Tun on host doesn't work when namespace is used

Operating system

Linux

System version

NixOS 26.11

Installation type

Original sing-box Command Line

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

No response

Version

sing-box version 1.14.1

Environment: go1.26.7 linux/amd64
Tags: with_gvisor,with_quic,with_dhcp,with_wireguard,with_utls,with_acme,with_clash_api,with_tailscale,with_ccm,with_ocm,with_cloudflared,with_usbip,with_openvpn,with_openconnect,badlinkname,tfogo_checklinkname0,with_naive_outbound
CGO: enabled

Description

The config:

{
  "inbounds": [
    {
      "address": [
        "172.19.0.1/30"
      ],
      "strict_route": false,
      "tag": "tun-in",
      "type": "tun"
    }
  ],
  "network_namespaces": [
    {
      "type": "unshare",
      "tag": "proxy",
      "pid_file": "proxy.pid"
    }
  ]
}

Get error

FATAL[0000] start service: start inbound/tun[tun-in]: configure tun interface: open tun: TUNSETIFF: operation not permitted

It works with a tun on host with on namespace or a tun in the namespace. But if there is a namespace then it can't setup the tun on host. It seems when it setup the namespace it loses all the caps.

Reproduction

Use

{
  "inbounds": [
    {
      "address": [
        "172.19.0.1/30"
      ],
      "strict_route": false,
      "tag": "tun-in",
      "type": "tun"
    }
  ],
  "network_namespaces": [
    {
      "type": "unshare",
      "tag": "proxy",
      "pid_file": "proxy.pid"
    }
  ]
}

without root.

Logs

Supporter

• I am a sponsor

Integrity requirements

• 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.
• 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.
• 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.
• 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.

Start by reproducing the minimal configuration with a tun inbound and an unshare network_namespaces entry on Linux without root. Trace how the host TUN interface is configured after namespace setup, comparing it with the working host-only and namespace-only cases; done means the host TUN initializes successfully while the namespace still behaves as configured.