When a request is received from a client, the request-transformer plugin has been configured to add a header which should be sent to the upstream service. When the upstream service receives the request, the header is missing
When adding a header using the request-tranformer the header is not sent to the upstream server
Uses:
Kong Gateway
TL;DR
Why isn’t a header added by the request-transformer plugin sent to the upstream server?
The request-transformer plugin processes headers in a fixed order: it removes headers listed in config.remove.headers, replaces headers listed in config.replace.headers, and only then adds headers from config.add.headers — but config.add.headers is skipped whenever the client already sent that header. To override a client-sent header, use config.replace.headers, or combine config.remove.headers with config.add.headers.
Cause
The behavior of the request-transformer plugin is documented. It evaluates header changes in a fixed order using three parameters:
config.remove.headers- List of header names. Unset the header(s) with the given name.config.add.headers- List ofheadername:valuepairs. If and only if the header is not already set, set a new header with the given value. Ignored if the header is already set.config.replace.headers- List ofheadername:valuepairs. If and only if the header is already set, replace its old value with the new one. Ignored if the header is not already set.
The plugin first gets a list of the request headers and performs a removal action. If a header is marked for removal (config.remove.headers), then the header value is set to nil and that header is marked for removal at a later time.
Next, the same list of headers is checked for the config.replace.headers list. If a match is found, then the value of that header sent by the client is changed to match the value as per the config.
The final check for headers to add is now done on the same list of request headers. Only if the header from the config.header.add list does not exist in the request headers sent by the client is it added.
The last step performed is to remove the headers that were previously flagged for removal.
Solution
In short, the parameters work as below:
config.remove.headers- remove a header that the client sends before sending the request upstreamconfig.replace.headers- change the value of an existing header sent by the client before sending the request upstreamconfig.add.headers- if the client does not send a header, then add a header before sending the request upstream
If you wish to change the value of a header sent by the client, you can use the config.replace.headers configuration. Alternatively, combining config.remove.headers with config.add.headers also correctly overrides a client-sent header value, since the header is removed before the add step evaluates whether the header already exists.