A very basic webserver that listens on port 80 and returns the apparent origin IP.

Built with gin.

Compiling requires Golang.

Clone the repo, move into the directory and run:

go build

This will compile to the binary file ipecho.

Port 80 requires root permissions, so you need to run the binary as root. For example:

# ./ipecho

Logging is done to STDOUT.

If you expect a reverse proxy to contact this application with headers that indicate the actual origin IP, such as X-Real-IP or X-Forwarded-For, you must indicate with a flag what the IP of this reverse proxy will be to mark it as trusted.

For example, if ipecho runs on 10.0.0.20 and a reverse proxy on 10.0.0.1, on the node that runs ipecho you would run:

# ./ipecho -proxy='10.0.0.1'

This flag can be specified multiple times to trust multiple proxies:

# ./ipecho -proxy='10.0.0.1' -proxy='10.0.0.2' (...)

$ curl -s localhost/json | jq
{
  "IP": "127.0.0.1",
  "USERAGENT": "curl/7.85.0"
}

$ curl -s localhost
127.0.0.1

In the same directory as Dockerfile, run:

$ docker build --tag ipecho .
(...)
Successfully built 2eeb58f23dac
Successfully tagged ipecho:latest

$ docker image ls
REPOSITORY   TAG               IMAGE ID       CREATED          SIZE
ipecho       latest            2eeb58f23dac   14 minutes ago   10.4MB

Start a container from the image (latest is implicit):

$ docker run --network=bridge -d ipecho
d240f5042683f299915b8c16f5c45cf2642ab4d69385c4c0c92a2523929ded1c

Figure out the IP and test it:

$ docker inspect d2 | grep IP
(...)
$ curl 172.17.0.2
172.17.0.1