Security Configuration Guide: Denial of Service Attack Prevention, Cisco IOS Release 15SY

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Configuring TCP Intercept (Preventing Denial-of-Service Attacks)

Configuring TCP Intercept (Preventing Denial-of-Service Attacks)

The TCP Intercept feature implements software to protect TCP servers from TCP SYN-flooding attacks, which are a type of denial-of-service attacks. The TCP Intercept feature helps prevent SYN-flooding attacks by intercepting and validating TCP connection requests.

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Restrictions for TCP Intercept

  • Do not configure the TCP Intercept feature with either NAT and/or the zone-based firewall or Context-Based Access Control (CBAC) firewall.

  • TCP options that are negotiated on a handshake (such as RFC 1323 about window scaling) are not renegotiated because the TCP intercept software does not know what a server can negotiate.

Information About TCP Intercept

TCP Intercept

The TCP Intercept feature implements software to protect TCP servers from TCP SYN-flooding attacks, which are a type of denial-of-service attacks.

A SYN-flooding attack occurs when a hacker floods a server with a barrage of requests for connection. Because these messages have unreachable return addresses, these connections cannot be established. The resulting volume of unresolved open connections eventually overwhelms the server and causes it to deny service to valid requests, thereby preventing legitimate users from connecting to websites, accessing e-mails, using FTP service, and so on.

The TCP Intercept feature helps prevent SYN-flooding attacks by intercepting and validating TCP connection requests. In intercept mode, the TCP intercept software intercepts TCP synchronization (SYN) packets that match an extended access list from clients to servers. The software establishes a connection with the client on behalf of the destination server, and if successful, establishes a connection with the server on behalf of the client and knits the two half connections transparently. Because of the intercept of SYN packets, connection attempts from unreachable hosts never reach the server. The software continues to intercept and forward packets throughout the duration of the connection. The number of SYN packets per second and the number of concurrent connections that are proxied depends on the platform, memory, processor, and so on.

In case of illegitimate requests, the configured timeouts for half-opened connections and the configured thresholds for TCP connection requests protect destination servers while still allowing valid requests.

When establishing a security policy using TCP intercept, you can choose to intercept either all requests or only those coming from specific networks or destined for specific servers. You can also configure the connection rate and the threshold for outstanding connections.

You can choose to operate TCP intercept in watch mode, as opposed to intercept mode. In watch mode, the software passively watches the connection requests flowing through a router. If a connection fails to get established in a configured interval, the software intervenes and terminates the connection attempt.

TCP Intercept and Watch Modes

The TCP Intercept feature can operate in either active intercept mode or passive watch mode. The default is intercept mode.

In intercept mode, the software actively intercepts each incoming connection request (SYN) and responds on behalf of the server with a SYN-ACK, then waits for an acknowledge (ACK) from the client. When the ACK is received, the original SYN is sent to the server and the software performs a three-way handshake with the server. When the three-way handshake is complete, the two half connections are joined.

In watch mode, connection requests are allowed to pass through the router to the server but are watched until they become established. If connection requests fail to establish within 30 seconds (configurable by using the ip tcp intercept watch-timeout command), the software sends a reset request to the server to clear up its state.

TCP Intercept Timers and Aggressive Thresholds

In the TCP Intercept feature, two factors determine when the aggressive behavior begins and ends: total number of incomplete connections and connection requests during the last one-minute sample period. Both these thresholds have default values that can be redefined. Use the ip tcp intercept max-incomplete and ip tcp intercept one-minute commands to configure aggressive thresholds.

When a threshold is exceeded, the TCP intercept assumes that the server is under attack and goes into aggressive mode. In aggressive mode, the following occurs:

  • Each newly arriving connection causes the oldest partial connection to be deleted. (You can change this setting to a random drop mode.)

  • The initial retransmission timeout is reduced by half to 0.5 seconds, which cuts the total time to establish a connection by half. (When not in aggressive mode, the initial retransmission timeout is 1 second. The subsequent timeouts are 2 seconds, 4 seconds, 8 seconds, and 16 seconds. The code retransmits four times before giving up, so it gives up after 31 seconds of no acknowledgment.)

  • In watch mode, the watch timeout is reduced by half. (If the default is in place, the watch timeout becomes 15 seconds.)

The drop strategy can be changed from the oldest connection to a random connection by using the ip tcp intercept drop-mode random command.

Use the ip tcp intercept max-incomplete command to change the threshold for triggering aggressive mode based on the total number of incomplete connections. The default values for low and high are 900 and 1100 incomplete connections, respectively.

Use the ip tcp intercept one-minute command to change the threshold for triggering aggressive mode based on the number of connection requests received in the last one-minute sample period. The default values for low and high are 900 and 1100 connection requests, respectively. When the high value is exceeded, the aggressive behavior begins. When quantities fall below the low value, the aggressive behavior ends.

How to Configure TCP Intercept


Note

Do not configure the TCP Intercept feature with either NAT and/or the zone-based firewall or Context-Based Access Control (CBAC) firewall.

Enabling TCP Intercept

You can define an access list to intercept either all requests or only those coming from specific networks or destined for specific servers. Typically, the access list will define the source as any and define specific destination networks or servers. Do not filter source addresses because you may not know the source from which to intercept packets. You must identify the destination addresses to protect destination servers.

