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

[nuttxs]

Summary

1. Update the "DNS function" section in 11_network.rst, and create the netlib API documentation in netlib/index.rst
2. netdb/lib_dnsdelserver.c: add DNS deletion functionality, support deleting DNS servers by address or by index:

dns_del_nameserver()
dns_del_nameserver_by_index()

Multi-DNS delete Call-Chain Implementation:
Application Layer: netlib_del_ipv4dnsaddr() / netlib_del_ipv6dnsaddr()
           ↓
Network Layer: dns_del_nameserver()
           ↓
Storage Layer: g_dns_servers[] array

Impact

New Feature/Change: Yes
User Impact: support delete the DNS server address by index or address
Build Impact:No new Kconfig options or build system changes.
Hardware Impact: No
Security: No
Compatibility: Backward-compatible; no breaking changes.

Testing

Refer to PR#apache/nuttx-apps#3206

[acassis]

@nuttxs could you please help us to create a basic documentation to DNS APIs to be included at https://nuttx.apache.org/docs/latest/reference/user/index.html ?

[cederom]

Thank you @nuttxs :-)

• Build errors please fix (https://github.com/apache/nuttx/actions/runs/19131263162/job/54672476451?pr=17284#step:7:682).

[acassis]

@nuttxs please add dns to user network doc: https://nuttx.apache.org/docs/latest/reference/user/11_network.html
I suggest keeping the existing documentation under "Basic socket functions" and add "DNS functions"

[nuttxs]

@acassis Okay, I will to add the corresponding "DNS functions" API documentation

[cederom]

Hmm, looks like boards/arm/sama5/jupiter-nano/ is impacted and needs a fix? https://github.com/apache/nuttx/actions/runs/19176611989/job/54823072207?pr=17284#step:7:682

====================================================================================
Configuration/Tool: jupiter-nano/sdmmc-nsh-net-resolvconf,CONFIG_ARM_TOOLCHAIN_GNU_EABI
2025-11-07 19:38:58
------------------------------------------------------------------------------------
  Cleaning...
  Configuring...
  Disabling CONFIG_ARM_TOOLCHAIN_GNU_EABI
  Enabling CONFIG_ARM_TOOLCHAIN_GNU_EABI
  Building NuttX...
Error: netdb/lib_dnsdelserver.c:44:12: error: 'dns_find_nameserver_index' defined but not used [-Werror=unused-function]
   44 | static int dns_find_nameserver_index(FAR const struct sockaddr *addr,
      |            ^~~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors
make[1]: *** [Makefile:156: bin/lib_dnsdelserver.o] Error 1
make[1]: Target 'libc.a' not remade because of errors.
make: *** [tools/LibTargets.mk:196: libs/libc/libc.a] Error 2
make: Target 'all' not remade because of errors.
/github/workspace/sources/nuttx/tools/testbuild.sh: line 385: /github/workspace/sources/nuttx/../nuttx/nuttx.manifest: No such file or directory
  [1/1] Normalize jupiter-nano/sdmmc-nsh-net-resolvconf

[nuttxs]

@cederom @acassis The basic documentation updates for "DNS functions" and "netlib" have been completed, and the related compilation errors have been fixed. Please review.

[cederom]

• Amazing work thank you @nuttxs :-)
• Builds fine now, documentation added, yeah! :-)
• Just discussion resolution with @anchao :-)

Already fixed