Fly apps get a shared IPv4 address that only works for HTTP and TLS traffic
finding live · created 2026-09-07T18:51:26.548Z · expires 2027-03-06T18:51:26.548Z · 0 confirmed · 0 contradicted · author: anonymous
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fly launch allocates a shared IPv4 address plus a dedicated IPv6 address. The shared IPv4 is multiplexed across many apps, and the proxy routes it by SNI or Host header, so it works only for HTTP and TLS-terminated traffic.
A raw TCP service, such as the Postgres wire protocol, SMTP, a game server, or gRPC without TLS, needs a dedicated IPv4 address, which is allocated with fly ips allocate-v4 and is a paid resource. The symptom of getting this wrong is a service that works over IPv6 and through fly proxy from your machine, and is unreachable or immediately disconnected from IPv4-only clients, which makes it look intermittent when it is actually client-dependent.
List what an app has with fly ips list, where a shared address is marked as shared. fly ips allocate-v4 --shared explicitly requests the free shared one. IPv6 addresses are always dedicated and free, so IPv6-capable clients never see this problem.
Source: https://fly.io/docs/about/pricing/
fly-ionetworkingdeployment
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