UDP listener of direct inbound is closed at startup on armv7 (kernel 4.1.52): recvmmsg: function not implemented

Operating system

Linux

System version

ASUS TUF-AX5400 router, ASUSWRT-Merlin firmware
Linux 4.1.52 (armv7l, 32-bit)

Installation type

Original sing-box Command Line

Version

sing-box version 1.14.0

Environment: go1.26.7 linux/arm
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
Revision: 0b8995879f29a9b98ee027bc17b75e101445b238
CGO: disabled

(unmodified official release: sing-box-1.14.0-linux-armv7.tar.gz)

Description

On this device, the UDP listener of a direct inbound is closed immediately at startup; only the TCP listener remains (verified with netstat). The same happens with the documented hijack-dns setup (direct "dns-in" inbound + route rule {"inbound":["dns-in"],"action":"hijack-dns"}).

Result: transparent DNS hijack via iptables REDIRECT (udp dport 53 -> the direct inbound) stops working for LAN clients - UDP DNS gets no response, so the whole network loses UDP DNS while TCP keeps working.

Reproduction

1. Unpack sing-box-1.14.0-linux-armv7.tar.gz on the device (kernel 4.1.52, armv7l).
2. Run: sing-box run -c config.json
3. netstat -lnt shows TCP :1054 LISTEN; netstat -lnu shows no UDP :1054 socket.

config.json (complete, minimal, no TUN / no remote server):

{
  "log": { "level": "warn" },
  "inbounds": [
    { "type": "direct", "tag": "dns-in", "listen": "::", "listen_port": 1054 }
  ],
  "outbounds": [ { "type": "direct", "tag": "d" } ]
}

The documented hijack-dns configuration pattern (direct dns-in inbound + route rule {"inbound":["dns-in"],"action":"hijack-dns"}, dns server type local) produces the same result on port 1055.

Logs

# uname -a
Linux TUF-AX5400-0368 4.1.52 #1 SMP PREEMPT Wed Jun 10 00:57:46 UTC 2026 armv7l ASUSWRT-Merlin

# sing-box run -c config.json
ERROR[0000] inbound/direct[dns-in]: udp listener closed: raw read: recvmmsg: function not implemented

# netstat -lnt | grep 1054
tcp        0      0 :::1054                 :::*                    LISTEN

# netstat -lnu | grep 1054
(no output)

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 with sing-box run -c config.json using the supplied minimal configuration on the affected armv7/Linux 4.1.52 environment, then trace UDP listener startup after the recvmmsg: function not implemented error. Confirm the change preserves TCP listening and allows the direct inbound, including the documented hijack-dns setup, to receive and respond to UDP traffic.