Connection Settings

This chapter describes how to configure connection settings for connections that go through the ASA, or for management connections that go to the ASA.

What Are Connection Settings?

Connection settings comprise a variety of features related to managing traffic connections, such as a TCP flow through the ASA. Some features are named components that you would configure to supply specific services.

Connection settings include the following:

  • Global timeouts for various protocols—All global timeouts have default values, so you need to change them only if you are experiencing premature connection loss.

  • Connection timeouts per traffic class—You can override the global timeouts for specific types of traffic using service policies. All traffic class timeouts have default values, so you do not have to set them.

  • Connection limits and TCP Intercept—By default, there are no limits on how many connections can go through (or to) the ASA. You can set limits on particular traffic classes using service policy rules to protect servers from denial of service (DoS) attacks. Particularly, you can set limits on embryonic connections (those that have not finished the TCP handshake), which protects against SYN flooding attacks. When embryonic limits are exceeded, the TCP Intercept component gets involved to proxy connections and ensure that attacks are throttled.

  • Dead Connection Detection (DCD)—If you have persistent connections that are valid but often idle, so that they get closed because they exceed idle timeout settings, you can enable Dead Connection Detection to identify idle but valid connections and keep them alive (by resetting their idle timers). Whenever idle times are exceeded, DCD probes both sides of the connection to see if both sides agree the connection is valid. The show service-policy command output includes counters to show the amount of activity from DCD. You can use the show conn detail command to get information about the initiator and responder and how often each has sent probes.

  • TCP sequence randomization—Each TCP connection has two initial sequence numbers (ISN): one generated by the client and one generated by the server. By default, the ASA randomizes the ISN of the TCP SYN passing in both the inbound and outbound directions. Randomization prevents an attacker from predicting the next ISN for a new connection and potentially hijacking the new session. However, TCP sequence randomization effectively breaks TCP SACK (Selective Acknowledgement), as the sequence numbers the client sees are different from what the server sees. You can disable randomization per traffic class if desired.

  • TCP Normalization—The TCP Normalizer protects against abnormal packets. You can configure how some types of packet abnormalities are handled by traffic class.

  • TCP State Bypass—You can bypass TCP state checking if you use asymmetrical routing in your network.

  • SCTP State Bypass—You can bypass Stream Control Transmission Protocol (SCTP) stateful inspection if you do not want SCTP protocol validation.

  • Flow offloading—You can identify select traffic to be offloaded to a super fast path, where the flows are switched in the NIC itself. Offloading can help you improve performance for data-intensive applications such as large file transfers.

  • IPsec flow offload—After the initial setup of an IPsec site-to-site VPN or remote access VPN security association (SA), IPsec connections are offloaded to the field-programmable gate array (FPGA) in the device, which should improve device performance. This feature is enabled by default on platforms that support it.

Configure Connection Settings

Connection limits, timeouts, TCP Normalization, TCP sequence randomization, and decrementing time-to-live (TTL) have default values that are appropriate for most networks. You need to configure these connection settings only if you have unusual requirements, your network has specific types of configuration, or if you are experiencing unusual connection loss due to premature idle timeouts.

Other connection-related features are not enabled. You would configure these services on specific traffic classes only, and not as a general service. These features include the following: TCP Intercept, TCP State Bypass, Dead Connection Detection (DCD), SCTP state bypass, flow offload.

The following general procedure covers the gamut of possible connection setting configurations. Pick and choose which to implement based on your needs.

Procedure

Step 1

Configure Global Timeouts. These settings change the default idle timeouts for various protocols for all traffic that passes through the device. If you are having problems with connections being reset due to premature timeouts, first try changing the global timeouts.

Step 2

Protect Servers from a SYN Flood DoS Attack (TCP Intercept). Use this procedure to configure TCP Intercept.

Step 3

Customize Abnormal TCP Packet Handling (TCP Maps, TCP Normalizer), if you want to alter the default TCP Normalization behavior for specific traffic classes.

Step 4

Bypass TCP State Checks for Asymetrical Routing (TCP State Bypass), if you have this type of routing environment.

Step 5

Disable TCP Sequence Randomization, if the default randomization is scrambling data for certain connections.

Step 6

Offload Large Flows, if you need to improve performance in a computing intensive data center.

Step 7

Configure Connection Settings for Specific Traffic Classes (All Services). This is a catch-all procedure for connection settings. These settings can override the global defaults for specific traffic classes using service policy rules. You also use these rules to customize TCP Normalizer, change TCP sequence randomization, decrement time-to-live on packets, and implement other optional features.

Step 8

Configure TCP Options, if you need to force resets or change some other standard TCP behavior.

Configure Global Timeouts

You can set the global idle timeout durations for the connection and translation slots of various protocols. If the slot has not been used for the idle time specified, the resource is returned to the free pool.

Changing the global timeout sets a new default timeout, which in some cases can be overridden for particular traffic flows through service policies.

Procedure

Use the timeout command to set global timeouts.

All timeout values are in the format hh:mm:ss , with a maximum duration of 1193:0:0 in most cases. Use the clear configure timeout command to reset all timeouts to their default values. If you want to simply reset one timer to the default, enter the timeout command for that setting with the default value.

Use 0 for the value to disable a timer.

You can configure the following global timeouts.

  • timeout conn hh:mm:ss —The idle time after which a connection closes, between 0:5:0 and 1193:0:0. The default is 1 hour (1:0:0).

  • timeout half-closed hh:mm:ss —The idle time until a TCP half-closed connection closes. A connection is considered half-closed if both the FIN and FIN-ACK have been seen. If only the FIN has been seen, the regular conn timeout applies. The minimum is 30 seconds. The default is 10 minutes.

  • timeout udp hh:mm:ss —The idle time until a UDP connection closes. This duration must be at least 1 minute. The default is 2 minutes.

  • timeout icmp hh:mm:ss —The idle time for ICMP, between 0:0:2 and 1193:0:0. The default is 2 seconds (0:0:2).

  • timeout icmp-error hh:mm:ss —The idle time before the ASA removes an ICMP connection after receiving an ICMP echo-reply packet, between 0:0:0 and 0:1:0 or the timeout icmp value, whichever is lower. The default is 0 (disabled). When this timeout is disabled, and you enable ICMP inspection, then the ASA removes the ICMP connection as soon as an echo-reply is received; thus any ICMP errors that are generated for the (now closed) connection are dropped. This timeout delays the removal of ICMP connections so you can receive important ICMP errors.

  • timeout sunrpc hh:mm:ss —The idle time until a SunRPC slot is freed. This duration must be at least 1 minute. The default is 10 minutes.

  • timeout H323 hh:mm:ss —The idle time after which H.245 (TCP) and H.323 (UDP) media connections close, between 0:0:0 and 1193:0:0. The default is 5 minutes (0:5:0). Because the same connection flag is set on both H.245 and H.323 media connections, the H.245 (TCP) connection shares the idle timeout with the H.323 (RTP and RTCP) media connection.

  • timeout h225 hh:mm:ss —The idle time until an H.225 signaling connection closes. The H.225 default timeout is 1 hour (1:0:0). To close a connection immediately after all calls are cleared, a value of 1 second (0:0:1) is recommended.

  • timeout mgcp hh:mm:ss —The idle time after which an MGCP media connection is removed, between 0:0:0 and 1193:0:0. The default is 5 minutes (0:5:0)

  • timeout mgcp-pat hh:mm:ss —The absolute interval after which an MGCP PAT translation is removed, between 0:0:0 and 1193:0:0. The default is 5 minutes (0:5:0). The minimum time is 30 seconds.

