sing-box failed to start when DNS is set as dhcp://auto

[ShadowOfTheDay]

Operating system

iOS

System version

18.0.1

Installation type

sing-box for iOS Graphical Client

If you are using a graphical client, please provide the version of the client.

1.9.7

Version

No response

Description

When dhcp://auto is set as one of the DNS servers, sing-box for iOS fails to start and throws the following Service Error:

(packet-tunnel) error: start service: initialize DNS server 0: write udp4
0.0.0.0:68->255.255.255.255:67：
sendto: no route to host

The error can be reproduced on my iPhone 100% of the time regardless of the network (Wi-Fi or cellular data). Interestingly, on my mac, on one day the same error is shown in 9/10 of the attempts, but the next day it runs just fine. I was not able to find a pattern.

Similar issues were raised by other users #2153 #1952 #1593 #1467 #1344 (some are in the replies) for different platforms, but most were marked as invalid or non-reproducible by developers.

Reproduction

Minimal configuration:

{
  "dns": {
    "servers": [
      {
        "tag": "local",
        "address": "dhcp://auto",
        //"address": "223.5.5.5",
        "detour": "direct"
      }
    ]
  }
}

Logs

The "crash log" is empty as sing-box fails to start. Below is the log: 

(packet-tunnel): Here I stand
INFO[0000] router: updated default interface pdp_ip0, index 13
INFO[0000] dns/transport[local]: dhcp: query DNS servers on pdp_ip0
(packet-tunnel) error: start service: initialize DNS server[0]: write udp4 0.0.0.0:68->255.255.255.255:67:
sendto: no route to host
(packet-tunnel) stopping, reason:
NEProviderStopReason (rawValue: 1)

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### Integrity requirements

- [X] I confirm that I have read the documentation, understand the meaning of all the configuration items I wrote, and did not pile up seemingly useful options or default values.
- [X] I confirm that I have provided the server and client configuration files and process that can be reproduced locally, instead of a complicated client configuration file that has been stripped of sensitive data.
- [X] I confirm that I have provided the simplest configuration that can be used to reproduce the error I reported, instead of depending on remote servers, TUN, graphical interface clients, or other closed-source software.
- [X] I confirm that I have provided the complete configuration files and logs, rather than just providing parts I think are useful out of confidence in my own intelligence.