Table Of Contents

Using Protection Tools

Preventing IP Spoofing

Configuring the Fragment Size

Show Fragment

Configuring TCP Options

TCP Reset Settings

Configuring IP Audit for Basic IPS Support

IP Audit Policy

Add/Edit IP Audit Policy Configuration

IP Audit Signatures

IP Audit Signature List

Using Protection Tools

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This chapter describes some of the many tools available to protect your network and includes the following sections:

•Preventing IP Spoofing

•Configuring the Fragment Size

•Configuring TCP Options

•Configuring IP Audit for Basic IPS Support

Preventing IP Spoofing

This section lets you enable Unicast Reverse Path Forwarding on an interface. Unicast RPF guards against IP spoofing (a packet uses an incorrect source IP address to obscure its true source) by ensuring that all packets have a source IP address that matches the correct source interface according to the routing table.

Normally, the ASA only looks at the destination address when determining where to forward the packet. Unicast RPF instructs the ASA to also look at the source address; this is why it is called Reverse Path Forwarding. For any traffic that you want to allow through the ASA, the ASA routing table must include a route back to the source address. See RFC 2267 for more information.

For outside traffic, for example, the ASA can use the default route to satisfy the Unicast RPF protection. If traffic enters from an outside interface, and the source address is not known to the routing table, the ASA uses the default route to correctly identify the outside interface as the source interface.

If traffic enters the outside interface from an address that is known to the routing table, but is associated with the inside interface, then the ASA drops the packet. Similarly, if traffic enters the inside interface from an unknown source address, the ASA drops the packet because the matching route (the default route) indicates the outside interface.

Unicast RPF is implemented as follows:

•ICMP packets have no session, so each packet is checked.

•UDP and TCP have sessions, so the initial packet requires a reverse route lookup. Subsequent packets arriving during the session are checked using an existing state maintained as part of the session. Non-initial packets are checked to ensure they arrived on the same interface used by the initial packet.

Configuration > Firewall > Advanced > Anti-Spoofing Fields

•Interface—Lists the interface names.

•Anti-Spoofing Enabled—Shows whether an interface has Unicast RPF enabled, Yes or No.

•Enable—Enables Unicast RPF for the selected interface.

•Disable—Disables Unicast RPF for the selected interface.

Configuring the Fragment Size

By default, the ASA allows up to 24 fragments per IP packet, and up to 200 fragments awaiting reassembly. You might need to let fragments on your network if you have an application that routinely fragments packets, such as NFS over UDP. However, if you do not have an application that fragments traffic, we recommend that you do not allow fragments through the ASA. Fragmented packets are often used as DoS attacks.

To modify the IP fragment database parameters of an interface, perform the following steps:

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Step 1 Choose the Configuration > Firewall > Advanced > Fragment pane, choose the interface to change in the Fragment table, and click Edit.

The Edit Fragment dialog box appears.

Step 2 In the Size field, set the maximum number of packets that can be in the IP reassembly database waiting for reassembly. The default is 200.

Step 3 In the Chain field, set the maximum number of packets into which a full IP packet can be fragmented. The default is 24 packets.

Step 4 In the Timeout field, set the maximum number of seconds to wait for an entire fragmented packet to arrive.

The timer starts after the first fragment of a packet arrives. If all fragments of the packet do not arrive by the number of seconds specified, all fragments of the packet that were already received will be discarded. The default is 5 seconds.

Step 5 Click OK.

Step 6 Click Apply.

Step 7 To view the fragment statistics, click Show Fragment. See the "Show Fragment" section for more information.

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Show Fragment

The Configuration > Properties > Fragment > Show Fragment pane displays the current IP fragment database statistics for each interface.

Fields

•Size—Display only. Displays the number of packets in the IP reassembly database waiting for reassembly. The default is 200.

•Chain—Display only. Displays the number of packets into which a full IP packet can be fragmented. The default is 24 packets.

•Timeout—Display only. Displays the number of seconds to wait for an entire fragmented packet to arrive. The timer starts after the first fragment of a packet arrives. If all fragments of the packet do not arrive by the number of seconds displayed, all fragments of the packet that were already received will be discarded. The default is 5 seconds.

•Threshold—Display only. Displays the IP packet threshold, or the limit after which no new chains can be created in the reassembly module.

•Queue—Display only. Displays the number of IP packets waiting in the queue for reassembly.

