Private Networking

Fly Apps are connected by a mesh of WireGuard tunnels using IPv6.

Applications within the same organization are assigned special addresses (6PN addresses) tied to the organization. Those applications can talk to each other because of those 6PN addresses, but applications from other organizations can’t; the Fly.io platform won’t forward between different 6PN networks.

This connectivity is always available to apps; you don’t have to do anything special to get it.

You can connect apps running outside of Fly.io to your 6PN network using WireGuard; for that matter, you can connect your dev laptop to your 6PN network. To do that, you’ll use flyctl, the Fly.io CLI, to generate a WireGuard configuration that has a 6PN address.

Discover apps through DNS on a Fly Machine

An app’s Fly Machines are configured with their DNS server pointing to fdaa::3. The DNS server on this address can resolve arbitrary DNS queries, so you can look up “google.com” with it. But it’s also aware of 6PN addresses, and, when queried from a Machine, will let you look up the addresses of other apps in your organization. Those addresses live under the synthetic top-level domain .internal.

Since this is the default configuration we set up for Machines on Fly.io, you probably don’t need to do anything special to make this work; if your app shares an organization with another app called random-potato-45, then you should be able to ping6 random-potato-45.internal.

If you want to get fancy, you can install dig on the Machine and query the DNS directly. For example:

root@f066b83b:/# dig +short aaaa my-app-name.internal @fdaa::3

fdaa:0:18:a7b:7d:f066:b83b:2

Discover apps through DNS on a WireGuard connection

The DNS server address is different on WireGuard connections than on Machines. That’s because you can run multiple WireGuard connections; your dev laptop could be WireGuard-connected to multiple organizations, but a Machine can’t be.

Your DNS server address for a WireGuard connection is part of the WireGuard tunnel configuration that flyctl generates. Your platform WireGuard tools might read and automatically configure DNS from that configuration, or they might not. Here’s an example of a WireGuard configuration file generated by the fly wireguard create command:

[Interface]
PrivateKey = [redacted]
Address = fdaa:0:18:a7b:d6b:0:a:2/120
DNS = fdaa:0:18::3

You guessed it; it’s the DNS line. Your DNS server address will also start with fdaa:, but the next two parts are unique to your organization’s network. All 6PN addresses are prefixed by the organization’s network ID; that’s the part of the address that locks it to your organization.

All our WireGuard DNS addresses follow this pattern: take the organization prefix, and tack ::3 onto the end. In this example, the Wireguard peer address is:

fdaa:0:18:a7b:d6b:0:a:2

The 6PN prefix is the first 3 :-separated parts, so the DNS server address for this example is:

fdaa:0:18::3

To use dig to probe DNS on a WireGuard connection, supply the DNS server address to it. For example:

root@f066b83b:/# dig +short aaaa my-app-name.internal @fdaa:0:18::3

fdaa:0:18:a7b:7d:f066:b83b:2

Connect to a running service via its 6PN address

In the /etc/hosts of a deployed Fly Machine, we alias the 6PN address of the Machine to fly-local-6pn.

For a service to be accessible via its 6PN address, it needs to bind to/listen on fly-local-6pn. For example, if you have a service running on port 8080, then you need to bind it to fly-local-6pn:8080 for it to be accessible at “[6PN_Address:8080]”.

Learn more about connecting to app services.

Fly.io .internal addresses

Fly.io .internal hostnames resolve to 6PN addresses (internal IPv6 addresses) associated with Fly Machines. You can use .internal addresses to connect to apps and Machines in your 6PN network by doing a DNS lookup for specific 6PN addresses and then using those addresses to send requests. You might want to use .internal addresses to connect your app to databases, API servers, or other apps in your 6PN network.

The .internal addresses can include qualifiers to return more specific addresses or info. For example, you can add a region name qualifier to return the 6PN addresses of an app’s Machines in a specific region: iad.my-app-name.internal. Querying this hostname returns the 6PN address (or addresses) of the my-app-name Machines in the iad region.

Some .internal hostnames return a TXT record with Machine, app, or region information. For example, if you request the TXT records using regions.my-app-name.internal, then you’ll get back a comma-separated list of regions that my-app-name is deployed in. And you can discover all the apps in the organization by requesting the TXT records associated with _apps.internal. This will return a comma-separated list of the app names.

The following table describes the information returned by each form of .internal address.

Examples of retrieving this information are in the fly-examples/privatenet repository.

Flycast - Private Fly Proxy services

A Flycast address is an app-wide IPv6 address that the Fly Proxy can route to privately. 6PN traffic doesn’t go through the Fly Proxy, so can’t use its features, like geographically aware load balancing and autostart/autostop based on traffic. Conversely, Flycast services can’t use internal DNS to target specific Machines or regions, and are not reachable on .internal addresses.

Use Flycast to do the following within the Fly.io private WireGuard mesh:

• Autostart and/or autostop Machines based on network requests.
• Use Fly Proxy’s geographically aware load balancing for private services.
• Connect to a service from another app that can’t use DNS.
• Connect from third-party software, like a database, that doesn’t support round-robin DNS entries.
• Access specific ports or services in your app from other Fly.io organizations.
• Use advanced proxy features like TLS termination or PROXY protocol support.

