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Exploring mTLS setup to ship a shopper certificates to the backend and OCSP validation | Azure Weblog and Updates

In our earlier weblog we mentioned what mutual transport layer safety (mTLS) is and what a few of its use circumstances are. On this weblog I wish to talk about two of these use circumstances. First, ship shopper certificates to the backend utility server and validate the setup by curl command and second arrange OCSP validation and do verification by openssl instructions.

Insert shopper certificates as HTTP header

In some circumstances, backend functions might have a shopper certificates that’s obtained by Utility Gateway. Consumer certificates can serve totally different functions as per the necessity of the backend functions. Some backend servers might have shopper certificates info for audit functions or might wish to situation token or cookie to a shopper certificates. In that case we might have to produce the shopper certificates to the backend. One method to resolve that is by supplying the certificates in base64 encoded format inside a nonstandard HTTP (Hypertext Switch Protocol) header. Please be aware, for safety functions and to forestall header injections, backend server should settle for the customized header from trusted Utility Gateway. Let’s talk about first ship shopper certificates to backend utility as customized http header. To attain you can arrange a rewrite rule to ship shopper certificates as HTTPS header.

Discover extra particulars on arrange a rewrite rule in our rewrite URL and question string with Azure Utility Gateway documentation.

Under is the rewrite rule you can create to ship shopper certificates to the backend as an HTTP header. Setup rewrite motion as under.

Screenshot of Create rewrite set explaining values that need to be populated for Rewrite rule.

Above is screenshot of Create rewrite set explaining values that should be populated for Rewrite rule.

As soon as a rewrite rule is created you’ll be able to confirm if the backend server is receiving shopper certificates within the HTTP header. To check the setup prerequisite is to have openssl and curl device put in in your machine. You need to have entry to the shopper certificates and shopper personal key.

Verification steps to examine shopper certificates in customized HTTP header:

Seize the shopper certificates output.

Screen shot showing client certificate output.

Above is display screen shot displaying shopper certificates output.

Run the next commad to ship a request to Utility Gateway:

  • curl -vk HTTPS://<> –key shopper.key –cert shopper.crt

Within the backend server you need to see the header you created within the Utility gateway rewrite rule. You’ll have to run community capturing instruments like tcpdump on the backend server.

Screenshot shows Client certificate that backend has received

Above screenshot exhibits Consumer certificates that backend has obtained.

Above you’ll be able to see the X-Consumer-cert header obtained by backend that now we have created within the rewrite rule. This header has the shopper certificates that now we have despatched. The backend server can extract this worth and use it based mostly on the specified use case.


On-line certificates standing protocol (OCSP) is now supported by Utility gateway. Let’s talk about right here setup OCSP and validate the setup with openssl command. With OCSP help you’ll be able to confirm the standing of the shopper certificates in actual time. This will forestall man-in-the-middle assaults by guaranteeing that the certificates being current remains to be legitimate and has not been compromised. You may get extra particulars about OCSP in RFC 2560. It’s simple to setup. When a shopper initiates a connection to an Utility Gateway configured with mutual TLS authentication, not solely can the certificates chain and issuer’s distinguished title be validated, however revocation standing of the shopper certificates will be checked with OCSP (On-line Certificates Standing Protocol). Throughout validation, the certificates introduced by the shopper shall be regarded up through the outlined OCSP responder outlined in its Authority Info Entry (AIA) extension. Within the occasion the shopper certificates has been revoked, the applying gateway will reply to the shopper with an HTTP 400 standing code and motive. If the certificates is legitimate, the request will proceed to be processed by utility gateway and forwarded on to the outlined backend pool.

Please examine this OCSP hyperlink to allow this functionality. I’ve summarized the PowerShell command to setup OCSP.

$AppGw = Get-AzApplicationGateway -Identify “ApplicationGateway01” -ResourceGroupName “ResourceGroup01”

$profile = Get-AzApplicationGatewaySslProfile -Identify “SslProfile01” -ApplicationGateway $AppGw

Set-AzApplicationGatewayClientAuthConfiguration -SslProfile $profile -VerifyClientCertIssuerDN -VerifyClientRevocation OCSP

Upon getting arrange OCSP, you’ll be able to confirm your shopper certificates with the OCSP endpoint utilizing openssl command.

  • openssl ocsp -issuer <ca-bundle> -cert shopper.crt -text -url <HTTP://FQDN>

Ca-bundle—certificates authority (CA) that has issued the certificates (uploaded per the hyperlink step 8 from our earlier weblog)

Consumer.crt—Consumer certificates

url—This shall be OCSP endpoint URL tackle. In the event you have no idea what the URL is yow will discover the OCSP endpoint of shopper certificates through the use of following command:

  • openssl x509 -in shopper.crt -text | grep -I OCSP


Screen shot of openssl command showing status of client certificate verification.

Above is display screen shot of openssl command displaying standing of shopper certificates verification.

You need to see the next response if certificates is legitimate:

Response confirm OK

shopper.crt: good

After verification of your shopper certificates by means of OCSP endpoint, you’ll be able to confirm the visitors by sending a request to Utility Gateway that has OCSP check-enabled.

  • curl -vk HTTPS:// –key shopper. Key –cert shopper.crt

In case the certificates isn’t a legitimate shopper certificates, OCSP will reply with both “revoked” or “unknown”. Under is the error for “unknown” certificates.


On this weblog now we have mentioned two circumstances that utility gateway helps. You could have realized ship shopper certificates to backend as HTTP header and confirm the setup through the use of curl command. Additionally, you’ve gotten realized arrange OCSP and confirm the setup by openssl command line.

Study extra and get began with Azure Utility Gateway



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