XDebug Plugin

The XDebug plugin allows HTTP clients to debug the operation of the Traffic Server cache using the default X-Debug header. The plugin is triggered by the presence of the X-Debug or the configured header in the client request. The contents of this header should be the names of the debug headers that are desired in the response. The XDebug plugin will inject the desired headers into the client response. In addition, a value of the form fwd=n may appear in the X-Debug header, where n is a nonnegative number. If n is zero, the X-Debug header will be deleted. Otherwise, n is decremented by 1. =n may be omitted, in which case the X-Debug header will not be modified or deleted.

XDebug is a global plugin. It is installed by adding it to the plugin.config file. It currently takes a single, optional configuration option, --header. E.g.


This overrides the default X-Debug header name.

Debugging Headers

The XDebug plugin is able to generate the following debugging headers:


If the Via header is requested, the XDebug plugin sets the proxy.config.http.insert_response_via_str configuration variable to 3 for the request.


If the Diags header is requested, the XDebug plugin enables the transaction specific diagnostics for the transaction. This also requires that proxy.config.diags.debug.enabled is set to 1.


All request and response headers are written to the response body. Because the body is altered, it disables writing to cache. In conjunction with the fwd tag, the response body will contain a chronological log of all headers for all transactions used for this response.


  • Request Headers from Client -> Proxy A

  • Request Headers from Proxy A -> Proxy B

  • Original content body

  • Response Headers from Proxy B -> Proxy A

  • Response Headers from Proxy A -> Client


The X-Cache-Key header contains the URL that identifies the HTTP object in the Traffic Server cache. This header is particularly useful if a custom cache key is being used.


The X-Cache header contains the results of any cache lookups.




No cache lookup was attempted.


The object was not found in the cache.


The object was found in the cache, but it was stale.


The object was fresh in the cache.


The cache lookup was skipped.

If a request goes through multiple proxies, each one appends its X-Cache header content the end of the existing X-Cache header. This is the same order as for the Via header.


The cache generation ID for this transaction, as specified by the proxy.config.http.cache.generation configuration variable.


The X-Milestones header contains detailed information about how long the transaction took to traverse portions of the HTTP state machine. The timing information is obtained from the TSHttpTxnMilestoneGet() API. Each milestone value is a fractional number of seconds since the beginning of the transaction.


A unique transaction ID, which identifies this request / transaction. This matches the log field format that is also available, %<cruuid>.


If the URL was remapped for a request, this header gives the to and from field from the line in remap.config that caused the URL to be remapped.


The X-ParentSelection-Key header contains the URL that is used to determine parent selection for an object in the Traffic Server. This header is particularly useful if a custom parent selection key is being used.