  • timeout sctp hh:mm:ss —The idle time until a Stream Control Transmission Protocol (SCTP) connection closes, between 0:1:0 and 1193:0:0. The default is 2 minutes (0:2:0).

  • timeout sip hh:mm:ss —The idle time until a SIP signaling port connection closes, between 0:5:0 and 1193:0:0. The default is 30 minutes (0:30:0).

  • timeout sip_media hh:mm:ss —The idle time until an SIP media port connection closes. This duration must be at least 1 minute. The default is 2 minutes. The SIP media timer is used for SIP RTP/RTCP with SIP UDP media packets, instead of the UDP inactivity timeout.

  • timeout sip-provisional-media hh:mm:ss —The timeout value for SIP provisional media connections, between 0:1:0 and 0:30:0. The default is 2 minutes.

  • timeout sip-invite hh:mm:ss —The idle time after which pinholes for PROVISIONAL responses and media xlates will be closed, between 0:1:0 and 00:30:0. The default is 3 minutes (0:3:0).

  • timeout sip-disconnect hh:mm:ss —The idle time after which a SIP session is deleted if the 200 OK is not received for a CANCEL or a BYE message, between 0:0:1 and 00:10:0. The default is 2 minutes (0:2:0).

  • timeout uath hh:mm:ss {absolute | inactivity} —The duration before the authentication and authorization cache times out and the user has to reauthenticate the next connection, between 0:0:0 and 1193:0:0. The default is 5 minutes (0:5:0). The default timer is absolute ; you can set the timeout to occur after a period of inactivity by entering the inactivity keyword. The uauth duration must be shorter than the xlate duration. Set to 0 to disable caching. Do not use 0 if passive FTP is used for the connection or if the virtual http command is used for web authentication.

  • timeout xlate hh:mm:ss —The idle time until a translation slot is freed. This duration must be at least 1 minute. The default is 3 hours.

  • timeout pat-xlate hh:mm:ss —The idle time until a PAT translation slot is freed, between 0:0:30 and 0:5:0. The default is 30 seconds. You may want to increase the timeout if upstream routers reject new connections using a freed PAT port because the previous connection might still be open on the upstream device.

  • timeout tcp-proxy-reassembly hh:mm:ss —The idle timeout after which buffered packets waiting for reassembly are dropped, between 0:0:10 and 1193:0:0. The default is 1 minute (0:1:0).

  • timeout floating-conn hh:mm:ss —When multiple routes exist to a network with different metrics, the ASA uses the one with the best metric at the time of connection creation. If a better route becomes available, then this timeout lets connections be closed so a connection can be reestablished to use the better route. The default is 0 (the connection never times out). To make it possible to use better routes, set the timeout to a value between 0:0:30 and 1193:0:0.

  • timeout conn-holddown hh:mm:ss —How long the system should maintain a connection when the route used by the connection no longer exists or is inactive. If the route does not become active within this holddown period, the connection is freed. The purpose of the connection holddown timer is to reduce the effect of route flapping, where routes might come up and go down quickly. You can reduce the holddown timer to make route convergence happen more quickly. The default is 15 seconds, the range is 00:00:00 to 00:00:15.

  • timeout igp stale-route hh:mm:ss —How long to keep a stale route before removing it from the router information base. These routes are for interior gateway protocols such as OSPF. The default is 70 seconds (00:01:10), the range is 00:00:10 to 00:01:40.

Protect Servers from a SYN Flood DoS Attack (TCP Intercept)

A SYN-flooding denial of service (DoS) attack occurs when an attacker sends a series of SYN packets to a host. These packets usually originate from spoofed IP addresses. The constant flood of SYN packets keeps the server SYN queue full, which prevents it from servicing connection requests from legitimate users.

You can limit the number of embryonic connections to help prevent SYN flooding attacks. An embryonic connection is a connection request that has not finished the necessary handshake between source and destination.

When the embryonic connection threshold of a connection is crossed, the ASA acts as a proxy for the server and generates a SYN-ACK response to the client SYN request using the SYN cookie method, so that the connection is not added to the SYN queue of the targeted host. The SYN cookie is the initial sequence number returned in the SYN-ACK that is constructed from MSS, time stamp, and a mathematical hash of other items to essentially create a secret. If the ASA receives an ACK back from the client with the correct sequence number and within the valid time window, it can then authenticate that the client is real and allow the connection to the server. The component that performs the proxy is called TCP Intercept.

The end-to-end process for protecting a server from a SYN flood attack involves setting connection limits, enabling TCP Intercept statistics, and then monitoring the results.

Before you begin

  • Ensure that you set the embryonic connection limit lower than the TCP SYN backlog queue on the server that you want to protect. Otherwise, valid clients can no longer access the server during a SYN attack. To determine reasonable values for embryonic limits, carefully analyze the capacity of the server, the network, and server usage.

  • Depending on the number of CPU cores on your ASA model, the maximum concurrent and embryonic connections can exceed the configured numbers due to the way each core manages connections. In the worst case scenario, the ASA allows up to n-1 extra connections and embryonic connections, where n is the number of cores. For example, if your model has 4 cores, if you configure 6 concurrent connections and 4 embryonic connections, you could have an additional 3 of each type. To determine the number of cores for your model, enter the show cpu core command.

Procedure

Step 1

Create an L3/L4 class map to identify the servers you are protecting. Use an access-list match. 


class-map name
match parameter

Example:


hostname(config)# access-list servers extended permit tcp any host 10.1.1.5 eq http
hostname(config)# access-list servers extended permit tcp any host 10.1.1.6 eq http
hostname(config)# class-map protected-servers 
hostname(config-cmap)# match access-list servers

Step 2

Add or edit a policy map that sets the actions to take with the class map traffic, and identify the class map. 


policy-map name
class name

Example:


hostname(config)# policy-map global_policy
hostname(config-pmap)# class protected-servers

In the default configuration, the global_policy policy map is assigned globally to all interfaces. If you want to edit the global_policy, enter global_policy as the policy name. For the class map, specify the class you created earlier in this procedure.

Step 3

Set the embryonic connection limits.

  • set connection embryonic-conn-max n —The maximum number of simultaneous embryonic TCP connections allowed, from 0 and 2000000. The default is 0, which allows unlimited connections.

  • set connection per-client-embryonic-max n —The maximum number of simultaneous embryonic TCP connections allowed per client, from 0 and 2000000. The default is 0, which allows unlimited connections.

  • set connection syn-cookie-mss n —The server maximum segment size (MSS) for SYN-cookie generation for embryonic connections upon reaching the embryonic connections limit, from 48 to 65535 . The default is 1380. This setting is meaningful only if you configure set connection embryonic-conn-max or per-client-embryonic-max .

Example:


hostname(config-pmap-c)# set connection embryonic-conn-max 1000
hostname(config-pmap-c)# set connection per-client-embryonic-max 50

Step 4

If you are editing an existing service policy (such as the default global policy called global_policy), you can skip this step. Otherwise, activate the policy map on one or more interfaces.

service-policy policymap_name {global | interface interface_name}

Example:


hostname(config)# service-policy global_policy global

The global keyword applies the policy map to all interfaces, and interface applies the policy to one interface. Only one global policy is allowed. You can override the global policy on an interface by applying a service policy to that interface. You can only apply one policy map to each interface.

Step 5

Configure threat detection statistics for attacks intercepted by TCP Intercept.

threat-detection statistics tcp-intercept [rate-interval minutes] [burst-rate attacks_per_sec] [average-rate attacks_per_sec]

Where:

  • rate-interval minutes sets the size of the history monitoring window, between 1 and 1440 minutes. The default is 30 minutes. During this interval, the ASA samples the number of attacks 30 times.

