X-Served-By
The HTTP X-Served-By response header is an unofficial HTTP header identifying the cache server or servers responsible for delivering the response.
Note
The X- prefix for non-standard headers is
deprecated per RFC 6648.
Usage
The X-Served-By header is added by CDN platforms to identify which edge or shield node processed a request. Fastly sets this header by default on all responses passing through its network.
The value contains one or more cache node identifiers. When a Fastly service uses shielding, the header lists multiple nodes separated by commas. The first entry is the shield node (closest to the origin server) and the second is the edge node (closest to the end user). This ordering mirrors the path the response traveled through the CDN.
The header works alongside X-Cache to provide a complete view of cache behavior. While X-Served-By names the nodes involved, X-Cache reports whether each node returned a cache hit or miss.
Values
Fastly cache node format
Fastly cache node identifiers follow the pattern
cache-[datacenter][number]-[CODE]. The three-letter suffix
is an IATA airport code representing the geographic location
of the point of presence (PoP). For example, SJC is San
Jose, JFK is New York, LGA is also New York (LaGuardia),
and PDX is Portland.
Generic server names
Outside of Fastly, other hosting providers and load balancers use X-Served-By with their own naming conventions. Values range from internal hostnames to datacenter identifiers.
Example
A single Fastly edge node served this response. The
identifier cache-sjc1000133-SJC shows the response came
from a San Jose PoP. The SJC suffix corresponds to the
San Jose International Airport IATA code.
X-Served-By: cache-sjc1000133-SJC
When shielding is active, two cache nodes appear. The first
entry is the shield node in New York (LGA) and the second
is the edge node in Portland (PDX). The response traveled
from origin to shield to edge before reaching the end user.
X-Served-By: cache-lga21984-LGA, cache-pdx12334-PDX
The header paired with X-Cache gives a full picture. Here the shield node had a cache miss while the edge node returned a hit from its local cache.
X-Served-By: cache-lga21984-LGA, cache-pdx12334-PDX
X-Cache: MISS, HIT
Takeaway
The X-Served-By header identifies the CDN cache nodes responsible for delivering a response. Fastly includes this header by default, using IATA airport codes to indicate the geographic location of each PoP in the delivery chain.