•Assembled—Display only. Displays the number of IP packets successfully reassembled.

•Fail—Display only. Displays the number of failed reassembly attempts.

•Overflow—Display only. Displays the number of IP packets in the overflow queue.

Configuring TCP Options

The Configuration > Properties > TCP Options pane lets you set parameters for TCP connections.

Fields

•Inbound and Outbound Reset—Sets whether to reset denied TCP connections for inbound and outbound traffic.

–Interface—Shows the interface name.

–Inbound Reset—Shows the interface reset setting for inbound TCP traffic, Yes or No. Enabling this setting causes the ASA to send TCP resets for all inbound TCP sessions that attempt to transit the ASA and are denied by the ASA based on access lists or AAA settings. Traffic between same security level interfaces is also affected. When this option is not enabled, the ASA silently discards denied packets.

–Outbound Reset—Shows the interface reset setting for outbound TCP traffic, Yes or No. Enabling this setting causes the ASA to send TCP resets for all outbound TCP sessions that attempt to transit the ASA and are denied by the ASA based on access lists or AAA settings. Traffic between same security level interfaces is also affected. When this option is not enabled, the ASA silently discards denied packets.

–Edit—Sets the inbound and outbound reset settings for the interface.

•Other Options—Sets additional TCP options.

–Send Reset Reply for Denied Outside TCP Packets—Enables resets for TCP packets that terminate at the least secure interface and are denied by the ASA based on access lists or AAA settings. When this option is not enabled, the ASA silently discards denied packets. If you enable Inbound Resets for the least secure interface (see TCP Reset Settings), then you do not also have to enable this setting; Inbound Resets handle to-the-ASA traffic as well as through the ASA traffic.

–Force Maximum Segment Size for TCP—Sets the maximum TCP segment size in bytes, between 48 and any maximum number. The default value is 1380 bytes. You can disable this feature by setting the bytes to 0. Both the host and the server can set the maximum segment size when they first establish a connection. If either maximum exceeds the value you set here, then the ASA overrides the maximum and inserts the value you set. For example, if you set a maximum size of 1200 bytes, when a host requests a maximum size of 1300 bytes, then the ASA alters the packet to request 1200 bytes.

–Force Minimum Segment Size for TCP—Overrides the maximum segment size to be no less than the number of bytes you set, between 48 and any maximum number. This feature is disabled by default (set to 0). Both the host and the server can set the maximum segment size when they first establish a connection. If either maximum is less than the value you set for the Force Minimum Segment Size for TCP Proxy field, then the ASA overrides the maximum and inserts the "minimum" value you set (the minimum value is actually the smallest maximum allowed). For example, if you set a minimum size of 400 bytes, if a host requests a maximum value of 300 bytes, then the ASA alters the packet to request 400 bytes.

–Force TCP Connection to Linger in TIME_WAIT State for at Least 15 Seconds—Forces each TCP connection to linger in a shortened TIME_WAIT state of at least 15 seconds after the final normal TCP close-down sequence. You might want to use this feature if an end host application default TCP terminating sequence is a simultaneous close. The default behavior of the ASA is to track the shutdown sequence and release the connection after two FINs and the ACK of the last FIN segment. This quick release heuristic enables the ASA to sustain a high connection rate, based on the most common closing sequence, known as the normal close sequence. However, in a simultaneous close, both ends of the transaction initiate the closing sequence, as opposed to the normal close sequence where one end closes and the other end acknowledges prior to initiating its own closing sequence (see RFC 793). Thus, in a simultaneous close, the quick release forces one side of the connection to linger in the CLOSING state. Having many sockets in the CLOSING state can degrade the performance of an end host. For example, some WinSock mainframe clients are known to exhibit this behavior and degrade the performance of the mainframe server. Using this feature creates a window for the simultaneous close down sequence to complete.

TCP Reset Settings

The Configuration > Properties > TCP Options > TCP Reset Settings dialog box sets the inbound and outbound reset settings for an interface.

Fields

•Send Reset Reply for Denied Inbound TCP Packets—Sends TCP resets for all inbound TCP sessions that attempt to transit the ASA and are denied by the ASA based on access lists or AAA settings. Traffic between same security level interfaces is also affected. When this option is not enabled, the ASA silently discards denied packets.