  • burst-rate attacks_per_sec sets the threshold for syslog message generation, between 25 and 2147483647. The default is 400 per second. When the burst rate is exceeded, syslog message 733104 is generated.

  • average-rate attacks_per_sec sets the average rate threshold for syslog message generation, between 25 and 2147483647. The default is 200 per second. When the average rate is exceeded, syslog message 733105 is generated.

Example:


hostname(config)# threat-detection statistics tcp-intercept

Step 6

Monitor the results with the following commands:

  • show threat-detection statistics top tcp-intercept [all | detail]—View the top 10 protected servers under attack. The all keyword shows the history data of all the traced servers. The detail keyword shows history sampling data. The ASA samples the number of attacks 30 times during the rate interval, so for the default 30 minute period, statistics are collected every 60 seconds.

  • clear threat-detection statistics tcp-intercept—Erases TCP Intercept statistics.

Example:


hostname(config)# show threat-detection statistics top tcp-intercept
Top 10 protected servers under attack (sorted by average rate)
Monitoring window size: 30 mins    Sampling interval: 30 secs
<Rank> <Server IP:Port> <Interface> <Ave Rate> <Cur Rate> <Total> <Source IP (Last Attack Time)>
----------------------------------------------------------------------------------
1    10.1.1.5:80 inside 1249 9503 2249245 <various> Last: 10.0.0.3 (0 secs ago)
2    10.1.1.6:80 inside 10 10 6080 10.0.0.200 (0 secs ago)

Customize Abnormal TCP Packet Handling (TCP Maps, TCP Normalizer)

The TCP Normalizer identifies abnormal packets that the ASA can act on when they are detected; for example, the ASA can allow, drop, or clear the packets. TCP normalization helps protect the ASA from attacks. TCP normalization is always enabled, but you can customize how some features behave.

The default configuration includes the following settings: 


no check-retransmission
no checksum-verification
exceed-mss allow
queue-limit 0 timeout 4
reserved-bits allow
syn-data allow
synack-data drop
invalid-ack drop
seq-past-window drop
tcp-options range 6 7 clear
tcp-options range 9 18 clear
tcp-options range 20 255 clear
tcp-options md5 allow
tcp-options mss allow
tcp-options selective-ack allow
tcp-options timestamp allow
tcp-options window-scale allow
ttl-evasion-protection
urgent-flag clear
window-variation allow-connection

To customize the TCP normalizer, first define the settings using a TCP map. Then, you can apply the map to selected traffic classes using service policies.

Procedure

Step 1

Create a TCP map to specify the TCP normalization criteria that you want to look for: tcp-map tcp-map-name

Step 2

Configure the TCP map criteria by entering one or more of the following commands. The defaults are used for any commands you do not enter. Use the no form of a command to disable the setting.

  • check-retransmission—Prevent inconsistent TCP retransmission. This command is disabled by default.

  • checksum-verification—Verify the TCP checksum, dropping packets that fail verification. This command is disabled by default.

  • exceed-mss {allow | drop}—Allow or drop packets whose data length exceeds the TCP maximum segment size. The default is to allow the packets.

  • invalid-ack {allow | drop}—Allow or drop packets with an invalid ACK. The default is to drop the packet, with the exception of WAAS connections, where they are allowed. You might see invalid ACKs in the following instances:

    • In the TCP connection SYN-ACK-received status, if the ACK number of a received TCP packet is not exactly the same as the sequence number of the next TCP packet sending out, it is an invalid ACK.

    • Whenever the ACK number of a received TCP packet is greater than the sequence number of the next TCP packet sending out, it is an invalid ACK.

  • queue-limit pkt_num [timeout seconds]—Set the maximum number of out-of-order packets that can be buffered and put in order for a TCP connection, between 1 and 250 packets. The default is 0, which means this setting is disabled and the default system queue limit is used depending on the type of traffic:

    • Connections for application inspection (the inspect command), and TCP check-retransmission (the TCP map check-retransmission command) have a queue limit of 3 packets. If the ASA receives a TCP packet with a different window size, then the queue limit is dynamically changed to match the advertised setting.

    • For other TCP connections, out-of-order packets are passed through untouched.

    If you set the queue-limit command to be 1 or above, then the number of out-of-order packets allowed for all TCP traffic matches this setting. For example, for application inspection and TCP check-retransmission traffic, any advertised settings from TCP packets are ignored in favor of the queue-limit setting. For other TCP traffic, out-of-order packets are now buffered and put in order instead of passed through untouched.

    The timeout seconds argument sets the maximum amount of time that out-of-order packets can remain in the buffer, between 1 and 20 seconds; if they are not put in order and passed on within the timeout period, then they are dropped. The default is 4 seconds. You cannot change the timeout for any traffic if the pkt_num argument is set to 0; you need to set the limit to be 1 or above for the timeout keyword to take effect.

  • reserved-bits {allow | clear | drop}—Set the action for reserved bits in the TCP header. You can allow the packet (without changing the bits), clear the bits and allow the packet, or drop the packet.

  • seq-past-window {allow | drop}—Set the action for packets that have past-window sequence numbers, namely the sequence number of a received TCP packet is greater than the right edge of the TCP receiving window. You can allow the packets only if the queue-limit command is set to 0 (disabled). The default is to drop the packets.

  • synack-data {allow | drop}—Allow or drop TCP SYNACK packets that contain data. The default is to drop the packet.

  • syn-data {allow | drop}—Allow or drop SYN packets with data. The default is to allow the packet.

  • tcp-options {md5 | mss | selective-ack | timestamp | window-scale | range lower upper} action —Set the action for packets with TCP options. These options are named: md5 , mss , selective-ack (selective acknowledgment mechanism), timestamp , and window-scale (window scale mechanism). For other options, you specify them by number on the range keyword, where the range limits are 6-7, 9-18, and 20-255. To target a single option by number, enter the same number for the lower and upper range. You can enter the command multiple times in a map to define your complete policy. Note that if a TCP connection is inspected, all options are cleared except the MSS and selective-acknowledgment (SACK) options, regardless of your configuration. Following are the possible actions:

    • allow [multiple] —Allow packets that contain a single option of this type. This is the default for all of the named options. If you want to allow packets even if they contain more than one instance of the option, add the multiple keyword. (The multiple keyword is not available with range .)

    • maximum limit —For mss only. Set the maximum segment size to the indicated limit, from 68-65535. The default TCP MSS is defined on the sysopt connection tcpmss command.

    • clear —Remove the options of this type from the header and allow the packet. This is the default for all of the numbered options. Note that clearing the timestamp option disables PAWS and RTT.

    • drop —Drop packets that contain this option. This action is available for md5 and range only.

  • ttl-evasion-protection—Have the maximum TTL for a connection be determined by the TTL in the initial packet. The TTL for subsequent packets can decrease, but it cannot increase. The system will reset the TTL to the lowest previously-seen TTL for that connection. This protects against TTL evasion attacks. TTL evasion protection is enabled by default, so you would only need to enter the no form of this command.

    For example, an attacker can send a packet that passes policy with a very short TTL. When the TTL goes to zero, a router between the ASA and the endpoint drops the packet. It is at this point that the attacker can send a malicious packet with a long TTL that appears to the ASA to be a retransmission and is passed. To the endpoint host, however, it is the first packet that has been received by the attacker. In this case, an attacker is able to succeed without security preventing the attack.

  • urgent-flag {allow | clear}—Set the action for packets with the URG flag. You can allow the packet, or clear the flag and allow the packet. The default is to clear the flag.

