Manual Chapter : Preventing DoS Attacks on Applications

Applies To:

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BIG-IP ASM

  • 15.0.0, 14.1.0
Manual Chapter

Preventing DoS Attacks on Applications

What is a DoS attack?

A
denial-of-service attack (DoS attack)
or
distributed denial-of-service attack (DDoS attack)
makes a victim's resource unavailable to its intended users, or obstructs the communication media between the intended users and the victimized site so that they can no longer communicate adequately. Perpetrators of DoS attacks typically target sites or services, such as banks, credit card payment gateways, and e-commerce web sites.
Application Security Manager (ASM) helps protect web applications from DoS attacks aimed at the resources that are used for serving the application: the web server, web framework, and the application logic. Advanced Firewall Manager (AFM) helps prevent network, SIP, and DNS DoS and DDoS attacks.
HTTP-GET attacks and page flood attacks are typical examples of application DoS attacks. These attacks are initiated either from a single user (single IP address) or from thousands of computers (distributed DoS attack), which overwhelms the target system. In page flood attacks, the attacker downloads all the resources on the page (images, scripts, and so on) while an HTTP-GET flood repeatedly requests specific URLs regardless of their place in the application.

About recognizing DoS attacks

Application Security Manager determines that traffic is a DoS attack based on calculations for transaction rates on the client side (TPS-based) or latency on the server side (stress-based). You can specify the thresholds that you want the system to use, or let the system automatically detect reasonable thresholds based on examining traffic patterns.
You can set up both methods of detection to work independently or you can set them up to work concurrently to detect attacks on both the client side and server side. Whichever method detects the attack handles DoS protection.
In addition, the system can protect web applications against DoS attacks on heavy URLs. Heavy URL protection implies that during a DoS attack, the system protects the heavy URLs that might cause stress on the server.
You can view details about DoS attacks that the system detected and logged in the event logs and DoS reports. You can also configure remote logging support for DoS attacks when creating a logging profile.

When to use different DoS protections

Application Security Manager provides several different types of DoS protections that you can set to protect applications. This table describes when it is most advantageous to use the different protections. You can use any combination of the protections.
DoS Protection
When to Use
TPS-based detection
To focus protection on the client side to detect an attack right away, mostly by looking at the requests per seconds thresholds.
Stress-based detection
To focus protection on the server side where attacks are detected when a server slowdown occurs. This protection provides more accurate DoS detection based on latency and requests per second thresholds.
Behavioral detection
To use behavioral analysis and machine learning of traffic flows to automatically discover and mitigate DoS attacks.
Heavy URL protection
If application users can query a database or submit complex queries that may slow the system down.
CAPTCHA challenge
To stop non-human attackers by presenting a character recognition challenge to suspicious users.

About configuring TPS-based DoS protection

When setting up DoS protection, you can configure the system to prevent DoS attacks based on transaction rates (TPS-based anomaly detection). If you use TPS-based anomaly protection, the system detects DoS attacks from the client side using the following calculations:
Transaction rate detection interval
A short-term average of recent requests per second (for a specific URL or from an IP address) that is updated every 10 seconds.
The averages for IP address and URL counts are done for each site, that is, for each virtual server and associated DoS profile. If one virtual server has multiple DoS profiles (implemented using a local traffic policy), then each DoS profile has its own statistics within the context of the virtual server.
Transaction rate history interval
A longer-term average of requests per second (for a specific URL or from an IP address) calculated for the past hour that is updated every 10 seconds.
If the ratio of the transaction rate detection interval to the transaction rate during the history interval is greater than the percentage indicated in the
TPS increased by
setting, the system considers the web site to be under attack, or the URL, IP address, or geolocation to be suspicious. In addition, if the transaction rate detection interval is greater than the
TPS reached
setting (regardless of the history interval), then again, the respective URL, IP address, or geolocation is suspicious or the site is being attacked.
Note that TPS-based protection might detect a DoS attack simply because many users are trying to access the server all at once, such as during a busy time or when a new product comes out. In this case, the attack might be a false positive because the users are legitimate. But the advantage of TPS-based DoS protection is that attacks can be detected earlier than when using stress-based protection. So it is important to understand the typical maximum peak loads on your system when setting up DoS protection, and to use the methods that are best for your application.

About configuring stress-based DoS protection

When setting up DoS protection, you can configure the system to prevent DoS attacks based on the server side (stress-based detection). In stress-based detection, it takes a latency increase and at least one suspicious IP address, URL, heavy URL, site-wide entry, or geolocation for the activity to be considered an attack.
The average latency is measured for each site, that is, for each virtual server and associated DoS profile. If one virtual server has multiple DoS profiles (implemented using a local traffic policy), then each DoS profile has its own statistics within the context of the virtual server.
Stress-based DoS protection also includes Behavioral DoS. When enabled, the system examines traffic behavior to automatically detect DoS attacks. Behavioral DoS reviews the offending traffic, and mitigates the attack with minimal user intervention required.
Stress-based protection is less prone to false positives than TPS-based protection because in a DoS attack, the server is reaching capacity and service/response time is slow: this is impacting all users. Increased latency can be used as a trigger step for detecting an L7 attack. Following the detection of a significant latency increase, it is important to determine whether you need further action. After examining the increase in the requests per second and by comparing these numbers with past activity, you can identify suspicious versus normal latency increases.

