Check_mk agent not listening on IPv4 port

Hey all,
I have just installed the agent but I am unable to determine why the service port is listening on tcp6 only? I have searched previous posts but no other occurances of this - its perhaps something I have overlooked… a 2nd set of eyes would be much appreciated;

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Component Version
LibreNMS 1.51-53-g9d56593
DB Schema 2019_02_10_220000_add_dates_to_fdb (132)
PHP 7.2.11
MySQL 5.5.60-MariaDB
RRDTool 1.4.8
SNMP NET-SNMP 5.7.2

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[OK] Composer Version: 1.8.5
[OK] Dependencies up-to-date.
[OK] Database connection successful
[OK] Database schema correct

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Firewall ports is open.
[root@ch-netmon-01 local]# firewall-cmd --list-all
public (active)
target: default
icmp-block-inversion: no
interfaces: ens160
sources:
services: ssh dhcpv6-client http https syslog snmp
ports: 9995/udp 3000/tcp 21/tcp 6556/tcp
protocols:
masquerade: no
forward-ports:
source-ports:
icmp-blocks:
rich rules:

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[root@ch-netmon-01 local]# netstat -tln
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address Foreign Address State
tcp 0 0 127.0.0.1:199 0.0.0.0:* LISTEN
tcp 0 0 0.0.0.0:3306 0.0.0.0:* LISTEN
tcp 0 0 127.0.0.1:6379 0.0.0.0:* LISTEN
tcp 0 0 0.0.0.0:111 0.0.0.0:* LISTEN
tcp 0 0 0.0.0.0:80 0.0.0.0:* LISTEN
tcp 0 0 0.0.0.0:21 0.0.0.0:* LISTEN
tcp 0 0 0.0.0.0:22 0.0.0.0:* LISTEN
tcp 0 0 0.0.0.0:3000 0.0.0.0:* LISTEN
tcp 0 0 127.0.0.1:25 0.0.0.0:* LISTEN
tcp 0 0 0.0.0.0:514 0.0.0.0:* LISTEN
tcp6 0 0 :::111 :::* LISTEN
tcp6 0 0 :::22 :::* LISTEN
tcp6 0 0 ::1:25 :::* LISTEN
tcp6 0 0 :::443 :::* LISTEN
tcp6 0 0 :::6556 :::* LISTEN

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[root@ch-netmon-01 local]# systemctl status check_mk.socket
● check_mk.socket - Check_MK LibreNMS Agent Socket
Loaded: loaded (/etc/systemd/system/check_mk.socket; enabled; vendor preset: disabled)
Active: active (listening) since Wed 2019-05-22 11:24:00 CDT; 30min ago
Listen: [::]:6556 (Stream)
Accepted: 7; Connected: 0

May 22 11:24:00 ch-netmon-01 systemd[1]: Listening on Check_MK LibreNMS Agent Socket.

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I have checked the perl output; but am unclear on what could be causing this…
[root@ch-netmon-01 local]# perl apache
Data fetch failure.

From the doc?

The agent uses TCP-Port 6556, please allow access from the LibreNMS host and poller nodes if you’re using the Distributed Polling setup.

https://docs.librenms.org/Extensions/Agent-Setup/

Does it help if you change the listen line in /etc/systemd/system/check_mk.socket

from:
ListenStream=6556

to:
ListenStream=0.0.0.0:6556 [::]:6556

?

The systemd/socket/listenstream documentation isn’t super clear, but it does say…

If the address string is a single number, it is read as port number to listen on via IPv6. Depending on the value of BindIPv6Only= (see below) this might result in the service being available via both IPv6 and IPv4 (default) or just via IPv6.

So setting BindIPv6Only to both may work as well.

bingo! It was the syntax that I was unable to locate yesterday.

Many thanks