"Log messages to logging server <FQDN>@<Port Number>@TCP/UDP cannot be delivered possibly due to an unresolvable FQDN" Appear in NSX Manager UI

Products

VMware NSX

Issue/Introduction

Alarms are seen on NSX UI with Event type "Remote Logging Server Error" with description:

"Log messages to logging server <FQDN>@<Port Number>@TCP cannot be delivered possibly due to an unresolvable FQDN , an invalid TLS certificate or missing NSX appliance iptables rule."

NSX version is 4.2.1.2 or 4.2.1.3
Alarms repeatedly get triggered and get resolved in the NSX Manager UI, these can be for Managers and edge nodes.
It may seem the alarms and errors started showing after an NSX upgrade. This is because the NSX version 3.x and 4.1.x versions are not impacted by this problem, and only may hit the issue after upgrading to 4.2.1.x or later versions.
The error can be seen in /var/log/syslog.log

/var/log/syslog:2025-06-18 ######## MONITORING [nsx@### alarmState="OPEN" comp="nsx-manager" errorCode="MP701099" eventFeatureName="audit_log_health" eventSev="CRITICAL" eventState="On" eventType="remote_logging_server_error" level="FATAL" nodeId="#####" subcomp="monitoring"] Log messages to logging server #########@######@UDP cannot be delivered possibly due to an unresolvable FQDN, an invalid TLS certificate or missing NSX appliance iptables rule.

Clearing of the message can also be seen in the logs:

2025-06-18 ######## [nsx@#### comp="nsx-manager" subcomp="nsx-sha" level="CRITICAL" eventFeatureName="audit_log_health" eventType="remote_logging_server_error" eventSev="critical" eventState="Off" Configuration for logging server #########@#####@UDP appear correct.

Checking FQDN of the destination logging server on the manager/edge is resolvable using, getent, nslookup, ping or dig commands:
- As admin user, check configured logging server:

get logging-servers

- Use the FQDN returned for the below commands, as root user:

getent ahosts <fqdn_mentioned_in_alarm>

nslookup <fqdn_mentioned_in_alarm>

ping <fqdn_mentioned_in_alarm>

dig <fqdn_mentioned_in_alarm>

The port numbers are confirmed to be accessible from NSX manager/edges:

nc -zv <fqdn_mentioned_in_alarm> <port_from_alarm>

Using tcpdump on the manager/edge as root user, we see syslog packets leaving to the destination syslog server:
tcpdump -i eth0 host <syslog-server-ip>
Check if the syslog has a configured IPTABLES rule, as root user run on manager/edge:

iptables -L OUTPUT --line-numbers

- You should see results showing an ACCEPT rule for UDP syslog, example below:

132 ACCEPT udp -- anywhere anywhere udp dpt:syslog owner UID match syslog

Delete and re-add of the syslog server as admin user on manager/edge does not resolve the issue:
- Remove:

del logging server <ip/fqdn:<port> proto udp level <log-level>

- Add:

set logging-server <ip/fqdn> proto udp level <log-level>

Packet capture as root user on the manager/edge management port shows syslog traffic leaving the edge and going to the configured syslog server, sample packet capture:

tcpdump -i eth0 ip dst <syslog-server-ip> dst port <port_from_alarm>

Environment

VMware NSX 4.2.x

Cause

This is a known issue impacting VMware NSX. If corresponding IP address of the logging server's FQDN changes, this results in NSX nodes incorrectly using the previous IP in the nodes' iptables rules.

Resolution

Workaround

Manually cleanup the logging server :
If CLI was initially used to configure the logging-server, it is recommended to use CLI to remove the logging-server (with admin user at NSX Manager/Edge):

del logging-server <hostname-or-ip-address[:port]

You can verify whether logging server got removed by the below command:

nsx> get logging-servers

If Node Profile (a.k.a Central Node Config) was used to add the logging-server, it can be removed from Node Profile at NSX UI.
Go to System > Fabric > Profiles > Node Profiles and then remove the logging-server from this list. It is required to remove this just once.
SSH to Manager/Edge node with root user. Go to /config/vmware/nsx-node-api/syslog/ directory and check if there is a file named exporter_firewall_rules file. If this file exists, then delete this file by using command:

rm -rf /config/vmware/nsx-node-api/syslog/exporter_firewall_rules

The above step needs to be done at all 3 Manager nodes, if manager nodes are in the alarm or impacted edge node, if edge nodes are in the alarm.
Confirm the port number NSX Manager/Edge uses to connect to the logging-server. This can also be found at the alarm. For example, if the alarm is "eventType="remote_logging_server_error" eventSev="critical" eventState="On" Log messages to logging server <FQDN>@99999@TCP cannot be delivered possibly due to an unresolvable FQDN, an invalid TLS certificate or missing NSX appliance iptables rule" , then the port number is 99999. [Here 99999 is a random port number used as an example ]
The below steps need to be executed at all the 3 Manager nodes, if manager nodes are in the alarm or impacted edge node, if edge nodes are in the alarm. Use the port number found at step 3 to manually remove any iptable rules for the logging server:

Get the iptable rule's linenumber:

iptables -L OUTPUT --line-numbers | grep 99999

Here is an example output:
1 ACCEPT udp -- anywhere <fqdn> udp dpt:99999 owner UID match syslog

This means this iptable rule's line number is 1

[Here 99999 is a random port number used as an example ]

Delete it by using below command

iptables -D OUTPUT 1

Re-check the line number by using "iptables -L OUTPUT --line-numbers | grep 99999 " and then do the deletion if there are multiple 99999 entries.

Caution: The sequence may change everytime a line is deleted. So deleting multiple lines together may result in deleting the wrong line. Remember to check "iptables -L OUTPUT --line-numbers | grep 99999 " every time and then perform the next deletion.
Add the logging-server again using "set logging-server" command following this official guide. Please add the logging server by specifying an IP address, instead of FQDN. Specifying a logging server using a FQDN may retrigger the issue or if it was added with Node Profile, add back in the Node Profile.