socket.if_nametoindex('no-such-iface') raises OSError() without setting errno

try:
    if_nametoindex('qweqweq')
except Exception as err:
    print(err.errno)

None

I think it's a bug because I can not distinguish from any other OS errors precisely (like EPERM vs ENOENT or so)

Linked PRs

gh-100218: correctly set errno when socket.if_{nametoindex,indextoname} raise OSError #140905
[3.14] gh-100218: correctly set errno when socket.if_{nametoindex,indextoname} raise OSError (GH-140905) #141284
[3.13] gh-100218: correctly set errno when socket.if_{nametoindex,indextoname} raise OSError (GH-140905) #141285

There's a comment in the source code: /* if_nametoindex() doesn't set errno */

Then it should not be OSError or it has to set some errno, like ENOENT or so.

static PyObject *
socket_if_nametoindex(PyObject *self, PyObject *args)
{
    PyObject *oname;
#ifdef MS_WINDOWS
    NET_IFINDEX index;
#else
    unsigned long index;
#endif
    if (!PyArg_ParseTuple(args, "O&:if_nametoindex",
                          PyUnicode_FSConverter, &oname))
        return NULL;

    index = if_nametoindex(PyBytes_AS_STRING(oname));
    Py_DECREF(oname);
    if (index == 0) {
        /* if_nametoindex() doesn't set errno */
        PyErr_SetString(PyExc_OSError, "no interface with this name");
        return NULL;
    }

    return PyLong_FromUnsignedLong(index);
}

Another example from the same file:

    if (h->h_addrtype != af) {
        /* Let's get real error message to return */
        errno = EAFNOSUPPORT;
        PyErr_SetFromErrno(PyExc_OSError);
        return NULL;
    }

I propose the same change. i.e. errno= ENOENT + PyErr_SetFromErrno(PyExc_OSError);

man if_nametoindex:

On success, if_nametoindex() returns the index number of the network interface;
on error, 0 is returned and errno is set to indicate the error.

It MUST set errno on error! I think there was some incorrect libc implementation when the code was written. So, a crutch like the one I described before is not ever required. As a solution, we can do the following :

errno = ENOENT; // for buggy if_nametoindex implementations, where they don't set errno.
index = if_nametoindex(PyBytes_AS_STRING(oname));
Py_DECREF(oname);
if (index == 0) {
    PyErr_SetFromErrno(PyExc_OSError);
    return NULL;
}

@sobolevn

@sobolevn

Sorry, I am not qualified enought in this module to make decisions :)
I've just highlighted that this is a known thing. Probably, you can go to the original comment PR and try to figure out what were the alternatives.

@gpshead

cc @gpshead as a network expert.

@socketpair

@socketpair: Ping on an old issue (!). What is your operating system? I cannot reproduce the issue on Linux.

FTR, I was able to reproduce this on an OpenSUSE 15.5 with Kernel 5.14.21-150500.55.88-default and libc 2.31.

Yes, it was linux x86, Fedora, but I don't know exact Fedora version and Python version

gh-100218: correctly set `errno` when `socket.if_{nametoindex,indexto…

pythongh-100218: correctly set `errno` when `socket.if_{nametoindex,i…

[3.13] pythongh-100218: correctly set `errno` when `socket.if_{nameto…

[3.14] gh-100218: correctly set `errno` when `socket.if_{nametoindex,…

[3.13] gh-100218: correctly set `errno` when `socket.if_{nametoindex,…

pythongh-100218: correctly set `errno` when `socket.if_{nametoindex,i…

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`socket.if_nametoindex('no-such-iface')` raises `OSError()` without setting `errno` #100218

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socket.if_nametoindex('no-such-iface') raises OSError() without setting errno #100218

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sobolevn commented on Dec 13, 2022

socketpair commented on Dec 13, 2022

socketpair commented on Dec 13, 2022

socketpair commented on Dec 13, 2022

sobolevn commented on Dec 13, 2022

arhadthedev commented on Feb 3, 2023

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socketpair commented on Nov 8, 2025

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`socket.if_nametoindex('no-such-iface')` raises `OSError()` without setting `errno` #100218