Zabbix Server (composed) problem with DNS resolution

[kingfisher77]

Zabbix Notification: PostgreSQL database "zabbix" on "postgres:5432" is not available: could not translate host name "postgres" to address: Name does not resolve

This is what we get randomly (1-3 times per day) in a Docker setup. This messages comes as a notification and as an entry in the Zabbix Server log. Postgres is running. System log and docker environment is inconspicuous.

Otherwise - the Zabbix server, the auto-registration, the templates, the active and passive elements, the triggers, the notifications, the graphical user interface - everything works fine.

The VM size at Hetzner Cloud is comfortable (ccx13) for what we do with Zabbix (~20 Linux servers, some network devices).

• Docker 24.0.5 (Ubuntu 22.4.3)
• Zabbix 6.4
• Postgres 14

The docker-compose.yml is based on the official example.

We run almost all of our services with Docker. The Zabbix setup is the first setup that has this error.

Does anyone have any idea what can cause this error?

services:

  postgres:
    hostname: postgres
    restart: always
    image: postgres:14-alpine
    volumes:
      - postgres:/var/lib/postgresql/data
    env_file:
      - ./env_vars/.env_pgsql
    secrets:
      - POSTGRES_USER
      - POSTGRES_PASSWORD
    healthcheck:
      test: ["CMD-SHELL", "pg_isready --dbname=$${POSTGRES_DB} --username=$${PGSQL_USER}"]
      interval: 60s
      timeout: 5s
      retries: 3
    stop_grace_period: 1m​

  server:
    hostname: zabbix
    image: zabbix/zabbix-server-pgsql:alpine-6.4-latest
    restart: always
    ports:
      - 10051:10051
    env_file:
      - ./env_vars/.env_srv
      - ./env_vars/.env_pgsql
    secrets:
      - POSTGRES_USER
      - POSTGRES_PASSWORD
    volumes:
      - server-usr-lib:/usr/lib/zabbix
      - server-var-lib:/var/lib/zabbix
    depends_on:
      - postgres
    #ulimits:
    #  nproc: 65535
    #  nofile:
    #    soft: 20000
    #    hard: 40000
    #deploy:
    #  resources:
    #    limits:
    #      cpus: '0.70'
    #      memory: 1G
    #    reservations:
    #      cpus: '0.5'
    #      memory: 512M
    sysctls:
      - net.ipv4.ip_local_port_range=1024 65000
      - net.ipv4.conf.all.accept_redirects=0
      - net.ipv4.conf.all.secure_redirects=0
      - net.ipv4.conf.all.send_redirects=0​

volumes:
  postgres:
  server-usr-lib:
  server-var-lib:

###
### Networks
###
networks:
  default:
    enable_ipv6: true
    ipam:
      driver: default
      config:
        - subnet: fd00:5c32:de45::/80
          gateway: fd00:5c32:de45::1
  traefik:
    external: true

secrets:
  POSTGRES_USER:
    file: ./env_vars/.POSTGRES_USER
  POSTGRES_PASSWORD:
    file: ./env_vars/.POSTGRES_PASSWORD​