diff --git a/docs/index.rst b/docs/index.rst
index c6d42c7be..025203dfa 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -735,7 +735,12 @@ Network
   - **speed**: the NIC speed expressed in mega bits (MB), if it can't be
     determined (e.g. 'localhost') it will be set to ``0``.
   - **mtu**: NIC's maximum transmission unit expressed in bytes.
-  - **flags**: a string of comma-separate flags on the interface (may be ``None``).
+  - **flags**: a string of comma-separated flags on the interface (may be the empty string).
+    Possible flags are: ``up``, ``broadcast``, ``debug``, ``loopback``,
+    ``pointopoint``, ``notrailers``, ``running``, ``noarp``, ``promisc``,
+    ``allmulti``, ``master``, ``slave``, ``multicast``, ``portsel``,
+    ``dynamic``, ``oactive``, ``simplex``, ``link0``, ``link1``, ``link2``,
+    and ``d2`` (some flags are only available on certain platforms).
 
   Example:
 
@@ -750,7 +755,7 @@ Network
 
   .. versionchanged:: 5.7.3 `isup` on UNIX also checks whether the NIC is running.
 
-  .. versionchanged:: 5.9.1 *flags* field was added.
+  .. versionchanged:: 5.9.1 *flags* field was added on POSIX.
 
 Sensors
 -------
diff --git a/psutil/_psaix.py b/psutil/_psaix.py
index 6e4a475ac..2391478c6 100644
--- a/psutil/_psaix.py
+++ b/psutil/_psaix.py
@@ -237,7 +237,8 @@ def net_if_stats():
     names = set([x[0] for x in net_if_addrs()])
     ret = {}
     for name in names:
-        isup, mtu = cext.net_if_stats(name)
+        mtu = cext_posix.net_if_mtu(name)
+        flags = cext_posix.net_if_flags(name)
 
         # try to get speed and duplex
         # TODO: rewrite this in C (entstat forks, so use truss -f to follow.
@@ -257,8 +258,10 @@ def net_if_stats():
                 speed = int(re_result.group(1))
                 duplex = re_result.group(2)
 
+        output_flags = ','.join(flags)
+        isup = 'running' in flags
         duplex = duplex_map.get(duplex, NIC_DUPLEX_UNKNOWN)
-        ret[name] = _common.snicstats(isup, duplex, speed, mtu, None)
+        ret[name] = _common.snicstats(isup, duplex, speed, mtu, output_flags)
     return ret
 
 
diff --git a/psutil/_psbsd.py b/psutil/_psbsd.py
index feb95e9c0..4a02c8314 100644
--- a/psutil/_psbsd.py
+++ b/psutil/_psbsd.py
@@ -390,13 +390,8 @@ def net_if_stats():
         else:
             if hasattr(_common, 'NicDuplex'):
                 duplex = _common.NicDuplex(duplex)
-            flag_list = []
-            for flagname, bit in _psposix.POSIX_NET_FLAGS:
-                if flags & (1 << bit):
-                    flag_list.append(flagname)
-
-            output_flags = ','.join(flag_list)
-            isup = 'running' in output_flags
+            output_flags = ','.join(flags)
+            isup = 'running' in flags
             ret[name] = _common.snicstats(isup, duplex, speed, mtu, output_flags)
     return ret
 
diff --git a/psutil/_pslinux.py b/psutil/_pslinux.py
index 4ac2112ad..1991af78e 100644
--- a/psutil/_pslinux.py
+++ b/psutil/_pslinux.py
@@ -1070,7 +1070,7 @@ def net_if_stats():
                 debug(err)
         else:
             output_flags = ','.join(flags)
-            isup = 'running' in output_flags
+            isup = 'running' in flags
             ret[name] = _common.snicstats(isup, duplex_map[duplex], speed, mtu, output_flags)
     return ret
 
