dnsdist fatal Lua error Caught exception: Error creating a xsk socket of if_name

[vincentmli]

Hi,

I have this dnsdist.conf using xsk

addCapabilitiesToRetain("CAP_SYS_ADMIN")
addCapabilitiesToRetain("CAP_NET_ADMIN")
addCapabilitiesToRetain("CAP_NET_RAW")

xsk = newXsk({ifName='green0', NIC_queue_id=0, frameNums=65536, xskMapPath='/sys/fs/bpf/dnsdist/xsk_map'})

addLocal("0.0.0.0:53", {xskSocket=xsk})

newServer("8.8.8.8:53")
newServer("1.1.1.1:53")

it works when I start dnsdist from command line like dnsdist --supervised &

but it failed when I use dnsdist init startup script, it appears Lua config parsing error

/etc/rc.d/init.d/dnsdist start

dnsdist log:

Jun 21 08:42:39 loongfire dnsdist[25410]: Configuration '/etc/dnsdist.conf' OK!
Jun 21 08:42:39 loongfire dnsdist[25415]: dnsdist 1.9.10 comes with ABSOLUTELY NO WARRANTY. This is free software, and you are welcome to redistribute it according to the terms of the GPL version 2
Jun 21 08:42:39 loongfire dnsdist[25415]: Fatal Lua error: [string "chunk"]:5: Caught exception: Error creating a xsk socket of if_name green0: Unknown error -16 stack traceback: ^I[C]: in function 'newXsk' ^I[string "chunk"]:5: in main chunk

the dnsdist init scropt below

. /etc/sysconfig/rc
. ${rc_functions}

[ -r "/etc/sysconfig/dnsdist" ] && . /etc/sysconfig/dnsdist

check_config() {
	if ! /usr/bin/dnsdist --check-config >/dev/null; then
		boot_mesg "dnsdist configuration file contains errors" "${FAILURE}"
		echo_failure
		return 1
	fi

	return 0
}

case "${1}" in
	start)
		if ! check_config; then
			exit 1
		fi

		boot_mesg "Starting dnsdist..."

		# Increasing maximum number of open files
		ulimit -n 65536

		# Starting daemon
	
	         /usr/bin/dnsdist --supervised >/dev/null &
		evaluate_retval
		;;

	stop)
		boot_mesg "Stopping dnsdist..."
		killproc /usr/bin/dnsdist
		;;

	reload)
		if ! check_config; then
			exit 1
		fi

		boot_mesg "Reloading dnsdist..."
		reloadproc /usr/bin/dnsdist
		;;

	restart)
		if ! check_config; then
			exit 1
		fi

		${0} stop
		sleep 1
		${0} start
		;;

	status)
		statusproc /usr/bin/dnsdist
		;;

	*)
		echo "Usage: ${0} {start|stop|reload|restart|status}"
		exit 1
		;;
esac

[rgacogne]

but it failed when I use dnsdist init startup script, it appears Lua config parsing error
[...]

Jun 21 08:42:39 loongfire dnsdist[25410]: Configuration '/etc/dnsdist.conf' OK!
Jun 21 08:42:39 loongfire dnsdist[25415]: dnsdist 1.9.10 comes with ABSOLUTELY NO WARRANTY. This is free software, and you are welcome to redistribute it according to the terms of the GPL version 2
Jun 21 08:42:39 loongfire dnsdist[25415]: Fatal Lua error: [string "chunk"]:5: Caught exception: Error creating a xsk socket of if_name green0: Unknown error -16 stack traceback: ^I[C]: in function 'newXsk' ^I[string "chunk"]:5: in main chunk

This is not a Lua parsing error. The creation of an AF_XDP socket object via xsk_socket__create() fails with -EBUSY which usually means Device or resource busy. My guess, based on the discussion in 1, is that the socket is created during the --check-config run, deleted, but not released by the kernel quickly enough before dnsdist is started. The options I see are:

• disable the config-check part of the startup
• do not create the XSK socket during config-check. You could try it like this:

addCapabilitiesToRetain("CAP_SYS_ADMIN")
addCapabilitiesToRetain("CAP_NET_ADMIN")
addCapabilitiesToRetain("CAP_NET_RAW")

if not inConfigCheck() then
  xsk = newXsk({ifName='green0', NIC_queue_id=0, frameNums=65536, xskMapPath='/sys/fs/bpf/dnsdist/xsk_map'})
  addLocal("0.0.0.0:53", {xskSocket=xsk})
else
  addLocal("0.0.0.0:53")
end

newServer("8.8.8.8:53")
newServer("1.1.1.1:53")

None of these options are very satisfying, but at the very least it would tell us if we are on the right track.

[vincentmli]

@rgacogne thanks! your suggestion resolves the problem