5.3. Installing RHEL with eDNS enabled using a custom bootable ISO


Create a custom bootable ISO to install RHEL with encrypted DNS (eDNS) enabled using a strict enforce policy. This method helps ensure that all DNS traffic is private and secure during and after the installation.

If you require a custom CA certificate bundle, you must install it by using the %certificate section in the Kickstart file. You then reference this Kickstart file in a script to build a new ISO, which includes kernel arguments to enforce a strict DoT policy. If your environment is configured to support a fallback to unencrypted DNS, you can perform a standard RHEL installation and configure eDNS afterwards.

Prerequisites

  • Commands that start with the # command prompt require administrative privileges provided by sudo or root user access. For information on how to configure sudo access, see Enabling unprivileged users to run certain commands.
  • You have downloaded the minimal installation Boot ISO image from the Product Downloads page.
  • You have a Kickstart file ready with a %certificate section if you need a custom CA bundle.
  • The lorax package is installed.

Procedure

  1. Optional: Create a Kickstart file with a %certificate section. Ensure the certificate is saved in a file named tls-ca-bundle.pem.

    %certificate --dir /etc/pki/dns/extracted/pem/ --filename tls-ca-bundle.pem
    -----BEGIN CERTIFICATE-----
    <Base64-encoded_certificate_content>
    -----END CERTIFICATE-----
    %end
  2. Add the Kickstart file and kernel arguments into the ISO:

    The following script example demonstrates how to create a custom bootable ISO with eDNS enabled. You must create a script file to automate this process.

    #!/bin/bash
    
    set -ex
    
    KERNELARGS=""
    
    # Enable network
    KERNELARGS+="ip=dhcp "
    
    # Set DoT DNS server
    KERNELARGS+="rd.net.dns=dns+tls://_<server_ip>_#_<dns_server_hostname>_ "
    
    # Set to 'exclusive' to disable fallback to unencrypted DNS. Other values: 'backup', 'prefer'.
    KERNELARGS+="rd.net.dns-resolve-mode=exclusive "
    
    # Set the dnsconfd plugin for NetworkManager
    KERNELARGS+="rd.net.dns-backend=dnsconfd "
    
    # Remove any existing ISO to prevent conflicts with the new build
    rm -f _<output_iso_filename>_
    
    # Create a new bootable ISO with the Kickstart config file and kernel arguments
    mkksiso --ks _<kickstart_file>_ --cmdline "$KERNELARGS" _<input_iso_filename>_ _<output_iso_filename>_
  3. Run the script.

    sh <script_filename>
  4. Install RHEL using the customized ISO file.

Verification

  • Verify your eDNS configuration:

    $ dnsconfd status

    Expected output:

    Running cache service:
    unbound
    Resolving mode: exclusive
    Config present in service:
    {
        ".": [
            "dns+tls://198.51.100.143#dot.dns.example.com"
        ]
    }
    State of Dnsconfd:
    RUNNING
    Info about servers: [
        {
            "address": "198.51.100.143",
            "port": 853,
            "name": "dot.dns.example.com",
            "routing_domains": [
                "."
            ],
            "search_domains": [],
            "interface": null,
            "protocol": "dns+tls",
            "dnssec": true,
            "networks": [],
            "firewall_zone": null
        }
    ]
  • Verify that DNS server is responsive by using nslookup:

    $ nslookup <domain_name>

    Replace the <domain_name> with the domain that you want to query.

Troubleshooting

  • Enable detailed logging in unbound:

    # unbound-control verbosity 5
  • Review logs for the relevant service:

    $ journalctl -xe -u <service_name>

    Replace <service_name> with NetworkManager, dnsconfd, or unbound.

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