About Behavioral DoS protection

Behavioral DoS
(BADoS) provides automatic protection against DDoS attacks by analyzing traffic behavior using machine learning and data analysis. Working together with other BIG-IP DoS protections, Behavioral DoS examines traffic flowing between clients and application servers in data centers, and automatically establishes the baseline traffic/flow profiles for Layer 7 (HTTP) and Layers 3 and 4.
For example, in the case of a DDoS attack from a botnet, each request may be completely legal but many requests all at once can slow down or crash the server. Behavioral DoS can mitigate the attack by slowing down the traffic no more than necessary to keep the server in good health.
Behavioral DoS continuously monitors server health and loading, by means of a customer feedback loop, to ensure the real-time correlations, and validate server conditions, attacks, and mitigations. Any subsequent anomalies are put on watch, and the system applies mitigations (slowdowns or blocks) as needed.
This is how Behavioral DoS works:
  • Learns typical behavior of normal traffic
  • Detects an attack based on current conditions (server health)
  • Finds behavior anomaly (what and who changed to cause congestion?)
  • Mitigates by slowing down suspicious clients
  • Improves with experience
You enable Behavioral DoS, which requires minimal configuration, in a DoS profile in the Stress-based detection settings. Because the system is tracking the traffic data, it adapts to changing conditions so there are no thresholds to specify. You set the level of mitigation that you want to occur, ranging from no mitigation (learning only) to aggressive protection (proactive DoS protection). The system can quickly detect Layer 7 DoS attacks, characterize the offending traffic, and mitigate the attack.
You can use a DoS profile that has Behavioral DoS enabled to protect one or, at most, two virtual servers.

About DoS mitigation methods

When setting up either transaction-based or stress-based DoS protection, you can specify
mitigation methods
that determine how the system recognizes and handles DoS attacks. You can use the following methods:
  • JavaScript challenges (also called Client-Side Integrity Defense)
  • CAPTCHA challenges
  • Request blocking (including Rate Limit or Block All)
You can configure the system to issue a JavaScript challenge to analyze whether the client is using a legal browser (that can respond to the challenge) when the system encounters a suspicious IP address, URL, geolocation, or site-wide criteria. If the client does execute JavaScript in response to the challenge, the system purposely slows down the interaction. The Client Side Integrity Defense mitigations are enacted only when the Operation Mode is set to blocking.
Based on the same suspicious criteria, the system can also issue a CAPTCHA (character recognition) challenge to determine whether the client is human or an illegal script. Depending on how strict you want to enforce DoS protection, you can limit the number of requests that are allowed through to the server or block requests that are deemed suspicious.
You can also use can use request blocking in the DoS profile to specify conditions for when the system blocks requests. Note that the system only blocks requests during a DoS attack when the Operation Mode for TPS-based or stress-based detection is set to Blocking. You can use request blocking to rate limit or block all requests from suspicious IP addresses, suspicious countries, or URLs suspected of being under attack. Site-wide rate limiting also blocks requests to web sites suspected of being under attack. If you block all requests, the system blocks suspicious IP addresses and geolocations except those on the whitelist. If you are using rate limiting, the system blocks some requests depending on the threshold detection criteria set in the DoS profile.
The mitigation methods that you select are used in the order they appear on the screen. The system enforces the methods only as needed if the previous method was not able to stem the attack.

About geolocation mitigation

You can mitigate DoS attacks based on geolocation by detecting traffic from countries sending suspicious traffic. This is part of the mitigation methods in the DoS profile for stress-based and TPS-based anomalies, and this method helps protect against unusual activity as follows:
  • Geolocation-based Client Side integrity: If traffic from countries matches the thresholds configured in the DoS profile, the system considers those countries suspicious, and sends a JavaScript challenge to each suspicious country.
  • Geolocation-based CAPTCHA challenge: If traffic from countries matches the thresholds configured in the DoS profile, the system considers those countries suspicious, and issues a CAPTCHA challenge to each suspicious country.
  • Geolocation-based request blocking: The system blocks all, or some, requests from suspicious countries.
In addition, you can add countries to a geolocation whitelist (traffic from these countries is never blocked) and a blacklist (traffic from these countries is always blocked when a DoS attack is detected).

About heavy URL protection

Heavy URLs
are URLs that may consume considerable server resources per request. Heavy URLs respond with low latency most of the time, but can easily reach high latency under specific conditions (such as DoS attacks). Heavy URLs are not necessarily heavy all the time, but tend to get heavy especially during attacks. Therefore, low rate requests to those URLs can cause significant DoS attacks and be hard to distinguish from legitimate clients.
Typically, heavy URLs involve complex database queries; for example, retrieving historical stock quotes. In most cases, users request recent quotes with weekly resolution, and those queries quickly yield responses. However, an attack might involve requesting five years of quotes with day-by-day resolution, which requires retrieval of large amounts of data, and consumes considerably more resources.
Application Security Manager (ASM) allows you to configure protection from heavy URLs in a DoS profile. You can specify a latency threshold for automatically detecting heavy URLs. If some of the web site's URLs could potentially become heavy URLs, you can manually add them so the system will keep an eye on them, and you can add URLs that should be ignored and not considered heavy.
ASM measures the tail latency of each URL and of the whole site for 24 hours to get a good sample of request behavior. A URL is considered
heavy
if its average tail latency is more than twice that of the site latency for the 24-hour period.