The general flow for setting up Flycast is:

1. Allocate a private IPv6 address for your app on one of your Fly.io organization networks.
2. Expose services in your app’s fly.toml [services] or [http_service] block; do not use force_https as Flycast is HTTP-only.
3. Deploy your app.
4. Access the services on the private IP from the target organization network.

Assign a Flycast address

By default, the Flycast IP address is allocated on an app’s parent organization network.

 fly ips allocate-v6 --private

VERSION IP                  TYPE    REGION  CREATED AT
v6      fdaa:0:22b7:0:1::3  private global  just now

If you want to expose services to another Fly.io organization you have access to, then use the --org flag.

 fly ips allocate-v6 --private --org my-other-org

VERSION IP                  TYPE    REGION  CREATED AT
v6      fdaa:0:22b7:0:1::3  private global  just now

DNS and Flycast

You can use appname.flycast domains. They behave like appname.internal domains, except they only return the app’s Flycast addresses (if you have any).

Private Network VPN

You can use the WireGuard VPN to connect to the 6PN private network. WireGuard is a flexible and secure way to plug into each one of your Fly.io organizations and connect to any app within that organization.

Set up a private network VPN

To set up your VPN, you’ll use flyctl to generate a tunnel configuration file with private keys already embedded. Then you can load that file into your local WireGuard application to create a tunnel. Activate the tunnel and you’ll be using the internal Fly.io DNS service which resolves .internal addresses - and passes on other requests to Google’s DNS for resolution.

1. Install your WireGuard App

Visit the WireGuard site for installation options. Install the software that is appropriate for your system. Windows and macOS have apps available to install. Linux systems have packages, typically named wireguard and wireguard-tools, you should install both.

2. Create your tunnel configuration

To create your tunnel, run:

fly wireguard create

Select the organization you want the WireGuard tunnel to work with:

? Select organization:  [Use arrows to move, type to filter]
> My Org (personal)
  Test Org (test-org)

The fly wireguard create command configures the WireGuard service and generates a tunnel configuration file, complete with private keys which cannot be recovered. This configuration file will be used in the next step. First, save the configuration file:

!!!! WARNING: Output includes private key. Private keys cannot be recovered !!!!
!!!! after creating the peer; if you lose the key, you’ll need to remove    !!!!
!!!! and re-add the peering connection.                                     !!!!
? Filename to store WireGuard configuration in, or 'stdout':  mypeer.conf
Wrote WireGuard configuration to 'mypeer.conf'; load in your WireGuard client

We suggest you name your saved configuration with the same name as the peer you have created. Add the extension .conf to ensure it will be recognized by the various WireGuard apps as a configuration file for a tunnel. Note that the name (excluding the .conf extension) shouldn’t exceed 15 characters since this is the maximum length for an interface name on Linux.

3. Import your tunnel

Windows

Run the WireGuard app. Click Import tunnel(s) from file. Select your configuration file. The WireGuard app will display the details of your tunnel. Click Activate to bring the tunnel online.

macOS

Run the WireGuard app. Click Import tunnel(s) from file. Select your configuration file and click Import. You might be prompted by the OS that WireGuard would like to add VPN configurations; click Allow. The WireGuard app will display the details of your tunnel. Click Activate to bring the tunnel online.

Ubuntu Linux

If you don’t have wg-quick installed, then run the command below. For Ubuntu 18.04 to 22.04, openresolv is also required.

sudo apt install wireguard-tools openresolv

Copy the configuration file to /etc/wireguard; you’ll need root/sudo permissions:

sudo cp basic.conf /etc/wireguard

Run wg-quick to bring up the connection by name (i.e. less the .conf extension). For example:

wg-quick up mypeer

[#] ip link add mypeer type wireguard
[#] wg setconf mypeer /dev/fd/63
[#] ip -6 address add fdaa:0:4:a7b:ab6:0:a:102/120 dev mypeer
[#] ip link set mtu 1420 up dev mypeer
[#] resolvconf -a tun.mypeer -m 0 -x
[#] ip -6 route add fdaa:0:4::/48 dev mypeer

Test the VPN tunnel

If you have the dig tool installed, a TXT query to _apps.internal will show all the apps in the organization you are connected to:

dig +noall +answer _apps.internal txt

_apps.internal.     5   IN  TXT "my-app-name,my-app-name-0,my-app-name-1"

You can also query for peer names and addresses:

dig +short txt _peer.internal @fdaa:0:18::3

"my-peer"

dig +short aaaa my-peer._peer.internal @fdaa:0:18::3

fdaa:0:18:a7b:7d:f066:b83b:102

Query for the 6PN addresses of all started Machines in an app:

dig +short aaaa my-app-name.internal

Manage WireGuard on Fly.io

List the tunnels

To list all the tunnels set up for an organization, run fly wireguard list. You can provide an organization on the command line or select an org when prompted.

Remove a tunnel

To remove a tunnel, run fly wireguard remove. You can specify the organization and tunnel name on the command line or be prompted for both.