netdb/lib_dnsdelserver.c: support delete the DNS server address

by index or address

 - dns_del_nameserver()
 - dns_del_nameserver_by_index()

Update the "DNS function" section in 11_network.rst, and create
the netlib API documentation in netlib/index.rst

Signed-off-by: nuttxs <zhaoqing.zhang@sony.com>

libs/libc/netdb/lib_dnsdelserver.c

@@ -0,0 +1,227 @@
+/****************************************************************************
+ * libs/libc/netdb/lib_dnsdelserver.c
+ *
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.  The
+ * ASF licenses this file to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance with the
+ * License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  See the
+ * License for the specific language governing permissions and limitations
+ * under the License.
+ *
+ ****************************************************************************/
+
+/****************************************************************************
+ * Included Files
+ ****************************************************************************/
+
+#include <nuttx/config.h>
+
+#include <stdbool.h>
+#include <stdio.h>
+#include <string.h>
+#include <errno.h>
+#include <debug.h>
+
+#include <nuttx/net/ip.h>
+#include <nuttx/net/dns.h>
+
+#include "netdb/lib_dns.h"
+
+#ifdef CONFIG_NETDB_DNSCLIENT
+
+/****************************************************************************
+ * Private Functions
+ ****************************************************************************/
+
+#ifndef CONFIG_NETDB_RESOLVCONF
+
+static int dns_find_nameserver_index(FAR const struct sockaddr *addr,
+                                     socklen_t addrlen)
+{
+  size_t cmplen = 0;
+  int i;
+
+#ifdef CONFIG_NET_IPv4
+  if (addr->sa_family == AF_INET)
+    {
+      cmplen = sizeof(struct sockaddr_in);
+    }
+#endif
+
+#ifdef CONFIG_NET_IPv6
+  if (addr->sa_family == AF_INET6)
+    {
+      cmplen = sizeof(struct sockaddr_in6);
+    }
+#endif
+
+  if (cmplen == 0)
+    {
+      /* Unsupported address family */
+
+      return -ENOSYS;
+    }
+
+  if (addrlen < cmplen)
+    {
+      return -EINVAL;
+    }
+
+  dns_lock();
+
+  /* Search for the matching nameserver in the array */
+
+  for (i = 0; i < g_dns_nservers; i++)
+    {
+      if (g_dns_servers[i].addr.sa_family == addr->sa_family)
+        {
+#ifdef CONFIG_NET_IPv4
+          if (addr->sa_family == AF_INET)
+            {
+              FAR struct sockaddr_in *in1 =
+                  (FAR struct sockaddr_in *)&g_dns_servers[i].addr;
+              FAR struct sockaddr_in *in2 =
+                  (FAR struct sockaddr_in *)addr;
+
+              /* Compare only the IP address part, ignore port and padding */
+
+              if (net_ipv4addr_cmp(in1->sin_addr.s_addr,
+                                   in2->sin_addr.s_addr))
+                {
+                  dns_unlock();
+                  return i;
+                }
+            }
+#endif
+
+#ifdef CONFIG_NET_IPv6
+          if (addr->sa_family == AF_INET6)
+            {
+              FAR struct sockaddr_in6 *in6_1 =
+                  (FAR struct sockaddr_in6 *)&g_dns_servers[i].addr;
+              FAR struct sockaddr_in6 *in6_2 =
+                  (FAR struct sockaddr_in6 *)addr;
+
+              /* Compare only the IPv6 address part */
+
+              if (net_ipv6addr_cmp(in6_1->sin6_addr.s6_addr,
+                                   in6_2->sin6_addr.s6_addr))
+                {
+                  dns_unlock();
+                  return i;
+                }
+            }
+#endif
+        }
+    }
+
+  dns_unlock();
+  return -ENOENT;
+}
+
+#endif /* !CONFIG_NETDB_RESOLVCONF */
+
+/****************************************************************************
+ * Public Functions
+ ****************************************************************************/
+
+/****************************************************************************
+ * Name: dns_del_nameserver
+ *
+ * Description:
+ *   Remove a DNS server from the list by matching the address
+ *
+ ****************************************************************************/
+
+#ifdef CONFIG_NETDB_RESOLVCONF
+
+int dns_del_nameserver(FAR const struct sockaddr *addr, socklen_t addrlen)
+{
+  /* Not implemented for CONFIG_NETDB_RESOLVCONF */
+
+  return -ENOSYS;
+}
+
+int dns_del_nameserver_by_index(int index)
+{
+  /* For resolv.conf mode, removing requires rewriting the file */
+
+  return -ENOSYS;
+}
+
+#else /* !CONFIG_NETDB_RESOLVCONF */
+
+int dns_del_nameserver(FAR const struct sockaddr *addr, socklen_t addrlen)
+{
+  int index;
+
+  if (addr == NULL)
+    {
+      return -EINVAL;
+    }
+
+  /* Find the nameserver in the array */
+
+  index = dns_find_nameserver_index(addr, addrlen);
+  if (index < 0)
+    {
+      return index;  /* Return the error code */
+    }
+
+  /* Remove the server by index */
+
+  return dns_del_nameserver_by_index(index);
+}
+
+int dns_del_nameserver_by_index(int index)
+{
+  int i;
+
+  if (index < 0 || index >= CONFIG_NETDB_DNSSERVER_NAMESERVERS)
+    {
+      return -EINVAL;
+    }
+
+  dns_lock();
+
+  if (index >= g_dns_nservers)
+    {
+      dns_unlock();
+      return -ENOENT;
+    }
+
+  /* Shift all subsequent entries down by one position */
+
+  for (i = index; i < g_dns_nservers - 1; i++)
+    {
+      memcpy(&g_dns_servers[i], &g_dns_servers[i + 1],
+             sizeof(union dns_addr_u));
+    }
+
+  memset(&g_dns_servers[g_dns_nservers - 1], 0, sizeof(union dns_addr_u));
+
+  /* Decrement the server count */
+
+  g_dns_nservers--;
+
+  dns_unlock();
+
+#if CONFIG_NETDB_DNSCLIENT_ENTRIES > 0
+  /* Clear the DNS cache after removing a server */
+
+  dns_clear_answer();
+#endif
+
+  return OK;
+}
+
+#endif /* CONFIG_NETDB_RESOLVCONF */
+#endif /* CONFIG_NETDB_DNSCLIENT */