Decode hostnames strictly as ASCII.

diff --git a/Doc/library/socket.rst b/Doc/library/socket.rst
--- a/Doc/library/socket.rst
+++ b/Doc/library/socket.rst
@@ -255,6 +255,8 @@ The module :mod:`socket` exports the fol
 
    .. versionchanged:: 3.2
       parameters can now be passed as single keyword arguments.
+      Raises :exc:`UnicodeDecodeError` if any of the returned names
+      contain non-ASCII byte values.
 
 .. function:: getfqdn([name])
 
@@ -286,6 +288,10 @@ The module :mod:`socket` exports the fol
    resolution, and :func:`getaddrinfo` should be used instead for IPv4/v6 dual
    stack support.
 
+   .. versionchanged:: 3.2
+      Raises :exc:`UnicodeDecodeError` if any of the returned names
+      contain non-ASCII byte values.
+
 
 .. function:: gethostname()
 
@@ -300,6 +306,9 @@ The module :mod:`socket` exports the fol
    Note: :func:`gethostname` doesn't always return the fully qualified domain
    name; use ``getfqdn()`` (see above).
 
+   .. versionchanged:: 3.2
+      Raises :exc:`UnicodeDecodeError` if the returned name contains
+      non-ASCII byte values.
 
 .. function:: gethostbyaddr(ip_address)
 
@@ -311,6 +320,10 @@ The module :mod:`socket` exports the fol
    domain name, use the function :func:`getfqdn`. :func:`gethostbyaddr` supports
    both IPv4 and IPv6.
 
+   .. versionchanged:: 3.2
+      Raises :exc:`UnicodeDecodeError` if any of the returned names
+      contain non-ASCII byte values.
+
 
 .. function:: getnameinfo(sockaddr, flags)
 
@@ -319,6 +332,10 @@ The module :mod:`socket` exports the fol
    or numeric address representation in *host*.  Similarly, *port* can contain a
    string port name or a numeric port number.
 
+   .. versionchanged:: 3.2
+      Raises :exc:`UnicodeDecodeError` if the returned name contains
+      non-ASCII byte values.
+
 
 .. function:: getprotobyname(protocolname)
 
diff --git a/Modules/socketmodule.c b/Modules/socketmodule.c
--- a/Modules/socketmodule.c
+++ b/Modules/socketmodule.c
@@ -739,6 +739,15 @@ PyThread_type_lock netdb_lock;
 #endif
 
 
+/* Return the string representation for the given hostname. */
+
+static PyObject *
+decode_hostname(const char *name)
+{
+    return PyUnicode_DecodeASCII(name, strlen(name), "strict");
+}
+
+
 /* Convert a string specifying a host name or one of a few symbolic
    names to a numeric IP address.  This usually calls gethostbyname()
    to do the work; the names "" and "<broadcast>" are special.
@@ -2983,7 +2992,7 @@ socket_gethostname(PyObject *self, PyObj
     if (res < 0)
         return set_error();
     buf[sizeof buf - 1] = '\0';
-    return PyUnicode_FromString(buf);
+    return decode_hostname(buf);
 }
 
 PyDoc_STRVAR(gethostname_doc,
@@ -3069,7 +3078,7 @@ gethost_common(struct hostent *h, struct
     if (h->h_aliases) {
         for (pch = h->h_aliases; *pch != NULL; pch++) {
             int status;
-            tmp = PyUnicode_FromString(*pch);
+            tmp = decode_hostname(*pch);
             if (tmp == NULL)
                 goto err;
 
@@ -3137,7 +3146,8 @@ gethost_common(struct hostent *h, struct
             goto err;
     }
 
-    rtn_tuple = Py_BuildValue("sOO", h->h_name, name_list, addr_list);
+    rtn_tuple = Py_BuildValue("NOO", decode_hostname(h->h_name),
+                              name_list, addr_list);
 
  err:
     Py_XDECREF(name_list);
@@ -3909,9 +3919,9 @@ socket_getaddrinfo(PyObject *self, PyObj
             makesockaddr(-1, res->ai_addr, res->ai_addrlen, protocol);
         if (addr == NULL)
             goto err;
-        single = Py_BuildValue("iiisO", res->ai_family,
+        single = Py_BuildValue("iiiNO", res->ai_family,
             res->ai_socktype, res->ai_protocol,
-            res->ai_canonname ? res->ai_canonname : "",
+            decode_hostname(res->ai_canonname ? res->ai_canonname : ""),
             addr);
         Py_DECREF(addr);
         if (single == NULL)
@@ -4011,7 +4021,7 @@ socket_getnameinfo(PyObject *self, PyObj
         set_gaierror(error);
         goto fail;
     }
-    ret = Py_BuildValue("ss", hbuf, pbuf);
+    ret = Py_BuildValue("Ns", decode_hostname(hbuf), pbuf);
 
 fail:
     if (res)