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HTTP PassThrough Transport is a non-blocking HTTP transport implementation based on HTTP Core NIO and specially designed for streaming messages. It is similar to the old message relay transport, but it does not care about the content type and simply streams all received messages through. It also , and is the default HTTP transport shipped with WSO2 ESB. Although the PassThrough Transport is somewhat similar to the NHTTP transport, it overcomes all the limitations of the NHTTP transport and provides a significant performance gain. The PassThrough Transport also has a simpler and cleaner model for forwarding messages back and forth. It can be used as an alternative to the NHTTP transport. . 

org.apache.synapse.transport.passthru.PassThroughHttpSSLListener is the listener class of the PassThrough Transport, and it receives HTTPS inbound requests.

org.apache.synapse.transport.passthru.PassThroughHttpSSLSender is the sender class of the PassThrough Transport, and it sends out HTTPS outbound requests. Both the listener and sender of the PassThrough Transport can verify certificate revocation. 

Verifying certificate revocation 

Anchor
revocation revocation

org.apache.synapse.transport.passthru.PassThroughHttpSSLListener as well as org.apache.synapse.transport.passthru.PassThroughHttpSSLSender can verify with the certificate authority whether a certificate is still trusted before it completes a SSL connection. If the certificate authority has revoked the certificate, a connection will not be completed. To enable this feature, you need to add the CertificateRevocationVerifier parameter to the receiver or sender in the <EI_HOME>/conf/axis2/axis2.xml file.

Setting the CertificateRevocationVerifier parameter at the transport listener allows you to verify client certificates when a client tries to make an HTTPS connection with the ESB. Following is a sample transport listener configuration that you can add in the axis2.xml file to enable certificate revocation verification:

<transportReceiver name="https" class="org.apache.synapse.transport.passthru.PassThroughHttpSSLListener">
...
   <parameter name="CertificateRevocationVerifier">true</parameter>
</transportReceiver>

Setting the CertificateRevocationVerifier parameter at the transport sender allows you to verify server certificates when the ESB tries to make an HTTPS connection with a backend server. Following is a sample transport sender configuration that you can add in the axis2.xml file to enable certificate revocation verification:

<transportSender name="https" class="org.apache.synapse.transport.passthru.PassThroughHttpSSLSender">
...
   <parameter name="CertificateRevocationVerifier">true</parameter>
</transportSender>

When the CertificateRevocationVerifier parameter is set to true, the ESB attempts to use the Online Certificate Status Protocol (OCSP) to verify with the certificate authority at the handshake phase of the SSL protocol. If the OCSP is not supported by the certificate authority, the ESB uses Certified Revocation Lists (CRL) instead. The verification process checks all the certificates in a certificate chain.

The response from the certificate authority includes the verification and the duration for which the verification is valid. To prevent any performance overhead of continuous HTTP calls, this verification response is cached for the duration specified by the certificate authority, so that subsequent verification calls are not required until the response has expired. There are two Least Recently Used (LRU) in-memory caches for OCSP and CRL, which are automatically managed by a dedicated CacheManager thread for each cache. These CacheManagers update expired cache entries and maintain the LRU cache replacement policy.