diff --git a/psutil/_psosx.py b/psutil/_psosx.py
index 9aebdf974..d5cbd8a01 100644
--- a/psutil/_psosx.py
+++ b/psutil/_psosx.py
@@ -273,7 +273,7 @@ def net_if_stats():
             if hasattr(_common, 'NicDuplex'):
                 duplex = _common.NicDuplex(duplex)
             output_flags = ','.join(flags)
-            isup = 'running' in output_flags
+            isup = 'running' in flags
             ret[name] = _common.snicstats(isup, duplex, speed, mtu, output_flags)
     return ret
 
diff --git a/psutil/_pssunos.py b/psutil/_pssunos.py
index 701471383..541c1aa4a 100644
--- a/psutil/_pssunos.py
+++ b/psutil/_pssunos.py
@@ -293,7 +293,7 @@ def net_if_stats():
         isup, duplex, speed, mtu = items
         if hasattr(_common, 'NicDuplex'):
             duplex = _common.NicDuplex(duplex)
-        ret[name] = _common.snicstats(isup, duplex, speed, mtu, None)
+        ret[name] = _common.snicstats(isup, duplex, speed, mtu, '')
     return ret
 
 
diff --git a/psutil/_psutil_posix.c b/psutil/_psutil_posix.c
index 877e81375..ddc3d6b89 100644
--- a/psutil/_psutil_posix.c
+++ b/psutil/_psutil_posix.c
@@ -9,6 +9,7 @@
 #include <Python.h>
 #include <errno.h>
 #include <stdlib.h>
+#include <stdbool.h>
 #include <sys/resource.h>
 #include <sys/types.h>
 #include <signal.h>
@@ -429,6 +430,24 @@ psutil_net_if_mtu(PyObject *self, PyObject *args) {
     return PyErr_SetFromErrno(PyExc_OSError);
 }
 
+static bool
+check_and_append_iff_flag(PyObject *py_retlist, short int flags, short int flag_to_check, const char * flag_name)
+{
+    PyObject *py_str = NULL;
+
+    if (flags & flag_to_check) {
+        py_str = PyUnicode_DecodeFSDefault(flag_name);
+        if (! py_str)
+            return false;
+        if (PyList_Append(py_retlist, py_str)) {
+            Py_DECREF(py_str);
+            return false;
+        }
+        Py_CLEAR(py_str);
+    }
+
+    return true;
+}
 
 /*
  * Get all of the NIC flags and return them.
@@ -440,89 +459,151 @@ psutil_net_if_flags(PyObject *self, PyObject *args) {
     int ret;
     struct ifreq ifr;
     PyObject *py_retlist = PyList_New(0);
-    PyObject *py_flag = NULL;
     short int flags;
 
     if (py_retlist == NULL)
         return NULL;
 
     if (! PyArg_ParseTuple(args, "s", &nic_name))
-        return NULL;
+        goto error;
 
     sock = socket(AF_INET, SOCK_DGRAM, 0);
-    if (sock == -1)
-        return NULL;
+    if (sock == -1) {
+        PyErr_SetFromErrno(PyExc_OSError);
+        goto error;
+    }
 
     PSUTIL_STRNCPY(ifr.ifr_name, nic_name, sizeof(ifr.ifr_name));
     ret = ioctl(sock, SIOCGIFFLAGS, &ifr);
-    if (ret == -1)
+    if (ret == -1) {
+        PyErr_SetFromErrno(PyExc_OSError);
         goto error;
+    }
 
     close(sock);
     sock = -1;
 
     flags = ifr.ifr_flags & 0xFFFF;
 
