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fix(net): fallback multicast join without adapters #2791

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fix(net): fallback multicast join without adapters #2791

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87dec47
fix(net): fallback multicast join without adapters
Skyline-23 Dec 5, 2025
ae6e10e
fix(net): avoid public IP for route probe
Skyline-23 Jan 23, 2026
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34 changes: 34 additions & 0 deletions pyatv/support/net.py
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Original file line number Diff line number Diff line change
Expand Up @@ -14,6 +14,9 @@

_LOGGER = logging.getLogger(__name__)

# Private RFC1918 target used only to select a default route.
_DEFAULT_ROUTE_PROBE = ("10.255.255.1", 1)


def unused_port() -> int:
"""Return a port that is unused on the current host."""
Expand Down Expand Up @@ -47,6 +50,26 @@ def mcast_socket(address: Optional[str], port: int = 0) -> socket.socket:
with suppress(OSError):
membership = socket.inet_aton("224.0.0.251") + socket.inet_aton(address)
sock.setsockopt(socket.IPPROTO_IP, socket.IP_ADD_MEMBERSHIP, membership)
else:
# Fallback for sandboxed platforms where ifaddr finds no adapters:
# detect default IPv4 via a dummy UDP connect and use it for multicast join.
try:
with socket.socket(socket.AF_INET, socket.SOCK_DGRAM) as probe:
probe.connect(_DEFAULT_ROUTE_PROBE)
Comment on lines +54 to +58
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Copilot AI Jan 23, 2026

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The UDP “default route probe” logic is duplicated in both mcast_socket and get_private_addresses. To avoid the two call sites drifting (e.g., different exception handling or filtering), consider extracting a small helper like _get_default_ipv4() and reusing it in both places.

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detected = probe.getsockname()[0]
sock.setsockopt(
socket.SOL_IP, socket.IP_MULTICAST_IF, socket.inet_aton(detected)
)
membership = socket.inet_aton("224.0.0.251") + socket.inet_aton(
detected
)
sock.setsockopt(socket.IPPROTO_IP, socket.IP_ADD_MEMBERSHIP, membership)
except OSError:
with suppress(OSError):
membership = socket.inet_aton("224.0.0.251") + socket.inet_aton(
"0.0.0.0"
)
sock.setsockopt(socket.IPPROTO_IP, socket.IP_ADD_MEMBERSHIP, membership)

_LOGGER.debug("Binding on %s:%d", address or "*", port)
sock.bind((address or "", port))
Expand Down Expand Up @@ -74,6 +97,17 @@ def get_private_addresses(include_loopback=True) -> List[IPv4Address]:
if ipaddr.is_private:
addresses.append(ipaddr)

# Fallback when no adapters are reported (e.g. sandboxed platforms).
if not addresses:
try:
Comment on lines +100 to +102
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Copilot AI Jan 23, 2026

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get_private_addresses fallback runs whenever addresses is empty, which also covers cases where adapters exist but only have public IPv4s. That expands behavior beyond the PR description (“when no adapters are reported”) and changes previous semantics for public-only hosts. Consider storing adapters = get_adapters() and only running the UDP-probe fallback when not adapters (or when no IPv4 addresses were enumerated).

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with socket.socket(socket.AF_INET, socket.SOCK_DGRAM) as probe:
probe.connect(_DEFAULT_ROUTE_PROBE)
Comment on lines +100 to +104
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Copilot AI Jan 23, 2026

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This new fallback changes observable behavior when get_adapters() returns an empty list (existing unit test tests/support/test_net.py::test_no_address currently expects []). Add/update tests to mock the UDP probe socket (connect/getsockname) so the fallback is deterministic and validated (both success and OSError paths).

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detected = IPv4Address(probe.getsockname()[0])
if include_loopback or not detected.is_loopback:
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In the get_private_addresses fallback, the detected address is appended without checking detected.is_private. This can cause the function to return a globally-routable IP, contradicting the docstring/previous behavior (only ipaddr.is_private values were returned). Apply the same is_private filter in the fallback path (while still honoring include_loopback).

Suggested change
if include_loopback or not detected.is_loopback:
if detected.is_private or (detected.is_loopback and include_loopback):

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addresses.append(detected)
except OSError:
pass
Comment on lines +108 to +109
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'except' clause does nothing but pass and there is no explanatory comment.

Suggested change
except OSError:
pass
except OSError as ex:
_LOGGER.debug(
"Unable to determine fallback local address via UDP probe: %s", ex
)

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return addresses


Expand Down
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