About site-wide DoS mitigation

In order to mitigate highly distributed DoS attacks, such as those instigated using large scale botnets attacking multiple URLs, you can specify when to use site-wide mitigation in a DoS profile. You can configure site-wide mitigation for either TPS-based or stress-based DoS protection. In this case, the whole site can be considered suspicious as opposed to a particular URL or IP address. Site-wide mitigation goes into effect when the system determines that the whole site is experiencing high-volume traffic but is not able to pinpoint and handle the problem.
The system implements site-wide mitigation method only as a last resort because it may cause the system to drop legitimate requests. However, it maintains, at least partially, the availability of the web site, even when it is under attack. When the system applies site-wide mitigation, it is because all other active detection methods were unable to stop the attack.
The whole site is considered suspicious when configured thresholds are crossed, and in parallel, specific IP addresses and URLs could also be found to be suspicious. The mitigation continues until the maximum duration elapses or when the whole site stops being suspicious. That is, there are no suspicious URLs, no suspicious IP addresses, and the whole site is no longer suspicious.

About CAPTCHA challenges in DoS detection

A CAPTCHA (or visual character recognition) challenge displays characters for a client to identify before they can access a web site or application. Whether the client can correctly identify the characters determines whether the client is human or is likely an illegal script. You can configure a CAPTCHA challenge as part of the mitigation policy for TPS-based DoS detection, stress-based DoS detection, or as part of proactive bot defense. If you have configured it, the system a CAPTCHA challenge to suspicious traffic.
The system provides a standard CAPTCHA response that clients will see. You can customize the response if you want.

About DoS protection and HTTP caching

HTTP caching enables the BIG-IP system to store frequently requested web objects (or static content) in memory to save bandwidth and reduce traffic load on web servers. The Web Acceleration profile has the settings to configure caching.
If you are using HTTP caching along with DoS protection, you need to understand how DoS protection for cached content works. In this case, URLs serving cached content are considered a DoS attack if they exceed the relative
TPS increased by
percentage (and not the explicit
TPS reached
number). Requests to static or cacheable URLs are always mitigated by rate limiting. This is true even during periods of mitigation using client-side integrity or CAPTCHA, and when those mitigations are not only URL-based.

Overview: Preventing DoS attacks on applications

You can configure the Application Security Manager to protect against DoS attacks on web applications. Depending on your configuration, the system detects DoS attacks based on transactions per second (TPS) on the client side, stress-based server latency, heavy URLs, geolocation, suspicious browsers, and failed CAPTCHA responses. Behavioral DoS (BADoS), part of stress-based detection, automatically discovers and mitigates DoS attacks using behavioral data.
You configure DoS protection for Layer 7 by creating a DoS profile with Application Security enabled. You then associate the DoS profile with one or more virtual servers representing applications that you want to protect. DoS protection is a system protection that is not part of a security policy.
The main factors in establishing the prevention policy are:
  • Attackers: The clients that initiate the actual attacks. They are represented by their IP addresses and the geolocations they come from.
  • Servers: The web application servers that are under attack. You can view them site-wide as the pairing of the virtual server and the DoS profile, by the URL, or as a pool member.
  • BIG-IP system: The middle tier that detects attacks and associated suspicious entities, then mitigates the attacks, or blocks or drops requests depending on the options you configure in the DoS profile.

Task Summary

Configuring DoS protection for applications

You can configure Application Security Manager to protect against and mitigate DoS attacks, and increase system security.
  1. On the Main tab, click
    Security
    DoS Protection
    Protection Profiles
    .
    The Protection Profiles list screen opens.
  2. Click
    Create
    .
    The Create New DoS Profile screen opens.
  3. In the
    Name
    field, type the name for the profile, then click
    Finished
    .
  4. In the list of DoS profiles, click the name of the profile you just created, and click the
    Application Security
    tab.
    This is where you set up application-level DoS protection.
  5. In the
    General Settings
    , for
    Application Security
    , click
    Edit
    and select the
    Enabled
    check box.
    General settings that you can configure are displayed.
  6. To configure
    Heavy URL Protection
    , edit the setting for which URLs to include or exclude, or use automatic detection.
    Another task describes heavy URL protection in more detail.
  7. To set up DoS protection based on the country where a request originates, edit the
    Geolocations
    setting, selecting countries to allow or disallow.
    1. Click
      Edit
      .
    2. Move the countries for which you want the system to block traffic during a DoS attack into the
      Geolocation Blacklist
      .
    3. Move the countries that you want the system to allow (unless the requests have other problems) into the
      Geolocation Whitelist
      .
    4. Use the Stress-based or TPS-based Detection settings to select appropriate mitigations by geolocation in the
      How to detect attackers and which mitigation to use
      settings.
    5. When done, click
      Close
      .
  8. If you have written an iRule to specify how the system handles a DoS attack and recovers afterwards, enable the
    Trigger iRule
    setting.
  9. To better protect an applications consisting of one page that dynamically loads new content, enable
    Single Page Application
    .
  10. If your application uses many URLs, in
    URL Patterns
    , you can create logical sets of similar URLs with the varying part of the URL acting like a parameter. Click
    Not Configured
    and type one or more URL patterns, for example,
    /product/*.php
    .
    The system then looks at the URL patterns that combine several URLs into one and can more easily recognize DoS attacks, for example, on URLs that might be less frequently accessed by aggregating the statistics from other similar URLs.
  11. If you want to use performance acceleration, in
    Performance acceleration
    , select the TCP fastL4 profile to use as the fast-path for acceleration.
    The profiles listed are those created in
    Local Traffic
    Profiles
    Protocol
    Fast L4
    .
  12. Click
    Update
    to save the DoS profile.
You have created a DoS profile that provides basic DoS protection including TPS-based detection and heavy URL detection (automatically enabled).
Next, consider configuring additional levels of DoS protection such as stress-based protection, single page applications, and geolocations. Look at the other options available under Application Security and adjust as needed. Also, you need to associate the DoS profile with a virtual server before it protects against DoS attacks.