If no access list match is found, the router allows the request to pass with no further action.

SUMMARY STEPS

    1.    enable

    2.    configure terminal

    3.    access-list access-list-number {deny | permit | remark} {host-ip-address | any | host}

    4.    ip tcp intercept list access-list-number

    5.    ip tcp intercept mode {intercept | watch}

    6.    ip tcp intercept drop-mode {oldest | random}

    7.    ip tcp intercept watch-timeout seconds

    8.    ip tcp intercept finrst-timeout seconds

    9.    ip tcp intercept connection-timeout seconds

    10.    ip tcp intercept max-incomplete low number high number

    11.    ip tcp intercept one-minute low number high number

    12.    exit

    13.    show tcp intercept connections

    14.    show tcp intercept statistics


DETAILED STEPS
     Command or ActionPurpose
    Step 1 enable


    Example: 
    Device> enable
     
    Enables privileged EXEC mode.

    • Enter your password if prompted.

     
    Step 2 configure terminal


    Example: 
    Device# configure terminal
     

    Enters global configuration mode.

     
    Step 3access-list access-list-number {deny | permit | remark} {host-ip-address | any | host}


    Example: 
    Device(config)# access-list 20 permit any
     

    Defines an extended IP access list.

     
    Step 4ip tcp intercept list access-list-number


    Example: 
    Device(config)# ip tcp intercept list 20
     

    Enables TCP intercept.

     
    Step 5ip tcp intercept mode {intercept | watch}


    Example: 
    Device(config)# ip tcp intercept mode intercept
     

    Changes the TCP intercept mode.

     
    Step 6ip tcp intercept drop-mode {oldest | random}


    Example: 
    Device(config)# ip tcp intercept drop-mode random
     

    Sets the TCP intercept drop mode.

     
    Step 7ip tcp intercept watch-timeout seconds


    Example: 
    Device(config)# ip tcp intercept watch-timeout 200
     

    Defines how long the software waits for a watched TCP intercept connection to reach the established state before sending a reset to the server.

     
    Step 8ip tcp intercept finrst-timeout seconds


    Example: 
    Device(config)# ip tcp intercept finrst-timeout 220
     

    Changes the time between receiving a reset or finish (FIN)-exchange and dropping the connection.

     
    Step 9ip tcp intercept connection-timeout seconds


    Example: 
    Device(config)# ip tcp intercept connection-timeout 180
     

    Changes the time a TCP connection is managed by TCP intercept after no activity.

     
    Step 10ip tcp intercept max-incomplete low number high number


    Example: 
    Device(config)# ip tcp intercept max-incomplete low 3220 high 4550
     
    Sets the threshold for the number of incomplete connections below which the software leaves aggressive mode or the maximum number of incomplete connections allowed before the software enters aggressive mode.

    • In Cisco IOS Release 12.4(15)T, the ip tcp intercept max-incomplete high and ip tcp intercept max-incomplete low commands were replaced by the ip tcp intercept max-incomplete low number high number command.

     
    Step 11ip tcp intercept one-minute low number high number


    Example: 
    Device(config)# ip tcp intercept one-minute low 234 high 456
     
    Sets the threshold for the number of connection requests received in the last one-minute below which the software leaves aggressive mode and the number of connection requests that can be received in the last one-minute before the software enters aggressive mode.

    • In Cisco IOS Release 12.4(15)T, the ip tcp intercept one-minute high and ip tcp intercept one-minute low commands were replaced by the ip tcp intercept one-minute low number high number command.

     
    Step 12exit


    Example: 
    Device(config)# exit
     

    Exits global configuration mode and enters privileged EXEC mode.

     
    Step 13show tcp intercept connections


    Example: 
    Device# show tcp intercept connections
     

    Displays incomplete and established TCP connections.

     
    Step 14show tcp intercept statistics


    Example: 
    Device# show tcp intercept statistics
     

    Displays TCP intercept statistics.

     

    Configuration Examples for TCP Intercept

    Example: Enabling TCP Intercept

    The following examples shows how to define the extended IP access list 101 and enable the intercept of packets for all TCP servers: 

    Router# configure terminal
Router(config)# access-list 101 permit any
Router(config)# ip tcp intercept list 101
Router(config)# ip tcp intercept mode intercept
Router(config)# ip tcp intercept drop-mode random
Router(config)# ip tcp intercept watch-timeout 200
Router(config)# ip tcp intercept finrst-timeout 220
Router(config)# ip tcp intercept connection-timeout 180
Router(config)# ip tcp intercept max-incomplete low 3220 high 4550
Router(config)# ip tcp intercept one-minute low 234 high 456

    Additional References

    Related Documents

    Related Topic

    Document Title

    Cisco IOS commands

    Cisco IOS Master Commands List, All Releases

    Security commands

    Standards and RFCs

    Standard/RFC

    Title

    RFC 1323

    TCP Extensions for High Performance

    MIBs

    MIB

    MIBs Link

    None

    To locate and download MIBs for selected platforms, Cisco software releases, and feature sets, use Cisco MIB Locator found at the following URL:

    http:/​/​www.cisco.com/​go/​mibs