http-dragonfly

This is a tiny service app to redirect, split or relay HTTP calls to one or more destinations based on flexible configuration.

Features

• Listen on one or more IP/ports pairs to serve calls. Each listener has its own configuration. Number of listeners is unlimited.
• Relay HTTP requests to one or more targets based on reach and flexible configuration set.
• Filter/restrict requests by methods, headers, body content and route it to all or conditionally selected targets.
• Transform request's headers, path and body in a flexible configurable way.
• Decide which response to send back based on the configured response strategy.
• Transform response headers and body.
• Propagate target's response status/headers/body or overwrite it.

Some typical use cases

Actually use cases are restricted by your fantasy only :)

Debugging:

• duplicate all queries from production environment to one or more test environments to evaluate new functionality with real production requests and workload
• duplicate specific conditionally selected queries to other target to analyze some issues dependent on content
• send web-hooks to several (test) environments simultaneously

Logging/observing/learning:

• duplicate queries to different log storages without affecting of normal request flow: i.e. log body to ElasticSearch, source IPs and user agent to some simple logger
• duplicate some conditionally selected queries (for example based on source IP or query string) to special endpoint to alert about abuse or intrusion attempt
• stream all (or selected) requests to machine learning system without affecting normal processing

Routing:

• route request to specific target based on query content

Usage

The easiest way to run exporter is to use Docker image. If you use Kubernetes to run workload you can use Helm chart to configure and deploy exporter. The third way to run exporter is to build native Rust binary using Cargo utility and run it.

Anyway, to run http-dragonfly we need a configuration file with listeners, targets, transformations and response strategy configured.

Docker image

Just use the following command to get usage help, the same as running it with --help command line option:

docker run --rm alexkarpenko/http-dragonfly:latest

Typical output is:

HTTP requests splitter/router/relay

Usage: http-dragonfly [OPTIONS] --config <CONFIG>

Options:
  -d, --debug
          Enable extreme logging (debug)
  -v, --verbose
          Enable additional logging (info)
  -j, --json-log
          Write logs in JSON format
  -c, --config <CONFIG>
          Path to config file
  -e, --env-mask <ENV_MASK>
          Allowed environment variables mask (regex) [default: ^HTTP_ENV_[a-zA-Z0-9_]+$]
  -p, --health-check-port <HEALTH_CHECK_PORT>
          Enable health check responder on the specified port
  -h, --help
          Print help
  -V, --version
          Print version

The only mandatory parameter is a path to configuration file. Detailed explanation of all possible configuration options is in the dedicated Concepts and Configuration section. Just for test purpose, there is an example minimal config file to listen on default 8080 port and forward requests to http://www.google.com/. To use it:

docker run --rm --name http-dragonfly -v $PWD/config.yaml:/config.yaml alex-karpenko/http-dragonfly:latest --config /config.yaml -v

Helm chart

To add Helm repository:

helm repo add alex-karpenko https://alex-karpenko.github.io/helm-charts
helm repo update

To deploy release create your own values file with overrides of the default values and your own config section and deploy Helm release to your K8s cluster:

helm install http-dragonfly alex-karpenko/http-dragonfly -f my-values.yaml

For example your values can be like below.

# info or debug, anything else - warning
logLevel: info

service:
  type: ClusterIP
  healthCheck:
    port: 3000 # actually this port is for health checks only
    expose: false
  listeners: # these ports are for listeners
  - port: 8000
    name: test-listener-1 # name is optional

config:
  listeners:
    - id: test-google
      listen_on: "*:8000"
      timeout: 5s
      methods:
      - GET
      strategy: failed_then_ok
      targets:
        - url: https://www.google.com/
          id: google
      response:
        override:
          headers:
          - add: x-response-target-id
            value: ${CTX_TARGET_ID}

Build your own binary

Since exported is written in Rust, you can use standard Rust tools to build binary for any platform you need. Of course, you have to have Rust tool-chain installed.

cargo build --release

And run it:

target/release/http-dragonfly --config ./config.yaml -v

Concepts and Configuration

Configuration is a yaml file with a list of listeners as a root element.

Listener

Each listener has a handler which listens on specific IP and port and do the following:

• accept incoming TCP connections
• get incoming request
• transform it if needed
• verify conditions
• send new request(s) to all configured and allowed targets
• wait for response(s) from all targets
• select one of the obtained or create new response according to configured strategy
• transform it if needed
• and finally send it back to the requester

Contexts

Context is a set of variables (like Unix environment variables) attached to each request. Those variables define request's environment that can be used almost everywhere in configuration file to substitute parameters and actually to impact configuration and request's content on the fly.