Creating a whitelist for DoS protection

You can specify IP addresses on a whitelist that the system does not check for DoS attacks. Addresses on the whitelist are trusted IP addresses that are never blocked.
Different types of whitelists are available depending on the hardware compatibility level of your system: whitelists (Level 1 or 2), rich whitelists (all levels), or extended whitelists (Level 2 only). You can create rich and extended whitelists when configuring Device Protection or creating a Protection Profile.
This task describes how to create a Whitelist Address List, which is configurable only if your system compatibility level is set to Level 1 or 2, and then add it to a DoS Protection Profile. You can check the compatibility level from the Advanced Menu at
System
Configuration
Device
General
.
  1. On the Main tab, click
    Security
    DoS Protection
    Protection Profiles
    .
    The Protection Profiles list screen opens.
  2. Click
    Create
    .
  3. In the
    Name
    field, type a name.
  4. To omit checking for DoS attacks on certain trusted addresses, click
    General Settings
    under
    Application Security
    .
  5. For the
    Source IP Address Whitelist
    field, click
    Edit
    and select a whitelist from the drop down list.
    If the drop down list is empty, click
    Manage Address List
    1. From the Main tab, click
      Shared Objects
      Address Lists
      .
      The Address Lists screen opens.
    2. Click
      Create
      .
    3. Enter the whitelist address name, description and adress(es). After each address click
      Add
      .
    4. When you are done, click
      Finished
      .
      The new whitelist is added to the Address Lists.
    5. To use the new whitelist, return to
      .
  6. When you are done, click
    Update
    .
The whitelist you created is created as a shared object that can be used for all DoS protection. You can use it in any DoS protection profile including those that contain DoS protection for applications, networks, SIP, and/or DNS.

Configuring TPS-based DoS detection

You can configure Application Security Manager to mitigate DoS attacks based on transaction rates using TPS-based DoS protection.
  1. On the Main tab, click
    Security
    DoS Protection
    Protection Profiles
    .
    The Protection Profiles list screen opens.
  2. Click the name of an existing DoS profile (or create a new one, then open it), and click the
    Application Security
    tab.
  3. On the left, under Application Security, click
    General Settings
    , and ensure that
    Application Security
    is enabled.
    If
    Application Security
    is disabled, click
    Enabled
    .
    The screen displays additional settings.
  4. On the left, under Application Security, click
    TPS-based Detection
    .
    The screen displays TPS-based DoS Detection settings.
  5. Click
    Edit All
    .
    You can also edit each setting separately instead of editing them all at once.
    The screen opens the settings for editing.
  6. For
    Operation Mode
    , select the option to determine how the system reacts when it detects a DoS attack.
    Transparent
    Displays data about DoS attacks on the DoS reporting screens, but does not block requests, or perform any of the mitigations.
    Blocking
    Applies the necessary mitigation steps to suspicious IP addresses, geolocations, URLs, or the entire site. Also displays information about DoS attacks on the DoS reporting screens.
    Select
    Off
    to turn this type of DoS Detection off.
    The screen displays additional configuration settings when you select an operation mode.
  7. For
    Thresholds Mode
    , select whether to let the system automatically determine thresholds (
    Automatic
    ) or to set the threshold values manually (
    Manual
    ) for the DoS profile.
    If you choose to set the values manually, more fields are shown in the
    How to detect attackers and which mitigation to use
    setting, and you can adjust the threshold values. The default values are reasonable for most installations. If using automatic thresholds, the system sets the values using a wide range to begin with, then calculates the values using 7 days of historical data and sets threshold values to the highest levels during normal activity (to minimize false positives). The system updates thresholds every 12 hours.
  8. For
    How to detect attackers and which mitigation to use
    , specify how to identify and stop DoS attacks. By default, source IP addresses and URLs are used to detect DoS attacks. You can specify other detection methods, and, if setting thresholds manually, adjust the thresholds for each of the settings as needed.
    By Source IP
    Specifies conditions for when to treat an IP address as an attacker. For automatic thresholds, one threshold is used for all source IP addresses.
    By Device ID
    Specifies conditions for when to treat a device as an attacker. For automatic thresholds, one threshold is used for all device IDs.
    By Geolocation
    Specifies when to treat a particular country as an attacker. If using automatic thresholds, the system calculates thresholds for the top 20 geolocations, setting different thresholds for every hour of the day. Thus, thresholds calculated at 9:00AM are based on data from 8:00-9:00AM, and are used at 8:00AM next day.
    By URL
    Specifies when the system treats a URL as under attack. For automatic thresholds, one threshold is used for all URLs. (Heavy URLs are not included in the calculations.)
    Site Wide
    Specifies conditions for how to determine when the entire web site is under attack. For automatic thresholds, one threshold value is used for the entire site.
    At least one mitigation method must be selected before you can edit the detection settings. If the specified thresholds in the settings are reached, the system limits the number of requests per second to the history interval and uses the selected mitigation methods described here.
    Client Side Integrity Defense
    Sends a JavaScript challenge to determine whether the client is a legal browser or an illegal script. Only used when the
    Operation Mode
    is set to
    Blocking
    .
    CAPTCHA Challenge
    Issues a CAPTCHA challenge to the traffic identified as suspicious by source IP address, geolocation, URL, or site wide.
    Request Blocking
    Specifies how and when to block (if the operation mode is set to
    Blocking
    ) or report (if the operation mode is set to
    Transparent
    ) suspicious requests. Select
    Block All
    to block all suspicious requests or
    Rate Limit
    to reduce the number of suspicious requests.
  9. For the
    Prevention Duration
    setting, specify the time spent in each mitigation step until deciding to move to the next mitigation step.
    Option
    Description
    Escalation Period
    Specifies the minimum time spent in each mitigation step before the system moves to the next step when preventing attacks against an attacker IP address or attacked URL. During a DoS attack, the system performs attack prevention for the amount of time configured here for the mitigation methods that are enabled. If after this period the attack is not stopped, the system enforces the next enabled prevention step. Type a number between
    1
    and
    3600
    . The default is
    120
    seconds.
    De-escalation Period
    Specifies the time spent in the final escalation step until retrying the steps using the mitigation methods that are enabled. Type a number (greater than the escalation period) between
    0
    (meaning the steps are never retried) and
    86400
    seconds. The default value is
    7200
    seconds (2 hours).
    DoS mitigation is reset after 2 hours, even if the detection criteria still hold, regardless of the value set for the
    De-escalation Period
    . If the attack is still taking place, a new attack occurs and mitigation starts over, retrying all the mitigation methods. If you set the
    De-escalation Period
    to less than 2 hours, the reset occurs more frequently.
  10. Click
    Update
    to save the DoS profile.
You have now configured a DoS profile to prevent DoS attacks based on the client side (TPS-based detection). When the system receives too many requests per second for a source IP address, device ID, URL, or site wide, it is considered suspicious. The attack starts if the system detects at least one suspicious entity. The attack ends when there are no suspicious entities for a period of two minutes.
Next, you need to associate the DoS profile with the application’s virtual server. You also have the option of configuring stress-based detection, heavy URL protection, or proactive bot defense in your DoS profile.

