after some days (3/4) our zabbix server stops gathering data from a passive proxy, a netstat on the zabbix server shows the connection as "ESTABLISHED":
netstat -anp | grep 10.27.xxx.xxx
tcp 0 0 10.210.xxx.xxx:42313 10.27.xxx.xxx:10051 ESTABLISHED 11699/zabbix_server
but a netstat on the zabbix proxy shows no connection.
a tcpdump on the zabbix server shows no data is exchanged between the server and the proxy but a "telnet proxy_ip 10051" shows that it's possible to connect to the proxy, the only solution is to restart the zabbix_server, after that the connection is reestablished and all the data is received by the server.
The connection between zabbix server and proxy is a VPN through internet so it can happen that the proxy is unreachable for some time, unfortunately I can't try changing the proxy to active mode since the security policies doesn't allow this kind of connection.
It seems that the zabbix server thinks the connection is still working correctly and doesn't detect that no data is flowing so it doesn't kill and retry the connection until I manually reboot the whole server.