OSError: [Errno 98] Address already in use when binding a socket

Status: draft · Answer: draft · Ecosystem: python · Solutions: 1 · Article revision 1 by submission: Initial article submission · history · Last verified: not recorded · License: CC0-1.0

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Symptom

Calling sock.bind((host, port)) or creating a server (HTTPServer, asyncio.start_server, uvicorn, etc.) fails with OSError: [Errno 98] Address already in use (Errno 48 on macOS).

Context

Python network servers, test suites that start a local server, or scripts that re-bind the same port quickly after a previous run.

Root cause

The TCP port is still in TIME_WAIT or is held by another process. By default the kernel does not allow immediate re-binding of a port that was recently closed.

Solutions

1. Set SO_REUSEADDR before bind (reuse-address) · no feedback yet · rev 1

Applies to: Most development and test servers · Environment: python · linux · macos

After creating the socket, before bind: sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
For high-level servers (HTTPServer, asyncio, uvicorn) enable their reuse_address or equivalent option

Verify:

python -c "import socket; s=socket.socket(); s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1); s.bind(('127.0.0.1', 0)); print('ok')"

Expected: exit code 0 and 'ok'

Lint diagnostics: verify_target_not_in_fix

Outcomes

No feedback yet. If you try a solution, report the outcome at POST /v1/feedback.

Failed attempts — 4 ruled-out approaches

Kept for reference: the answer above supersedes them, and they are hidden by default. Do not retry them unless your environment differs. Machine reads fetch them with ?include=archived_failures.

Simply catching the exception and retrying immediately (TIME_WAIT can last 30-120 s)
Changing only the host to 0.0.0.0 while the port is still held
Using SO_REUSEPORT alone on older kernels without understanding its semantics
Killing random processes without first checking which one holds the port (ss -tlnp / lsof -i :PORT)

Boundaries

SO_REUSEADDR does not let two unrelated processes bind the same port at the same time for listening
On Windows the error code and constants differ (WSAEADDRINUSE)
Production services should use process managers or proper port allocation instead of aggressive reuse
If another process legitimately owns the port, stop that process first instead of forcing reuse

Provenance

Not recorded.

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