Application Request Routing – Part 2(Reverse Proxy and Troubleshooting ARR, URLRewrite Issues)

Application Request Routing – Part 2(Reverse Proxy and Troubleshooting ARR, URLRewrite Issues)

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In the last blog we had discussed how we can leverage ARR as a load balancer. Over here in this blog we will see how we can use ARR as a reverse proxy when the content server is not exposed to the outside world. We will also discuss how we can go on about troubleshooting errors and issues like 502.3 which is the most common error while using ARR. You can use even server farms to configure as reverse proxy by adding a single server to the server farm.

Before we go on about how we can configure ARR as reverse proxy Lets check some of the options available in ARR Reverse proxy.

Go to inetmgr UI and click on the server name and you will find the option ‘Application Request Routing Cache’. Below is how the UI looks.


Below are the options and its intended purpose.

1) Cache Configuration:

This option lets you configure proxy cache settings for your ARR server.


Cache clean-up interval: By default it is 5 minutes. This setting specifies the time in interval in minutes, at which the cache will check for files or invalidate the cache files and delete it if requires.

Disk usage high threshold: This setting specifies the maximum percentage of configured disk space for cache content to be used before the cached files are deleted to make up more space.

Disk usage low threshold: This setting specifies percentage of configured disk space that will be reached once files are deleted.

Byte range segment size: This is similar to chunking. This setting specifies the maximum size of the chunks that be created by segmenting a single request. Default is 256kb.

Compression: This is used to compress responses into a format that the servers are capable of handling such as gzip.

2) Cache control rules:

This section lets you configure rules to manage the cache control behavior.



You can create rules to be executed when either cache control directive is not present in the headers of the response sent by server or the requests or you can have the rule execute always independent of the cache control header by selecting the item in the drop down

Do not cache: you can select this if you don’t want specific contents to be cached based on the rule.

Cache: select this option to cache based on the condition.

Host name: this specifies for which host name the content should be either cached or not.

URL: this is a very good option where you can specify a frequently accessed content which can be cached.

For eg: you can choose to cache images i.e., */images/*.jpg all the image contents which is definitely a good rule to improve performance.

3) Add Drive and Add secondary drive:

Using this setting you can change your disk cache location to a required custom drive may be D:\ and also limit the amount of space you can use.


4) Server Proxy Settings:



Reverse rewrite host in response headers: This option might not be of much value over here but it’s a very important setting while having reverse proxy. Imagine the internet exposed URL is and you have the backend servers’ contoso1 and contoso2. Consider an example where you have a redirect status set and in the response location tag is set to http://contoso1/redirectedpage.aspx in web server in respect to the request forwarded from ARR server and this has to be notified to the end client. The client doesn't know who contoso1 is. So before sending the complete response to the client, the ARR server rewrites the host name in the location tag as

Setting up ARR as a reverse proxy


Imagine you have a site with external URL and the actual content is hosted on IIS server which is not exposed to the internet world and internal/ intranet URL is http://contososerver/.*

So over here we will forward the requests to the ARR server and inturn internally route it to the backend server. Below is how the configuration will look like.

At the server level go to Application Request Routing Cache->Server proxy settings




When you specify the server you want to reroute the requests ARR will automatically create inbound rules for you as below in the URLRewrite module at server level as below.




<rule name="ARR_server_proxy" enabled="true" patternSyntax="Wildcard" stopProcessing="true">

<match url="*" />

<action type="Rewrite" url="http://contososerver/{R:0}" />




You can modify the rules on the go based on the requirement. Also you can add multiple inbound rules to reverse proxy the requests to different backend servers based on different conditions such as Hostnames. When you are using ARR as reverse proxy to route the requests to other servers or different sites within the same server, the urlrewrite rules should be created at the server level, not specific to any site.

Troubleshooting errors and issues with ARR

The most common error you run into when using ARR is the 502.3 error.

The best tools that you can use to troubleshoot ARR and 502.3 errors are below.

1) FREB tracing(Failed request tracing)

2) Fiddler tracing (you can install fiddler from

3) Winhttp tracing and netmon traces

FREB tracing:

This is one of the beautiful tool inbuilt in IIS which will get you more info on why you are getting 502.3 errors. You can configure failed request tracing at the default web site which is running on port 80 at the site level as below.

Go to Failed request tracing at site level->Edit site tracing->Enable


Again Failed request tracing at site level->Add




Try and reproduce the issue. When you encounter a 502.3 error next time you will have the freb xml files in C:\inetpub\logs\FailedReqLogFiles\W3SVC1. Open the xml file in internet explorer. If you are unable to open it in IE, make sure you go to and check internet options->security->internet->scripting is enabled.

Below is one of the scenarios when you get a 502.3 error and what you see in FREB

502.3 “The connection with the server was terminated abnormally”

– This happens if a request is in progress to a content server and the application pool serving that request terminates suddenly (for example due to a web application on the content server throwing an unhandled exception on a non-request thread).

