Fix inconsistent IPv6 dual-stack behavior between single-worker and multi-worker modes - #3070
Fix inconsistent IPv6 dual-stack behavior between single-worker and multi-worker modes#3070aleks-drozy wants to merge 3 commits into
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Config.bind_socket() (used by the reload and multi-worker supervisors) never set IPV6_V6ONLY on IPv6 sockets, so dual-stack behavior silently depended on the OS default (e.g. Linux's net.ipv6.bindv6only sysctl). The single-worker path, however, delegates to asyncio's loop.create_server(host=..., port=...), which always forces IPV6_V6ONLY=True on sockets it creates via getaddrinfo, making the socket IPv6-only regardless of platform. So a server bound to "::" would silently drop IPv4 connectivity in single-worker mode while accepting IPv4 in multi-worker/reload mode (or vice versa, depending on the host's sysctl default). Fix both sides so they consistently bind dual-stack: - Config.bind_socket() now explicitly sets IPV6_V6ONLY=False after creating an AF_INET6 socket, instead of relying on the OS default. - Server.startup()'s single-worker "standard case" now binds its own dual-stack socket for IPv6 hosts and passes it to loop.create_server(sock=...), bypassing asyncio's own getaddrinfo path (and its hardcoded IPV6_V6ONLY=True) entirely. Fixes Kludex#2945
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Included review availability: Your plan includes up to 10 reviews per rolling hour; 9 remain after this review. 📝 WalkthroughWalkthroughThe change makes IPv6 wildcard sockets dual-stack in configuration and single-worker startup. It adds shared IPv6 capability detection and regression tests for socket options and IPv4 connectivity. ChangesIPv6 dual-stack behavior
Estimated code review effort: 3 (Moderate) | ~20 minutes Merge Risk: ⚪ Minimal · up to The change makes IPv6 wildcard binding consistently dual-stack and adds regression coverage for both socket binding and single-worker connectivity; no actionable merge-blocking risk remains beyond normal checks and review. Sequence Diagram(s)sequenceDiagram
participant Server.startup
participant IPv6Socket
participant asyncio
participant IPv4Client
Server.startup->>IPv6Socket: create dual-stack socket
Server.startup->>IPv6Socket: bind IPv6 wildcard address
Server.startup->>asyncio: create server from bound socket
IPv4Client->>asyncio: send HTTP request over IPv4
asyncio-->>IPv4Client: return HTTP 204
🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
✨ Finishing Touches🧪 Generate unit tests (beta)
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Inline comments:
In `@tests/test_config.py`:
- Around line 265-274: Update test_ipv6_socket_bind_is_dual_stack to perform the
existing successful IPv6 bind probe used by
test_run_ipv6_dual_stack_single_worker before calling Config.bind_socket(), and
skip the test when the probe cannot bind ::. Preserve the current dual-stack
assertions and test flow when IPv6 is available.
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Merging this PR will degrade performance by 10.06%
Warning Please fix the performance issues or acknowledge them on CodSpeed. Performance Changes
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Re-trigger cubic
- Move _has_ipv6 test helper into tests/utils.py as has_ipv6() and add a skipif guard to test_ipv6_socket_bind_is_dual_stack, since not all CI runners have IPv6 available (CodeRabbit + cubic-dev-ai). - Move the IPV6_V6ONLY setsockopt call inside the existing try/except OSError block in Config.bind_socket() so failures are handled consistently with the bind() call (cubic-dev-ai). - Avoid leaking the IPv6 socket in Server.startup() if bind()/ create_server() raises OSError, by tracking sock and closing it in the except branch (cubic-dev-ai). Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
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Inline comments:
In `@tests/utils.py`:
- Around line 24-25: Update has_ipv6() to catch only OSError around the socket
operation, allowing TypeError and other unrelated exceptions to propagate, while
preserving the existing socket cleanup behavior.
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…o OSError Rename the shadowing `sock` local in the IPv6 dual-stack bind branch to `ipv6_sock` to resolve a mypy no-redef error against the earlier `for sock in sockets` loop, and narrow the bare `except Exception` in tests/utils.py has_ipv6() to `except OSError` per review feedback.
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Summary
Fixes #2945.
A server bound to an IPv6 wildcard host (
--host ::) behaved inconsistently depending on how it was run:--reload:Config.bind_socket()creates the socket manually and only setsSO_REUSEADDR. It never setsIPV6_V6ONLY, so dual-stack behavior silently depended on the OS default (e.g. Linux'snet.ipv6.bindv6onlysysctl, commonly0→ dual-stack).Server.startup()'s "standard case" delegates toasyncio.loop.create_server(host=..., port=...). Perasyncio/base_events.py, asyncio always setsIPV6_V6ONLY=TrueonAF_INET6sockets it creates through this path, regardless of the platform — making the socket IPv6-only unconditionally.The net effect: on a host where dual-stack is the OS default (e.g. Linux), a single-worker server bound to
::silently drops IPv4 connectivity, while the same app run with--workers 2accepts IPv4 fine — or vice versa depending on the sysctl. This is confusing and, as reported, breaks real-world setups such as rootless Podman with pasta networking, which relies on the server accepting both address families.Root cause
uvicorn/config.py,Config.bind_socket(): creates the IPv6 socket but never callssetsockopt(IPPROTO_IPV6, IPV6_V6ONLY, ...).uvicorn/server.py,Server.startup()"standard case": relies onloop.create_server(host=.., port=..), whose internalgetaddrinfo-based socket creation in CPython unconditionally forcesIPV6_V6ONLY=Truefor everyAF_INET6socket, bypassing any platform default.Fix
Make both code paths agree on dual-stack-by-default, explicitly and deterministically (not relying on the OS default):
Config.bind_socket()now explicitly setsIPV6_V6ONLY=Falseright after creating anAF_INET6socket.Server.startup()'s single-worker "standard case" now, for IPv6 hosts, binds its own socket (SO_REUSEADDR+IPV6_V6ONLY=False) and passes it toloop.create_server(sock=...)instead ofhost=/port=, bypassing asyncio'sgetaddrinfopath (and its hardcodedIPV6_V6ONLY=True) entirely. This is the same technique already used by the reload/multiprocess supervisors viabind_socket().Non-IPv6 hosts, UDS, and
--fdpaths are untouched.Testing
test_ipv6_socket_bind_is_dual_stackintests/test_config.py: binds viaConfig.bind_socket()withhost="::"and assertsIPV6_V6ONLY == 0.test_run_ipv6_dual_stack_single_workerintests/test_main.py: runs a real single-worker server bound to::and confirms an IPv4 (127.0.0.1) client can connect successfully — this is the exact regression from the issue.httpx.ConnectError: All connection attempts failedfor the single-worker case, andIPV6_V6ONLY == 1for thebind_socket()case) and pass with it.tests/test_config.py,tests/test_main.py, andtests/test_server.pyslices locally (Windows): all passing, no regressions.ruff format --check,ruff check, andmypyall clean on the changed files (pre-existingmypyfindings on Windows are unrelated stdlib-stub platform mismatches — e.g.AF_UNIX/SIGHUPnot being present in thewin32stubs — and not on any line touched by this change).🤖 Generated with Claude Code
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