    The URG flag is used to indicate that the packet contains information that is of higher priority than other data within the stream. The TCP RFC is vague about the exact interpretation of the URG flag, therefore end systems handle urgent offsets in different ways, which may make the end system vulnerable to attacks.

  • window-variation {allow | drop}—Allow or drop a connection that has changed its window size unexpectedly. The default is to allow the connection.

    The window size mechanism allows TCP to advertise a large window and to subsequently advertise a much smaller window without having accepted too much data. From the TCP specification, “shrinking the window” is strongly discouraged. When this condition is detected, the connection can be dropped.

Step 3

Apply the TCP map to a traffic class using a service policy.

  1. Define the traffic class with an L3/L4 class map and add the map to a policy map. 

    
class-map name
match parameter
policy-map name
class name

    Example:

    
hostname(config)# class-map normalization
hostname(config-cmap)# match any
hostname(config)# policy-map global_policy
hostname(config-pmap)# class normalization

    In the default configuration, the global_policy policy map is assigned globally to all interfaces. If you want to edit the global_policy, enter global_policy as the policy name. For information on matching statements for class maps, see Create a Layer 3/4 Class Map for Through Traffic.

  2. Apply the TCP map: set connection advanced-options tcp-map-name

    Example:

    
hostname(config-pmap-c)# set connection advanced-options tcp_map1

  3. If you are editing an existing service policy (such as the default global policy called global_policy), you are done. Otherwise, activate the policy map on one or more interfaces.

    service-policy policymap_name {global | interface interface_name}

    Example:

    
hostname(config)# service-policy global_policy global

    The global keyword applies the policy map to all interfaces, and interface applies the policy to one interface. Only one global policy is allowed. You can override the global policy on an interface by applying a service policy to that interface. You can only apply one policy map to each interface.

Examples

For example, to allow urgent flag and urgent offset packets for all traffic sent to the range of TCP ports between the well known FTP data port and the Telnet port, enter the following commands: 


hostname(config)# tcp-map tmap
hostname(config-tcp-map)# urgent-flag allow
hostname(config-tcp-map)# class-map urg-class
hostname(config-cmap)# match port tcp range ftp-data telnet
hostname(config-cmap)# policy-map pmap
hostname(config-pmap)# class urg-class
hostname(config-pmap-c)# set connection advanced-options tmap
hostname(config-pmap-c)# service-policy pmap global

Bypass TCP State Checks for Asymetrical Routing (TCP State Bypass)

If you have an asymetrical routing environment in your network, where the outbound and inbound flow for a given connection can go through two different ASA devices, you need to implement TCP State Bypass on the affected traffic.

However, TCP State Bypass weakens the security of your network, so you should apply bypass on very specific, limited traffic classes.

The following topics explain the problem and solution in more detail.

The Asymetrical Routing Problem

By default, all traffic that goes through the ASA is inspected using the Adaptive Security Algorithm and is either allowed through or dropped based on the security policy. The ASA maximizes the firewall performance by checking the state of each packet (new connection or established connection) and assigning it to either the session management path (a new connection SYN packet), the fast path (an established connection), or the control plane path (advanced inspection).

TCP packets that match existing connections in the fast path can pass through the ASA without rechecking every aspect of the security policy. This feature maximizes performance. However, the method of establishing the session in the fast path using the SYN packet, and the checks that occur in the fast path (such as TCP sequence number), can stand in the way of asymmetrical routing solutions: both the outbound and inbound flow of a connection must pass through the same ASA device.

For example, a new connection goes to Security Appliance 1. The SYN packet goes through the session management path, and an entry for the connection is added to the fast path table. If subsequent packets of this connection go through Security Appliance 1, then the packets match the entry in the fast path, and are passed through. But if subsequent packets go to Security Appliance 2, where there was not a SYN packet that went through the session management path, then there is no entry in the fast path for the connection, and the packets are dropped. The following figure shows an asymmetric routing example where the outbound traffic goes through a different ASA than the inbound traffic:

Figure 1. Asymmetric Routing

If you have asymmetric routing configured on upstream routers, and traffic alternates between two ASA devices, then you can configure TCP state bypass for specific traffic. TCP state bypass alters the way sessions are established in the fast path and disables the fast path checks. This feature treats TCP traffic much as it treats a UDP connection: when a non-SYN packet matching the specified networks enters the ASA device, and there is not a fast path entry, then the packet goes through the session management path to establish the connection in the fast path. Once in the fast path, the traffic bypasses the fast path checks.

Guidelines and Limitations for TCP State Bypass

TCP State Bypass Unsupported Features

The following features are not supported when you use TCP state bypass:

  • Application inspection—Inspection requires both inbound and outbound traffic to go through the same ASA, so inspection is not applied to TCP state bypass traffic.

  • AAA authenticated sessions—When a user authenticates with one ASA, traffic returning via the other ASA will be denied because the user did not authenticate with that ASA.

  • TCP Intercept, maximum embryonic connection limit, TCP sequence number randomization—The ASA does not keep track of the state of the connection, so these features are not applied.

  • TCP normalization—The TCP normalizer is disabled.

  • Stateful failover.

TCP State Bypass NAT Guidelines

Because the translation session is established separately for each ASA, be sure to configure static NAT on both devices for TCP state bypass traffic. If you use dynamic NAT, the address chosen for the session on Device 1 will differ from the address chosen for the session on Device 2.

Configure TCP State Bypass

To bypass TCP state checking in asymetrical routing environments, carefully define a traffic class that applies to the affected hosts or networks only, then enable TCP State Bypass on the traffic class using a service policy. Because bypass reduces the security of the network, limit its application as much as possible.

Before you begin

If there is no traffic on a given connection for 2 minutes, the connection times out. You can override this default using the set connection timeout idle command for the TCP state bypass traffic class. Normal TCP connections timeout by default after 60 minutes.

Procedure

Step 1

Create an L3/L4 class map to identify the hosts that require TCP State Bypass. Use an access-list match to identify the source and destination hosts. 


class-map name
match parameter
Example:

hostname(config)# access-list bypass extended permit tcp host 10.1.1.1 host 10.2.2.2
hostname(config)# class-map bypass-class 
hostname(config-cmap)# match access-list bypass

Step 2

Add or edit a policy map that sets the actions to take with the class map traffic, and identify the class map. 


policy-map name
class name
Example:

hostname(config)# policy-map global_policy
hostname(config-pmap)# class bypass-class

In the default configuration, the global_policy policy map is assigned globally to all interfaces. If you want to edit the global_policy, enter global_policy as the policy name. For the class map, specify the class you created earlier in this procedure.

Step 3

Enable TCP State Bypass on the class: set connection advanced-options tcp-state-bypass

Step 4

If you are editing an existing service policy (such as the default global policy called global_policy), you are done. Otherwise, activate the policy map on one or more interfaces.

service-policy policymap_name {global | interface interface_name}

Example:

hostname(config)# service-policy global_policy global

The global keyword applies the policy map to all interfaces, and interface applies the policy to one interface. Only one global policy is allowed. You can override the global policy on an interface by applying a service policy to that interface. You can only apply one policy map to each interface.

Example

The following is a sample configuration for TCP state bypass: 


hostname(config)# access-list tcp_bypass extended permit tcp 10.1.1.0 255.255.255.224 any

hostname(config)# class-map tcp_bypass
hostname(config-cmap)# description "TCP traffic that bypasses stateful firewall"
hostname(config-cmap)# match access-list tcp_bypass

hostname(config-cmap)# policy-map tcp_bypass_policy
hostname(config-pmap)# class tcp_bypass
hostname(config-pmap-c)# set connection advanced-options tcp-state-bypass

hostname(config-pmap-c)# service-policy tcp_bypass_policy interface outside

Disable TCP Sequence Randomization

Each TCP connection has two ISNs: one generated by the client and one generated by the server. The ASA randomizes the ISN of the TCP SYN passing in both the inbound and outbound directions.

