HTTP header manipulation

The HTTP connection manager manipulates several HTTP headers both during decoding (when the request is being received) as well as during encoding (when the response is being sent).


The user-agent header may be set by the connection manager during decoding if the add_user_agent option is enabled. The header is only modified if it is not already set. If the connection manager does set the header, the value is determined by the --service-cluster command line option.


The server header will be set during encoding to the value in the server_name option.


If an external client sets this header, Envoy will join the provided trace ID with the internally generated x-request-id. x-client-trace-id needs to be globally unique and generating a uuid4 is recommended. If this header is set, it has similar effect to x-envoy-force-trace. See the tracing.client_enabled runtime configuration setting.


Internal services often want to know which service is calling them. This header is cleaned from external requests, but for internal requests will contain the service cluster of the caller. Note that in the current implementation, this should be considered a hint as it is set by the caller and could be easily spoofed by any internal entity. In the future Envoy will support a mutual authentication TLS mesh which will make this header fully secure. Like user-agent, the value is determined by the --service-cluster command line option. In order to enable this feature you need to set the user_agent option to true.


Internal services may want to know the downstream node request comes from. This header is quite similar to x-envoy-downstream-service-cluster, except the value is taken from the --service-node option.


It is a common case where a service wants to perform analytics based on the client IP address. Per the lengthy discussion on XFF, this can get quite complicated. A proper implementation involves forwarding XFF, and then choosing the first non RFC1918 address from the right. Since this is such a common occurrence, Envoy simplifies this by setting x-envoy-external-address during decoding if and only if the request ingresses externally (i.e., it’s from an external client). x-envoy-external-address is not set or overwritten for internal requests. This header can be safely forwarded between internal services for analytics purposes without having to deal with the complexities of XFF.


If an internal request sets this header, Envoy will modify the generated x-request-id such that it forces traces to be collected. This also forces x-request-id to be returned in the response headers. If this request ID is then propagated to other hosts, traces will also be collected on those hosts which will provide a consistent trace for an entire request flow. See the tracing.global_enabled and tracing.random_sampling runtime configuration settings.


It is a common case where a service wants to know whether a request is internal origin or not. Envoy uses XFF to determine this and then will set the header value to true.

This is a convenience to avoid having to parse and understand XFF.


x-forwarded-client-cert (XFCC) is a proxy header which indicates certificate information of part or all of the clients or proxies that a request has flowed through, on its way from the client to the server. A proxy may choose to sanitize/append/forward the XFCC header before proxying the request.

The XFCC header value is a comma (“,”) separated string. Each substring is an XFCC element, which holds information added by a single proxy. A proxy can append the current client certificate information as an XFCC element, to the end of the request’s XFCC header after a comma.

Each XFCC element is a semicolon “;” separated string. Each substring is a key-value pair, grouped together by an equals (“=”) sign. The keys are case-insensitive, the values are case-sensitive. If “,”, “;” or “=” appear in a value, the value should be double-quoted. Double-quotes in the value should be replaced by backslash-double-quote (“).

The following keys are supported:

  1. By The Subject Alternative Name (SAN) of the current proxy’s certificate.
  2. Hash The SHA 256 diguest of the current client certificate.
  3. SAN The SAN field (URI type) of the current client certificate.
  4. Subject The Subject field of the current client certificate. The value is always double-quoted.

Some examples of the XFCC header are:

  1. x-forwarded-client-cert: By=;Hash=468ed33be74eee6556d90c0149c1309e9ba61d6425303443c0748a02dd8de688;Subject="/C=US/ST=CA/L=San Francisco/OU=Lyft/CN=Test Client";SAN=
  2. x-forwarded-client-cert: By=;Hash=468ed33be74eee6556d90c0149c1309e9ba61d6425303443c0748a02dd8de688;SAN=,By=;Hash=9ba61d6425303443c0748a02dd8de688468ed33be74eee6556d90c0149c1309e;SAN=

How Envoy processes XFCC is specified by the forward_client_cert and the set_current_client_cert_details HTTP connection manager options. If forward_client_cert is unset, the XFCC header will be sanitized by default.


x-forwarded-for (XFF) is a standard proxy header which indicates the IP addresses that a request has flowed through on its way from the client to the server. A compliant proxy will append the IP address of the nearest client to the XFF list before proxying the request. Some examples of XFF are:

  1. x-forwarded-for: (single client)
  2. x-forwarded-for:, (external proxy hop)
  3. x-forwarded-for:, (internal proxy hop)

Envoy will only append to XFF if the use_remote_address HTTP connection manager option is set to true. This means that if use_remote_address is false, the connection manager operates in a transparent mode where it does not modify XFF. This is needed for certain types of mesh deployments depending on whether the Envoy in question is an edge node or an internal service node.

