Oskar Okuno's Blog

Syncthing Discovery Server in Docker with Caddy


On a journey to discover your own infrastructure

'big 1700s victorian sailing ship with a captain using binocular, other victorian ships in background, in the style of FAMOUS 19TH-CENTURY painter, impressionist oil painting'

Why run your own Discovery Server?

Short answer: Well, why not? Self hosting! Slightly longer answer: In theory it could speed up client discovery since you don't need to query the same instance(s) as everyone else on the planet. And it can also be more secure to opt out of the global discovery server and only rely on your own server to make your digital footprint slightly smaller. The official guide can be found here. Here is further reading on the security behind the Syncthing infrastructure.

1. Run Discovery Server as Docker Container

The docker container can be found in Docker Hub under the namesyncthing/discosrv. I am running it using the following bash script:


docker run \
  --name $NAME \
  -d \
  -p \
  --restart=unless-stopped \
  --network shared_docker-network \
  syncthing/discosrv:$VERSION -http \

Note that I am adding this container to a specific network and I am only exposing the port to localhost. In this network I am also running a caddy server which will be running a reverse proxy and handling the TLS/HTTPS requests. The discovery server needs TLS in order to operate securely. Since I am lazy I will use Caddy to solve this issue for me. All I need to do is to setup an A or CNAME record which points to my server and Caddy handles the certificates.

2. Setup Caddy as Reverse Proxy

As mentioned above, Caddy will handle TLS. The following Caddyfile will reverse proxy all request with your.server.com to the Syncthing Discover Server running as a Docker Container in the same Docker network as the Caddy Docker Container. This Caddyfile is inspired by this old Github comment. But, one of the header is outdated now. The new header_up line to be used is: header_up X-Tls-Client-Cert-Der-Base64 {http.request.tls.client.certificate_der_base64}. This is pointed out in a newer comment on a PR. Please see the whole file below

your.server.com {
  reverse_proxy syncthing-discovery-server:8443 {
    header_up X-Forwarded-For {http.request.remote.host}
    header_up X-Client-Port {http.request.remote.port}
    header_up X-Tls-Client-Cert-Der-Base64 {http.request.tls.client.certificate_der_base64}

  tls {
    client_auth {
      mode request

3. Point your Syncthing Client to your Discovery Server

Go to Settings > Connections > Global Discovery Servers and make a comma and add the full URL (e.g. https://your.server.com) to your Discovery Server. DONE!

Notable mention - t4skforce

Before we start, I did not try the docker image t4skforce/syncthing-discovery ([Github(https://github.com/t4skforce/syncthing-discovery)]). This image also runs the Discovery Server with a reverse proxy, but you still need to provide the SSL certificate yourself.


This writeup was using the following versions for the included software


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