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Preserving Client IP in Kubernetes

When deploying applications in Kubernetes it's not uncommon to want to preserve the client's source IP address.

Given that you likely have a Service in front of your Pod it may not come as a surprise that preserving the client address isn't always trivial. This can often result in the Pod application seeing local network IPs as the client IP.

Preserved Client IP Support across vendors

IP preservation is something addressed in the Kubernetes documentation on external load balancers but there's a note that this may only be possible on GKE.

I'm currently using Digital Ocean, it turned out that enabling externalTrafficPolicy on my Service did not do what I wanted. Internal network IPs show up on my applications.

Digital Ocean are clearly aware of this need and have built a feature in to their platform to address this. This is detailed on their documentation on load balancers.

This is done by adding the following annotation to your service:

metadata:
  name: proxy-protocol
  annotations:
    service.beta.kubernetes.io/do-loadbalancer-enable-proxy-protocol: "true"

The bad news is that whilst this feature will cause the external Service to pass on the client IP address, it does so using the PROXY Protocol. If you're using nginx or something else that speaks PROXY, then you can stop reading - this should work with a quick setting tweak.

If you have an application that doesn't speak PROXY then read on.

Cue mmproxy

After some searching I was pleased to discover that someone else had had this problem and solved it!

The open source project mmproxy tackles exactly this challenge. It acts as a go between - understanding PROXY protocol and doing some iptables tricks to pass it on to the server.

But can we make it work in a kubernetes cluster? After some experimenting, I'm pleased to report that I was able to get this working on a normal Digital Ocean Kubernetes Cluster. Here's some config that worked for me.

This creates a service set up to use the PROXY protocol (because of annotations and externalTrafficPolicy):

apiVersion: v1
kind: Service
metadata:
  name: mmproxy
  annotations:
    service.beta.kubernetes.io/do-loadbalancer-enable-proxy-protocol: "true"
spec:
  type: LoadBalancer
  ports:
  - port: 9001
    protocol: TCP
    targetPort: 9001

  externalTrafficPolicy: Local
  selector:
    app: mmproxy

Next we create a deployment to receive the traffic to mmproxy and forward onward within the same pod:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: mmproxy
spec:
  replicas: 1
  selector:
    matchLabels:
      app: mmproxy
  template:
    metadata:
      labels:
        app: mmproxy
    spec:
      initContainers:
      - name: setup
        image: docker.pkg.github.com/andrewmichaelsmith/mmproxy/mmproxy:latest
        command: [ "/bin/bash", "-cx" ]
        args:
          # Source: https://github.com/cloudflare/mmproxy
          - echo 1 > /proc/sys/net/ipv4/conf/eth0/route_localnet;
            iptables -t mangle -I PREROUTING -m mark --mark 123 -m comment --comment mmproxy -j CONNMARK --save-mark;
            ip6tables -t mangle -I PREROUTING -m mark --mark 123 -m comment --comment mmproxy -j CONNMARK --save-mark;
            iptables -t mangle -I OUTPUT -m connmark --mark 123 -m comment --comment mmproxy -j CONNMARK --restore-mark;
            ip6tables -t mangle -I OUTPUT -m connmark --mark 123 -m comment --comment mmproxy -j CONNMARK --restore-mark;
            ip rule add fwmark 123 lookup 100;
            ip -6 rule add fwmark 123 lookup 100;
            ip route add local 0.0.0.0/0 dev lo table 100;
            ip -6 route add local ::/0 dev lo table 100;
            exit 0 ; # XXX hack
        securityContext:
          privileged: True
      containers:
      - name: netcat
        image: docker.pkg.github.com/andrewmichaelsmith/mmproxy/mmproxy:latest
        command: [ "/bin/bash", "-cx" ]
        args:
          - apt-get install -y netcat;
            while true; do nc -l -vv -p 9002; done;
      - name: mmproxy
        image: docker.pkg.github.com/andrewmichaelsmith/mmproxy/mmproxy:latest
        command: [ "/bin/bash", "-cx" ]
        args:
          - echo "0.0.0.0/0" > allowed-networks.txt;
            /mmproxy/mmproxy --allowed-networks allowed-networks.txt -l 0.0.0.0:9001 -4 127.0.0.1:9002 -6 [::1]:9002;
        securityContext:
          privileged: True

So here we have:

  • init container (setup): perform mmproxy setup (as per https://github.com/cloudflare/mmproxy).
  • proxy container (mmprxoy): run mmproxy, listen on 9001 forward to 9002.
  • app container (netcat): listen on 9002 and log connections.

Does it work? First we find the IP of our service:

$ kubectl get svc | grep mmproxy
mmproxy         LoadBalancer   10.245.126.223   157.245.27.182   9001:30243/TCP 5m27s

Then try and connect to it:

$ nc -v 157.245.27.182 9001   
Connection to 157.245.27.182 9001 port [tcp/*] succeeded!
HELLO

And check the logs:

+ nc -l -vv -p 9002
listening on [any] 9002 ...
connect to [127.0.0.1] from 44.125.114.38 59630
HELLO

Hurray! The actual client IP is preserved.

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