Cisco ASA 5500 Series Configuration Guide using ASDM, 6.4 and 6.6
Getting Started With Application Layer Protocol Inspection
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Getting Started with Application Layer Protocol Inspection

Table Of Contents

Getting Started with Application Layer Protocol Inspection

Information about Application Layer Protocol Inspection

How Inspection Engines Work

When to Use Application Protocol Inspection

Guidelines and Limitations

Default Settings

Configuring Application Layer Protocol Inspection


Getting Started with Application Layer Protocol Inspection


This chapter describes how to configure application layer protocol inspection. Inspection engines are required for services that embed IP addressing information in the user data packet or that open secondary channels on dynamically assigned ports. These protocols require the ASA to do a deep packet inspection instead of passing the packet through the fast path (see the "Stateful Inspection Overview" section for more information about the fast path). As a result, inspection engines can affect overall throughput. Several common inspection engines are enabled on the ASA by default, but you might need to enable others depending on your network.

This chapter includes the following sections:

Information about Application Layer Protocol Inspection

Guidelines and Limitations

Default Settings

Configuring Application Layer Protocol Inspection

Information about Application Layer Protocol Inspection

This section includes the following topics:

How Inspection Engines Work

When to Use Application Protocol Inspection

How Inspection Engines Work

As illustrated in Figure 46-1, the ASA uses three databases for its basic operation:

Access lists—Used for authentication and authorization of connections based on specific networks, hosts, and services (TCP/UDP port numbers).

Inspections—Contains a static, predefined set of application-level inspection functions.

Connections (XLATE and CONN tables)—Maintains state and other information about each established connection. This information is used by the Adaptive Security Algorithm and cut-through proxy to efficiently forward traffic within established sessions.

Figure 46-1 How Inspection Engines Work

In Figure 46-1, operations are numbered in the order they occur, and are described as follows:

1. A TCP SYN packet arrives at the ASA to establish a new connection.

2. The ASA checks the access list database to determine if the connection is permitted.

3. The ASA creates a new entry in the connection database (XLATE and CONN tables).

4. The ASA checks the Inspections database to determine if the connection requires application-level inspection.

5. After the application inspection engine completes any required operations for the packet, the ASA forwards the packet to the destination system.

6. The destination system responds to the initial request.

7. The ASA receives the reply packet, looks up the connection in the connection database, and forwards the packet because it belongs to an established session.

The default configuration of the ASA includes a set of application inspection entries that associate supported protocols with specific TCP or UDP port numbers and that identify any special handling required.

When to Use Application Protocol Inspection

When a user establishes a connection, the ASA checks the packet against access lists, creates an address translation, and creates an entry for the session in the fast path, so that further packets can bypass time-consuming checks. However, the fast path relies on predictable port numbers and does not perform address translations inside a packet.

Many protocols open secondary TCP or UDP ports. The initial session on a well-known port is used to negotiate dynamically assigned port numbers.

Other applications embed an IP address in the packet that needs to match the source address that is normally translated when it goes through the ASA.

If you use applications like these, then you need to enable application inspection.

When you enable application inspection for a service that embeds IP addresses, the ASA translates embedded addresses and updates any checksum or other fields that are affected by the translation.

When you enable application inspection for a service that uses dynamically assigned ports, the ASA monitors sessions to identify the dynamic port assignments, and permits data exchange on these ports for the duration of the specific session.

Guidelines and Limitations

This section includes the guidelines and limitations for this feature.

Context Mode Guidelines

Supported in single and multiple context mode.

Firewall Mode Guidelines

Supported in routed and transparent firewall mode.

Failover Guidelines

State information for multimedia sessions that require inspection are not passed over the state link for stateful failover. The exception is GTP, which is replicated over the state link.

IPv6 Guidelines

Supports IPv6 for the following inspections:

FTP

HTTP

ICMP

SIP

SMTP

IPsec pass-through

Additional Guidelines and Limitations

Some inspection engines do not support PAT, NAT, outside NAT, or NAT between same security interfaces. See "Default Settings" for more information about NAT support.

For all the application inspections, the adaptive security appliance limits the number of simultaneous, active data connections to 200 connections. For example, if an FTP client opens multiple secondary connections, the FTP inspection engine allows only 200 active connections and the 201 connection is dropped and the adaptive security appliance generates a system error message.

Inspected protocols are subject to advanced TCP-state tracking, and the TCP state of these connections is not automatically replicated. While these connections are replicated to the standby unit, there is a best-effort attempt to re-establish a TCP state.

Inspection Reset Behavior

When you configure an inspection engine to use a reset action and a packet triggers a reset, the ASA sends a TCP reset under the following conditions:

The ASA sends a TCP reset to the inside host when the service resetoutbound command is enabled. (The service resetoutbound command is disabled by default.)

The ASA sends a TCP reset to the outside host when the service resetinbound command is enabled. (The service resetinbound command is disabled by default.)

For more information, see the service command in the ASA command reference.

This behavior ensures that a reset action will reset the connections on the ASA and on inside servers; therefore countering denial of service attacks. For outside hosts, the ASA does not send a reset by default and information is not revealed through a TCP reset.