You can investigate this further to check if there were any issues on the content server and investigate why the connection was terminated. You can even collect netmon traces.

Fiddler tracing

If you want to see how the requests are routed to the backend server from ARR and to see what the exact headers are forwarded to the content server and if you are getting any errors in HTTP status codes then Fiddler tracing would be a good bet. You can attach Fiddler to ARR server and get the requests going out of ARR and the responses coming to the ARR server from the backend server.

To attach fiddler to the ARR server to record requests follow the below steps.

1) Install fiddler on the ARR server.

2) Fiddler runs on port 8888, so you need to attach ARR to route the requests to backend server through port 8888 so that fiddler can record the requests and responses. To do that go Application Request Routing Cache at the server level ->Server Proxy settings and add forward proxy value as localhost:8888 as below.


Consider you are getting a 404 error from the backend server to ARR server, this is how the fiddler trace will look like. You can see the successful and failure requests and responses obtained from the backend server as below.


WinHttp tracing:

Once the ARR server routes the request and gets the response back from the backend server, ARR then repackages the response to send it back to the client. For this purpose of repackaging ARR uses WinHTTP interface. If you don’t see much info in the FREB traces or fiddler traces then there might be a possibility that request is failing at WinHTTP level. To find out exactly if it is failing at WinHTTP level and where exactly it is failing, you can enable WinHTTP traces. Collecting WinHTTP traces and the way to run the trace differs on the platform and the version of the operating system we are using on the ARR server. You can refer Richard Marr’s blog where he gives a very good description on how to collect these traces. Below are the steps how we can collect winhttp traces from Richard Marr’s blog.

Windows 2008

1. Start the Tracing. From a command prompt run the following command:

netsh winhttp set tracing trace-file-prefix="C:\TEMP\WinHttpLog" level=verbose format=hex state=enabled max-trace-file-size=1048576000

2. Recycle the IIS Application Pool.

3. Reproduce the issue.

4. Stop the Tracing. From a command prompt run the following command:

netsh winhttp set tracing state=disabled

5. Review the trace with Notepad or any Text editor.

NOTE: The Identity of the IIS application pool will require write access to the  log location  c:\Temp in this example:

This type of tracing is process bitness specific, so if you are looking at a 32 bit process running from 64 bit OS, you need to use: c:\windows\syswow64\cmd.exe, rather than using the regular 64 bit cmd.exe (start a run a cmd.exe)

Windows 2008 R2

Method 1

This method will output the Winhttp API calls , but not raw data for network communication. From a command prompt run the following command:

1. Start the tracing

netsh winhttp set tracing trace-file-prefix="C:\Temp\Test3" level=verbose format=hex

netsh winhttp set tracing output=file max-trace-file-size=512000 state=enabled

2. Recycle the IIS Application Pool.

3. Reproduce the issue.

4. Stop the Tracing. From a command prompt run the following command:

netsh winhttp set tracing state=disabled

5. Review the trace with Notepad or any Text editor.

NOTE: The Identity of the IIS application pool will require write access to the log location c:\Temp in this example:

This type of tracing is process bitness specific, so if you are looking at a 32 bit process running from 64 bit OS, you need to use: c:\windows\syswow64\cmd.exe, rather than using the regular 64 bit cmd.exe (start a run a cmd.exe)

Method 2

To get the raw data communication at network layer and the Winhttp  Api calls.

1. Start the tracing: From a command prompt run the following command:

netsh trace start scenario=InternetClient capture=yes report=yes

Note the etl file location for example:

  Trace File:         C:\Users\<your user name>\AppData\Local\Temp\NetTraces\NetTrace.etl

2.Recycle the IIS Application Pool.

3. Reproduce the issue.

4. Stop the tracing: From a command prompt run the following command:

netsh trace stop

5. Read the Trace by opening it in Netmon 3.4. 

Method 3

The ETW format for winhttp API is available on windows 2008 R2 and win7 via the Event Viewer

1.  Open event viewer. Go to “View” menu --> make sure “Show Analytic and debug logs” is checked.

2. Open “Applications and Services logs” -- > Open “Microsoft” -- > Open “Windows –> Winhttp –> Diagnostic.


3. Highlight “Diagnostic” under Winhttp tree and right click mouse, then click “enable log”.

4.  Reproduce the issue then you can review the logs.

Below is one of the sample winhttp output which shows an error 502.3 which is happening because of timeout at ARR level. The ARR server is waiting for the data from the backend server and timing out as below.

