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).

user-agent

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.

server

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

x-client-trace-id

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.

x-envoy-downstream-service-cluster

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.

x-envoy-downstream-service-node

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.

x-envoy-external-address

It is a common case where a service wants to perform analytics based on the origin client’s IP address. Per the lengthy discussion on XFF, this can get quite complicated, so Envoy simplifies this by setting x-envoy-external-address to the trusted client address if the request is 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.

x-envoy-force-trace

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.

x-envoy-internal

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-envoy-original-dst-host

The header used to override destination address when using the Original Destination load balancing policy.

It is ignored, unless the use of it is enabled via use_http_header.

x-forwarded-client-cert

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 (URI type) of the current proxy’s certificate.
  2. Hash The SHA 256 diguest of the current client certificate.
  3. Cert The entire client certificate in URL encoded PEM format.
  4. Subject The Subject field of the current client certificate. The value is always double-quoted.
  5. URI The URI type Subject Alternative Name field of the current client certificate.
  6. DNS The DNS type Subject Alternative Name field of the current client certificate. A client certificate may contain multiple DNS type Subject Alternative Names, each will be a separate key-value pair.

A client certificate may contain multiple Subject Alternative Name types. For details on different Subject Alternative Name types, please refer RFC 2459.

Some examples of the XFCC header are:

  1. For one client certificate with only URI type Subject Alternative Name: x-forwarded-client-cert: By=http://frontend.lyft.com;Hash=468ed33be74eee6556d90c0149c1309e9ba61d6425303443c0748a02dd8de688;Subject="/C=US/ST=CA/L=San Francisco/OU=Lyft/CN=Test Client";URI=http://testclient.lyft.com
  2. For two client certificates with only URI type Subject Alternative Name: x-forwarded-client-cert: By=http://frontend.lyft.com;Hash=468ed33be74eee6556d90c0149c1309e9ba61d6425303443c0748a02dd8de688;URI=http://testclient.lyft.com,By=http://backend.lyft.com;Hash=9ba61d6425303443c0748a02dd8de688468ed33be74eee6556d90c0149c1309e;URI=http://frontend.lyft.com
  3. For one client certificate with both URI type and DNS type Subject Alternative Name: x-forwarded-client-cert: By=http://frontend.lyft.com;Hash=468ed33be74eee6556d90c0149c1309e9ba61d6425303443c0748a02dd8de688;Subject="/C=US/ST=CA/L=San Francisco/OU=Lyft/CN=Test Client";URI=http://testclient.lyft.com;DNS=lyft.com;DNS=www.lyft.com

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

x-forwarded-for

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: 50.0.0.1 (single client)
  2. x-forwarded-for: 50.0.0.1, 40.0.0.1 (external proxy hop)
  3. x-forwarded-for: 50.0.0.1, 10.0.0.1 (internal proxy hop)

Envoy will only append to XFF if the use_remote_address HTTP connection manager option is set to true and the skip_xff_append <envoy_api_field_config.filter.network.http_connection_manager.v2.HttpConnectionManager.skip_xff_append> is set false: This means that if use_remote_address is false (which is the default) or skip_xff_append is true, the connection manager operates in a transparent mode where it does not modify XFF.

Attention

In general, use_remote_address should be set to true when Envoy is deployed as an edge node (aka a front proxy), whereas it may need to be set to false when Envoy is used as an internal service node in a mesh deployment.

The value of use_remote_address controls how Envoy determines the trusted client address. Given an HTTP request that has traveled through a series of zero or more proxies to reach Envoy, the trusted client address is the earliest source IP address that is known to be accurate. The source IP address of the immediate downstream node’s connection to Envoy is trusted. XFF sometimes can be trusted. Malicious clients can forge XFF, but the last address in XFF can be trusted if it was put there by a trusted proxy.

Envoy’s default rules for determining the trusted client address (before appending anything to XFF) are:

  • If use_remote_address is false and an XFF containing at least one IP address is present in the request, the trusted client address is the last (rightmost) IP address in XFF.
  • Otherwise, the trusted client address is the source IP address of the immediate downstream node’s connection to Envoy.