-    if (flags & IFF_UP) {
-        py_flag = PyUnicode_DecodeFSDefault("up");
-        if (PyList_Append(py_retlist, py_flag))
-            goto error;
-    }
-    if (flags & IFF_BROADCAST) {
-        py_flag = PyUnicode_DecodeFSDefault("broadcast");
-        if (PyList_Append(py_retlist, py_flag))
-            goto error;
-    }
-    if (flags & IFF_DEBUG) {
-        py_flag = PyUnicode_DecodeFSDefault("debug");
-        if (PyList_Append(py_retlist, py_flag))
-            goto error;
-    }
-    if (flags & IFF_LOOPBACK) {
-        py_flag = PyUnicode_DecodeFSDefault("loopback");
-        if (PyList_Append(py_retlist, py_flag))
-            goto error;
-    }
-    if (flags & IFF_POINTOPOINT) {
-        py_flag = PyUnicode_DecodeFSDefault("pointopoint");
-        if (PyList_Append(py_retlist, py_flag))
-            goto error;
-    }
-    if (flags & IFF_NOTRAILERS) {
-        py_flag = PyUnicode_DecodeFSDefault("notrailers");
-        if (PyList_Append(py_retlist, py_flag))
-            goto error;
-    }
-    if (flags & IFF_RUNNING) {
-        py_flag = PyUnicode_DecodeFSDefault("running");
-        if (PyList_Append(py_retlist, py_flag))
-            goto error;
-    }
-    if (flags & IFF_NOARP) {
-        py_flag = PyUnicode_DecodeFSDefault("noarp");
-        if (PyList_Append(py_retlist, py_flag))
-            goto error;
-    }
-    if (flags & IFF_PROMISC) {
-        py_flag = PyUnicode_DecodeFSDefault("promisc");
-        if (PyList_Append(py_retlist, py_flag))
-            goto error;
-    }
-    if (flags & IFF_ALLMULTI) {
-        py_flag = PyUnicode_DecodeFSDefault("allmulti");
-        if (PyList_Append(py_retlist, py_flag))
-            goto error;
-    }
-    if (flags & IFF_MULTICAST) {
-        py_flag = PyUnicode_DecodeFSDefault("multicast");
-        if (PyList_Append(py_retlist, py_flag))
-            goto error;
-    }
+    // Linux/glibc IFF flags: https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/gnu/net/if.h;h=251418f82331c0426e58707fe4473d454893b132;hb=HEAD
+    // macOS IFF flags: https://opensource.apple.com/source/xnu/xnu-792/bsd/net/if.h.auto.html
+    // AIX IFF flags: https://www.ibm.com/support/pages/how-hexadecimal-flags-displayed-ifconfig-are-calculated
+    // FreeBSD IFF flags: https://www.freebsd.org/cgi/man.cgi?query=if_allmulti&apropos=0&sektion=0&manpath=FreeBSD+10-current&format=html
+
+#ifdef IFF_UP
+    // Available in (at least) Linux, macOS, AIX, BSD
+    if (!check_and_append_iff_flag(py_retlist, flags, IFF_UP, "up"))
+        goto error;
+#endif
+#ifdef IFF_BROADCAST
+    // Available in (at least) Linux, macOS, AIX, BSD
+    if (!check_and_append_iff_flag(py_retlist, flags, IFF_BROADCAST, "broadcast"))
+        goto error;
+#endif
+#ifdef IFF_DEBUG
+    // Available in (at least) Linux, macOS, BSD
+    if (!check_and_append_iff_flag(py_retlist, flags, IFF_DEBUG, "debug"))
+        goto error;
+#endif
+#ifdef IFF_LOOPBACK
+    // Available in (at least) Linux, macOS, BSD
+    if (!check_and_append_iff_flag(py_retlist, flags, IFF_LOOPBACK, "loopback"))
+        goto error;
+#endif
+#ifdef IFF_POINTOPOINT
+    // Available in (at least) Linux, macOS, BSD
+    if (!check_and_append_iff_flag(py_retlist, flags, IFF_POINTOPOINT, "pointopoint"))
+        goto error;
+#endif
+#ifdef IFF_NOTRAILERS
+    // Available in (at least) Linux, macOS, AIX
+    if (!check_and_append_iff_flag(py_retlist, flags, IFF_NOTRAILERS, "notrailers"))