Randomizing the ISN of the protected host prevents an attacker from predicting the next ISN for a new connection and potentially hijacking the new session. However, TCP sequence randomization effectively breaks TCP SACK (Selective Acknowledgement), as the sequence numbers the client sees are different from what the server sees.

You can disable TCP initial sequence number randomization if necessary, for example, because data is getting scrambled. For example:

  • If another in-line firewall is also randomizing the initial sequence numbers, there is no need for both firewalls to be performing this action, even though this action does not affect the traffic.

  • If you use eBGP multi-hop through the ASA, and the eBGP peers are using MD5. Randomization breaks the MD5 checksum.

  • You use a WAAS device that requires the ASA not to randomize the sequence numbers of connections.

  • You enable hardware bypass for the ISA 3000, and TCP connections are dropped when the ISA 3000 is no longer part of the data path.


Note

We do not recommend disabling TCP sequence randomization when using clustering. There is a small chance that some TCP sessions won't be established, because the SYN/ACK packet might be dropped.

Procedure

Step 1

Create an L3/L4 class map to identify the traffic whose TCP sequence numbers should not be randomized. The class match should be for TCP traffic; you can identify specific hosts (with an ACL), do a TCP port match, or simply match any traffic. 


class-map name
match parameter

Example:


hostname(config)# access-list preserve-sq-no extended permit tcp any host 10.2.2.2
hostname(config)# class-map no-tcp-random 
hostname(config-cmap)# match access-list preserve-sq-no

Step 2

Add or edit a policy map that sets the actions to take with the class map traffic, and identify the class map. 


policy-map name
class name

Example:


hostname(config)# policy-map global_policy
hostname(config-pmap)# class no-tcp-random

In the default configuration, the global_policy policy map is assigned globally to all interfaces. If you want to edit the global_policy, enter global_policy as the policy name. For the class map, specify the class you created earlier in this procedure.

Step 3

Disable TCP sequence number randomization on the class:

set connection random-sequence-number disable

If you later decide to turn it back on, replace “disable “with enable.

Step 4

If you are editing an existing service policy (such as the default global policy called global_policy), you are done. Otherwise, activate the policy map on one or more interfaces.

service-policy policymap_name {global | interface interface_name}

Example:


hostname(config)# service-policy global_policy global

The global keyword applies the policy map to all interfaces, and interface applies the policy to one interface. Only one global policy is allowed. You can override the global policy on an interface by applying a service policy to that interface. You can only apply one policy map to each interface.

Offload Large Flows

If you deploy the ASA on supported devices in a data center, you can identify select traffic to be offloaded to a super fast path, where traffic is switched in the NIC itself. Offloading can help you improve performance for data-intensive applications such as large file transfers.

  • High Performance Computing (HPC) Research sites, where the ASA is deployed between storage and high compute stations. When one research site backs up using FTP file transfer or file sync over NFS, the large amount of data traffic affects all contexts on the ASA. Offloading FTP file transfer and file sync over NFS reduces the impact on other traffic.

  • High Frequency Trading (HFT), where the ASA is deployed between workstations and the Exchange, mainly for compliance purposes. Security is usually not a concern, but latency is a major concern.

Before being offloaded, the ASA first applies normal security processing, such as access rules and inspection, during connection establishment. The ASA also does session tear-down. But once a connection is established, if it is eligible to be offloaded, further processing happens in the NIC rather than the ASA.

Offloaded flows continue to receive limited stateful inspection, such as basic TCP flag and option checking, and checksum verification if you configure it. The system can selectively escalate packets to the firewall system for further processing if necessary.

To identify flows that can be offloaded, you create a service policy rule that applies the flow offloading service. A matching flow is then offloaded if it meets the following conditions:

  • IPv4 addresses only.

  • TCP, UDP, GRE only.

  • Standard or 802.1Q tagged Ethernet frames only.

  • (Transparent mode only.) Multicast flows for bridge groups that contain two and only two interfaces.

Reverse flows for offloaded flows are also offloaded.

Flow Offload Limitations

Not all flows can be offloaded. Even after offload, a flow can be removed from being offloaded under certain conditions. Following are some of the limitations:

Device Limitations

The feature is supported on the following devices:

  • Firepower 4100/9300 running FXOS 1.1.3 or higher.

  • Secure Firewall 4200

  • Secure Firewall 3100

Flows that cannot be offloaded

The following types of flows cannot be offloaded.

  • Any flows that do not use IPv4 addressing, such as IPv6 addressing.

  • Flows for any protocol other than TCP, UDP, and GRE.


    Note

    PPTP GRE connections cannot be offloaded.

  • Flows that require inspection. In some cases, such as FTP, the secondary data channel can be offloaded although the control channel cannot be offloaded.

  • IPsec and TLS/DTLS VPN connections that terminate on the device.

  • Multicast flows in routed mode.

  • Multicast flows in transparent mode for bridge groups that have three or more interfaces.

  • TCP Intercept flows.

  • TCP state bypass flows. You cannot configure flow offload and TCP state bypass on the same traffic.

  • AAA cut-through proxy flows.

  • Vpath, VXLAN related flows.

  • Flows tagged with security groups.

  • Reverse flows that are forwarded from a different cluster node, in case of asymmetric flows in a cluster.

  • Centralized flows in a cluster, if the flow owner is not the control unit.

Additional Limitations

  • Flow offload and Dead Connection Detection (DCD) are not compatible. Do not configure DCD on connections that can be offloaded.

  • If more than one flow that matches flow offload conditions are queued to be offloaded at the same time to the same location on the hardware, only the first flow is offloaded. The other flows are processed normally. This is called a collision. Use the show flow-offload flow command in the CLI to display statistics for this situation.

  • Although offloaded flows pass through FXOS interfaces, statistics for these flows do not appear on the logical device interface. Thus, logical device interface counters and packet rates do not reflect offloaded flows.

Conditions for reversing offload

After a flow is offloaded, packets within the flow are returned to the ASA for further processing if they meet the following conditions:

  • They include TCP options other than Timestamp.

  • They are fragmented.

  • They are subject to Equal-Cost Multi-Path (ECMP) routing, and ingress packets move from one interface to another.

Configure Flow Offload

To configure flow offload, you must enable the service and then create service policies to identify the traffic that is eligible for offloading. Enabling or disabling the service requires a reboot. However, adding or editing service policies does not require a reboot.

Procedure

Step 1

Enable the flow offload service.

flow-offload enable

Example:

ciscoasa(config)# flow-offload enable

Step 2

Create the service policy rule that identifies traffic that is eligible for offload.

  1. Create an L3/L4 class map to identify the traffic that is eligible for flow offload. Matching by access-list or port would be the most typical options. 

    
class-map name
match parameter
    Example:
    
hostname(config)# access-list offload permit tcp 10.1.1.0 255.255.255.224 any
hostname(config)# class-map flow_offload 
hostname(config-cmap)# match access-list offload

  2. Add or edit a policy map that sets the actions to take with the class map traffic, and identify the class map. 

    
policy-map name
class name
    Example:
    
hostname(config)# policy-map offload_policy 
hostname(config-pmap)# class flow_offload

    In the default configuration, the global_policy policy map is assigned globally to all interfaces. If you want to edit the global_policy, enter global_policy as the policy name. For the class map, specify the class you created earlier in this procedure.

