azure APIM client certificate towards SAP gateway

0
votes

We're trying to use a client certificate to authenticate when calling an OData service in SAP S/4HANA. And we're calling from an azure APIM instance. As certificate we've created a self-signed certificate and configured SAP S/4HANA according to this blog (https://blogs.sap.com/2020/05/03/x.509-certificate-based-logon-to-odata-services/) Then we test this from the browser it works like a charm. But calling from azure APIM we get the following response from SAP S/4HANA:

<?xml version="1.0" encoding="utf-8"?> <error xmlns:xsi="http://www.w3.org/2001/XMLSchema-Instance">
<code>HTTP/404/E/Not Found</code>
<message> Service /sap/opu/odata/sap/xxxxyyyy/xxyyzz call was terminated because the corresponding service is not available.The termination occurred in system UFI with error code 404 and for the reason Not found. Please select a valid URL. If it is a valid URL, check whether service /sap/opu/odata/sap/xxxxyyyy/xxyyzz is active in transaction SICF. If you do not yet have a user ID, contact your system administrator. </message>

SAP S/4HANA support says that then calling from browser they can 'see' certificate in payload but then calling from APIM, the payload is 'empty'.

I've got the trace logs from the SAP S/4HANA gateway server and I've noticed this subtly difference calling from browser vs calling from APIM:

Browser call (successfull): [Thr 140708195055360] HttpModGetDefRules: determined the defactions: COPY_CERT_TO_MPI (1)

APIM call (failed): [Thr 140708197697280] HttpModGetDefRules: determined the defactions: NOTHING (0)

So the certificate is obviously reaching SAP S/4HANA gateway server but not the SAP S/4HANA Odata server. So somehow, for some reason it's lost on the SAP S/4HANA gateway server only then it comes from azure APIM.

I've tried to make the calls 100% identical (same headers same values) but I can't control the way the certificate is added in azure apim or can one ?

I read that one can set the certificate body using policy below:

<authentication-certificate body="@(context.Variables.GetValueOrDefault<byte[]>("byteCertificate"))" password="optional-certificate-password" />

but I can't figure out how to get a proper value for "byteCertificate".

Has anyone done this? Or has anyone had a similar issue?

1
azuresslazure-api-managementclient-certificatess4hana

1 Answers

1
votes

We finally found the solution! Thanks to microsoft APIM support team, thanks a lot :)

APIM acts like a reverse proxy and adds headers related to this role. The header "X-Forwarded-For" causes SAP to deny the request with the above misleading error message. We found a solution that SAP could configure:

ICM parameter "icm/HTTPS/accept_ccert_for_x_forwarded_for_requests" has to be set to "true" - per default it's set to "false".

(The header can't be deleted with a policy on APIM side.)