netutils/netlib: support delete the DNS server address by index or address

[nuttxs]

Summary

netlib/netlib_delipv4dnsaddr.c: add DNS deletion functionality, support deleting DNS servers by address or by index:

Modify the DHCP-client data structure to support multiple DNS addresses.
Enhance DHCP-option parsing to extract all DNS-server addresses.
Set all received DNS-server addresses.
Fix some example case compiler error.

netlib_del_ipv4dnsaddr()
netlib_del_ipv4dnsaddr_by_index()
netlib_del_ipv6dnsaddr()
netlib_del_ipv6dnsaddr_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

Depend on PR#apache/nuttx#17284

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

Test process example

// Add the first DNS server
netlib_set_ipv4dnsaddr(&dns1);

// Add the second DNS server
netlib_set_ipv4dnsaddr(&dns2);

// List all DNS servers
dns_foreach_nameserver(print_dns_callback, &count);

// Delete DNS server by index
netlib_del_ipv4dnsaddr_by_index(0);

// Delete DNS server by address
netlib_del_ipv4dnsaddr(&dns2);

[cederom]

Thank you @nuttxs :-) Some issues:

• Build errors, please fix.
• PR and Commit topic does not match content, it also changes DHCP functionalities, please update (i.e. netutils/netlib: DHCP and DNS related updates. then bullet point changes already there).

[xiaoxiang781216]

Thank you @nuttxs :-) Some issues:

* Build errors, please fix.

* PR and Commit topic does not match content, it also changes DHCP functionalities, please update (i.e. netutils/netlib: DHCP and DNS related updates. then bullet point changes already there).

apache/nuttx#17284 need be merged first.

[cederom]

Thanks @xiaoxiang781216 I missed the dependency on apache/nuttx#17284 sorry :-)

@nuttxs please update git commit message to:

netutils/netlib: DHCP and DNS related updates.

* new functionality: delete the DNS server address by index or address.
* Modify the DHCP-client data structure to support multiple DNS addresses.
* Enhance DHCP-option parsing to extract all DNS-server addresses.
* Set all received DNS-server addresses.

Signed..

[acassis]

@nuttxs could you please help to create some basic Documentation about netlib: https://nuttx.apache.org/docs/latest/applications/netutils/netlib/index.html ?

[nuttxs]

The basic documentation update for “netlib” has been completed—please verify it (https://github.com/apache/nuttx/pull/17284/files#diff-a3d43ff48c0c7f39a7306020ac95d948edde9aebb9a3d30cdd13fcdff2d0b206) meets the requirements.
As for the compilation errors, I will push update again once PR #17284 (apache/nuttx#17284) is merged.

[cederom]

Amazing work thank you @nuttxs :-)

NOTE: This PR depends on apache/nuttx#17284 and should be verified again after dependency merge :-)

[xiaoxiang781216]

@nuttxs please fix the compiler error:

====================================================================================
Configuration/Tool: stm3240g-eval/discover,CONFIG_ARM_TOOLCHAIN_GNU_EABI
2025-11-09 10:25:57
------------------------------------------------------------------------------------
  Cleaning...
  Configuring...
  Disabling CONFIG_ARM_TOOLCHAIN_GNU_EABI
  Enabling CONFIG_ARM_TOOLCHAIN_GNU_EABI
  Building NuttX...
chip/stm32_gpio.c:44:11: note: '#pragma message: CONFIG_STM32_USE_LEGACY_PINMAP will be deprecated migrate board.h see tools/stm32_pinmap_tool.py'
   44 | #  pragma message "CONFIG_STM32_USE_LEGACY_PINMAP will be deprecated migrate board.h see tools/stm32_pinmap_tool.py"
      |           ^~~~~~~
discover_main.c: In function 'discover_main':
Error: discover_main.c:155:21: error: '(struct in_addr *)&ds.dnsaddr' is a pointer; did you mean to use '->'?
  155 |       if (ds.dnsaddr.s_addr != 0)
      |                     ^
      |                     ->
Error: discover_main.c:157:34: error: passing argument 1 of 'netlib_set_ipv4dnsaddr' from incompatible pointer type [-Werror=incompatible-pointer-types]
  157 |           netlib_set_ipv4dnsaddr(&ds.dnsaddr);
      |                                  ^~~~~~~~~~~
      |                                  |
      |                                  struct in_addr (*)[1]
In file included from discover_main.c:37:
/github/workspace/sources/apps/include/netutils/netlib.h:484:54: note: expected 'const struct in_addr *' but argument is of type 'struct in_addr (*)[1]'
  484 | int netlib_set_ipv4dnsaddr(FAR const struct in_addr *inaddr);
      |                                ~~~~~~~~~~~~~~~~~~~~~~^~~~~~