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examples/c: reject unknown sockfilter interfaces - #377
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danielocfb merged 1 commit intoOct 20, 2026
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if_nametoindex() returns zero when the requested interface does not exist. Passing that index to bind() on an AF_PACKET socket selects all interfaces instead of reporting an error. Reject a failed lookup and close the socket before returning. Signed-off-by: Eduard Zateev <hackerowskiy@gmail.com>
sairex53
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October 18, 2026 06:20
danielocfb
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Oct 20, 2026
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Seems fine to me, thanks
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sockfilter -i <missing-interface>passes the zero returned byif_nametoindex()tobind(). For an AF_PACKET socket, index zero means all interfaces, so a misspelled interface silently broadens capture.Reject a failed lookup and close the socket before returning.
Validation on Fedora 44 aarch64:
sockfilterwith Clang 22 and the installed bpftool 7.6.0 (make -C examples/c CC=clang BPFTOOL=/usr/bin/bpftool sockfilter). The bundled bpftool build hits the existing Build fails with -Werror=discarded-qualifiers in libbpf.c #373.open_raw_sock()implementation with real AF_PACKET sockets in an isolated user/network namespace. Before this change, a nonexistent interface succeeded with ifindex 0; after it, the lookup fails and repeated failures leave the descriptor count unchanged. The existinglointerface still binds correctly.Privileged BPF load/attach was not tested; passwordless sudo is unavailable in this environment.