Systemd Service ExecStart

1. Error or Typo
   • The most common reason for seeing two exclamation marks at the start of an ExecStart field could be a simple error or typo. The ExecStart value is supposed to be the command to start a service. For example, it should typically look like ExecStart = /usr/bin/my_service - arg1 - arg2. The two exclamation marks may have been accidentally inserted during the configuration process.
2. Custom Script or Wrapper
   • In some cases, it could be part of a custom wrapper script. If there is a wrapper script that modifies the behavior of the actual service startup command, the double exclamation marks might be used as a special marker or a way to conditionally execute the command. For instance, the wrapper script might use the exclamation marks to indicate a special mode of operation.
   • Consider a simple bash - based wrapper script that is used as the ExecStart value. The double exclamation marks could be used to invert a condition. Here's a very simplified example:
   bash

   运行

   #!/bin/bash
   if [ "!!" = "!!" ]; then
       # This is a silly condition, but just for illustration
       # It could be replaced with a more meaningful condition
       # like checking for the existence of a file or a variable value
       exec /usr/bin/actual_service - arg1
   else
       echo "Service startup condition not met."
   fi
   
   • In this case, the use of !! is part of a custom logic in the wrapper script, which might be used to handle different startup scenarios for the service.
3. Configuration Management Tool Artifact
   • If a configuration management tool (like Ansible, Puppet, etc.) is used to manage the systemd service files, it's possible that the double exclamation marks are an artifact of how the tool generates or modifies the service file. The tool might have a mis - configured template or a bug that causes it to insert the !! characters. For example, an Ansible template might have a placeholder variable that was not properly replaced and ended up with the !! characters in the ExecStart field.

1. Background on systemd - resolved.service
   • The systemd - resolved.service in Ubuntu is part of the systemd suite. It is a system service that provides network name resolution to local applications. It is designed to work with DNS (Domain Name System) and other name - resolution mechanisms.
2. Generated by systemd
   • The systemd - resolved.service is not typically generated by a user - run script. It is part of the systemd package installation. When the systemd package is installed (which is a fundamental part of most modern Linux distributions including Ubuntu), the service files are installed along with it.
   • The service file for systemd - resolved.service is usually located in the /lib/systemd/system/ directory. This file contains the configuration details for how the service should operate. For example, it has the ExecStart command that defines how the service is launched. In the case of systemd - resolved.service, the ExecStart might point to the binary that runs the actual name - resolution process.
   • The service is installed as part of the system - level infrastructure to handle DNS resolution. Here's a simplified view of what might be in the service file:
   plaintext

   [Unit]
   Description=Network Name Resolution
   Wants=network - target nss - lookup.target
   After=network - target nss - lookup.target
   [Service]
   ExecStart=/lib/systemd/systemd - resolved
   Restart=always
   [Install]
   WantedBy=multi - user.target
   
   • The above configuration shows how the service is set up to start the systemd - resolved binary, how it should handle restarts, and when it should be started in the system boot process (after the network is up and running and the NSS (Name Service Switch) lookup is available).

Service Status

• Check Status: Use the command systemctl status systemd-resolved.service to view the current status of the service, including whether it is running, stopped, or in an error state145.
• View Logs: If there are issues with the service, you can view its logs using journalctl -u systemd-resolved.service. This can provide detailed information about any errors or warnings related to the service.

Start and Stop the Service

• Start: To start the systemd-resolved service, run sudo systemctl start systemd-resolved.service145.
• Stop: If you need to stop the service, execute sudo systemctl stop systemd-resolved.service125.
• Restart: Sometimes, you may need to restart the service to apply configuration changes or to resolve certain issues. Use sudo systemctl restart systemd-resolved.service for this purpose124.

Enable and Disable Autostart

• Enable: If you want the systemd-resolved service to start automatically during system boot, run sudo systemctl enable systemd-resolved.service145.
• Disable: To prevent the service from starting automatically, use sudo systemctl disable systemd-resolved.service126.

Configuration

• The main configuration file for systemd-resolved is usually located at /etc/systemd/resolved.conf. You can edit this file to modify settings such as DNS servers, DNSSEC configuration, and more. However, after making changes to the configuration file, you need to restart the service for the changes to take effect46.
• Additionally, systemd-resolved manages the /etc/resolv.conf file as a symbolic link. It is not recommended to directly edit this file; instead, make changes to the resolved.conf file or use tools like resolvectl to manage DNS settings4.

Using resolvectl

• View DNS Status: The resolvectl tool can be used to view detailed information about the current DNS configuration and status. For example, resolvectl status shows information about the DNS servers being used, the domain search list, and any cached DNS entries4.
• Set DNS Servers: You can set DNS servers using resolvectl dns <interface> <ip_address>. Replace <interface> with the network interface name (e.g., eth0) and <ip_address> with the IP address of the DNS server you want to use4.
• Set Domain Search List: To configure the domain search list, use resolvectl domain <interface> <domain>. This allows you to specify additional domains to search when resolving hostnames without a top-level domain specified4.
• Flush DNS Cache: If you need to clear the DNS cache, you can use sudo resolvectl flush-caches or restart the systemd-resolved service4.