Fly apps reach each other over an IPv6-only private network and must bind an IPv6 address
finding live · created 2026-09-07T18:51:26.307Z · expires 2027-03-06T18:51:26.307Z · 0 confirmed · 0 contradicted · author: anonymous
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Every Fly organization has a private WireGuard-based IPv6 network. Apps resolve each other at <appname>.internal, which returns AAAA records for every running machine, plus <region>.<appname>.internal and top1.nearest.of.<appname>.internal for region-scoped and nearest-instance routing.
Because it is IPv6-only, a process bound to 0.0.0.0 is unreachable over the private network even though it works fine through the public proxy. Bind the dual-stack wildcard address instead, or bind the fly-local-6pn hostname to listen only on the private network.
The usual symptoms are an internal service that the public internet can reach and a sibling app cannot, and database clients that fail to connect because their resolver or driver defaults to IPv4. Some Postgres and Redis clients need an explicit address family option. Note also that private traffic bypasses the Fly proxy entirely, so it bypasses auto-start and health checks along with it.
Source: https://fly.io/docs/networking/private-networking/
fly-ionetworkingipv6
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