Just few examples how to use contexts:

• insert Authorization header from OS environment variable instead of committing secret value to config.
• change target's URI based on original URI parameters or headers like add path or query string.
• insert some information about query into request or response headers: selected target, source IP, etc.
• create request or response body based on request headers or OS environment

There are four contexts depending on query stage:

Context type	Variables	Description
Application	CTX_APPLICATION_NAME	Name of this app, http-dragonfly
	CTX_APPLICATION_VERSION	Version of the app
	OS environment variables	All OS environment variables which names satisfy restriction mask from the command line (default mask is ^HTTP_ENV_[a-zA-Z0-9_]+$])
Request	CTX_LISTENER_NAME	ID of the listener which accepted the request
	CTX_REQUEST_SOURCE_IP	Client's source IP address
	CTX_REQUEST_METHOD	Request method
	CTX_REQUEST_HOST	URL host name from the original request
	CTX_REQUEST_PATH	URL path from the original request
	CTX_REQUEST_QUERY	URL query string from the original request
	CTX_REQUEST_HEADERS_<UPPERCASE_HEADER_NAME>	Each request's header has it's context variable
Target	CTX_TARGET_ID	ID of the target which response will be returned back
	CTX_TARGET_HOST	Host name of the selected target
Response	CTX_RESPONSE_HEADERS_<UPPERCASE_HEADER_NAME>	Each response's header has it's context variable
	CTX_RESPONSE_STATUS	Status returned by target query

To use context variables in the config just specify it similar to the bash variables (all shell expressions work). Few obvious examples, more realistic examples you can see in this file below:

• ${CTX_APPLICATION_NAME}
• ${CTX_REQUEST_QUERY:-}
• ${CTX_TARGET_ID:-unknown}

Important notes:

• Target and responses context's variables are undefined in response context for strategies like *_override because there is no response target exists.
• Each context includes context of previous stage: request includes application, target includes request, response includes target (except above note).
• Using of OS environment variables is restricted to specified mask to avoid including of all app environment to the context and unintentional exposing of the run-time environment state.
• If context variable is not defined and there is no default value in the expression then error won't be raised and variable won't be substituted and whole expression will be left as is.
• There are some special cases where context variables can't be used and will be ignored (lft as is), see notes in particular configuration sections.

Variables substitution stages:

• application context applies to whole config file just after loading before parsing and validating.
• request context applies to each request after receiving before headers and body transformations.
• target context applies to it's target before targe's headers, body and URL transformation.
• response context applies during response override process before headers and body transformation.

Listener configuration

Each listener has the following configuration parameters:

• id: unique name of the listener.
• listen_on: IP address and port to listen on.
• timeout: time to wait for request/headers/body.
• methods: list of allowed HTTP methods to pass through this listener.
• strategy: response strategy to select which target(s) to use and which response to send back.
• headers: list of transformations to apply to request headers before pass it to targets.
• targets: list of targets to query for responses.
• response: specification of response transformations.

Listener: id

Format: string.

Default: LISTENER-<IP>:<PORT> where IP and PORT are values from listen_on parameter.

Each listener has to have its own unique name (ID) to distinguish listeners at least in logs. This is an optional parameter with default to LISTENER-<IP>:<PORT> where IP and PORT are values from listener's listen_on parameter.

So if you omit this parameter it'll be defaulted to unique value but if you prefer to see some reasonable value in logs and config you should set this parameter.

Listener: listen_on

Format: <IP>:<PORT>, IP - any valid IP v4, or 0.0.0.0 or * for all host's IP addresses; port is an integer in the range 1..65535.

Default: 0.0.0.0:8080

It's obvious that each listener accepts connections on its own IP and port. If you have more than one listener in the config you have to specify this parameter at least for all non-default listeners.

Listener: timeout

Format: human readable time interval, like 5s, 1m30s, etc.

Default: 10s

This time is an interval between accepting incoming connection and getting request's data like headers and/or body. If remote side hasn't sent any data during this interval connection will be dropped without response.