+        goto error;
+#endif
+#ifdef IFF_RUNNING
+    // Available in (at least) Linux, macOS, AIX, BSD
+    if (!check_and_append_iff_flag(py_retlist, flags, IFF_RUNNING, "running"))
+        goto error;
+#endif
+#ifdef IFF_NOARP
+    // Available in (at least) Linux, macOS, BSD
+    if (!check_and_append_iff_flag(py_retlist, flags, IFF_NOARP, "noarp"))
+        goto error;
+#endif
+#ifdef IFF_PROMISC
+    // Available in (at least) Linux, macOS, BSD
+    if (!check_and_append_iff_flag(py_retlist, flags, IFF_PROMISC, "promisc"))
+        goto error;
+#endif
+#ifdef IFF_ALLMULTI
+    // Available in (at least) Linux, macOS, BSD
+    if (!check_and_append_iff_flag(py_retlist, flags, IFF_ALLMULTI, "allmulti"))
+        goto error;
+#endif
+#ifdef IFF_MASTER
+    // Available in (at least) Linux
+    if (!check_and_append_iff_flag(py_retlist, flags, IFF_MASTER, "master"))
+        goto error;
+#endif
+#ifdef IFF_SLAVE
+    // Available in (at least) Linux
+    if (!check_and_append_iff_flag(py_retlist, flags, IFF_SLAVE, "slave"))
+        goto error;
+#endif
+#ifdef IFF_MULTICAST
+    // Available in (at least) Linux, macOS, BSD
+    if (!check_and_append_iff_flag(py_retlist, flags, IFF_MULTICAST, "multicast"))
+        goto error;
+#endif
+#ifdef IFF_PORTSEL
+    // Available in (at least) Linux
+    if (!check_and_append_iff_flag(py_retlist, flags, IFF_PORTSEL, "portsel"))
+        goto error;
+#endif
+#ifdef IFF_AUTOMEDIA
+    // Available in (at least) Linux
+    if (!check_and_append_iff_flag(py_retlist, flags, IFF_AUTOMEDIA, "automedia"))
+        goto error;
+#endif
+#ifdef IFF_DYNAMIC
+    // Available in (at least) Linux
+    if (!check_and_append_iff_flag(py_retlist, flags, IFF_DYNAMIC, "dynamic"))
+        goto error;
+#endif
+#ifdef IFF_OACTIVE
+    // Available in (at least) macOS, BSD
+    if (!check_and_append_iff_flag(py_retlist, flags, IFF_OACTIVE, "oactive"))
+        goto error;
+#endif
+#ifdef IFF_SIMPLEX
+    // Available in (at least) macOS, AIX, BSD
+    if (!check_and_append_iff_flag(py_retlist, flags, IFF_SIMPLEX, "simplex"))
+        goto error;
+#endif
+#ifdef IFF_LINK0
+    // Available in (at least) macOS, BSD
+    if (!check_and_append_iff_flag(py_retlist, flags, IFF_LINK0, "link0"))
+        goto error;
+#endif
+#ifdef IFF_LINK1
+    // Available in (at least) macOS, BSD
+    if (!check_and_append_iff_flag(py_retlist, flags, IFF_LINK1, "link1"))
+        goto error;
+#endif
+#ifdef IFF_LINK2
+    // Available in (at least) macOS, BSD
+    if (!check_and_append_iff_flag(py_retlist, flags, IFF_LINK2, "link2"))
+        goto error;
+#endif
+#ifdef IFF_D2
+    // Available in (at least) AIX
+    if (!check_and_append_iff_flag(py_retlist, flags, IFF_D2, "d2"))
+        goto error;
+#endif
 
     return py_retlist;
 
 error:
-    Py_XDECREF(py_flag);
     Py_DECREF(py_retlist);
     if (sock != -1)
         close(sock);
diff --git a/psutil/_pswindows.py b/psutil/_pswindows.py
index 57ace466b..7d882b774 100644
--- a/psutil/_pswindows.py
+++ b/psutil/_pswindows.py
@@ -386,7 +386,7 @@ def net_if_stats():
         isup, duplex, speed, mtu = items
         if hasattr(_common, 'NicDuplex'):
             duplex = _common.NicDuplex(duplex)
-        ret[name] = _common.snicstats(isup, duplex, speed, mtu, None)
+        ret[name] = _common.snicstats(isup, duplex, speed, mtu, '')
     return ret