In an environment where there are one or more trusted proxies in front of an edge Envoy instance, the xff_num_trusted_hops configuration option can be used to trust additional addresses from XFF:

  • If use_remote_address is false and xff_num_trusted_hops is set to a value N that is greater than zero, the trusted client address is the (N+1)th address from the right end of XFF. (If the XFF contains fewer than N+1 addresses, Envoy falls back to using the immediate downstream connection’s source address as trusted client address.)
  • If use_remote_address is true and xff_num_trusted_hops is set to a value N that is greater than zero, the trusted client address is the Nth address from the right end of XFF. (If the XFF contains fewer than N addresses, Envoy falls back to using the immediate downstream connection’s source address as trusted client address.)

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

Example 1: Envoy as edge proxy, without a trusted proxy in front of it
Settings:
use_remote_address = true
xff_num_trusted_hops = 0
Request details:
Downstream IP address = 192.0.2.5
XFF = “203.0.113.128, 203.0.113.10, 203.0.113.1”
Result:
Trusted client address = 192.0.2.5 (XFF is ignored)
X-Envoy-External-Address is set to 192.0.2.5
XFF is changed to “203.0.113.128, 203.0.113.10, 203.0.113.1, 192.0.2.5”
X-Envoy-Internal is removed (if it was present in the incoming request)
Example 2: Envoy as internal proxy, with the Envoy edge proxy from Example 1 in front of it
Settings:
use_remote_address = false
xff_num_trusted_hops = 0
Request details:
Downstream IP address = 10.11.12.13 (address of the Envoy edge proxy)
XFF = “203.0.113.128, 203.0.113.10, 203.0.113.1, 192.0.2.5”
Result:
Trusted client address = 192.0.2.5 (last address in XFF is trusted)
X-Envoy-External-Address is not modified
X-Envoy-Internal is removed (if it was present in the incoming request)
Example 3: Envoy as edge proxy, with two trusted external proxies in front of it
Settings:
use_remote_address = true
xff_num_trusted_hops = 2
Request details:
Downstream IP address = 192.0.2.5
XFF = “203.0.113.128, 203.0.113.10, 203.0.113.1”
Result:
Trusted client address = 203.0.113.10 (2nd to last address in XFF is trusted)
X-Envoy-External-Address is set to 203.0.113.10
XFF is changed to “203.0.113.128, 203.0.113.10, 203.0.113.1, 192.0.2.5”
X-Envoy-Internal is removed (if it was present in the incoming request)
Example 4: Envoy as internal proxy, with the edge proxy from Example 3 in front of it
Settings:
use_remote_address = false
xff_num_trusted_hops = 2
Request details:
Downstream IP address = 10.11.12.13 (address of the Envoy edge proxy)
XFF = “203.0.113.128, 203.0.113.10, 203.0.113.1, 192.0.2.5”
Result:
Trusted client address = 203.0.113.10
X-Envoy-External-Address is not modified
X-Envoy-Internal is removed (if it was present in the incoming request)
Example 5: Envoy as an internal proxy, receiving a request from an internal client
Settings:
use_remote_address = false
xff_num_trusted_hops = 0
Request details:
Downstream IP address = 10.20.30.40 (address of the internal client)
XFF is not present
Result:
Trusted client address = 10.20.30.40
X-Envoy-External-Address remains unset
X-Envoy-Internal is set to “true”
Example 6: The internal Envoy from Example 5, receiving a request proxied by another Envoy
Settings:
use_remote_address = false
xff_num_trusted_hops = 0
Request details:
Downstream IP address = 10.20.30.50 (address of the Envoy instance proxying to this one)
XFF = “10.20.30.40”
Result:
Trusted client address = 10.20.30.40
X-Envoy-External-Address remains unset
X-Envoy-Internal is set to “true”

A few very important notes about XFF:

  1. If use_remote_address is set to true, Envoy sets the x-envoy-external-address header to the trusted client address.
  1. XFF is what Envoy uses to determine whether a request is internal origin or external origin. If use_remote_address is set to true, the request is internal if and only if the request contains no XFF and the immediate downstream node’s connection to Envoy has an internal (RFC1918 or RFC4193) source address. If use_remote_address is false, the request is internal if and only if XFF contains a single RFC1918 or RFC4193 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.
  2. Testing IPv6 in a large multi-hop system can be difficult from a change management perspective. For testing IPv6 compatibility of upstream services which parse XFF header values, represent_ipv4_remote_address_as_ipv4_mapped_ipv6 can be enabled in the v2 API. Envoy will append an IPv4 address in mapped IPv6 format, e.g. ::FFFF:50.0.0.1. This change will also apply to x-envoy-external-address.

x-forwarded-proto

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.

