Thank you for your report, reproduced also for passive check without discovery.

Here is how it worked previously:
Frontend:

Received empty response from Zabbix Agent at [127.0.0.1]. Assuming that agent dropped connection because of access permissions.

Server log with warning level:

 31294:20180222:095319.526 Zabbix agent item "agent.ping" on host "Zabbix server" failed: first network error, wait for 15 seconds
 31296:20180222:095334.562 Zabbix agent item "agent.ping" on host "Zabbix server" failed: another network error, wait for 15 seconds
 31296:20180222:095349.670 Zabbix agent item "agent.ping" on host "Zabbix server" failed: another network error, wait for 15 seconds
 31296:20180222:095404.720 temporarily disabling Zabbix agent checks on host "Zabbix server": host unavailable

For discovery
Server log with debug level

 31860:20180222:095920.818 discovery: item [system.uname] error: Received empty response from Zabbix Agent at [127.0.0.1]. Assuming that agent dropped connection because of access permissions.

Old behavior must be restored.

Fixed in development branch:
svn://svn.zabbix.com/branches/dev/ZBX-13523

thank you for doing a more throughout testing.

could you please clarify on the changes - to what extent was the old behaviour restored ?
that is, will discovery stop logging these messages ?

what will happen if a passive agent version 1.0 is queried by the server ?

vso yes, it will stop logging the messages, behavior will be same as before, I am not sure about 1.0 do you have one to try ?

<richlv> not right now, but if there was a specification, what would it say about such a scenario ?

• server connects to agent, sends "HEADERsomekey"
• agent responds with "1"

vso I think in that case agent would think that only ZBXD\1\6 was received but I am not sure if Zabbix server 4.0 works with agent 1.0 at all.

Successfully tested

Fixed in:

• pre-4.0.0alpha5 (trunk) r78137

I have installed zabbix agent in version 4.0.0alpha7 and server is 3.0.18. After configuration passive monitoring I see this error message in zabbix agent log

Message from $ZABBIX_SERVER_IP is missing header. Message ignored.

There is no compatibility between 4.0 and 3.0?

Any update?

Server: Zabbix 3.0.22

Client: 4.0.0beta2

Result: Message from XXX is missing header. Message ignored.

This is documented in upgrade notes:

Plain text protocol dropped

Support for the plain text protocol has been dropped and a header is now mandatory. A header has been added to Zabbix get requests, Zabbix server/proxy passive check requests and frontend requests to Zabbix server.

As a consequence, Zabbix agents that are older than version 1.4 are no longer supported. Also, messages from self-written senders will be rejected if the header is absent. Whereas previously Zabbix trappers would accept messages without headers as well as messages with headers, now they will only accept messages with protocol header.

I'm affected by this as well (Zabbix Server: 3.0, one of the agents in 4.0).
What I don't understand, the above quoted note says "agents that are older than version 1.4" but we are talking about the other way round: agent is newer than Zabbix server.

IMO this scenario might affect more users where the server is still 3.x while some of the monitored agents get updated before the server and then monitoring will break.

Wouldn't it be possible to add some compatibility layer for exactly this case?