Configuring behavioral & stress-based DDoS protection

You can configure Application Security Manager to mitigate Layer 7 DDoS attacks based on server latency and traffic behavior. Behavioral DDoS protection is based on continuous machine learning of traffic and flow characteristics, and detects attacks very quickly.
  1. On the Main tab, click
    Security
    DoS Protection
    Protection Profiles
    .
    The Protection Profiles list screen opens.
  2. Click the name of an existing DoS profile (or create a new one, then open it), and click the
    Application Security
    tab.
  3. On the left, under Application Security, click
    General Settings
    , and ensure that
    Application Security
    is enabled.
    The screen displays additional settings.
  4. On the left, under Application Security, click
    Behavioral & Stress-based Detection
    .
    The screen displays Behavioral & Stress-based DoS Detection settings.
  5. Click
    Edit All
    .
    You can also edit each setting separately instead of editing them all at once.
    The screen opens the settings for editing.
  6. For
    Operation Mode
    , select the option to determine how the system reacts when it detects a DoS attack.
    Transparent
    Displays data about DoS attacks on the DoS reporting screens, but does not block requests, or perform any of the mitigations.
    Blocking
    Applies the necessary mitigation steps to suspicious IP addresses, geolocations, URLs, or the entire site. Also displays information about DoS attacks on the DoS reporting screens.
    Select
    Off
    to turn this type of DoS Detection off.
    The screen displays additional configuration settings when you select an operation mode.
  7. For
    Thresholds Mode
    , select whether to let the system automatically determine thresholds (
    Automatic
    ) or to set the threshold values manually (
    Manual
    ) for the DoS profile.
    If you choose to set the values manually, more fields are shown in the
    Stress-based Detection and Mitigation
    setting, and you can adjust the threshold values. The default values are reasonable for most installations. If using automatic thresholds, the system sets the values using a wide range to begin with, then calculates the values using 7 days of historical data and sets threshold values to the highest levels during normal activity (to minimize false positives). Thereafter, the system updates thresholds every 12 hours.
  8. For
    Stress-based Detection and Mitigation
    , specify how to identify and stop DoS attacks. By default, source IP addresses and URLs are enabled to detect DoS attacks. You can specify other detection methods, and, if setting thresholds manually, adjust the thresholds for each of the settings as needed.
    By Source IP
    Specifies conditions for when to treat an IP address as an attacker. The system calculates one automatic threshold for the most accessed source IP addresses, and another threshold for the rest.
    By Device ID
    Specifies conditions for when to treat a device as an attacker. For automatic thresholds, one threshold is calculated for highly accessed device IDs, and another for the rest.
    By Geolocation
    Specifies when to treat a particular country as an attacker. If using automatic thresholds, the system calculates thresholds for the top 20 geolocations, setting different thresholds for every hour of the day. Thus, thresholds calculated at 9:00AM are based on data from 8:00-9:00AM, and are used at 8:00AM next day.
    By URL
    Specifies when the system treats a URL as under attack. For automatic thresholds, one threshold is calculated for highly accessed URLs, and another for the rest. (Heavy URLs are not included in the calculations.)
    Site Wide
    Specifies conditions for how to determine when the entire web site is under attack. For automatic thresholds, one threshold is used sitewide.
    At least one mitigation method must be selected before you can edit the detection settings. If the specified thresholds in the settings are reached, the system limits the number of requests per second to the history interval and uses the selected mitigation methods described here. These methods do not apply to Behavioral DoS.
    Client Side Integrity Defense
    Sends a JavaScript challenge to determine whether the client is a legal browser or an illegal script. Only used when the
    Operation Mode
    is set to
    Blocking
    .
    CAPTCHA Challenge
    Issues a CAPTCHA challenge to the traffic identified as suspicious by source IP address, geolocation, URL, or site wide.
    Request Blocking
    Specifies how and when to block (if the operation mode is set to
    Blocking
    ) or report (if the operation mode is set to
    Transparent
    ) suspicious requests. Select
    Block All
