This article describes how to implement exponential back-off algorithm on a NetScaler appliance.

Background

Exponential time-block algorithm is about mitigating dictionary attacks. Dictionary attacks are mostly brute force attacks in which the attacker tries a random combination of username/password to complete a successful authentication on an interface. This is completed generally in a loop, firing many requests every second till a successful combination is found. Exponential back-off algorithm blocks an attacker from issuing anymore authentication requests for a time that depends on the number of requests the attacker has made in the past and the time elapsed since then. Following is the general formula:

E(c) = 1/2 * (2(pow,c) - 1)

where,

c is the number of attempts

E(c) is the time in seconds for which you must block the user.

Attempts	Lockout (s)	Lockout (h, m, s)
1	0	0s
2	2	2s
3	4	4s
4	8	8s
5	16	16s
6	32	32s
7	64	1m 4s
8	128	2m 8s
9	256	4m 16s
10	512	8m 32s
11	1024	17m 4s
12	2048	34m 8s
13	4096	1h 8m 16s
14	8192	2h 16m 32s
15	16384	4h 33m 4s

The general purpose this algorithm serves is that it takes more and more time for an attacker to issue a next authentication request after the last attack has failed. Because brute force dictionary attacks are highly dependent on large number of random requests sent, this algorithm serves to stop them.

How Does a NetScaler Appliance Implement Exponential Back-off Algorithm

To implement this algorithm, you can use the built-in NetScaler HTTP Callouts feature. See Citrix Documentation - HTTP Callouts for more information.

The following basic components are required to implement exponential back-off algorithm or any such algorithm:

After the script responds back, the connection is either allowed or disallowed. The block is maintained for the time calculated according to the formula given earlier.

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Instructions

To implement exponential back-off algorithm, complete the following procedure:

1. Add an HTTP Callout which makes a callout to a Web resource (executable):
   set policy httpCallout log_ip -IPAddress 192.168.101.56 -port 80 -returnType BOOL -hostExpr 192.168.101.56 -urlStemExpr "\"/exponential.php\"" -headers My-Header("This is my callout") -parameters ip(CLIENT.IP.SRC) -resultExpr "HTTP.RES.BODY(1000).AFTER_STR(\"action:\").EQ(\"block\")"

2. Match a failed authentication, which generally is a 401 message, with a rewrite policy and bind it at the global level:
   add rewrite policy unauth_rew_policy "HTTP.RES.STATUS.EQ(404) && SYS.HTTP_CALLOUT(log_ip)" RESET
   bind rewrite global unauth_rew_policy 1 END -type RES_DEFAULT

3. Upload the script that implements the algorithm to a Web server. The following is a sample file for Exponential Back-off algorithm (exponential.php):
   You can download the sample exponential.php script attached in this article.

   Sample ns.conf for Exponential Back-off setup

   add ns ip 192.168.101.121 255.255.255.0 -type snip
   set ns ip 192.168.101.121 -mgmtAccess enabled
   add policy httpCallout log_ip
   set policy httpCallout log_ip -IPAddress 192.168.101.121 -port 80 -hostExpr '"nitro.example.net"' -returnType BOOL -httpMethod POST -urlStemExpr "\"/exponential.php\"" -headers Content-Type("application/x-www-form-urlencoded") Authorization("Basic bnNyb290Om5zcm9vdA==") -parameters ip=192.168.101.26 -resultExpr q/HTTP.RES.BODY(1000).AFTER_STR("message\": \"").BEFORE_STR("\"").EQ("Done")/
   add responder policy call_log_ip "HTTP.REQ.METHOD.EQ(GET) && SYS.HTTP_CALLOUT(log_ip)" NOOP
   set responder policy call_log_ip -rule "HTTP.REQ.METHOD.EQ(GET) && SYS.HTTP_CALLOUT(log_ip)" -action NOOP
   add lb vserver v1 http 192.168.101.125 80
   add service s1 192.168.101.87 http 80
   bind lb vserver v1 s1
   bind lb vserver v1 -policyName call_log_ip -priority 100 -gotoPriorityExpression NEXT

The above mentioned sample code is provided to you as is with no representations, warranties or conditions of any kind. You may use, modify and distribute it at your own risk. CITRIX DISCLAIMS ALL WARRANTIES WHATSOEVER, EXPRESS, IMPLIED, WRITTEN, ORAL OR STATUTORY, INCLUDING WITHOUT LIMITATION WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, TITLE AND NONINFRINGEMENT. Without limiting the generality of the foregoing, you acknowledge and agree that (a) the sample code may exhibit errors, design flaws or other problems, possibly resulting in loss of data or damage to property; (b) it may not be possible to make the sample code fully functional; and (c) Citrix may, without notice or liability to you, cease to make available the current version and/or any future versions of the sample code. In no event should the code be used to support ultra-hazardous activities, including but not limited to life support or blasting activities. NEITHER CITRIX NOR ITS AFFILIATES OR AGENTS WILL BE LIABLE, UNDER BREACH OF CONTRACT OR ANY OTHER THEORY OF LIABILITY, FOR ANY DAMAGES WHATSOEVER ARISING FROM USE OF THE SAMPLE CODE, INCLUDING WITHOUT LIMITATION DIRECT, SPECIAL, INCIDENTAL, PUNITIVE, CONSEQUENTIAL OR OTHER DAMAGES, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. Although the copyright in the code belongs to Citrix, any distribution of the sample code should include only your own standard copyright attribution, and not that of Citrix. You agree to indemnify and defend Citrix against any and all claims arising from your use, modification or distribution of the sample code.

This article describes how to implement exponential back-off algorithm on a NetScaler appliance.

Wikipedia - Exponential backoff

Citrix Documentation - HTTP Callout

Citrix Blog - Dynamic and runtime configuration in NetScaler using HTTP callout and NITRO

Advantages of Using a NetScaler Appliance

More Callouts

Sample Exponential Back-off Script

Download the attachment for this article which is the sample exponential back-off script.