  3. Enable flow offload on the class: set connection advanced-options flow-offload

  4. If you are editing an existing service policy (such as the default global policy called global_policy), you are done. Otherwise, activate the policy map on one or more interfaces.

    service-policy policymap_name {global | interface interface_name}

    Example:
    
hostname(config)# service-policy offload_policy interface outside

    The global keyword applies the policy map to all interfaces, and interface applies the policy to one interface. Only one global policy is allowed. You can override the global policy on an interface by applying a service policy to that interface. You can only apply one policy map to each interface.

Example

The following example classifies all TCP traffic from the 10.1.1.0 255.255.255.224 subnet as eligible for offload and attaches the policy to the outside interface. 


hostname(config)# access-list offload permit tcp 10.1.1.0 255.255.255.224 any
hostname(config)# class-map flow_offload 
hostname(config-cmap)# match access-list offload 
hostname(config)# policy-map offload_policy 
hostname(config-pmap)# class flow_offload 
hostname(config-pmap-c)# set connection advanced-options flow-offload 
hostname(config)# service-policy offload_policy interface outside

IPsec Flow Offload

You can configure supporting device models to use IPsec flow offload. After the initial setup of an IPsec site-to-site VPN or remote access VPN security association (SA), IPsec connections are offloaded to the field-programmable gate array (FPGA) in the device, which should improve device performance.

Offloaded operations specifically relate to the pre-decryption and decryption processing on ingress, and the pre-encryption and encryption processing on egress. The system software handles the inner flow to apply your security policies.

IPsec flow offload is enabled by default, and applies to the following device types:

  • Secure Firewall 3100

  • Secure Firewall 4200

IPsec flow offload is also used when the device's VTI loopback interface is enabled.

Limitations for IPsec Flow Offload

The following IPsec flows are not offloaded:

  • IKEv1 tunnels. Only IKEv2 tunnels will be offloaded. IKEv2 supports stronger ciphers.

  • Flows that have volume-based rekeying configured.

  • Flows that have compression configured.

  • Transport mode flows. Only tunnel mode flows will be offloaded.

  • AH format. Only ESP/NAT-T format will be supported.

  • Flows that have post-fragmentation configured.

  • Flows that have anti-replay window size other than 64bit and anti-replay is not disabled.

  • Flows that have firewall filter enabled.

Configure IPsec Flow Offload

IPsec flow offload is enabled by default on hardware platforms that support the feature. However, egress optimization is not enabled by default, so you need to configure it if you want the feature.

Before you begin

IPsec flow offload is configured globally. You cannot configure it for selected traffic flows.

Use the no form of these commands to disable the features.

To see the current configuration state, use the show flow-offload ipsec info command.

Procedure

Step 1

Enable IPsec flow offload.

flow-offload-ipsec

Step 2

Enable egress optimization to optimize the data path to enhance performance for single tunnel flows.

flow-offload-ipsec egress-optimization

The configuration for egress optimization is separate from flow offload. However, even if enabled, it is effective only if you also enable IPsec flow offload. Egress optimization is not enabled by default.

Configure Connection Settings for Specific Traffic Classes (All Services)

You can configure different connection settings for specific traffic classes using service policies. Use service policies to:

  • Customize connection limits and timeouts used to protect against DoS and SYN-flooding attacks.

  • Implement Dead Connection Detection so that valid but idle connections remain alive.

  • Disable TCP sequence number randomization in cases where you do not need it.

  • Customize how the TCP Normalizer protects against abnormal TCP packets.

  • Implement TCP State Bypass for traffic subject to asymetrical routing. Bypass traffic is not subject to inspection.

  • Implement Stream Control Transmission Protocol (SCTP) State Bypass to turn off SCTP stateful inspection.

  • Implement flow offload to improve performance on supported hardware platforms.

  • Decrement time-to-live (TTL) on packets so that the ASA will show up on trace route output.


    Note

    If you decrement time to live, packets with a TTL of 1 will be dropped, but a connection will be opened for the session on the assumption that the connection might contain packets with a greater TTL. Note that some packets, such as OSPF hello packets, are sent with TTL = 1, so decrementing time to live can have unexpected consequences for transparent mode ASA devices. The decrement time-to-live settings does not impact the OSPF process when ASA is operating in a routed mode.

You can configure any combination of these settings for a given traffic class, except for TCP State Bypass and TCP Normalizer customization, which are mutually exclusive.


Tip

This procedure shows a service policy for traffic that goes through the ASA. You can also configure the connection maximum and embryonic connection maximum for management (to the box) traffic.

Before you begin

If you want to customize the TCP Normalizer, create the required TCP Map before proceeding.

The set connection command (for connection limits and sequence randomization) and set connection timeout commands are described here separately for each parameter. However, you can enter the commands on one line, and if you enter them separately, they are shown in the configuration as one command.

Procedure

Step 1

Create an L3/L4 class map to identify the traffic for which you want to customize connection settings. 


class-map name
match parameter

Example:


hostname(config)# class-map CONNS
hostname(config-cmap)# match any

For information on matching statements, see Create a Layer 3/4 Class Map for Through Traffic.

Step 2

Add or edit a policy map that sets the actions to take with the class map traffic, and identify the class map. 


policy-map name
class name

Example:


hostname(config)# policy-map global_policy
hostname(config-pmap)# class CONNS

In the default configuration, the global_policy policy map is assigned globally to all interfaces. If you want to edit the global_policy, enter global_policy as the policy name. For the class map, specify the class you created earlier in this procedure.

Step 3

Set connection limits and TCP sequence number randomization. (TCP Intercept.)

By default, there are no connection limits. If you implement limits, the system must start tracking them, which can increase CPU and memory usage and result in operational problems for systems under heavy load, especially in a cluster.

  • set connection conn-max n—(TCP, UDP, SCTP.) The maximum number of simultaneous connections that are allowed, between 0 and 2000000, for the entire class. The default is 0, which allows unlimited connections. For TCP connections, this applies to established connections only.

    • If two servers are configured to allow simultaneous connections, the connection limit is applied to each configured server separately.

    • Because the limit is applied to a class, one attack host can consume all the connections and leave none for the rest of the hosts that are matched to the class.

  • set connection per-client-max n—(TCP, UDP, SCTP.) The maximum number of simultaneous connections allowed per client, between 0 and 2000000. The default is 0, which allows unlimited connections. This argument restricts the maximum number of simultaneous connections that are allowed for each host that is matched to the class. For TCP connections, this includes established, half-open, and half-closed connections.

  • set connection embryonic-conn-max n—The maximum number of simultaneous embryonic TCP connections allowed, between 0 and 2000000. The default is 0, which allows unlimited connections. By setting a non-zero limit, you enable TCP Intercept, which protects inside systems from a DoS attack perpetrated by flooding an interface with TCP SYN packets. Also set the per-client options to protect against SYN flooding.

  • set connection per-client-embryonic-max n—The maximum number of simultaneous embryonic TCP connections allowed per client, between 0 and 2000000. The default is 0, which allows unlimited connections.

  • set connection syn-cookie-mss n —The server maximum segment size (MSS) for SYN-cookie generation for embryonic connections upon reaching the embryonic connections limit, from 48 to 65535 . The default is 1380. This setting is meaningful only if you configure set connection embryonic-conn-max or per-client-embryonic-max .

  • set connection random-sequence-number {enable | disable}—Whether to enable or disable TCP sequence number randomization. Randomization is enabled by default.

Example:


hostname(config-pmap-c)# set connection conn-max 256 random-sequence-number disable

Step 4

Set connection timeouts and Dead Connection Detection (DCD).

