Configure ICAP servers

Configure API Gateway to connect to an ICAP server.

5 minute read

The Internet Content Adaptation Protocol (ICAP) is a lightweight HTTP-based protocol used to optimize proxy servers, which frees up resources and standardizes how features are implemented. For example, ICAP is typically used to implement features such as virus scanning, content filtering, ad insertion, or language translation in the HTTP proxy server cache. Content Adaptation refers to performing a specific value-added service (for example, virus scanning) for a specific client request and/or response.

You can configure ICAP servers under the Environment Configuration > External Connections tree node, which you can then specify in an ICAP filter later. To configure an ICAP server, right-click the ICAP Servers node, and select Add an ICAP Server to display the ICAP Server Settings dialog.

Server settings

Configure the following settings on the Server tab:

Host: The machine name or IP address of the remote ICAP host. Defaults to the localhost (

Port: The port on which the ICAP server is listening. Defaults to 1344.

Request Service: The relative path to the service exposed by the ICAP server that handles Request Modification (REQMOD) requests. The default value is /request.

Response Service: The relative path to the service exposed by the ICAP server that handles Response Modification (RESPMOD) requests. The default value is /response.

Options Service: The relative path to the service exposed by the ICAP server that handles OPTIONS requests. OPTIONS requests enable server capabilities to be queried. The default value is /options.

The paths for each of these services should be given by your ICAP server’s documentation.

Security settings

The following settings on the Security tab enable you to secure the connection to the ICAP server:

Trusted Certificates: When the API Gateway connects to the ICAP server over SSL, it must decide whether to trust the ICAP server’s SSL certificate. You can select a list of CA or server certificates on the Trusted Certificates tab that are considered trusted by API Gateway when connecting to the ICAP server. The table displayed on the Trusted Certificates tab lists all certificates imported into the API Gateway Certificate Store. To trust a certificate for this particular connection, select the box next to the certificate in the table.

Client SSL Authentication: In cases where the destination ICAP server requires clients to authenticate to it using an SSL certificate, you must select a client certificate on the Client SSL Authentication tab. Select the check box next to the client certificate that you want to use to authenticate to the ICAP server.

Advanced: The Ciphers field enables you to specify the ciphers that API Gateway supports. API Gateway sends this list of supported ciphers to the destination ICAP server, which then selects the highest strength common cipher as part of the SSL handshake. The selected cipher is used to encrypt the data when it is sent over the secure channel. The default value is FIPS:!SSLv3:!aNULL.

Advanced settings

Select one of the following ICAP server modes on the Advanced tab:

Request Modification Mode (REQMOD): Specifies that the ICAP filter in the API Gateway sends a Request Modification (REQMOD) request to the ICAP server. The ICAP server returns a modified version of the request, an HTTP response, or a 204 response code indicating that no modification is required. This mode protects your own API service providers from malicious clients requests, for example, requests from unknown sources. This mode is selected by default.

Response Modification Mode (RESPMOD): Specifies that the ICAP filter in the API Gateway sends a Response Modification (RESPMOD) request to the ICAP server. For example, the API Gateway sends an origin server’s HTTP response to the ICAP server. The response from the ICAP server can be an adapted HTTP response, an error, or a 204 response code indicating that no adaptation is required. Use this mode when you are a client yourself and you want to communicate, for example, with a cloud application. Here you want to make sure that the response you receive does not contain malicious code..

Custom headers

The ICAP filter supports custom headers. Custom header support allows for integration with additional ICAP servers, for example, OPSWAT.

You can configure custom headers in the Advanced tab for the above REQMOD and RESPMOD options. When the ICAP filter is executed, the specified custom headers are added to outgoing REQMOD and RESPMOD requests.

Custom headers override default headers with the same name, so if a custom header has the same name as a header that is automatically added by the filter implementation, only the custom header value is sent.

Message Preview

Setting a message preview length notifies the ICAP server to only expect the specified number of bytes, after which point the ICAP server determines whether or not the remainder of the message body needs to be sent. Previewing a message can yield performance improvements for certain use cases, in particular where large payloads are being sent. For example, virus checkers can usually certify payloads as being ‘clean’ simply by looking at the first few bytes.

To enable Message Preview for REQMOD/RESPMOD requests, go to the Advanced tab, click Add from the Custom ICAP Headers table, and select Preview from the dropdown list. Enter a numeric value for the preview length.

After you configure the Preview header, API Gateway initially sends the configured preview of the message body, then it waits for further instruction. The ICAP server sends a 100 Continue response if it requires more data, a 204 response if the message body does not need to be modified, or a specific REQMOD/RESPMOD response in any other case.

The ICAP server can control the length of the preview message by including a Preview header in response to an ICAP OPTIONS request. This notifies API Gateway that the ICAP server can only support a message preview of this length. If the preview length configured in API Gateway is greater than that specified by the ICAP server, API Gateway downgrades its preview length to match the value specified by the ICAP server

For more information, see RFC 3507 Section 4.5.

Last modified August 17, 2022: Update ICAP documentation (#2392) (74a96559)