Validate Dns hostname for embedded null characters

src/libraries/System.Net.NameResolution/src/Resources/Strings.resx

@@ -72,6 +72,9 @@
   <data name="net_invalid_ip_addr" xml:space="preserve">
     <value>IPv4 address 0.0.0.0 and IPv6 address ::0 are unspecified addresses that cannot be used as a target address.</value>
   </data>
+  <data name="net_hostname_invalid_character" xml:space="preserve">
+    <value>The host name cannot contain a null character.</value>
+  </data>
   <data name="SystemNetNameResolution_PlatformNotSupported" xml:space="preserve">
     <value>System.Net.NameResolution is not supported on this platform.</value>
   </data>

src/libraries/System.Net.NameResolution/src/System/Net/Dns.cs

@@ -895,6 +895,13 @@ private static void ValidateHostName(string hostName)
                 throw new ArgumentOutOfRangeException(nameof(hostName),
                     SR.Format(SR.net_toolong, nameof(hostName), MaxHostName.ToString(NumberFormatInfo.CurrentInfo)));
             }
+
+            // The hostname is passed to native APIs that treat '\0' as the end of the string,
+            // so embedded null characters would silently truncate the name. Reject them up front.
+            if (hostName.Contains('\0'))
+            {
+                throw new ArgumentException(SR.net_hostname_invalid_character, nameof(hostName));
+            }
         }
 
         private static bool LogFailure(object hostNameOrAddress, in NameResolutionActivity activity, Exception exception)

src/libraries/System.Net.NameResolution/tests/FunctionalTests/GetHostEntryTest.cs

@@ -255,6 +255,20 @@ public async Task DnsGetHostEntry_BadName_ThrowsArgumentOutOfRangeException_Obso
             await Assert.ThrowsAnyAsync<ArgumentOutOfRangeException>(() => Task.Factory.FromAsync(Dns.BeginGetHostEntry, Dns.EndGetHostEntry, hostNameOrAddress, null));
         }
 
+        [Theory]
+        [InlineData("\0")]
+        [InlineData("\0host")]
+        [InlineData("host\0")]
+        [InlineData("ho\0st")]
+        [InlineData("host\0name")]
+        public async Task DnsGetHostEntry_NullCharacterInName_ThrowsArgumentException(string hostNameOrAddress)
+        {
+            Assert.Throws<ArgumentException>(() => Dns.GetHostEntry(hostNameOrAddress));
+            Assert.Throws<ArgumentException>(() => Dns.GetHostAddresses(hostNameOrAddress));
+            await Assert.ThrowsAsync<ArgumentException>(() => Dns.GetHostEntryAsync(hostNameOrAddress));
+            await Assert.ThrowsAsync<ArgumentException>(() => Dns.GetHostAddressesAsync(hostNameOrAddress));
+        }
+
         [Theory]
         [InlineData(0)]
         [InlineData(1)]