Listener: methods

Format: list, allowed values are GET, POST, PUT, PATCH, DELETE, OPTIONS, HEAD

Default: empty - all method are allowed, although there is no option like ANY or ALL

This is first filter for incoming requests: all request with method not in the list will be rejected with 405 status (method not allowed). If you want to accept all methods on the listener just don't specify this parameter, that means everything is allowed.

Example:

methods:
  - GET
  - POST
  - OPTIONS

Listener: strategy

Format: see below list of allowed values.

Default: failed_then_override

Strategy is about how to decide which target(s) to query and which response to send back. This is one of the crucial listener's config parameter. Generally all strategies can be divided into four groups by prefixes:

• always - regardless of any obtained responses from the targets we should always send back something else (override) or unconditional (e.g. response form some specified target)
• ok - we respond with any successful response if we got at least one successful status from any target, but if all targets are failed (regardless of kind of failure) we should return something else
• failed - like previous but vise versa: we respond with any failed response if we got at least one failure status from any target, but if all targets are ok we should return something else
• conditional_routing - we query single target only which satisfies some condition (see below) and return its response.

Strategy name	How it works
always_override	Query all allowed targets (see explanation of allowed targets below the table) but return response defined in response.override section (see below)
always_target_id	Query all allowed targets but return response from one specific target regardless of it's status
ok_then_failed	Query all allowed targets, if at least on response is successful - return any successful one, if all responses are failed - return any failed response
ok_then_target_id	Query all allowed targets, if at least on response is successful - return any successful one, if all responses are failed - return response from one specific target regardless of it's status
ok_then_override	Query all allowed targets, if at least on response is successful - return any successful one, if all responses are failed - return response defined in response.override section (see below)
failed_then_ok	Query all allowed targets, if at least one query is failed - return any failed response, if all responses are successful - return any successful one
failed_then_target_id	Query all allowed targets, if at least one query is failed - return any failed response, if all responses are successful - return response from one specific target regardless of it's status
failed_then_override	Query all allowed targets, if at least one query is failed - return any failed response, if all responses are successful - return response defined in response.override section (see below). This is default behavior: query everything, return fail if failed or return some predefined OK response if everything is good.
conditional_routing	Select single target to query based on conditions (see targets config below), query it and return it's response.

Any target may have condition parameter which restricts allowance of the target to query it. This condition is predicate based on request's headers or body content. If target's condition is false that target will be excluded from allowed list and won't be queried. Exception is conditional_routing strategy where all targets must have conditions but after evaluation of all conditions only single one can be true and only this one target will be queried for response. More about conditions configuration is in the targets config section.

Listener: headers

Format: list of objects.

Default: empty (don't touch headers except Host).

This parameter is a list of transformations which should be applied to the original request's headers before proceeding to querying and evaluating targets. Using this config we can add, change or drop some headers from request. Each element of list is a single transformation action, actions will be applied in the list order.

By default all (untransformed) headers will be passed to targets without changes.

Possible transformations:

• add - creates new header with specified name and value, if header already exists - transformation will be ignored.
• update - change value of existing header with specified name, if header doesn't exist - transformation will be ignored.
• drop - drops header with specified name if it exists. Special case is * name which drops all headers.

Examples:

headers:
  - add: X-Some-Fun
    value: "123"
  - drop: Content-Type
  - add: Accept
    value: "*"

Important note:

• if you strictly need to set header to some specific value regardless of header's presence you should add two consecutive actions - add and update with the same value, order doesn't matter: if you try to update non-existent header - new one won't appear, if you try to add existing header - value won't be changed.
• if you need to guarantee some stable set of headers instead of requested, just drop all headers (drop: "*") as first action and add all needed ones as following actions.

Listener: targets

Format: list of object.

Default: no defaults, at least one target should be defined.

Target config includes following parameters:

• id: unique (among the listener's targets) target name/ID, default is TARGET-<url>
• url: full URL of the target
• headers: target's headers transformations, like request's config, empty by default
• body: create new body if defined, or pass original body by default
• timeout: time to wait for response from the target, like listener's config, default is 60s
• on_error: what to do if error occurred during request, default is propagate, see explanation below
• error_status: what status should be returned from the target if on_error set to status, usually (but not mandatory) this is something like 500
• condition: predicate expression to calculate before request, if value is false this target will be excluded from the list of allowed targets, default is true, see details below

Listener: targets.on_error

Parameter on_error defines target's behavior in case of any error like request timeout, network error, application problems or any other cases when getting response (even unsuccessful) is impossible. Possible values:

• propagate: return reasonable status code which points to cause of error (if possible), this is some 5xx status.
• status: regardless of error nature return some specific status code which should be defined in error_status parameter.
• drop: remove this target from the list of responses - it won't be even considered as possible response during post-processing of results.

