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.

--header=ATS-My-Debug

This overrides the default X-Debug header name.

All the debug headers are disabled by default, and you need to enable them selectively by passing header names to --enable option.

--enable=x-remap,x-cache

This enables X-Remap and X-Cache. If a client’s request has X-Debug: x-remap, x-cache, probe, XDebug will only inject X-Remap and X-Cache.

To enable the JSON transaction header probe functionality:

--enable=probe-full-json

This allows clients to request X-Debug: probe-full-json to receive request and response header information in a structured JSON format.

Debugging Headers

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

Via

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.

Diags

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.

Probe

All request and response headers are written to the response body. Because the body is altered, it disables writing to cache. Further, the Content-Type value is modified to application/json and an X-Original-Content-Type response header is added to indicate the original response Content-Type value.

In conjunction with the fwd tag, the response body will contain a chronological log of all headers for all transactions used for this response.

Layout:

  • 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

Probe-Full-JSON

Similar to Probe but formats the output as a complete JSON object containing request and response headers. The response body is modified to include client request headers, proxy request headers, the original server response body, server response headers, and proxy response headers in a structured JSON format. In contrast to Probe, the response content with this feature is parsable with JSON parsing tools like jq. Because the body is altered, it disables writing to cache and changes the Content-Type to application/json. However, as with the probe header, a X-Original-Content-Type response header is added to indicate the original response Content-Type value.

JSON Nodes:

  • client-request: Headers from the client to the proxy.

  • proxy-request: Headers from the proxy to the origin server (if applicable).

  • server-body: The original response body content from the origin server.

  • server-response: Headers from the origin server to the proxy.

  • proxy-response: Headers from the proxy to the client.

For the server-body value, by default the plugin chooses an encoding based on the original response Content-Type:

  • Textual content types (e.g. text/*, and types containing json, xml, html, csv, or javascript) are JSON-escaped.

  • Other content types are hex-encoded.

You can override the derived encoding by providing an option with the probe-full-json header value:

  • X-Debug: probe-full-json=escape forces JSON escaping of the origin body.

  • X-Debug: probe-full-json=hex forces hex encoding of the origin body.

  • X-Debug: probe-full-json=nobody omits the origin body entirely.

Here’s an example of the JSON output:

$ curl -s -H"uuid: 1" -H "Host: example.com" -H "X-Debug: probe-full-json" http://127.0.0.1:61003/test | jq
{
    "client-request": {
        "start-line": "GET http://127.0.0.1:61000/test HTTP/1.1",
        "uuid": "1",
        "host": "127.0.0.1:61000",
        "x-request": "from-client"
    },
    "proxy-request": {
        "start-line": "GET /test HTTP/1.1",
        "uuid": "1",
        "host": "127.0.0.1:61000",
        "x-request": "from-client",
        "Client-ip": "127.0.0.1",
        "X-Forwarded-For": "127.0.0.1",
        "Via": "http/1.1 traffic_server[f47ffc16-0a20-441e-b17d-6e3cb044e025] (ApacheTrafficServer/10.2.0)"
    },
    "server-body": "Original server response",
    "server-response": {
        "start-line": "HTTP/1.1 200 ",
        "content-type": "text/html",
        "content-length": "24",
        "x-response": "from-origin",
        "Date": "Tue, 19 Aug 2025 15:02:07 GMT"
    },
    "proxy-response": {
        "start-line": "HTTP/1.1 200 OK",
        "content-type": "application/json",
        "x-response": "from-origin",
        "Date": "Tue, 19 Aug 2025 15:02:07 GMT",
        "Age": "0",
        "Transfer-Encoding": "chunked",
        "Connection": "keep-alive",
        "Server": "ATS/10.2.0",
        "X-Original-Content-Type": "text/html"
    }
}
X-Cache-Key

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.

X-Cache

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

Value

Description

none

No cache lookup was attempted.

miss

The object was not found in the cache.

hit-stale

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

hit-fresh

The object was fresh in the cache.

skipped

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.

X-Cache-Generation

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

X-Milestones

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.

X-Transaction-ID

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

X-Remap

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.

X-Effective-URL

If the URL was remapped for a request, this header gives the URL resulting from the remapping. Note that if there are multiple remaps, this header aggregates the URLs, space-comma-separated. The URLs are inside doublequotes.

X-ParentSelection-Key

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.