    to block all suspicious requests or
    Rate Limit
    to reduce the number of suspicious requests.
  9. For the
    Behavioral Detection and Mitigation
    settings, specify how to mitigate DDoS attacks discovered based on behavior.
    Bad actors behavior detection
    Lets the system identify IP addresses of bad actors by examining traffic behavior and anomaly detection.
    Request signatures detection
    Examines requests and creates behavioral signatures describe patterns found in attacks the system has identified. Select
    Use approved signatures only
    if you want to verify that the system-generated signatures are valid before letting the system use them.
    Mitigation
    Specifies the level of mitigation to perform for attacks discovered using behavioral DoS.
    • Conservative Protection
      : If
      Bad actors behavior detection
      is enabled, slows down and rate limits requests from anomalous IP addresses based on anomaly detection confidence and server health. If
      Request signatures detection
      is enabled, blocks requests that match behavioral signatures.
    • Standard Protection
      : If
      Bad actors behavior detection
      is enabled, slows down requests from anomalous IP addresses based on its anomaly detection confidence and server health. Rate limits requests from anomalous IP addresses and, if necessary, rate limits all requests based on server health. Limits the number of concurrent connections from anomalous IP addresses and, if necessary, limits the number of all concurrent connections based on server health. If
      Request signatures detection
      is enabled, blocks requests that match behavioral signatures.
    • Aggressive Protection
      : If
      Bad actors behavior detection
      is enabled, does all that standard protection does plus it proactively performs all protection actions (even before an attack). Increases the impact of the protection techniques. If
      Request signatures detection
      is enabled, blocks requests that match behavioral signatures. Increases the impact of blocked requests.
    • No Mitigation
      : Learns and monitors traffic behavior, but takes no action.
  10. For the
    Prevention Duration
    setting, specify the time spent in each mitigation step until deciding to move to the next mitigation step.
    Option
    Description
    Escalation Period
    Specifies the minimum time spent in each mitigation step before the system moves to the next step when preventing attacks against an attacker IP address or attacked URL. During a DoS attack, the system performs attack prevention for the amount of time configured here for the mitigation methods that are enabled. If after this period the attack is not stopped, the system enforces the next enabled prevention step. Type a number between
    1
    and
    3600
    . The default is
    120
    seconds.
    De-escalation Period
    Specifies the time spent in the final escalation step until retrying the steps using the mitigation methods that are enabled. Type a number (greater than the escalation period) between
    0
    (meaning the steps are never retried) and
    86400
    seconds. The default value is
    7200
    seconds (2 hours).
    DoS mitigation is reset after 2 hours, even if the detection criteria still hold, regardless of the value set for the
    De-escalation Period
    . If the attack is still taking place, a new attack occurs and mitigation starts over, retrying all the mitigation methods. If you set the
    De-escalation Period
    to less than 2 hours, the reset occurs more frequently.
  11. Click
    Update
    to save the DoS profile.
You have now configured a DoS profile to prevent DoS attacks automatically based on server health and/or Behavioral DoS. The BIG-IP system monitors server health and estimates the server load based on Layer 7 statistics including TPS, pending transactions, request drop rate, and so on. If the system detects potential attack conditions, the mitigation starts working (depending on the level of behavioral protection you selected) seconds after an attack begins.
If using stress-based or behavioral DoS protection, the system may falsely detect an attack in the event of a runtime failure (such as a backend server being down) or a configuration issue (such as the system having no pool or the pool having no pool members).
The mitigation process starts with the list of suspicious IP addresses and slows down suspicious clients. The system may also perform ingress rate shaping. The suspicious clients are tagged as a result of the Layer 7 behavioral analysis.
Next, associate the DoS profile with the application’s virtual server. You also have the option of configuring TPS-based detection, proactive bot defense, or heavy URL protection in your DoS profile.