Default Settings

By default, the configuration includes a policy that matches all default application inspection traffic and applies inspection to the traffic on all interfaces (a global policy). Default application inspection traffic includes traffic to the default ports for each protocol. You can only apply one global policy, so if you want to alter the global policy, for example, to apply inspection to non-standard ports, or to add inspections that are not enabled by default, you need to either edit the default policy or disable it and apply a new one.

Table 46-1 lists all inspections supported, the default ports used in the default class map, and the inspection engines that are on by default, shown in bold. This table also notes any NAT limitations.

Table 46-1 Supported Application Inspection Engines 

Application 1
Default Port
NAT Limitations
Standards 2
Comments

CTIQBE

TCP/2748

No extended PAT.

DCERPC

TCP/135

DNS over UDP

UDP/53

No NAT support is available for name resolution through WINS.

RFC 1123

No PTR records are changed.

FTP

TCP/21

RFC 959

GTP

UDP/3386
UDP/2123

No extended PAT.

Requires a special license.

H.323 H.225 and RAS

TCP/1720 UDP/1718
UDP (RAS) 1718-1719

No NAT on same security interfaces.

No static PAT.

No extended PAT.

ITU-T H.323, H.245, H225.0, Q.931, Q.932

HTTP

TCP/80

RFC 2616

Beware of MTU limitations stripping ActiveX and Java. If the MTU is too small to allow the Java or ActiveX tag to be included in one packet, stripping may not occur.

ICMP

All ICMP traffic is matched in the default class map.

ICMP ERROR

All ICMP traffic is matched in the default class map.

ILS (LDAP)

TCP/389

No extended PAT.

Instant Messaging (IM)

Varies by client

No extended PAT.

RFC 3860

IP Options

RFC 791, RFC 2113

All IP Options traffic is matched in the default class map.

MGCP

UDP/2427, 2727

No extended PAT.

RFC 2705bis-05

MMP

TCP 5443

No extended PAT.

NetBIOS Name Server over IP

UDP/137, 138 (Source ports)

No extended PAT.

NetBIOS is supported by performing NAT of the packets for NBNS UDP port 137 and NBDS UDP port 138.

PPTP

TCP/1723

RFC 2637

RADIUS Accounting

1646

RFC 2865

RSH

TCP/514

No PAT

Berkeley UNIX

RTSP

TCP/554

No extended PAT.

No outside NAT.

RFC 2326, 2327, 1889

No handling for HTTP cloaking.

SIP

TCP/5060
UDP/5060

No outside NAT.

No NAT on same security interfaces.

No extended PAT.

RFC 2543

SKINNY (SCCP)

TCP/2000

No outside NAT.

No NAT on same security interfaces.

No extended PAT.

Does not handle TFTP uploaded Cisco IP Phone configurations under certain circumstances.

SMTP and ESMTP

TCP/25

RFC 821, 1123

SNMP

UDP/161, 162

No NAT or PAT.

RFC 1155, 1157, 1212, 1213, 1215

v.2 RFC 1902-1908; v.3 RFC 2570-2580.

SQL*Net

TCP/1521

No extended PAT.

v.1 and v.2.

Sun RPC over UDP and TCP

UDP/111

No extended PAT.

The default rule includes UDP port 111; if you want to enable Sun RPC inspection for TCP port 111, you need to create a new rule that matches TCP port 111 and performs Sun RPC inspection.

TFTP

UDP/69

RFC 1350

Payload IP addresses are not translated.

WAAS

No extended PAT.

XDCMP

UDP/177

No extended PAT.

1 Inspection engines that are enabled by default for the default port are in bold.

2 The ASA is in compliance with these standards, but it does not enforce compliance on packets being inspected. For example, FTP commands are supposed to be in a particular order, but the ASA does not enforce the order.


Configuring Application Layer Protocol Inspection

This feature uses Security Policy Rules to create a service policy. Service policies provide a consistent and flexible way to configure ASA features. For example, you can use a service policy to create a timeout configuration that is specific to a particular TCP application, as opposed to one that applies to all TCP applications. See Chapter 36 "Configuring a Service Policy," for more information.

Inspection is enabled by default for some applications. See the "Default Settings" section for more information. Use this section to modify your inspection policy.

Detailed Steps


Step 1 Choose Configuration > Firewall > Service Policy Rules.

Step 2 Add or edit a service policy rule according to the "Adding a Service Policy Rule for Through Traffic" section.

If you want to match non-standard ports, then create a new rule for the non-standard ports. See the "Default Settings" section for the standard ports for each inspection engine. You can combine multiple rules in the same service policy if desired, so you can create one rule to match certain traffic, and another to match different traffic. However, if traffic matches a rule that contains an inspection action, and then matches another rule that also has an inspection action, only the first matching rule is used.

Step 3 In the Edit Service Policy Rule > Rule Actions dialog box, click the Protocol Inspection tab.

For a new rule, the dialog box is called Add Service Policy Rule Wizard - Rule Actions.

Step 4 Select each inspection type that you want to apply.

Step 5 (Optional) Some inspection engines let you control additional parameters when you apply the inspection to the traffic. Click Configure for each inspection type to configure an inspect map.

You can either choose an existing map, or create a new one. You can predefine inspect maps in the Configuration > Firewall > Objects > Inspect Maps pane.

Step 6 You can configure other features for this rule if desired using the other Rule Actions tabs.

Step 7 Click OK (or Finish from the wizard).