I’m using Traefik v2.2 as a reverse proxy for my docker containers. Basically all HTTP or HTTPS traffic is handled by Traefik as an ingress container and then routing according to rules defined in my docker-compose file to the appropriate internal container.

Something that I’ve needed to do for a project is add a header to an inbound request in order to identify that the request has been processed by traefik. I tried to follow the documentation but found it… lacking. For anyone interested in the official documentation for adding headers to Traefik you can find it here: https://docs.traefik.io/middlewares/headers/

What I’ve ended up with is a service container (nginx in my case) that looks like this:

nginx:
        image: nginx:stable-alpine
        volumes:
            - ./src:/var/www
            - ./.docker/nginx/default.conf:/etc/nginx/conf.d/default.conf
        labels:
            - "traefik.enable=true"
            - "traefik.http.routers.router1.rule=Host(`localhost`)"
            - "traefik.http.middlewares.testHeader.headers.customrequestheaders.test-header=new-header"
            - "traefik.http.routers.router1.middlewares=testHeader"
    php:
        image: php:7.4-fpm
        volumes:
            - ./src:/var/www

Now something that threw me was the header “test_header” doesn’t exist in requests handled by nginx. A linked PHP container simply running print_r($_SERVER) dumps all the variables to the page. It’s only at this point I discovered that Traefik adds HTTP_ to the header. So instead of :

test-header = new-header

you get:

HTTP_TEST_HEADER = new-header

I think this is one of those things that you can waste alot of time on if you don’t know that Traefik is re-writing the header values in this way.

Looking back at the Traefik configuration, to add a header you basically have two steps:

First, create a new header. This is done in the following format:

traefik.http.middlewares.{your header group name}.headers.customrequestheaders.{header key}={header value}

As an example:

- "traefik.http.middlewares.testHeader.headers.customrequestheaders.test-header=new-header"

This creates a header `HTTP_TEST_HEADER` and assigns it to the `testHeader` middleware group.

Secondly, assign this middleware group to the router you’re using for your service. In my case that’s an nginx container on a router `router1`. This takes the following format:

traefik.http.routers.{router name}.middlewares={middleware name}

As an example:

- "traefik.http.routers.router1.middlewares=testHeader"

If you’re not sure what this looks like in the context of the other containers check the first, more complete example or see the entire docker-compose file below (NOTE: this file exposes the Traefik API for debugging purposes, so don’t blindly deploy it to production):

version: '3'

services:
    proxy:
        image: traefik:v2.2
        command:
            - "--providers.docker=true"
            - "--providers.docker.exposedbydefault=false"
            - "--entrypoints.web.address=:80"
            - "--api.insecure=true"
        ports:
            - "80:80"
            - "8080:8080"
        volumes:
            - "/var/run/docker.sock:/var/run/docker.sock:ro"

    nginx:
        image: nginx:stable-alpine
        volumes:
            - ./src:/var/www
            - ./.docker/nginx/default.conf:/etc/nginx/conf.d/default.conf
        labels:
            - "traefik.enable=true"
            - "traefik.http.routers.router1.rule=Host(`localhost`)"
            - "traefik.http.middlewares.testHeader.headers.customrequestheaders.test-header=new-header"
            - "traefik.http.routers.router1.middlewares=testHeader"
            - "traefik.http.middlewares.test-ratelimit.ratelimit.average=1"
            - "traefik.http.middlewares.test-ratelimit.ratelimit.period=1m"

    php:
        image: php:7.4-fpm
        volumes:
            - ./src:/var/www