vCenter Server fails to send email alerts to SMTP server with "host name lookup failure" error
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vCenter Server fails to send email alerts to SMTP server with "host name lookup failure" error

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Article ID: 392300

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Updated On:

Products

VMware vCenter Server

Issue/Introduction

  • vCenter Server fails to send email notifications for triggered alarms. While the alarms activate as expected, Sendmail does not successfully deliver the alert emails to the specified recipients.

  • Below is a screenshot of a vCenter alarm configuration. The alarm successfully triggered internally, but the configured email alert was never sent externally.

  • On vCenter server /var/log/vmware/vpxd/vpxd.log, the below excerpts are seen: 

YYYY-MM DD <vCenter server name> vpxd[6##9]: Event [20##10] [1-1] [YYYY-MM-DD] [vim.event.AlarmActionTriggeredEvent] [info] [] [<Cluster name>] [20##10] [Alarm '<Alarm name>' on <VM name> triggered an action]
YYYY-MM DD <vCenter server name> vpxd[6##9]: Event [20##18] [1-1] [YYYY-MM-DD] [vim.event.AlarmEmailCompletedEvent] [info] [] [<Cluster name>] [20##18] [Alarm '<Alarm name>' on <VM name> sent email to ####@example.com]
YYYY-MM DD <vCenter server name> vpxd[6##9]: Event [22##91] [1-1] [YYYY-MM-DD] [vim.event.EventEx] [info] [] [<Cluster name>] [224691] [Alarm '<Alarm name>' on <VM name> triggered by event 22##90 '<VM name>' on example.domain in <Cluster name> is powered off']
YYYY-MM DD <vCenter server name> vpxd[6##9]: Event [22##92] [1-1] [YYYY-MM-DD] [vim.event.AlarmActionTriggeredEvent] [info] [] [<Cluster name>] [22##92] [Alarm '<Alarm name>' on <VM name> triggered an action]
YYYY-MM DD <vCenter server name> vpxd[6##9]: Event [22##99] [1-1] [YYYY-MM-DD] [vim.event.AlarmEmailCompletedEvent] [info] [] [<Cluster name>] [22##99] [Alarm '<Alarm name>' on <VM name> sent email to ####@example.com]
YYYY-MM DD <vCenter server name> vpxd[6##9]: Event [22##41] [1-1] [YYYY-MM-DD] [vim.event.EventEx] [info] [] [<Cluster name>] [22##41] [Alarm '<Alarm name>' on <VM name> triggered by event 22##40

  • On vCenter server /var/log/vmware/messages, the below excerpts are seen:

YYYY-MM-DD <vCenter name> sendmail[363##79]: 52L7r###631276: to=<####@example.com>, delay=00:00:00, xdelay=00:00:00, mailer=relay, pri=12##00, relay=<SMTP server name>, dsn=4.4.3, stat=Deferred: Name server: example.domain: host name lookup failure
YYYY-MM-DD <vCenter name> sendmail[363##81]: 52L7r###631277: to=<####@example.com>, delay=00:00:00, xdelay=00:00:00, mailer=relay, pri=12##00, relay=<SMTP server name>, dsn=4.4.3, stat=Deferred: Name server: example.domain: host name lookup failure
sendmail -v -q : Running /var/spool/mqueue/56HIJ####1902 (sequence 1 of 3) <####@example.com>... Connecting to <SMTP server name> port 25 via relay... <####@example.com>... Deferred: Name server: <SMTP server name>: host name lookup failure

  • Running sendmail queue manually from the vCenter shell to process any pending messages and provide verbose output : 

sendmail -v -q

Running /var/spool/mqueue/56HIJ####1902 (sequence 1 of 3)

<####@example.com>... Connecting to <SMTP server name> port 25 via relay...
<####@example.com>... Deferred: Name server: <SMTP server name>: host name lookup failure

Cause

The core issue is that sendmail fails to resolve the DNS hostname of the mail relay server. This prevents sendmail from establishing a connection, causing email deliveries to be deferred and email notifications to stop. 

Resolution

Note: Before making any changes, take a snapshot of the vCenter Server and offline snapshots of all vCenters if in linked mode, refer: Snapshot Best practices for vCenter Server Virtual Machines

  1. SSH to the vCenter server and login as root 

  2. Type shell and press enter

  3. Validate the name resolution of the SMTP server for both forward and reverse lookup using nslookup.

    1. Example:

      1. nslookup example.smtp.server 8.8.8.8 (example primary DNS server) 

      2. nslookup example.smtp.server 8.8.4.4 (example secondary DNS server) 

    2. If the name resolution succeeds, check if the loopback IP address is included in the /etc/resolv.conf on the vCenter server to enable local DNS caching. If missing, include the loopback IP address 127.0.0.1 to enable local DNS caching using the VAMI configuration utility with the command:

      1. /opt/vmware/share/vami/vami_config_net

      2. Select option 4 to configure DNS servers.

      3. Set DNS Server 1 to 127.0.0.1 (localhost) and DNS Server 2 to your primary DNS IP (e.g., 8.8.8.8).

      4. (Optional) Add a domain (e.g. example.org) as the search domain. 

  4. Restart Services and Validate:

    1. Restart the Sendmail Service using:
          systemctl restart sendmail

    2. Test mail sending and DNS resolution with:
          echo "Subject: sendmail test" | sendmail -v [email protected]

    3. Check Sendmail Service status:
          systemctl status sendmail

    •  

Additional DNS Configuration:

Add or update secondary DNS server using VAMI: How to Change or Update DNS Server IP Address in VAMI

Additional Information

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