Listener: targets.condition

Obviously this parameter defines some conditional expression (predicate) which should be true to allow querying of this particular target. So condition expression have to be evaluated to boolean only. Expression syntax is the same as jq utility has. Root contains the following objects:

• body: JSON body on original request (before body processing if target.body is defined)
• env: target request context - list of name/value pairs of environment variables (before applying target's context)
• request: complex object with original request's attributes (before applying of any target's transformation)
  • headers: list of name/value pairs with request headers (headers names are in lower case)
  • uri: complex object
    • full: full URI staring
    • host: host part of URI
    • path: path
    • query: query string of URI

Special case of condition expression (and actually default value) is word default instead of predicate, that means this condition is true and target have to be queried anyway. This condition expression should be used with conditional_routing strategy to mark target which will be queried in case if all other conditions are false. If strategy is conditional_routing there can be only one target with default condition.

Some additional explanation:

• if strategy is any except conditional_routing:
  • condition is empty (absent) or default - target will be queried anyway
  • condition is defined as jq expression - target will be queried in case of expression evaluates to true only
• if strategy is conditional_routing:
  • only one condition is true - this target will be queried
  • all conditions are false and there is one target with default condition - this default will be queried
  • all conditions are false and there is no targets with default condition - error will be propagated
  • more than one condition is true - error will be propagated

In other words conditional_routing ensure querying of single target only which satisfy it's condition.

Condition examples:

• .env["CTX_REQUEST_HOST"] == "www.google.com"
• .request.headers["x-auth-token"] != ""
• .body.some.body.int.value == 5
• .body.data.products[]|length > 0
• default

Listener: target config examples

Query www.example.com if request has any non-empty path and forward all requests to logger unconditionally:

listeners:
- targets:
  - url: https://www.example.com${CTX_REQUEST_PATH:-"/"}
    id: query
    condition: .env[CTX_REQUEST_PATH] != ""
  - url: http://query-logger:9090/

Query www.example.com if request'a header X-Route-To-Query is true and forward all requests to logger unconditionally:

listeners:
- targets:
  - url: https://www.example.com${CTX_REQUEST_PATH:-"/"}
    id: query
    condition: .request.headers["x-route-to-query"] == "true"
  - url: http://query-logger:9090/

Conditional routing depending on value in body with logging of unknown queries and transformation of queries:

listeners:
- strategy: conditional_routing
  targets:
  - url: https://www.example.com/path-1
    condition: .body.data.value == 1
    id: query-1
    body: |
      {
        "query": "${CTX_REQUEST_QUERY:-}",
        "path": "${CTX_REQUEST_PATH:-}"
      }
    headers:
    - drop: content-length
    - drop: content-type
    - add: content-type
      value: application/json
  - url: https://www.example.com/path-2
    id: query-2
    condition: .body.data.value == 2
    headers:
    - update: User-Agent
      value: ${CTX_APP_NAME}/${CTX_APP_VERSION}
  - url: https://www.example.com/path-3
    id: query-3
    condition: .body.data.value == 3
    on_error: status
    error_status: 200
  - url: http://bad-queries-logger:3333/
    headers:
    - drop: Authorization
    condition: default

Important notes:

• if you change request body don't forget to drop content-length header and add/update content-type header, otherwise request handler will panic due to request inconsistency.

Listener: response

Format: object definition.

Default:

response:
  failed_status_regex: "4\d{2}|5\d{2}"
  no_targets_status: 500

This parameter defines how to transform or override (create) response before returning it to the requester. Allowed parameters are:

• target_selector: target ID to select for response in case of *_target_id strategy is configured, this parameter is mandatory for such strategies and allowed in this case only.
• failed_status_regex: regex to asses if response status should be interpreted as failed, reasonable default includes all 4xx and 5xx statuses.
• no_targets_status: which status code should be returned in case when no targets to query (all conditions are false) or all responses were dropped due to on_error: drop target's parameter and strategy is *_target_id or *_ok.
• override: response override config (see below), optional

Response override config intended to provide custom (overridden) response parts such as body, headers and status code. So you can define three parameters here:

• body: overrides body content in response
• headers: defines headers transformations similar to this
• status: set particular response status instead of original value

Important notes:

• default behavior of those overrides is to pass original content of body and headers and status. But for all *_override strategies defaults is empty body, empty headers and status 200. So if you need to create exactly new response instead of the one obtained from some target, you can (or have to) define those overrides. If you omit this config than *_override strategy returns empty response with 200 status.
• if you change response body don't forget to drop content-length header and add/update content-type header, otherwise request handler will panic due to response inconsistency.