You might want to explicitly send resets for inbound traffic if you need to reset identity request (IDENT) connections. When you send a TCP RST (reset flag in the TCP header) to the denied host, the RST stops the incoming IDENT process so that you do not have to wait for IDENT to time out. Waiting for IDENT to time out can cause traffic to slow because outside hosts keep retransmitting the SYN until the IDENT times out, so the service resetinbound command might improve performance.

•Send Reset Reply for Denied Outbound TCP Packets—Sends TCP resets for all outbound TCP sessions that attempt to transit the ASA and are denied by the ASA based on access lists or AAA settings. Traffic between same security level interfaces is also affected. When this option is not enabled, the ASA silently discards denied packets. This option is enabled by default. You might want to disable outbound resets to reduce the CPU load during traffic storms, for example.

Configuring IP Audit for Basic IPS Support

The IP audit feature provides basic IPS support for the ASA that does not have an AIP SSM. It supports a basic list of signatures, and you can configure the ASA to perform one or more actions on traffic that matches a signature.

This section includes the following topics:

•IP Audit Policy

•Add/Edit IP Audit Policy Configuration

•IP Audit Signatures

•IP Audit Signature List

IP Audit Policy

The Configuration > Firewall > Advanced > IP Audit > IP Audit Policy pane lets you add audit policies and assign them to interfaces. You can assign an attack policy and an informational policy to each interface. The attack policy determines the action to take with packets that match an attack signature; the packet might be part of an attack on your network, such as a DoS attack. The informational policy determines the action to take with packets that match an informational signature; the packet is not currently attacking your network, but could be part of an information-gathering activity, such as a port sweep. For a complete list of signatures, see the IP Audit Signature List.

Fields

•Name—Shows the names of the defined IP audit policies. Although the default actions for a named policy are listed in this table ("--Default Action--"), they are not named policies that you can assign to an interface. Default actions are used by named policies if you do not set an action for the policy. You can modify the default actions by selecting them and clicking the Edit button.

•Type—Shows the policy type, either Attack or Info.

•Action—Shows the actions taken against packets that match the policy, Alarm, Drop, and/or Reset. Multiple actions can be listed.

•Add—Adds a new IP audit policy.

•Edit—Edits an IP audit policy or the default actions.

•Delete—Deletes an IP audit policy. You cannot delete a default action.

•Policy-to-Interface Mappings—Assigns an attack and informational policy to each interface.

–Interface—Shows the interface name.

–Attack Policy—Lists the attack audit policy names available. Assign a policy to an interface by clicking the name in the list.

–Info Policy—Lists the informational audit policy names available. Assign a policy to an interface by clicking the name in the list.

Add/Edit IP Audit Policy Configuration

The Configuration > Firewall > Advanced > IP Audit > IP Audit Policy > Add/Edit IP Audit Policy Configuration dialog box lets you add or edit a named IP audit policy that you can assign to interfaces, and lets you modify the default actions for each signature type.

Fields

•Policy Name—Sets the IP audit policy name. You cannot edit the name after you add it.

•Policy Type—Sets the policy type. You cannot edit the policy type after you add it.

–Attack—Sets the policy type as attack.

–Information—Sets the policy type as informational.

•Action—Sets one or more actions to take when a packet matches a signature. If you do not choose an action, then the default policy is used.

–Alarm—Generates a system message showing that a packet matched a signature. For a complete list of signatures, see IP Audit Signature List.

–Drop—Drops the packet.

–Reset—Drops the packet and closes the connection.

IP Audit Signatures

The Configuration > Firewall > Advanced > IP Audit > IP Audit Signatures pane lets you disable audit signatures. You might want to disable a signature if legitimate traffic continually matches a signature, and you are willing to risk disabling the signature to avoid large numbers of alarms.

For a complete list of signatures, see the "IP Audit Signature List" section.

Fields

•Enabled—Lists the enabled signatures.

•Disabled—Lists the disabled signatures.

•Disable—Moves the selected signature to the Disabled pane.

•Enable—Moves the selected signature to the Enabled pane.

IP Audit Signature List

Table 78-1 lists supported signatures and system message numbers.