The defaults described below assume you have not changed the global defaults for these behaviors using the timeout command; the global defaults override the ones described here. Enter 0 to disable the timer, so that a connection never times out.

  • set connection timeout embryonic hh:mm:ss—The timeout period until a TCP embryonic (half-open) connection is closed, between 0:0:5 and 1193:00:00. The default is 0:0:30.

  • set connection timeout idle hh:mm:ss [reset]—The idle timeout period after which an established connection of any protocol closes, between 0:0:1 and 1193:0:0. The default is 1:0:0. For TCP traffic, the reset keyword sends a reset to TCP endpoints when the connection times out.

    The default udp idle timeout is 2 minutes. The default icmp idle timeout is 2 seconds. The default esp and ha idle timeout is 30 seconds. For all other protocols, the default idle timeout is 2 minutes.

  • set connection timeout half-closed hh:mm:ss—The idle timeout period until a half-closed connection is closed, between 0:5:0 (for 9.1(1) and earlier) or 0:0:30 (for 9.1(2) and later) and 1193:0:0. The default is 0:10:0. Half-closed connections are not affected by DCD. Also, the ASA does not send a reset when taking down half-closed connections.

  • set connection timeout dcd [retry-interval [max_retries]]—Enable Dead Connection Detection (DCD). Before expiring an idle connection, the ASA probes the end hosts to determine if the connection is valid. If both hosts respond, the connection is preserved, otherwise the connection is freed. When operating in transparent firewall mode, you must configure static routes for the endpoints. You cannot configure DCD on connections that are also offloaded, so ensure DCD and flow offload traffic classes do not overlap. Use the show conn detail command to track how many DCD probes have been sent by the initiator and responder.

    The retry-interval sets the time duration in hh:mm:ss format to wait after each unresponsive DCD probe before sending another probe, between 0:0:1 and 24:0:0. The default is 0:0:15. The max-retries sets the number of consecutive failed retries for DCD before declaring the connection as dead. The minimum value is 1 and the maximum value is 255. The default is 5.

    For systems that are operating in a cluster or high-availability configuration, we recommend that you do not set the interval to less than one minute (0:1:0). If the connection needs to be moved between systems, the changes required take longer than 30 seconds, and the connection might be deleted before the change is accomplished.

Example:


hostname(config-pmap-c)# set connection timeout idle 2:0:0 embryonic 0:40:0
half-closed 0:20:0 dcd

Step 5

Decrement time-to-live (TTL) on packets that match the class: set connection decrement-ttl

This command, along with the icmp unreachable command, is required to allow a traceroute through the ASA that shows the ASA as one of the hops.

Example:


hostname(config)# class-map global-policy
hostname(config-cmap)# match any
hostname(config-cmap)# exit
hostname(config)# policy-map global_policy
hostname(config-pmap)# class global-policy
hostname(config-pmap-c)# set connection decrement-ttl
hostname(config-pmap-c)# exit
hostname(config)# icmp unreachable rate-limit 50 burst-size 6

Step 6

Set advanced connection options.

Advanced options are special purpose configurations that are not needed under normal circumstances. You configure them with the set connection advanced-options command.

  • set connection advanced-options tcp_map_name —Customize TCP Normalizer behavior by applying a TCP map. For detailed information, see Customize Abnormal TCP Packet Handling (TCP Maps, TCP Normalizer).

  • set connection advanced-options tcp-state-bypass —Implement TCP State Bypass. For detailed information, see Bypass TCP State Checks for Asymetrical Routing (TCP State Bypass).

  • set connection advanced-options sctp-state-bypass —Implement SCTP State Bypass to turn off SCTP stateful inspection. For more information, see SCTP Stateful Inspection.

  • set connection advanced-options flow-offload —(ASA on the Firepower 4100/9300 chassis, FXOS 1.1.3 or later, only.) Implement flow offloading. Eligible traffic is offloaded to a super fast path, where the flows are switched in the NIC itself. You must also enter the flow-offload enable command, which is not part of the service policy.

Example:


hostname(config-pmap-c)# set connection advanced-options tcp_map1

Step 7

If you are editing an existing service policy (such as the default global policy called global_policy), you are done. Otherwise, activate the policy map on one or more interfaces.

service-policy policymap_name {global | interface interface_name}

Example:


hostname(config)# service-policy global_policy global

The global keyword applies the policy map to all interfaces, and interface applies the policy to one interface. Only one global policy is allowed. You can override the global policy on an interface by applying a service policy to that interface. You can only apply one policy map to each interface.

Example

The following example sets the connection limits and timeouts for all traffic: 


hostname(config)# class-map CONNS
hostname(config-cmap)# match any
hostname(config-cmap)# policy-map CONNS
hostname(config-pmap)# class CONNS
hostname(config-pmap-c)# set connection conn-max 1000 embryonic-conn-max 3000
hostname(config-pmap-c)# set connection timeout idle 2:0:0 embryonic 0:40:0
half-closed 0:20:0 dcd
hostname(config-pmap-c)# service-policy CONNS interface outside

You can enter set connection commands with multiple parameters or you can enter each parameter as a separate command. The ASA combines the commands into one line in the running configuration. For example, if you entered the following two commands in class configuration mode: 


hostname(config-pmap-c)# set connection conn-max 600
hostname(config-pmap-c)# set connection embryonic-conn-max 50

The output of the show running-config policy-map command would display the result of the two commands in a single, combined command: 


set connection conn-max 600 embryonic-conn-max 50

Configure TCP Options

You can configure options to control some aspects of TCP behavior. The defaults for these settings are appropriate for most networks.

Procedure

Step 1

(CLI). Configure TCP reset behavior.

service { resetinbound [ interface interface_name ] | resetoutbound [ interface interface_name ] | resetoutside }

  • resetinbound . 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. The ASA also sends resets for packets that are allowed by an access list or AAA, but do not belong to an existing connection and are denied by the stateful firewall. Traffic between same security level interfaces is also affected. When this option is not enabled, the ASA silently discards denied packets. If you do not specify an interface, then this setting applies to all interfaces.

  • resetoutbound . 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. The ASA also sends resets for packets that are allowed by an access list or AAA, but do not belong to an existing connection and are denied by the stateful firewall. 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.

  • resetoutside . 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. The ASA also sends resets for packets that are allowed by an access list or AAA, but do not belong to an existing connection and are denied by the stateful firewall. When this option is not enabled, the ASA silently discards the packets of denied packets.

    We recommend that you use the this option with interface PAT. This option allows the ASA to terminate the IDENT from an external SMTP or FTP server. Actively resetting these connections avoids the 30-second timeout delay.

Step 2

Set TCP MSS to ensure that the maximum TCP segment size for through traffic does not exceed the value you set and that the maximum is not less than a specified size.

sysopt connection tcpmss [ minimum ] bytes

Without minimum keyword. 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 bytes to 0.

minimum . Overrides the maximum segment size to be no less than the specified bytes, between 48 and 65535 bytes. This feature is disabled by default (set to 0).

Step 3

Set TCP connection time wait.

sysopt connection timewait

Use this command to force 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.

Step 4

Set the maximum number of TCP unprocessed segments.

sysopt connection tcp-max-unprocessed-seg segments

Sets the maximum number of TCP unprocessed segments, from 6 to 24. The default is 6. If you find that SIP phones are not connecting to the call manager, you can try increasing the maximum number of unprocessed TCP segments.

Monitoring Connections

You can use the following commands to monitor connections:

  • show conn [detail]

    Shows connection information. Detailed information uses flags to indicate special connection characteristics. For example, the “b” flag indicates traffic subject to TCP State Bypass.

