Zabbix 4.4.5: Slack Media Type: TCP CLOSE-WAIT

[sscheib]

Hello community,

I have setup the new Slack media type as described here: https://git.zabbix.com/projects/ZBX/...es/media/slack
It seems to be working - somewhat. I get a message whenever a trigger gets fired and additionally the message gets updated, if the issue is resolved again.
The only issue I encounter is, that whenever a new notification gets send out via Slack it leaves (for some reason) a TCP socket behind in a CLOSE-WAIT state. This is actually reproducable: Every single time a trigger gets fired a couple of seconds later a new TCP socket in CLOSE-WAIT state is appearing.
Once I disable the media type Slack and restart the zabbix-server, the TCP sockets in CLOSE-WAIT state are gone again and do not appear again. This is only the case for the media type Slack.

Please see the following output

root@zabbix:/home/steffen# ss | grep WAIT              
tcp   CLOSE-WAIT 78     0                                            10.10.20.8:41342                                  13.226.147.73:https                      
tcp   CLOSE-WAIT 78     0                                            10.10.20.8:44734                                143.204.214.159:https                      
tcp   CLOSE-WAIT 78     0                                            10.10.20.8:39360                                  13.35.250.164:https                      
tcp   CLOSE-WAIT 78     0                                            10.10.20.8:44940                                143.204.214.159:https

Unfortunately, in the Zabbix server log, there are no entries regarding Slack. The IPs are all just cloudfront addresses, but as they only appear once Slack is activated and right after a trigger has fired, I am pretty sure they will refer to Slack.


Some background information (I don't know whether its useful, but I'll simply post everything which is somewhat relevant):
{$ZABBIX_URL} global macro value: https://zabbix.lan (also tried zabbix.lan and http://zabbix.lan)
Zabbix server version: 4.4.5
OS: Raspbian 10.2 (buster), with latest updates installed
Slack: free plan
Hardware: Raspberry Pi 3 Model B Plus Rev 1.3:
Guide used: https://git.zabbix.com/projects/ZBX/...es/media/slack

Does anybody else experience the same issue or might even have a solution to this?
Thanks!