- header-parse-error-handler #!optional HANDLERparameter
When there is an error parsing a given header, this parameter's procedure will be invoked.
HANDLER is a procedure accepting four values: the header name, the header contents, the current headers and the exception object. The procedure must return the new headers. Defaults to a procedure that simply returns the current headers. When an error occurs while parsing the header line itself (for example when a colon is missing between the header name and contents), the error will not be caught.
In such a case, Servers should return a 400 Bad Request error and clients should error out. The reason that malformed error lines are ignored is that there are several servers and clients that send headers content values that are slightly off, even though the rest of the request is OK. In the interest of the "robustness principle", it's best to simply ignore these headers with "bad" content values.