IIS URL rewrite custom server variable Bug

Markus Sc 1

Hello,
I have a custom server variable in the IIS URL Rewrite module let's call it CUSTOMER in which I store the value of {PATH_INFO} in an inbound rule.
Because I am only interested in the part of the path identifying the customer i made an outbound rule to rewrite the variable to get only the customer part (Matching scope: Server Variable).

The full path is like:

/relay/CustomerId/Controller/Action

my pattern is:

^/relay/([^/]+)/.*

the rewrite rule is:

{R:1}

which should work and I double checked it with regex101.com.

Due to the presumed bug I get the full unaltered value of the original path.
As soon as you add just any character or a second group to the rewrite rule the behavior is as expected.

So for me the workaround is that (by adding the group (.)) I changed the pattern to:

^/relay/(.)([^/]+)/.*

with this rewrite rule:

{R:1}{R:2}

Such a solution is not optimal and may, in certain circumstances, lead to unforeseeable erroneous behavior some time later in production. So I think this bug should be fixed.

At least I hope to help someone else facing this issue struggling with finding a solution.

Sam Wu-MSFT 7,036 Reputation points Microsoft Vendor

2022-08-26T09:29:54.37+00:00

@Markus Sc

According to your description, I couldn’t understand your requirement clearly. what do you want to capture by pattern? the following is the result of my test, the result of {R:1}{R:2} is CustomerId. please post more details information about your requirement.
Markus Sc 1 Reputation point

2022-08-26T10:18:30.573+00:00

Thanks for your answer,
just to make clear, the screenshot you provided shows the regex pattern of my workaround.

But the regex pattern is never the problem.
^/relay/([^/]+)/.* Is exactly what I would need and I double checked it on regex101.com.

If you look at my screenshot you will see the non working rewrite rule (Value in Action). As I said the regex pattern is correct here.

The problem is that only using a single matching group like {R:1} as Value in Action does not work. As soon as you add any character or any second matching group (even the same one a second time) everything is working as expected. But I do not need any additional characters so I searched for a workaround which I fortunately found.

By "does not work" I mean that the custom server variable is not rewritten and keeps the original value.

Non working rewrite rule:

I hope I could make the issue clearer now.

I would see this as a bug but If someone knows a good reason for the behavior I would appreciate to be enlightened.
Sam Wu-MSFT 7,036 Reputation points Microsoft Vendor

2022-08-29T02:15:05.203+00:00

@Markus Sc

^/relay/([^/]+)/.* Is exactly what I would need and I double checked it on regex101.com.

I don't know what your test result in regex101.com is and I don't care about it, but I still recommend you to test in iis because it is the most correct way.

And your regex above is ^/relay/(.)([^/]+)/.*, but now it becomes ^/relay/([^/]+)/.*.

If you look at my screenshot you will see the non working rewrite rule (Value in Action). As I said the regex pattern is correct here.
By "does not work" I mean that the custom server variable is not rewritten and keeps the original value.

You can try to use the failed request trace to see the detailed request process, this will generate detail log file, which will help you to identify the problem.
Markus Sc 1 Reputation point

2022-08-31T13:46:45.613+00:00

@Sam Wu-MSFT

And your regex above is ^/relay/(.)([^/]+)/., but now it becomes ^/relay/([^/]+)/..

^/relay/([^/]+)/.* is what I really needed and ^/relay/(.)([^/]+)/.* is my workaround to make the rewrite module do what I want.

As I said before: But the regex pattern is never the problem. I also tested it in IIS and it worked

The issue lies within the action rule. I don't know how to clear things up. I tried already to explain it twice.

Let me try it a third time:

When you use the action rule {R:1} (without any additional prefix or suffix) you will get no change of the custom variable.

When you use the action rule {R:1}abc (or any other addition to {R:1}) the custom variable is changed to what you would expect. But obviously I do not need abc

Explicit example:

original value of custom variable:
/relay/123456/Home/Page

pattern: ^/relay/([^/]+)/.*

action rule: {R:1}
result: /relay/123456/Home/Page What is going on here?

action rule: {R:1}abc
result: 123456abc

best regards
Markus
Sam Wu-MSFT 7,036 Reputation points Microsoft Vendor

2022-09-01T03:21:01.537+00:00

@Markus Sc

As I mentioned above, you can use failed request tracking to see detailed request information and you will find out cause.