    When you use the detail keyword, you can see information about Dead Connection Detection (DCD) probing, which shows how often the connection was probed by the initiator and responder. For example, the connection details for a DCD-enabled connection would look like the following: 

    
TCP dmz: 10.5.4.11/5555 inside: 10.5.4.10/40299,
    flags UO , idle 1s, uptime 32m10s, timeout 1m0s, bytes 11828, 
cluster sent/rcvd bytes 0/0, owners (0,255)
  Traffic received at interface dmz
        Locally received: 0 (0 byte/s)
  Traffic received at interface inside
        Locally received: 11828 (6 byte/s)
  Initiator: 10.5.4.10, Responder: 10.5.4.11
  DCD probes sent: Initiator 5, Responder 5

  • show flow-offload {info [detail] | cpu | flow [count | detail] | statistics}

    Shows information about the flow offloading, including general status information, CPU usage for offloading, offloaded flow counts and details, and offloaded flow statistics.

  • show service-policy

    Shows service policy statistics, including Dead Connection Detection (DCD) statistics.

  • show threat-detection statistics top tcp-intercept [all | detail]

    View the top 10 protected servers under attack. The all keyword shows the history data of all the traced servers. The detail keyword shows history sampling data. The ASA samples the number of attacks 30 times during the rate interval, so for the default 30 minute period, statistics are collected every 60 seconds.


Note

In the ASA configuration, embryonic connections—connection requests that have not yet completed the three-way handshake process—are closed quickly and not synchronized between the active and standby devices. This design ensures HA system efficiency and security. For this reason, there might be a difference in the number of connections on both ASAs, which is to be expected.

History for Connection Settings

Feature Name

Platform Releases

Description

TCP state bypass

8.2(1)

This feature was introduced. The following command was introduced: set connection advanced-options tcp-state-bypass.

Connection timeout for all protocols

8.2(2)

The idle timeout was changed to apply to all protocols, not just TCP.

The following command was modified: set connection timeout

Timeout for connections using a backup static route

8.2(5)/8.4(2)

When multiple static routes exist to a network with different metrics, the ASA uses the one with the best metric at the time of connection creation. If a better route becomes available, then this timeout lets connections be closed so a connection can be reestablished to use the better route. The default is 0 (the connection never times out). To take advantage of this feature, change the timeout to a new value.

We modified the following command: timeout floating-conn.

Configurable timeout for PAT xlate

8.4(3)

When a PAT xlate times out (by default after 30 seconds), and the ASA reuses the port for a new translation, some upstream routers might reject the new connection because the previous connection might still be open on the upstream device. The PAT xlate timeout is now configurable, to a value between 30 seconds and 5 minutes.

We introduced the following command: timeout pat-xlate.

This feature is not available in 8.5(1) or 8.6(1).

Increased maximum connection limits for service policy rules

9.0(1)

The maximum number of connections for service policy rules was increased from 65535 to 2000000.

We modified the following commands: set connection conn-max, set connection embryonic-conn-max, set connection per-client-embryonic-max, set connection per-client-max.

Decreased the half-closed timeout minimum value to 30 seconds

9.1(2)

The half-closed timeout minimum value for both the global timeout and connection timeout was lowered from 5 minutes to 30 seconds to provide better DoS protection.

We modified the following commands: set connection timeout half-closed, timeout half-closed.

Connection holddown timeout for route convergence.

9.4(3)

9.6(2)

You can now configure how long the system should maintain a connection when the route used by the connection no longer exists or is inactive. If the route does not become active within this holddown period, the connection is freed. You can reduce the holddown timer to make route convergence happen more quickly. However, the 15 second default is appropriate for most networks to prevent route flapping.

We added the following command: timeout conn-holddown .

SCTP idle timeout and SCTP state bypass

9.5(2)

You can set an idle timeout for SCTP connections. You can also enable SCTP state bypass to turn off SCTP stateful inspection on a class of traffic.

We added or modified the following commands: timeout sctp , set connection advanced-options sctp-state-bypass .

Flow offload for the ASA on the Firepower 9300.

9.5(2.1)

You can identify flows that should be offloaded from the ASA and switched directly in the NIC (on the Firepower 9300). This provides improved performance for large data flows in data centers.

This feature requires FXOS 1.1.3.

We added or modified the following commands: clear flow-offload , flow-offload enable , set-connection advanced-options flow-offload , show conn detail , show flow-offload .

Flow offload support for the ASA on the Firepower 4100 series.

9.6(1)

You can identify flows that should be offloaded from the ASA and switched directly in the NIC for the Firepower 4100 series.

This feature requires FXOS 1.1.4.

There are no new commands or ASDM screens for this feature.

Flow offload support for multicast connections in transparent mode.

9.6(2)

You can now offload multicast connections to be switched directly in the NIC on transparent mode Firepower 4100 and 9300 series devices. Multicast offload is available for bridge groups that contain two and only two interfaces.

There are no new commands or ASDM screens for this feature.

Changes in TCP option handling.

9.6(2)

You can now specify actions for the TCP MSS and MD5 options in a packet’s TCP header when configuring a TCP map. In addition, the default handling of the MSS, timestamp, window-size, and selective-ack options has changed. Previously, these options were allowed, even if there were more than one option of a given type in the header. Now, packets are dropped by default if they contain more than one option of a given type. For example, previously a packet with 2 timestamp options would be allowed, now it will be dropped.

You can configure a TCP map to allow multiple options of the same type for MD5, MSS, selective-ack, timestamp, and window-size. For the MD5 option, the previous default was to clear the option, whereas the default now is to allow it. You can also drop packets that contain the MD5 option. For the MSS option, you can set the maximum segment size in the TCP map (per traffic class). The default for all other TCP options remains the same: they are cleared.

We modified the following command: timeout igp stale-route .

Stale route timeout for interior gateway protocols

9.7(1)

You can now configure the timeout for removing stale routes for interior gateway protocols such as OSPF.

We added the following command: timeout igp stale-route .

Global timeout for ICMP errors

9.8(1)

You can now set the idle time before the ASA removes an ICMP connection after receiving an ICMP echo-reply packet. When this timeout is disabled (the default), and you enable ICMP inspection, then the ASA removes the ICMP connection as soon as an echo-reply is received; thus any ICMP errors that are generated for the (now closed) connection are dropped. This timeout delays the removal of ICMP connections so you can receive important ICMP errors.

We added the following command: timeout icmp-error

Default idle timeout for TCP state bypass

9.10(1)

The default idle timeout for TCP state bypass connections is now 2 minutes instead of 1 hour.

Initiator and responder information for Dead Connection Detection (DCD), and DCD support in a cluster.

9.13(1)

If you enable Dead Connection Detection (DCD), you can use the show conn detail command to get information about the initiator and responder. Dead Connection Detection allows you to maintain an inactive connection, and the show conn output tells you how often the endpoints have been probed. In addition, DCD is now supported in a cluster.

New/Modified commands: show conn (output only).

Configure the maximum segment size (MSS) for embryonic connections.

9.16(1)

You can configure a service policy to set the server maximum segment size (MSS) for SYN-cookie generation for embryonic connections upon reaching the embryonic connections limit. This is meaningful for service policies where you are also setting embryonic connection maximums.

New or changed commands: set connection syn-cookie-mss .

IPsec flow offload.

9.18(1)

On the Secure Firewall 3100, IPsec flows are offloaded by default. After the initial setup of an IPsec site-to-site VPN or remote access VPN security association (SA), IPsec connections are offloaded to the field-programmable gate array (FPGA) in the device, which should improve device performance.

We added the following commands: clear flow-offload-ipsec , flow-offload-ipsec , show flow-offload-ipsec