X-DC
The HTTP X-DC response header is an unofficial HTTP header identifying the data center routing path a request followed through a CDN or cloud infrastructure.
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.
A typical three-value format represents three tiers of infrastructure: the edge node closest to the client, a mid-tier cache, and the origin data center. 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 tiers in the routing path resolve to the same cloud region. The request entered and was served entirely within one geographic location. This is the most common pattern and indicates optimal routing with minimal cross-region latency.
Multi-region path
The tiers in the routing path include different regions. The edge node receiving the request is in one region, while the mid-tier cache or origin server is in another. This happens when the nearest edge does not have a cached copy and the request is forwarded to a different region for fulfillment.
Example
A request handled entirely within a single US East region. All three routing tiers resolved to the same data center, meaning the edge, mid-tier cache, and origin were 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
Takeaway
The X-DC header traces the data center routing path across edge, mid-tier, and origin infrastructure tiers. Comparing the region values reveals whether a request stayed local or crossed geographic boundaries during fulfillment.