x-request-id

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:

x-ot-span-context

The x-ot-span-context HTTP header is used by Envoy to establish proper parent-child relationships between tracing spans when used with the LightStep tracer. 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.

x-b3-traceid

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 <https://github.com/openzipkin/b3-propagation>.

x-b3-spanid

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 <https://github.com/openzipkin/b3-propagation>.

x-b3-parentspanid

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 <https://github.com/openzipkin/b3-propagation>.

x-b3-sampled

The x-b3-sampled HTTP header is used by the Zipkin tracer in Envoy. When the Sampled flag is either not specified or set to 1, the span 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 <https://github.com/openzipkin/b3-propagation>.

x-b3-flags

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 <https://github.com/openzipkin/b3-propagation>.

b3

The b3 HTTP header is used by the Zipkin tracer in Envoy. Is a more compressed header format. See more on zipkin tracing here <https://github.com/openzipkin/b3-propagation#single-header>.

x-datadog-trace-id

The x-datadog-trace-id HTTP header is used by the Datadog tracer in Envoy. The 64-bit value represents the ID of the overall trace, and is used to correlate the spans.

x-datadog-parent-id

The x-datadog-parent-id HTTP header is used by the Datadog tracer in Envoy. The 64-bit value uniquely identifies the span within the trace, and is used to create parent-child relationships between spans.

x-datadog-sampling-priority

The x-datadog-sampling-priority HTTP header is used by the Datadog tracer in Envoy. The integer value indicates the sampling decision that has been made for this trace. A value of 0 indicates that the trace should not be collected, and a value of 1 requests that spans are sampled and reported.

Custom request/response headers

Custom request/response headers can be added to a request/response at the weighted cluster, route, virtual host, and/or global route configuration level. See the v2 API documentation.

No :-prefixed pseudo-header may be modified via this mechanism. The :path and :authority headers may instead be modified via mechanisms such as prefix_rewrite and host_rewrite.

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

Envoy supports adding dynamic values to request and response headers. The percent symbol (%) is used to delimit variable names.

Attention

If a literal percent symbol (%) is desired in a request/response header, it must be escaped by doubling it. For example, to emit a header with the value 100%, the custom header value in the Envoy configuration must be 100%%.

Supported variable names are:

%DOWNSTREAM_REMOTE_ADDRESS_WITHOUT_PORT%

Remote address of the downstream connection. If the address is an IP address the output does not include port.

Note

This may not be the physical remote address of the peer if the address has been inferred from proxy proto or x-forwarded-for.

%DOWNSTREAM_LOCAL_ADDRESS%
Local address of the downstream connection. If the address is an IP address it includes both address and port. If the original connection was redirected by iptables REDIRECT, this represents the original destination address restored by the Original Destination Filter using SO_ORIGINAL_DST socket option. If the original connection was redirected by iptables TPROXY, and the listener’s transparent option was set to true, this represents the original destination address and port.
%DOWNSTREAM_LOCAL_ADDRESS_WITHOUT_PORT%
Same as %DOWNSTREAM_LOCAL_ADDRESS% excluding port if the address is an IP address.
%PROTOCOL%
The original protocol which is already added by Envoy as a x-forwarded-proto request header.
%UPSTREAM_METADATA([“namespace”, “key”, …])%
Populates the header with EDS endpoint metadata from the upstream host selected by the router. Metadata may be selected from any namespace. In general, metadata values may be strings, numbers, booleans, lists, nested structures, or null. Upstream metadata values may be selected from nested structs by specifying multiple keys. Otherwise, only string, boolean, and numeric values are supported. If the namespace or key(s) are not found, or if the selected value is not a supported type, then no header is emitted. The namespace and key(s) are specified as a JSON array of strings. Finally, percent symbols in the parameters do not need to be escaped by doubling them.
%PER_REQUEST_STATE(reverse.dns.data.name)%
Populates the header with values set on the stream info filterState() object. To be usable in custom request/response headers, these values must be of type Envoy::Router::StringAccessor. These values should be named in standard reverse DNS style, identifying the organization that created the value and ending in a unique name for the data.
%START_TIME%

Request start time. START_TIME can be customized with specifiers as specified in access log format rules.

An example of setting a custom header with current time in seconds with the milliseconds resolution:

route:
  cluster: www
  request_headers_to_add:
    - header:
        key: "x-request-start"
        value: "%START_TIME(%s.%3f)%"
      append: true