Public Network Services

Fly.io has public and private network services available. The public network services connect applications to the wider public internet, while the private network services allow application instances to communicate with other application instances within the Fly.io private network.

IP addresses

Anycast

We announce global IP blocks from all of our datacenters over BGP, otherwise known as anycast. Anycast is a core internet routing mechanism that connects clients to the “nearest” server advertising a block of IPs. You can read all about it on Wikipedia.

IPv6

A new Fly App running a public service automatically gets a dedicated anycast IPv6 address when it’s first deployed.

Shared IPv4

Along with the dedicated IPv6, apps configured to handle either

  • HTTP on port 80, or
  • TLS & HTTP on port 443

(or both) are automatically given a shared anycast IPv4 address on the first deployment.

Shared IPs are recommended unless you have an explicit need for your own IP. These IPs are shared across many apps and organizations. Routing is based on your app’s domain. One reason to use a dedicated IP is if your app needs to do something over UDP.

If you want to allocate a shared IPv4 to an app without a public IPv4 address, this is possible (with flyctl v0.0.439 and newer) using

fly ips allocate-v4 --shared

Dedicated IPv4

Allocating a dedicated IPv4 anycast address is now opt-in only, but can still be done manually with

fly ips allocate-v4

when needed; for example, if you want your app to handle TCP directly.

If your app has a shared IPv4 address and you allocate a dedicated one, the shared IPv4 is released automatically.

IPv6 addresses and shared IPv4 anycast addresses are free. Dedicated IPv4 addresses are billed monthly.

Switch from dedicated IPv4 to shared IPv4

The following steps should allow you to switch an app with a custom domain from a dedicated IPv4 address to a free shared IPv4 without downtime.

  1. Add a shared IPv4 address via flyctl ips allocate-v4 --shared
  2. If you don’t use a CNAME to .fly.dev, add the shared IPv4 as an A record.
  3. Confirm your app works via shared IP: curl -Iv http://<your-hostname> --resolve <your-hostname>:80:<new-shared-ipv4>. You should receive a 301 redirect response.
  4. If you are using a custom domain and don’t already have an SSL certificate for this app, create one. The cert is required for routing to your app via the custom domain, even for HTTP connections.
  5. Wait for DNS caches to clear. Five minutes is likely enough, but this varies wildly. This is determined by the larger of your DNS record’s TTL and that of our <your-app>.fly.dev record.
  6. Remove the dedicated IPv4 from the app using fly ips release <ipv4-address-to-release>. You can view your app’s IP addresses using fly ips list.
  7. Remove the unwanted IPv4 address from your DNS if it was set manually as an A record.

If you do not have a custom domain (your app is accessed by its .fly.dev address), you don’t have to change any DNS records yourself, and you don’t need to add your own SSL cert.

Connection handlers

Set the handlers config setting in the services.ports section of fly.toml to specify which middleware applies to incoming TCP connections for each port you accept external connections on. Use these to convert TCP connections into something your application can handle.

Valid handler strings:

  • "http": Convert TCP connection to HTTP
  • "tls": Convert TLS connection to unencrypted TCP
  • "pg_tls": Handle TLS for PostgreSQL connections
  • "proxy_proto": Wrap TCP connection in PROXY protocol

TCP pass through

If you don’t specify handlers, we just forward TCP to your application as-is. This is useful if you want to handle TLS termination yourself, for example.

HTTP

Many applications have limited HTTP support, the HTTP middleware normalizes HTTP connections and sends HTTP 1.1 requests to the application process. This is roughly how nginx and other reverse proxies work, and allows your application to globally accept modern HTTP protocols (like HTTP/2) without extra complexity.

If your application stack has good HTTP/2 support (like Go), you will get better performance accepting TCP connections directly, and using the TLS handler to terminate SSL. Your application does need to understand h2c for this to work, however.

The HTTP handler adds a number of standard HTTP headers to requests, and a few Fly.io-specific headers for convenience:

Header Description
Fly-Client-IP The IP address Fly.io accepted a connection from
X-Forwarded-For A comma separated list of proxy servers the request passed through. MDN has full documentation for this header
X-Forwarded-Proto Original client protocol, either http or https
X-Forwarded-SSL Indicates if client connected over SSL, either on or off
X-Forwarded-Port Original connection port, header may be set by client
Fly-Forwarded-Port Original connection port, always set by Fly.io
Fly-Region Original incoming connection region

You can set a preference on HTTP requests for which region you would like to connect to:

Fly-Prefer-Region: region-code

TLS

The TLS middleware terminates TLS using Fly.io-managed application certificates, then forwards a plaintext connection to the application process. This is useful for running TCP services and offloading TLS to the Fly Proxy.

For performance purposes, the Fly Proxy will terminate TLS on the host a client connects to, and then forward the connection to the nearest available application instance.

Note: the TLS handler includes ALPN negotiation for HTTP/2. When possible, applications will connect to these kinds of Fly.io services using HTTP/2, and we will forward an unencrypted HTTP/2 connection (h2c) to the application process.

Postgres connections

The pg_tls handler manages Postgres connections, so that you can expose your Postgres databases over the proxy securely. Some interesting notes by @glebarez on why standard TLS termination won’t work for Postgres: https://github.com/glebarez/pgssl#readme

PROXY protocol

The proxy_proto handler adds information about the original connection, including client IP + port and server IP + port (from the client’s perspective). Most applications need additional logic to accept the proxy protocol, either using a prebuilt library or implementing the proxy protocol directly.

Fly.io supports version 1 (human-readable header format) of the PROXY protocol by default with the proxy_proto handler. You can configure the proxy_proto handler to use version 2 (binary header format) in the services.ports.proxy_proto_options section of your fly.toml file.

HTTP to HTTPS redirects

The force_https configuration option automatically redirects HTTP to HTTPS. When enabled, all HTTP requests return a redirect response with a 301 status code. This option can only be set on HTTP handlers - deployments will fail if set on other handlers.

Outbound IP addresses

Fly Machiness have IPv6 addresses from which they make requests to the wider Internet without going through the Fly Proxy.

We don’t offer static IPs or regional IP ranges, and we discourage the use of our outbound IPs to bypass firewalls. A Machine’s outbound IP is liable to change without notice. This shouldn’t be a daily occurrence, but it will happen if a Machine is moved for whatever reason, such as a load-balancing of Fly Machines between servers in one region.

If you depend on knowing this address for some reason, it’s up to you to monitor it for changes.

Find your Machine’s outbound IP

The Machine’s outbound IPv6 address is available in the FLY_PUBLIC_IP environment variable. Use fly ssh console -s to get an interactive shell on the Machine of your choice.

/ # echo $FLY_PUBLIC_IP
2605:4c40:92:520a:0:8c93:3d8a:1

You can also call out to an external service that’ll tell you your IP. For example:

/ # curl text.ipv6.wtfismyip.com
2605:4c40:92:520a:0:8c93:3d8a:1

or

/ # dig +short txt o-o.myaddr.l.google.com @ns1.google.com
"2605:4c40:92:520a:0:8c93:3d8a:1"

So this Machine’s outbound IPv6 address is 2605:4c40:92:520a:0:8c93:3d8a:1.

You may have to install software on the Machine to use dig or curl. On Debian, dig belongs to the dnsutils package.