Discovery rule doesn't work with specific proxy

[cjbruck]

I started my Zabbix environment with a single server that monitored everything. As I started to overwhelm it with requests, I added proxy servers at all sites with servers to spread the load out. We added a server at another site and I added a Zabbix proxy to that server as well. However, for some reason this proxy in particular will not work with discovery rules. It never populates devices in Monitoring>Discovery. I can assign the same discovery rule to the main server or any of the other proxies and it works fine. I've been passively researching this for several months and have been unsuccessful in finding the issue. Finally, I deleted the Zabbix proxy VM and recreated it from scratch (Ubuntu Server 24.04 is the platform), but I'm having the same issue. I then created a VM in the same network as the main server to rule out firewall/ACL blocks and it didn't work either. I did a manual snmpwalk on the proxy and confirmed that I'm able to get a correct response for the query I'm making. I'm beginning to think that when I created the other proxies I did something in the configuration that I'm forgetting to do now. Any help in diagnosing the issue is appreciated.

Main server config: https://pastebin.com/SZZ0xRQ6
Example working proxy config: https://pastebin.com/btWSTFWb
Non-working proxy config: https://pastebin.com/kiatRsZz

[cjbruck]

So, I discovered something (pun intended). I changed the maximum concurrent checks per type from the default "Unlimited" to "One" and discovery began working on the affected proxy. Can anyone explain why this might be the case? I forgot to mention that I'm doing SNMPv3 discovery. I did notice in the documentation that SNMPv3 discovery always uses one worker, but that setting it to "Unlimited" should still work; it would just fall back from whatever number of start discoverers you have set in the configuration to 1:

Maximum concurrent checks per type

Set the maximum number of discovery threads (workers) per service check for parallel processing of discovery checks: One - one thread Unlimited - unlimited number of threads (but no more than in the StartDiscoverers parameter) Custom - set a custom number of threads (0-999) Note that all discovery rules with SNMPv3 asynchronous service checks are processed by one worker due to the peculiarities of libsnmp implementation, i.e. increasing the number of workers will not increase the discovery speed.