Add Match localnetwork / getifaddrs support on Windows

[steffen-heil-secforge]

Tracking issue: PowerShell/Win32-OpenSSH#2440

Summary

Adds support for the Match localnetwork ssh_config criterion on Windows by implementing POSIX getifaddrs()/freeifaddrs() over the Win32 GetAdaptersAddresses() API.

Previously check_match_ifaddrs() (readconf.c) and the BindInterface interface enumeration (sshconnect.c) were gated on HAVE_IFADDRS_H, which the Windows build never defined, and win32compat had no getifaddrs() — so Match localnetwork always returned "not supported on this platform".

Changes

• contrib/win32/win32compat/inc/ifaddrs.h (new): struct ifaddrs, IFF_* flags (Windows has no <net/if.h>), getifaddrs/freeifaddrs prototypes.
• contrib/win32/win32compat/getifaddrs.c (new): implementation over GetAdaptersAddresses().
• contrib/win32/openssh/config.h.vs: define HAVE_IFADDRS_H and HAVE_GETIFADDRS.
• contrib/win32/openssh/win32iocompat.vcxproj (+ .filters): build the new sources into posix_compat.lib.
• contrib/win32/openssh/ssh.vcxproj: link Iphlpapi.lib.
• regress/unittests/win32compat/getifaddrs_tests.c (new): unit tests, registered in tests.c/tests.h and unittest-win32compat.

Notes

• Match localnetwork is client-only in 10.0 (readconf.c / ssh_config).
• ifa_name is the adapter friendly name (what BindInterface matches on Windows).
• Disabled/disconnected adapters are returned without IFF_UP, so the existing consumers skip them (matches POSIX getifaddrs(3) semantics).

Testing

Built on Windows x64 (Release). ssh.exe (OpenSSH_for_Windows_10.0p2) builds and links; Match localnetwork 127.0.0.0/8 matches the loopback (enumerated via getifaddrs), 203.0.113.0/24 does not, and the "not supported on this platform" path is gone. The getifaddrs unit checks pass. Sources also cross-compile for ARM64.

[Copilot]

Pull request overview

Adds Windows support for ssh_config’s Match localnetwork (and the BindInterface address enumeration path) by providing a Win32-backed getifaddrs()/freeifaddrs() implementation in win32compat, then enabling and wiring it into the Visual Studio build and unit tests.

Changes:

• Introduces contrib/win32/win32compat/getifaddrs.c plus a Windows ifaddrs.h shim (including minimal IFF_* flags).
• Enables the feature in the Windows build (HAVE_IFADDRS_H, HAVE_GETIFADDRS) and builds/links the new code (adds Iphlpapi.lib where needed).
• Adds a Win32compat unit test and registers it in the test runner.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
regress/unittests/win32compat/tests.h	Registers the new getifaddrs unit test symbol.
regress/unittests/win32compat/tests.c	Runs the new getifaddrs unit test.
regress/unittests/win32compat/getifaddrs_tests.c	Adds unit tests for the Windows getifaddrs/freeifaddrs shim.
contrib/win32/win32compat/inc/ifaddrs.h	Adds a Windows-compatible struct ifaddrs + prototypes + minimal IFF_* flags.
contrib/win32/win32compat/getifaddrs.c	Implements getifaddrs/freeifaddrs over GetAdaptersAddresses().
contrib/win32/openssh/win32iocompat.vcxproj.filters	Adds the new source/header to the VS filters.
contrib/win32/openssh/win32iocompat.vcxproj	Builds the new getifaddrs.c into posix_compat.lib.
contrib/win32/openssh/unittest-win32compat.vcxproj	Builds the new unit test and links Iphlpapi.lib.
contrib/win32/openssh/ssh.vcxproj	Links Iphlpapi.lib for the client.
contrib/win32/openssh/config.h.vs	Defines HAVE_IFADDRS_H and HAVE_GETIFADDRS for Windows builds.

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[steffen-heil-secforge]

Thanks for the review. Both points are addressed in 98cbc60 (kept as a separate commit on top of the original):

1. getifaddrs.c — ifa_name can be NULL: ifa_name is now guaranteed to be set. GetAdaptersAddresses always provides AdapterName, so a NULL name in practice means an allocation failure; that case is now treated as ENOMEM and fails the call, rather than emitting a NULL-name entry. This keeps the getifaddrs(3) contract and prevents the consumers (readconf.c, sshconnect.c) from silently skipping addresses.

2. getifaddrs_tests.c — brittle assumptions: dropped the non-empty-list and IPv4-loopback/IFF_UP expectations (an empty list is a valid result, and IPv4 may be disabled). The test now asserts only the call result and per-entry invariants (ifa_name/ifa_addr non-NULL, address family is AF_INET/AF_INET6).