Huge configuration example

Below is an example of almost all possible configuration parameters with some explanations.

listeners:
  - id: Listener-8080 # default is LISTENER-<on value>
    listen_on: "*:8080" # or ip:port like 1.2.3.4:1234, or just port number
    timeout: 10s
    methods: # default is empty list that means "any method"
      - GET
      - POST
    strategy: failed_then_override # default

    headers:
      - drop: "*" # special case, default is preserve everything except Host
      - add: X-Added-Header
        value: something
      - update: Authorization
        value: ${HTTP_ENV_SOME_AUTH_TOKEN}
      - drop: X-Forwarded-For
      - update: User-Agent
        value: "this was ${CTX_REQUEST_HEADERS_USER_AGENT}"

    targets:
      - id: Target-0 # default is TARGET-<url value>
        url: https://qqq.www.com/
        timeout: 60s
        body: '{"method": "${CTX_REQUEST_METHOD}"}'
        headers:
          - drop: content-length
          - drop: content-type
          - add: content-type
            value: application/json
        on_error: status
        error_status: 555
      - id: Target-1
        condition: .body.target == "1"
        url: https://qqq.www.com/${CTX_REQUEST_PATH}
      - id: Target-2
        url: https://qqq.www.com/${CTX_REQUEST_PATH}?${CTX_REQUEST_QUERY}
        headers:
          - update: Authorization
            value: ${HTTP_ENV_TARGET_2_AUTH_TOKEN}
          - drop: X-Added-Header

    response:
      failed_status_regex: "3\\d{2}|4\\d{2}|5\\d{2}"
      no_targets_status: 500

      override:
        # "200 OK" is default for *_override
        # for other strategies - no default, so status will be preserved from response or overridden if value is defined
        status: 200
        # empty is default for *_override
        # ${CTX_RESPONSE_BODY} is for other strategies
        body: |
          {"status": "ok"}
        headers: # default is to preserve original headers or empty if strategy is one of *_override
          - drop: content-length
          - drop: content-type
          - add: content-type
            value: application/json
          - add: X-Http-Dragonfly-Version
            value: ${CTX_APP_VERSION}
          - add: X-Http-Dragonfly-Response-Source
            value: ${CTX_TARGET_ID:-unknown}

  - id: Condition-plus-target_id-8081
    listen_on: "*:8081"
    timeout: 5s
    strategy: always_target_id
    methods:
      - PUT
      - POST
    targets:
      - id: Target-0
        condition: .body.target == "0"
        url: https://qqq.www.com/
        timeout: 60s
      - id: Target-1
        condition: .headers["qqq"] == "WWW"
        url: https://qqq.www.com/${CTX_REQUEST_PATH}
      - id: Target-2
        condition: default
        url: https://qqq.www.com/${CTX_REQUEST_PATH}?${CTX_REQUEST_QUERY}
        headers:
          - add: X-Http-Gorgona
            value: default target
    response:
      target_selector: Target-0

      override:
        headers:
          - add: X-Http-Conditional-Response-Source
            value: ${CTX_TARGET_ID}

  - id: Router-8082
    listen_on: "*:8082"
    strategy: conditional_routing
    targets:
      - id: Target-0
        condition: .body.target == "0" and .env["CTX_REQUEST_METHOD"] != "POST"
        url: https://qqq.www.com/
        timeout: 60s
      - id: Target-1
        condition: .headers["qqq"] == "WWW" and .env["CTX_REQUEST_METHOD"] != "POST"
        url: https://qqq.www.com/${CTX_REQUEST_PATH}
      - id: Target-2
        condition: .env["CTX_REQUEST_METHOD"] == "POST"
        url: https://qqq.www.com/${CTX_REQUEST_PATH}?${CTX_REQUEST_QUERY}
    response:
      no_targets_status: 200

      override:
        headers:
          - add: X-Http-Conditional-Response-Source
            value: ${CTX_TARGET_ID}