Table 78-1 Signature IDs and System Message Numbers 

Signature ID	Message Number	Signature Title	Signature Type	Description
1000	400000	IP options-Bad Option List	Informational	Triggers on receipt of an IP datagram where the list of IP options in the IP datagram header is incomplete or malformed. The IP options list contains one or more options that perform various network management or debugging tasks.
1001	400001	IP options-Record Packet Route	Informational	Triggers on receipt of an IP datagram where the IP option list for the datagram includes option 7 (Record Packet Route).
1002	400002	IP options-Timestamp	Informational	Triggers on receipt of an IP datagram where the IP option list for the datagram includes option 4 (Timestamp).
1003	400003	IP options-Security	Informational	Triggers on receipt of an IP datagram where the IP option list for the datagram includes option 2 (Security options).
1004	400004	IP options-Loose Source Route	Informational	Triggers on receipt of an IP datagram where the IP option list for the datagram includes option 3 (Loose Source Route).
1005	400005	IP options-SATNET ID	Informational	Triggers on receipt of an IP datagram where the IP option list for the datagram includes option 8 (SATNET stream identifier).
1006	400006	IP options-Strict Source Route	Informational	Triggers on receipt of an IP datagram in which the IP option list for the datagram includes option 2 (Strict Source Routing).
1100	400007	IP Fragment Attack	Attack	Triggers when any IP datagram is received with an offset value less than 5 but greater than 0 indicated in the offset field.
1102	400008	IP Impossible Packet	Attack	Triggers when an IP packet arrives with source equal to destination address. This signature will catch the so-called Land Attack.
1103	400009	IP Overlapping Fragments (Teardrop)	Attack	Triggers when two fragments contained within the same IP datagram have offsets that indicate that they share positioning within the datagram. This could mean that fragment A is being completely overwritten by fragment B, or that fragment A is partially being overwritten by fragment B. Some operating systems do not properly handle fragments that overlap in this manner and may throw exceptions or behave in other undesirable ways upon receipt of overlapping fragments, which is how the Teardrop attack works to create a DoS.
2000	400010	ICMP Echo Reply	Informational	Triggers when a IP datagram is received with the protocol field of the IP header set to 1 (ICMP) and the type field in the ICMP header set to 0 (Echo Reply).
2001	400011	ICMP Host Unreachable	Informational	Triggers when an IP datagram is received with the protocol field of the IP header set to 1 (ICMP) and the type field in the ICMP header set to 3 (Host Unreachable).
2002	400012	ICMP Source Quench	Informational	Triggers when an IP datagram is received with the protocol field of the IP header set to 1 (ICMP) and the type field in the ICMP header set to 4 (Source Quench).
2003	400013	ICMP Redirect	Informational	Triggers when a IP datagram is received with the protocol field of the IP header set to 1 (ICMP) and the type field in the ICMP header set to 5 (Redirect).
2004	400014	ICMP Echo Request	Informational	Triggers when a IP datagram is received with the protocol field of the IP header set to 1 (ICMP) and the type field in the ICMP header set to 8 (Echo Request).
2005	400015	ICMP Time Exceeded for a Datagram	Informational	Triggers when a IP datagram is received with the protocol field of the IP header set to 1 (ICMP) and the type field in the ICMP header set to 11(Time Exceeded for a Datagram).
2006	400016	ICMP Parameter Problem on Datagram	Informational	Triggers when a IP datagram is received with the protocol field of the IP header set to 1 (ICMP) and the type field in the ICMP header set to 12 (Parameter Problem on Datagram).
2007	400017	ICMP Timestamp Request	Informational	Triggers when a IP datagram is received with the protocol field of the IP header set to 1 (ICMP) and the type field in the ICMP header set to 13 (Timestamp Request).
2008	400018	ICMP Timestamp Reply	Informational	Triggers when a IP datagram is received with the protocol field of the IP header set to 1 (ICMP) and the type field in the ICMP header set to 14 (Timestamp Reply).
2009	400019	ICMP Information Request	Informational	Triggers when a IP datagram is received with the protocol field of the IP header set to 1 (ICMP) and the type field in the ICMP header set to 15 (Information Request).
2010	400020	ICMP Information Reply	Informational	Triggers when a IP datagram is received with the protocol field of the IP header set to 1 (ICMP) and the type field in the ICMP header set to 16 (ICMP Information Reply).
2011	400021	ICMP Address Mask Request	Informational	Triggers when a IP datagram is received with the protocol field of the IP header set to 1 (ICMP) and the type field in the ICMP header set to 17 (Address Mask Request).