Configuring heavy URL protection

To use heavy URL protection, F5 recommends that you configure stress-based anomaly settings in the DoS profile. That way the system can detect low-volume attacks on heavy URLs when no other high-volume attacks are underway. Also, you must enable at least one of the URL-based prevention policy methods in the TPS-based Anomaly or stress-based Anomaly settings in the DoS profile.
You can configure Application Security Manager (ASM) to prevent DoS attacks on heavy URLs. Heavy URLs are URLs on your application web site that may consume considerable resources under certain conditions. By tracking URLs that are potentially heavy, you can mitigate DoS attacks on these URLs before response latency exceeds a specific threshold.
  1. On the Main tab, click
    Security
    DoS Protection
    Protection Profiles
    .
    The Protection Profiles list screen opens.
  2. Click the name of an existing DoS profile (or create a new one, then open it), and click the
    Application Security
    tab.
  3. On the left, under Application Security, click
    General Settings
    , and ensure that
    Application Security
    is enabled.
    The screen displays additional settings.
  4. In the General Settings, next to
    Heavy URL Protection
    , click
    Edit
    .
  5. To have the system automatically detect heavy URLs, select
    A URL is considered heavy if its portion of transactions with latency above this threshold is higher than usual for this site
    , and adjust the latency threshold if necessary.
    The default value is
    1000
    milliseconds.
    The system detects heavy URLs by measuring the latency tail ratio, which is the number of transactions whose latency is consistently greater than the threshold.
  6. If you expect certain URLs to be heavy (have high latency) at times, add them to the
    Configure a list of Heavy URLs
    setting:
    1. Type each URL in the form
      /query.html
      . The URLs in this list may include wildcards, such as
      /product/*
      .
    2. Type the threshold in milliseconds at which point you want the URL to be considered heavy.
    3. Click
      Add
      , adding as many URLs as you need to.
    If you are not sure which URLs to add to the Heavy URLs list, leave this setting unconfigured and let the system automatically detect heavy URLs.
    If you want to add a wildcard URL to the heavy URL list, you also must add the wildcard URL to the
    URL Patterns
    field on this screen. A wildcard not added to the URL Patterns will not function as a wildcard.
  7. In the
    Configure a list of URLs which are excluded from being automatically detected as Heavy URLs
    setting, type the URLs not to consider heavy, and click
    Add
    .
    The URLs in this list may include wildcards, such as
    /product/*
    .
  8. Click
    Update
    to save the DoS profile.
You have now configured a DoS profile that includes heavy URL protection. Heavy URLs are detected based on reaching higher latency under certain conditions. ASM tracks the probability distribution of server latency, which is called heavy tailed.
To validate automatic detection, you can view the URL Latencies report (
Security
Reporting
DoS
URL Latencies
) periodically to check that the latency threshold that you used is close to the value in the latency histogram column for all traffic. You should set the latency threshold so that approximately 95% of the requests for the virtual server have lower latency.
By reviewing the URL Latencies report and sorting the URLs listed by latency, you can make sure that the URLs that you expect to be heavy are listed in the DoS profile. Also, if the system detects too many (or too few) heavy URLs, you can increase (or decrease) the latency threshold.

Recording traffic during DoS attacks

If you have DoS protection enabled, you can configure the system to record traffic during DoS attacks. By reviewing the recorded traffic in the form of a TCP dump, you can diagnose the attack vectors and attackers, observe whether and how the attack was mitigated, and determine whether you need to change the DoS protection configuration.
  1. On the Main tab, click
    Security
    DoS Protection
    Protection Profiles
    .
    The Protection Profiles list screen opens.
  2. Click the name of an existing DoS profile (or create a new one, then open it), and click the
    Application Security
    tab.
  3. On the left, under Application Security, click
    General Settings
    , and ensure that
    Application Security
    is enabled.
    The screen displays additional settings.
  4. On the left, under Application Security, click
    Record Traffic
    .
  5. For
    Record Traffic During Attacks
    , click
    Edit
    , then select the
    Enabled
    check box.
    The screen displays additional configuration settings.
  6. For
    Maximum TCP Dump Duration
    , click
    Edit
    , then type the maximum number of seconds (from 1 - 300) for the system to record traffic during a DoS attack.
    The default value is
    30
    seconds.
  7. For
    Maximum TCP Dump Size
    , type the maximum size (from 1 - 50) allowed for the TCP dump.
    When the maximum size is reached, the dump is complete. The default value is
    10
    MB.
  8. For
    TCP Dump Repetition
    , specify how often to perform TCP dumps during a DoS attack:
    • To record traffic once during an attack, select
      Dump once per attack
      .
    • To record traffic periodically during an attack, select
      Repeat dump after
      and type the number of seconds (between 1 - 3600) for how long to wait after completing a TCP dump before starting the next one.
  9. Click
    Update
    to save the DoS profile.
When the system detects a DoS attack, it performs a TCP dump to record the traffic on the virtual server where the attack occurred. The files are located on the system in
/shared/dosl7/tcpdumps
. The name of the file has the format:
<yyyy_mm_dd_hh:mm:ss>-<attack_ID>-<seq_num>.pcap
, including the time the dump started, the ID of the attack in logs and reports, and the number of the TCP dump since the attack started. If traffic being recorded is SSL traffic, it is recorded encrypted.
If working with F5 support, you can collect the TCP dump files into a QuickView file so that support personnel can help determine the cause of the DoS attack, and recommend ways of preventing future attacks.

