo traffic, remove
it.
Without Teredo connectivity, DirectAccess clients that are located behind network address translation (NAT) devices will use IP-HTTPS for IPv6 connectivity to the Forefront UAG DirectAccess server, but be aware that IP-HTTPS-based connections have lower performance and higher overhead than Teredo-based connections.