Fake EC2 Metadata Service

You’re developing an app that will run in a VM locally but relies on the EC2 Metadata Service for some part of its behavior when running in Amazon’s cloud. How do you run the same thing locally?

Enter the Fake EC2 Metadata Service, a simple Sinatra app that exposes some of the functionality running on http://169.254.169.254.

Setup on Linux

Create a loopback interface bound to 169.254.169.254 (this probably needs to be run with root privileges):

ifconfig lo:0 169.254.169.254 netmask 255.255.255.255 up

Now run the app:

docker-compose up

Setup on Mac

Docker for Mac doesn’t support the same type of networking features that it does on Linux, so we need a different process.

On El Capitan and Sierra, we can redirect outgoing traffic intended for 169.254.169.254 back to the port forwarding set up by Docker.

+---------------------------------------------+          +-------------------------------------+
|HTTP Client                                  |          |pf                                   |
|  GET /latest/meta-data/local-ipv4 HTTP/1.1  +----------> Reroute outgoing traffic from       |
|  Host: 169.254.169.254                      |          | 169.254.169.254:80 to 127.0.0.1:80  |
|                                             |          +-------------------------+-----------+
+---------------------------------------------+                                    |
                                                                                   |
                                                                                   |
+---------------------------------------------+          +-------------------------v-------+
|Docker port forwarding                       |          |pf                               |
|  Forward traffic from 127.0.0.1:8169 to     <----------+ Redirect traffic from           |
|  published Container Port                   |          | 127.0.0.1:80 to 127.0.0.1:8169  |
|                                             |          +---------------------------------+
+---------------------+-----------------------+
                      |
                      |
       +--------------v---------------+
       |Fake EC2 Metadata Service     |
       |   200 OK                     |
       |   …                          |
       |                              |
       +------------------------------+

Create a file in /etc/pf.anchors/fake-ec2-metadata-service containing the following:

Packets = "proto tcp from any to 169.254.169.254 port 80"
rdr pass log on lo0 $Packets -> 127.0.0.1 port 8169
pass out log route-to lo0 inet $Packets keep state

Then, at the very bottom of /etc/pf.conf, load the pf rules:

load anchor "fake-ec2-metadata-service" from "/etc/pf.anchors/fake-ec2-metadata-service"

Immediately after the first Apple anchor, include the fake-ec2-metadata-service filter rules:

anchor "fake-ec2-metadata-service"

Immediately after the first Apple rdr-anchor, include the fake-ec2-metadata-service redirection rules:

rdr-anchor "fake-ec2-metadata-service"

The entire file should look something like this:

scrub-anchor "com.apple/*"
nat-anchor "com.apple/*"
rdr-anchor "com.apple/*"
rdr-anchor "fake-ec2-metadata-service"
dummynet-anchor "com.apple/*"
anchor "com.apple/*"
anchor "fake-ec2-metadata-service"
load anchor "com.apple" from "/etc/pf.anchors/com.apple"
load anchor "fake-ec2-metadata-service" from "/etc/pf.anchors/fake-ec2-metadata-service"

Load and enable the pf rules by executing

sudo pfctl -F all -f /etc/pf.conf
sudo pfctl -E

Run the container by executing

docker-compose up

Example Requests

curl http://169.254.169.254/latest/meta-data/local-ipv4
curl http://169.254.169.254/latest/meta-data/local-hostname
curl http://169.254.169.254/latest/meta-data/iam/security-credentials/
curl http://169.254.169.254/latest/meta-data/iam/security-credentials/default