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If you’re trying to set up multiple domain names in your AWS environment using DHCP options, you might run into some challenges. Currently, AWS VPC DHCP Option Sets only support a single domain name because of how DHCP options are designed. They use DHCP Option 15, which, according to the official standards, can only support one domain name.
In the past, with Amazon Linux 2, it was possible to add multiple domain names by passing a space-separated list to the domain-name parameter when creating DHCP options. This would make the /etc/resolv.conf file include multiple domain entries. However, this was not an intended feature, just an unintended side effect, and it can cause issues with newer Linux distributions.
Modern systems like Amazon Linux 2023 and Ubuntu/Debian distributions often now use systemd-networkd for network configurations. These systems follow the standards more strictly. Since spaces aren’t valid in domain names, the space characters get encoded as \032, which results in mashed-up or malformed domain names like foo.example.com\032bar.example.com. This leads to search domains not working as expected.
What does this mean for you? It means that AWS’s current setup only supports one domain name per DHCP option, which the documentation clearly states. While some Linux systems accept multiple domain names separated by spaces, most do not. Windows and many Linux systems follow the standards and will treat the value as a single domain only, which can cause unexpected results if you try to add multiple.
The true solution to this problem is to use DHCP Option 119, which is designed to support multiple domain names according to RFC 3397. This option allows you to specify several domains properly, avoiding the encoding issues and ensuring all your systems recognize the multiple domains correctly.
If you need this feature, the best step is to submit your request to AWS support or your AWS account manager. They can take your feedback, track it, and potentially include it in future updates. This way, you’re helping improve AWS for everyone who needs more flexible DNS options in their network setup.
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