15:57:26.662 ::Completing WinHttpReadData() with error ERROR_WINHTTP_TIMEOUT (12002); Request 
Handle = 1C8457F0, Context = 1C5F0E70
15:57:26.662 ::sys-recver processing WebReceiveHttpResponseEntity completion 
(error-cdoe = ? (0x5b4), #bytes = 0, overlapped = 1B775260)
15:57:26.662 ::sys-recver failed to read data; error = ? (1460)
15:57:26.662 ::ERROR_WINHTTP_FROM_WIN32 mapped (?) 1460 to (ERROR_WINHTTP_TIMEOUT) 12002
15:57:26.662 ::sys-req completing a read-data call(error = ERROR_WINHTTP_TIMEOUT(12002),cbRead =0)
15:57:26.662 ::usr-req 1C9B4F80 received OnReadData() callback; error = ERROR_WINHTTP_TIMEOUT 
(12002), ulNumberOfBytesTransferred = 0, dwptrContext = 1C380160

Netmon traces will further help you to get the exact timeout. As a workaround for this issue you can increase the timeout value for ARR at the server proxy settings or investigate why it is taking a long time in the backend server.

Hope this one helps Smile


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  • Thank you for this blog post! I have a problem with ARR that you might be able to help me with! I'm trying to redirect requests through my IIS with ARR to a tomcat service by using URL Rewrite. The pattern it should match is jft.generatepdf.localhost/* and I've tested it against an expected URL String (it matches). The problem is that I'm getting page not found (404) when trying to access it. It worked before when I tried with the pattern .*, but that I can't do as I have 2 more services to use on the same port (port 80). Do you have any ideas?

  • @Lind: if I am correct jft.generatepdf.localhost is the hostname. We cant check for Hostname in the pattern section. your pattern section should be either *(if wildcard matching) or .* (if you are using regular expressions) then under the conditions section you can check the hostname in {HTTP_HOST} matches jft.generatepdf.localhost. The explanation what we need to check in each section of a inbound rule is in below link

    Please let me know in case oa ny follow up questions.

  • @Chiranth Ramaswamy Thank you for your reply! I'm in desperate need of fixing this so really appreciate it:) I assume that I have to add jft.generatepdf.localhost to my hostfile while I'm using my local machine for testing right?

    So adding:   jft.generatepdf.localhost

    should be required`?

    I'm about to test this and I'll get back to you with the results!

  • @Chiranth Ramaswamy, I tried this but with no luck. This is my setup for the URL Rewrite:

    Name: Tomcat

    Matches the pattern using Regular Expression

    Pattern: *


     Input: {HTTP_HOST}

     Type: Matches the pattern

     Pattern: jft.generatepdf.localhost

    Action type: Rewrite

    Rewrite URL: http://localhost:8080/{R:0}

    In the host file I added: jft.generatepdf.localhost

    I still get page not found when trying to access with this string:


    Any ideas?

  • @Lind: Please provide me the below information.

    URL or hostname mapped to IIS: xxxxxx(Binding for the site in IIS)

    Direct URL when you hit the TOMCAT server:xxxxx

  • @Chiranth Ramaswamy

    Binding to IIS: I have no binding set up (that I know of) to the tomcat service (let's call it generatepdf). Is this the same type of binding you can set up for a "native" iis service?

    Direct URL When I hit the Tomcat Server: Sorry for my lack of knowledge here but do you mean an example I can use to access the service through tomcat directly without the proxy?

    If that's the case here it is:


  • @Lind: Create a site in IIS and have a binding for the iis site mapped to IP,PORT and the hostname. Lets keep the hostname as jft.generatepdf.localhost. After doing this follow the below steps.

    1) In ARR server proxy settings at server level check enable proxy check box

    2) Disable the other rules that you might have created.

    3) In hosts file add the entry to map jft.generatepdf.localhost to the IP you are using in the site. probably bind it to a specific ip rather than loopback address (

    4) At the site level go to URL rewrite->Add inbound rule->Blank rule

    For below steps I am assuming we using 'regular expressions'

    5) Pattern section should be .*

    6) Under conditions->add condition->Condition Input will be {HTTP_HOST}and the pattern section will be jft.generatepdf.localhost

    7) Action type will be Rewrite

    8) Action URL will be http://localhost:8080/{R:0}

    9) Now try to access jft.generatepdf.localhost/.../GeneratePDFServlet locally and see if it works

    let me know in case of any queries

  • @Chiranth Ramaswamy I Thank you a million times over! you found the missing piece!

    However, I have two more services on the IIS (native IIS services) that I need to be able to access through the same port (port 80). When I added bindings for them to port 80, something crashed with the setup. It still seemed to access the tomcat service through IIS as intended but the db access that I perform did no longer work.

    I don't expect you to know how to solve that problem as it is most likley highly individual but maybe you know if this is supposed to be possible? (meaning to have 2 native IIS Services on the same port as this proxy using bindings?)

  • @Lind: you can run multiple IIS services on the same port provided you have a unique combination of IP:PORT:Hostname. so if you want the IP and port to be the same then you need to add different hostnames for different services

  • @Chiranth Ramaswamy, Ok then I know it is possible, I just need to find out what might be causing my issues. I'm using unique hostnames so the problem is probably elsewhere then.

    Thanks again for all your help! I've been trying to get this working for over a week without success so I am ever so grateful for you taking time to help me!

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