2012	400022	ICMP Address Mask Reply	Informational	Triggers when a IP datagram is received with the protocol field of the IP header set to 1 (ICMP) and the type field in the ICMP header set to 18 (Address Mask Reply).
2150	400023	Fragmented ICMP Traffic	Attack	Triggers when a IP datagram is received with the protocol field of the IP header set to 1 (ICMP) and either the more fragments flag is set to 1 (ICMP) or there is an offset indicated in the offset field.
2151	400024	Large ICMP Traffic	Attack	Triggers when a IP datagram is received with the protocol field of the IP header set to 1(ICMP) and the IP length > 1024.
2154	400025	Ping of Death Attack	Attack	Triggers when a IP datagram is received with the protocol field of the IP header set to 1(ICMP), the Last Fragment bit is set, and (IP offset * 8) + (IP data length) > 65535 that is to say, the IP offset (which represents the starting position of this fragment in the original packet, and which is in 8 byte units) plus the rest of the packet is greater than the maximum size for an IP packet.
3040	400026	TCP NULL flags	Attack	Triggers when a single TCP packet with none of the SYN, FIN, ACK, or RST flags set has been sent to a specific host.
3041	400027	TCP SYN+FIN flags	Attack	Triggers when a single TCP packet with the SYN and FIN flags are set and is sent to a specific host.
3042	400028	TCP FIN only flags	Attack	Triggers when a single orphaned TCP FIN packet is sent to a privileged port (having port number less than 1024) on a specific host.
3153	400029	FTP Improper Address Specified	Informational	Triggers if a port command is issued with an address that is not the same as the requesting host.
3154	400030	FTP Improper Port Specified	Informational	Triggers if a port command is issued with a data port specified that is <1024 or >65535.
4050	400031	UDP Bomb attack	Attack	Triggers when the UDP length specified is less than the IP length specified. This malformed packet type is associated with a denial of service attempt.
4051	400032	UDP Snork attack	Attack	Triggers when a UDP packet with a source port of either 135, 7, or 19 and a destination port of 135 is detected.
4052	400033	UDP Chargen DoS attack	Attack	This signature triggers when a UDP packet is detected with a source port of 7 and a destination port of 19.
6050	400034	DNS HINFO Request	Informational	Triggers on an attempt to access HINFO records from a DNS server.
6051	400035	DNS Zone Transfer	Informational	Triggers on normal DNS zone transfers, in which the source port is 53.
6052	400036	DNS Zone Transfer from High Port	Informational	Triggers on an illegitimate DNS zone transfer, in which the source port is not equal to 53.
6053	400037	DNS Request for All Records	Informational	Triggers on a DNS request for all records.
6100	400038	RPC Port Registration	Informational	Triggers when attempts are made to register new RPC services on a target host.
6101	400039	RPC Port Unregistration	Informational	Triggers when attempts are made to unregister existing RPC services on a target host.
6102	400040	RPC Dump	Informational	Triggers when an RPC dump request is issued to a target host.
6103	400041	Proxied RPC Request	Attack	Triggers when a proxied RPC request is sent to the portmapper of a target host.
6150	400042	ypserv (YP server daemon) Portmap Request	Informational	Triggers when a request is made to the portmapper for the YP server daemon (ypserv) port.
6151	400043	ypbind (YP bind daemon) Portmap Request	Informational	Triggers when a request is made to the portmapper for the YP bind daemon (ypbind) port.
6152	400044	yppasswdd (YP password daemon) Portmap Request	Informational	Triggers when a request is made to the portmapper for the YP password daemon (yppasswdd) port.
6153	400045	ypupdated (YP update daemon) Portmap Request	Informational	Triggers when a request is made to the portmapper for the YP update daemon (ypupdated) port.
6154	400046	ypxfrd (YP transfer daemon) Portmap Request	Informational	Triggers when a request is made to the portmapper for the YP transfer daemon (ypxfrd) port.
6155	400047	mountd (mount daemon) Portmap Request	Informational	Triggers when a request is made to the portmapper for the mount daemon (mountd) port.
6175	400048	rexd (remote execution daemon) Portmap Request	Informational	Triggers when a request is made to the portmapper for the remote execution daemon (rexd) port.
6180	400049	rexd (remote execution daemon) Attempt	Informational	Triggers when a call to the rexd program is made. The remote execution daemon is the server responsible for remote program execution. This may be indicative of an attempt to gain unauthorized access to system resources.
6190	400050	statd Buffer Overflow	Attack	Triggers when a large statd request is sent. This could be an attempt to overflow a buffer and gain access to system resources.