X-DC
The HTTP X-DC unofficial response header identifies the data center routing path a request followed through a CDN or cloud infrastructure, expressed as comma-separated region identifiers.
Usage
The X-DC header is a non-standard header added by CDN and cloud infrastructure providers to trace which data centers handled a request. The value is a comma-separated list of cloud region identifiers representing the routing tiers the request passed through on its way from edge to origin.
No vendor documents the value structure. A common observed format carries three comma-separated region identifiers, consistent with a routing path across infrastructure tiers such as an edge node, a mid-tier cache, and the origin data center, though the meaning of each position is undocumented. When all three values match, the request was handled entirely within one region. When the values differ, the request crossed regions between tiers. This information helps diagnose latency, verify geographic routing policies, and confirm Caching infrastructure is directing traffic as expected.
Note
The "X-" naming convention for HTTP headers, "X" referring to "experimental", has been deprecated and needs to be transitioned to the formal naming convention for HTTP headers.
Values
Single-region path
All identifiers in the value resolve to the same cloud region, consistent with a request entering and being served entirely within one geographic location. This is the most common pattern observed and suggests routing with minimal cross-region latency.
Multi-region path
The identifiers in the value include different regions, consistent with an edge node in one region forwarding the request to a cache or origin in another. Under the observed-format reading, this pattern appears when the nearest edge lacks a cached copy and the request crosses regions for fulfillment.
Example
A request handled entirely within a single US East region. All three values resolve to the same data center, consistent with every routing tier being co-located. This is the fastest routing scenario.
X-DC: gcp-us-east1,gcp-us-east1,gcp-us-east1
A request entering through a US West edge node but routed to US East for the mid-tier and origin tiers. This cross-region path adds some latency compared to a single-region response, and indicates the content or origin server lives in the eastern region.
X-DC: gcp-us-west1,gcp-us-east1,gcp-us-east1
A request served from a European data center. The
gcp-europe-west1 region identifier shows the
infrastructure is running on Google Cloud Platform in
Western Europe.
X-DC: gcp-europe-west1,gcp-europe-west1,gcp-europe-west1