Configuring CAPTCHA for DoS protection

You can configure a CAPTCHA challenge as part of the mitigation policy for TPS-based DoS detection, behavioral & stress-based DoS detection, or as part of proactive bot defense. A CAPTCHA (or visual character recognition) challenge determines whether the client is human or an illegal script.
  1. On the Main tab, click
    Security
    DoS Protection
    Protection Profiles
    .
    The Protection Profiles list screen opens.
  2. Click the name of an existing DoS profile (or create a new one, then open it), and click the
    Application Security
    tab.
  3. On the left, under Application Security, click
    General Settings
    , and ensure that
    Application Security
    is enabled.
    The screen displays additional settings.
  4. On the left, under Application Security, select an option to configure TPS-based, behavioral & stress-based, or proactive bot defense, and select CAPTCHA as one of the mitigation methods.
    1. For
      TPS-based Detection
      , in the
      How to detect attackers and which mitigation to use
      setting, edit the source IP, device ID, geolocation, URL, or site-wide mitigation, and select
      CAPTCHA Challenge
      .
    2. For
      Behavioral & Stress-based Detection
      , in the
      Stress-based Detection and Mitigation
      setting, edit the source IP, device ID, geolocation, URL, or site-wide mitigation, and select
      CAPTCHA Challenge
      .
    3. For
      Proactive Bot Defense
      , in the
      Block requests from suspicious browsers
      setting, select
      CAPTCHA Challenge
      to challenge a suspected bot.
  5. To customize the CAPTCHA response that the system sends as a challenge to suspicious users, click
    CAPTCHA Settings
    .
    The
    CAPTCHA Settings
    link is only available after you select a CAPTCHA challenge.
    The Application Security General Settings area opens and displays the CAPTCHA Response.
  6. In the
    CAPTCHA Response
    setting, specify the text the system sends as a challenge to users.
    This setting appears only if one or more of the CAPTCHA Challenge options is selected.
    1. From the
      First Response Type
      list, select
      Custom
      .
    2. Edit the text (HTML) in the
      First Response Body
      field.
      You can use the following variables within the challenge or response.
      Variable
      Use
      %DOSL7.captcha.image%
      Displays the CAPTCHA image in data URI format.
      %DOSL7.captcha.change%
      Displays the change CAPTCHA challenge icon.
      %DOSL7.captcha.solution%
      Displays the solution text box.
      %DOSL7.captcha.submit%
      Displays the
      Submit
      button.
    3. Click
      Show
      to see what it looks like.
  7. In the
    CAPTCHA Response
    setting, specify the text the system sends to users if they fail to respond correctly to the CAPTCHA challenge.
    1. From the
      Failure Response Type
      list, select
      Custom
      if you want to change the text.
    2. If customizing the text, edit the text in the
      Failure Response Body
      field.
      You can use the same variables in the text to send a second challenge.
    3. Click
      Show
      to see what it looks like.
  8. Click
    Update
    to save the DoS profile.
You have now configured a CAPTCHA challenge for potential DoS attackers that helps with filtering out bots. The system sends a character recognition challenge only on the first request of a client session. If it is solved correctly, the request is sent to the server. Subsequent requests in the session do not include the challenge. If the client fails the first challenge, the CAPTCHA response is sent. If that also fails, the client is handled according to the mitigation methods selected in the DoS profile.

Associating a protection profile with a protected object

You must add the DoS protection profile to the protected object to provide enhanced protection from DoS attacks, and track anomalous activity on the BIG-IP system.
  1. On the Main tab, click
    Security
    DoS Protection
    Protected Objects
    .
  2. Click the name of the protected object (virtual server) to which you want to assign a protection profile.
    The Properties pane opens on the right.
  3. In the Protection Settings area, from the
    Protection Profile
    list, select the name of the protection profile to assign.
    Ensure a Service Profile is selected to enable the protected object to process application traffic.
  4. Click
    Save
    .
The DoS protection profile is associated with the protected object and DoS protection is now enabled.

Implementation Result

When you have completed the steps in this implementation, you have configured the Application Security Manager (ASM) to protect against L7 DoS attacks. If using proactive bot defense, ASM protects against DDoS, web scraping, and brute force attacks (on the virtual servers that use this DoS profile) before the attacks can harm the system. Depending on the configuration, the system may also detect DoS attacks based on transactions per second (TPS) on the client side, server latency, or both.
In TPS-based detection mode, if the ratio of the transaction rate during the history interval is greater than the TPS increased by percentage, the system considers the URL to be under attack, the IP address or country to be suspicious, or possibly the whole site to be suspicious.
In stress-based detection mode, if there is a latency increase and at least one suspicious IP address, country, URL, or heavy URL, the system considers the URL to be under attack, the IP address or country to be suspicious, or possibly the whole site to be suspicious.
If you enabled heavy URL protection, the system tracks URLs that consume higher than average resources and mitigates traffic that is going to those URLs.
If you chose the blocking operation mode, the system applies the necessary mitigation steps to suspicious IP addresses, URLs, or geolocations, or applies them site-wide. If using the transparent operation mode, the system reports DoS attacks but does not block them.
If you are using iRules® to customize reaction to DoS attacks, when the system detects a DoS attack based on the configured conditions, it triggers an iRule and responds to the attack as specified in the iRule code.
After traffic is flowing to the system, you can check whether DoS attacks are being prevented, and investigate them by viewing DoS event logs and reports.