Windows IP Security Filters

October 23, 2002

Joe Klemencicjklemenc@fnal.govFermilab Business Services

What are IP Security Filters?

New feature in Windows 2000, XP & .NET Initially created to authenticate and encrypt

communications Packet Filtering method adopted by

Microsoft during Windows 2000 beta testing

Intended Usage

Authenticate host connections Negotiate encryption schemes Filter access to services by host/subnet

It is important to note that the Authentication schemes are not User based authentication,

but Machine based authentication.

Default Defined Policies

Client (Respond Only) Request Security (Server) Require Security (Server)

Clients attempting to connect to resources that require Authentication and/or Encryption must have

an appropriately configured response policy.

IP Security Policy Creation Group Policy Editor (gpedit.msc) MMC Snap-In GUI manager

Group Policy Editor within an Active Directory OU Properties

IPSECPOL.EXE command line utility from the W2K Resource Kit (Windows 2000)

IPSECCMD.EXE command line utility from the Support Tools on the media CD (XP & .NET)

Proper planning and testing is the key to a successful policy creation and implementation

Anatomy of an IP Security Policy An IP Security Policy consists of:

– IP Filters• Define who, what, where

– Source IP/Network Address– Destination IP/Network Address– Protocol/Port/Service

– IP Filter Actions• Define how

– Permit– Block– Negotiate Security (Authenticate/Encrypt)

• Kerberos (Requires W2K Domain)• PKI• Shared Key

IP Security Filter Modes

Dynamic Mode– Adds anonymous rules to the policy agent– Can co-exist with a DS based policy

Static Mode– Creates or modifies the stored policy– Overwrites current named policy or activates

new named policy

IP Filter Evaluation Order

Rule Evaluation is from Most Granular to Least Specific

1. My IP Address

2. Specific IP Address Defined

3. Specific IP Subnet

4. Any IP Address

A. Specific Protocol/Port combination

B. Specific Protocol/Any Port

C. Any Protocol

IP Filter Evaluation Order (cont)

Visualize filter rule processing by applying weightsSource/Destination Addresses:

My IP Address: 3Specific IP Address: 2Specific Network: 1Any Address: 0

Protocol Used: Specified (TCP/UDP/ICMP/RAW/…): 1Any Protocol: 0

Source/Destination Service Ports:Specified (23/80/135/137/139/443/445…): 1Any Port: 0

IP Filter Evaluation Order (cont)Source Src Port Dest Dest Port Protocol Action

Any Any MyIP Any Any Block

0 0 3 0 0 = 3

Any Any MyIP 80 TCP Permit

0 0 3 1 1 = 5

10.1.1.0 Any MyIP Any Any Permit

1 0 3 0 0 = 4

10.1.1.0 Any MyIP 139 TCP Block

1 0 3 1 1 = 6

In this example, all traffic from the 10.1.1.0 network, except TCP/139 would be allowed. Also allow TCP/80 traffic from anywhere would be allowed. All other traffic will be blocked.

Mirroring vs. Reverse Rules

The Mirror Rule option is only activated when defining Authentication or Encryption Filter Actions

When creating simple Permit/Block Packet Filters, always create the reverse rule at the same time to prevent inadvertent denial of legitimate traffic

BUT…. The reverse rule may inadvertently allow unsolicited connectivity

Reverse Rule

Consider the following:

Src Src-Port Dst Dst-Port Prot Action

MyIP Any Any 80 TCP Permit

Any 80 MyIP Any TCP Permit

Any Any MyIP Any Any Block

In this ruleset, we are allowing the local machine to surf the Internet while prohibiting all other communication. An attacker could use a port-

redirector and still connect to the local machine’s NetBios service as long as they source their connection from TCP/80.

Sample Implementation Scenarios

Internet

Allow Web services from everywhere, but restrict FTP from only certain hosts

Simple Packet Filtering

Src Src-Port Dst Dst-Port Prot Action

Any Any MyIP 80 TCP Permit

MyIP 80 Any Any TCP Permit

MyNet Any MyIP 21 TCP Permit

MyNet Any MyIP 20 TCP Permit

MyIP 21 MyNet Any TCP Permit

MyIP 20 MyNet Any TCP Permit

Any Any MyIP 21 TCP Block

Any Any MyIP 20 TCP Block

Sample Implementation Scenarios

Wireless

Service Authentication

Allow normal traffic from network, but request a Host to Kerberos authenticate from Wireless network

This authentication is separate from the Application Authentication Mechanism

Src Src-Port Dst Dst-Port Prot Action

WLAN Any MyIP Any Any Kerberos Auth (auto-mirror)

Sample Implementation ScenariosService Encryption

Encrypt communications between servers while allowing for unencrypted traffic from workstations. Also, block communications from non-local workstations

Src Src-Port Dst Dst-Port Prot ActionDC1 Any MyIP Any Any Kerberos Auth & Encrypt (auto-mirror)MyIP Any DC1 Any Any Kerberos Auth & Encrypt (auto-mirror)MyNet Any MyIP Any Any PermitMyIP Any MyNet Any Any PermitAny Any MyIP Any Any Block

Usage Caveats

Certain traffic is not inspected by an IP Policy- Anything with a source port of 88 (Kerberos)*- IKE- Multicast Traffic- Broadcast Traffic- RSVP/Quality of Service

*This behavior may be changed by setting the following value in the Registry:

HKLM\SYSTEM\CurrentControlSet\Services\IPSEC\NoDefaultExempt DWORD=1

See Microsoft Q253169 article for more information

Usage Caveats (cont)

No logging available when using as a Packet Filter

Because of Rule Evaluation Order, an ill-defined rule may allow traffic intended to be blocked

Difficult to implement Packet Filtering on General Use Workstations, but optimal for Kiosks and Servers

Further ReadingSecuring W2K with IP Filters: Part 1 (Step-by-Step How-To Guide)

http://online.securityfocus.com/infocus/1559

Securing W2K with IP Filters: Part 2 (Implementing Encryption)

http://online.securityfocus.com/infocus/1566

Using IPSEC to Lock Down a Server

http://www.microsoft.com/serviceproviders/columns/using_ipsec.asp

Active Directory Replication Over Firewalls

http://www.microsoft.com/serviceproviders/columns/config_ipsec_P63623.asp

Microsoft Q254949 Article: Domain Controller IPSEC Support

http://support.microsoft.com/default.aspx?scid=kb;en-us;Q254949

How to Enable IPSEC Through a Firewall

http://support.microsoft.com/default.aspx?scid=kb;en-us;Q233256