Envoy uses the final XFF contents to determine whether a request originated externally or internally. This influences whether the x-envoy-internal header is set.

A few very important notes about XFF:

  1. Since IP addresses are appended to XFF, only the last address (furthest to the right) can be trusted. More specifically, the first external (non RFC1918) address from the right is the only trustable addresses. Anything to the left of that can be spoofed. To make this easier to deal with for analytics, etc., front Envoy will also set the x-envoy-external-address header.
  2. XFF is what Envoy uses to determine whether a request is internal origin or external origin. It does this by checking to see if XFF contains a single IP address which is an RFC1918 address.
    • NOTE: If an internal service proxies an external request to another internal service, and includes the original XFF header, Envoy will append to it on egress if use_remote_address is set. This will cause the other side to think the request is external. Generally, this is what is intended if XFF is being forwarded. If it is not intended, do not forward XFF, and forward x-envoy-internal instead.
    • NOTE: If an internal service call is forwarded to another internal service (preserving XFF), Envoy will not consider it internal. This is a known “bug” due to the simplification of how XFF is parsed to determine if a request is internal. In this scenario, do not forward XFF and allow Envoy to generate a new one with a single internal origin IP.


It is a common case where a service wants to know what the originating protocol (HTTP or HTTPS) was of the connection terminated by front/edge Envoy. x-forwarded-proto contains this information. It will be set to either http or https.


The x-request-id header is used by Envoy to uniquely identify a request as well as perform stable access logging and tracing. Envoy will generate an x-request-id header for all external origin requests (the header is sanitized). It will also generate an x-request-id header for internal requests that do not already have one. This means that x-request-id can and should be propagated between client applications in order to have stable IDs across the entire mesh. Due to the out of process architecture of Envoy, the header can not be automatically forwarded by Envoy itself. This is one of the few areas where a thin client library is needed to perform this duty. How that is done is out of scope for this documentation. If x-request-id is propagated across all hosts, the following features are available:


The x-ot-span-context HTTP header is used by Envoy to establish proper parent-child relationships between tracing spans. This header can be used with both LightStep and Zipkin tracers. For example, an egress span is a child of an ingress span (if the ingress span was present). Envoy injects the x-ot-span-context header on ingress requests and forwards it to the local service. Envoy relies on the application to propagate x-ot-span-context on the egress call to an upstream. See more on tracing here.


The x-b3-traceid HTTP header is used by the Zipkin tracer in Envoy. The TraceId is 64-bit in length and indicates the overall ID of the trace. Every span in a trace shares this ID. See more on zipkin tracing here <>.


The x-b3-spanid HTTP header is used by the Zipkin tracer in Envoy. The SpanId is 64-bit in length and indicates the position of the current operation in the trace tree. The value should not be interpreted: it may or may not be derived from the value of the TraceId. See more on zipkin tracing here <>.


The x-b3-parentspanid HTTP header is used by the Zipkin tracer in Envoy. The ParentSpanId is 64-bit in length and indicates the position of the parent operation in the trace tree. When the span is the root of the trace tree, the ParentSpanId is absent. See more on zipkin tracing here <>.


The x-b3-sampled HTTP header is used by the Zipkin tracer in Envoy. When the Sampled flag is 1, the soan will be reported to the tracing system. Once Sampled is set to 0 or 1, the same value should be consistently sent downstream. See more on zipkin tracing here <>.


The x-b3-flags HTTP header is used by the Zipkin tracer in Envoy. The encode one or more options. For example, Debug is encoded as X-B3-Flags: 1. See more on zipkin tracing here <>.

Custom request/response headers

Custom request/response headers can be added to a request/response that matches a specific route at the route, virtual host, and global route configuration level. See the relevant v1 and v2 API documentation.

Note: Headers are appended to requests/responses in the following order: route level headers, virtual host level headers and finally global level headers.

Envoy additionally supports adding dynamic values to the request headers. Supported dynamic values are:

The original client IP which is already added by Envoy as a x-forwarded-for request header.
The original protocol which is already added by Envoy as a x-forwarded-proto request header.