Using custom domains

Intermediate

Tutorial

Overview

By default all canisters on the Internet Computer are accessible through icp0.io and their canister ID. In addition to that default domain, one can also make a canister accessible under a custom domain. This guide explains how to do that.

There are, essentially, two approaches to make a canister accessible under a custom domain:

• Register the domain with the boundary nodes.
• Hosting the domain on your own infrastructure.

For both approaches, you need to acquire a domain through any registrar (pick your favorite one).

The two approaches differ in the ease of use and the configurability. When registering the domain with the boundary nodes, you simply have to configure the DNS records of the custom domain and the boundary nodes take care of obtaining a certificate, renewing the certificate before its expiration, and SEO. When hosting the domain on your own infrastructure, you need to obtain and renew the certificates, and provide your own infrastructure with support for the HTTP gateway protocol. However, you are also more flexible in how you configure your domain.

You may need to specify a host in your frontend code when you are using a custom domain, as the HttpAgent may not be able to automatically infer the host like it can on icp0.io and ic0.app. To configure your agent, it will look something like this:

// Point to icp-api for the mainnet. Leaving host undefined will work for localhost
const host = isProduction ? 'https://icp-api.io' : undefined;
const agent = new HttpAgent({ host });

Custom domains on the boundary nodes

In the following, first all steps necessary to register your custom domain with the boundary nodes are listed. Then, it is explained how one can update and remove a registration.

First registration

By following the steps below, you can host your canister under your custom domain using the boundary nodes. The steps are first outlined, then can be seen through a concrete example to illustrate these steps, followed by some instructions on troubleshooting.

• Step 1: Configure the DNS record of your domain, which is denoted with CUSTOM_DOMAIN.

  Add a CNAME entry for your domain pointing to CUSTOM_DOMAIN.icp1.io such that all the traffic destined to your domain is redirected to the boundary nodes; Add a TXT entry containing the canister ID to the _canister-id-subdomain of your domain (e.g., _canister-id.CUSTOM_DOMAIN); Add a CNAME entry for the _acme-challenge-subdomain (e.g., _acme-challenge.CUSTOM_DOMAIN) pointing to _acme-challenge.CUSTOM_DOMAIN.icp2.io in order for the boundary nodes to acquire the certificate.

In many cases, it is not possible to set a CNAME record for the top of a domain, the Apex record. In this case, DNS providers support so-called CNAME flattening. To this end, these DNS providers offer flattened record types, such as ANAME or ALIAS records, which can be used instead of the CNAME to CUSTOM_DOMAIN.icp1.io.

In some cases (like AWS:Route53), the format of the TXT record can depend on whether it is APEX or not. For APEX, the name has to be empty, with the value containing the key-value pair: "_canister-id:CANISTER_ID". For non-apex (e.g. www), the name should be the standard “_canister-id.www” and value "CANISTER_ID".

---

• Step 2: Create a file named ic-domains in your canister under the .well-known directory containing the custom domain.

The .well-known directory with the ic-domains file should be placed within the canister files, not at the root. The location of the files should allow your frontend framework to include the files during the build process.

By default, dfx excludes all files and directories whose names start with a . from the asset canister. Hence, to include the ic-domains-file, you need to create an additional file, called .ic-assets.json.

Create a new file with the name .ic-assets.json inside the same top-level directory as the .well-known directory.

To use multiple custom domains and their subdomains with a single canister, simply list each one of them on a newline in the ic-domains-file:

custom-domain1.com
subdomain1.custom-domain1.com
subdomain2.custom-domain1.com
custom-domain2.com
subdomain1.custom-domain3.com
custom-domain4.com

In the above example, subdomain1 could, for example, stand for www.

Configure the .well-known directory to be included by writing the following configuration into the .ic-assets.json-file:

[
    {
        "match": ".well-known",
        "ignore": false
    }
]

Deploy the updated canister.

---

• Step 3: Register the domain with the boundary nodes by issuing the following command and replacing CUSTOM_DOMAIN with your custom domain.

curl -sLv -X POST \
    -H 'Content-Type: application/json' \
    https://icp0.io/registrations \
    --data @- <<EOF
{
    "name": "CUSTOM_DOMAIN"
}
EOF

If the call was successful, you will get a JSON response that contains the request ID in the body, which you can use to query the status of your registration request:

{"id":"REQUEST_ID"}

In case the call failed, you will get an error message indicating the reason for the failure:

• Missing DNS CNAME record: the CNAME entry for the _acme-challenge-subdomain is missing.
• Existing DNS TXT challenge record: the DNS record already contains a TXT entry for the _acme-challenge-subdomain. Remove it and try again.
• Missing DNS TXT record: the TXT entry for the _canister-id-subdomain is missing.
• Invalid DNS TXT record: the content of the TXT entry is not a valid canister ID.
• More than one DNS TXT record: there are multiple TXT entries for the _canister-id-subdomain. Remove them and keep only one.
• Failed to retrieve known domains: the ic-domains-file is not accessible under .well-known/ic-domains.
• Domain is missing from list of known domains: the custom domain is missing from the ic-domains-file.
• Rate limit exceeded for apex domain: too many registration requests have been submitted for the apex domain. Try again later. You can submit at most 5 registration requests per apex domain and hour.

---

• Step 4: Processing the registration can take several minutes.

  Track the progress of your registration request by issuing the following command and replacing REQUEST_ID with the ID you received in the previous step.

curl -sLv -X GET \
    https://icp0.io/registrations/REQUEST_ID

The status will be one of the following:

• PendingOrder: the registration request has been submitted and is waiting to be picked up.
• PendingChallengeResponse: the certificate has been ordered.
• PendingAcmeApproval: the challenge has been completed.
• Available: the registration request has been successfully processed.
• Failed: the registration request failed.

---

• Step 5: Once your registration request becomes available, wait a few minutes for the certificate to become available on all boundary nodes.

  After that, you should be able to access your canister using the custom domain.

Concrete example

Imagine you wanted to register your domain foo.bar.com for your canister with the canister ID hwvjt-wqaaa-aaaam-qadra-cai.

DNS configuration

Record Type	Host	Value
CNAME	@	foo.bar.com.icp1.io
TXT	_canister-id	hwvjt-wqaaa-aaaam-qadra-cai
CNAME	_acme-challenge	_acme-challenge.foo.bar.com.icp2.io

Some DNS providers do not require you to specify the main domain. For example, you would just have to specify foo instead of foo.bar.com, _canister-id.foo instead of _canister-id.foo.bar.com, and _acme-challenge.foo instead of _acme-challenge.foo.bar.com.

.well-known/ic-domains

• Step 1: Create the ic-domains file with the following content in the .well-known directory:

  foo.bar.com

• Step 2: Create the .ic-assets.json file at the root of the canister source:

[
    {
        "match": ".well-known",
        "ignore": false
    }
]

• Step 3: Deploy the updated canister.

Start the registration process

curl -sLv -X POST \
    -H 'Content-Type: application/json' \
    https://icp0.io/registrations \
    --data @- <<EOF
{
    "name": "foo.bar.com"
}
EOF

In the following document, this guide provides detailed instructions to configure DNS records on the example of two popular domain registrars.

Troubleshooting

When you are running into issues trying to register your custom domains, make the following checks:

• Check your DNS configuration using a tool like dig or nslookup. To check, for example, the TXT record with the canister ID, you can run dig TXT _canister-id.CUSTOM_DOMAIN. In particular, make sure that there are no extra entries (e.g., multiple TXT records for the _canister-id-subdomain).
• Check that there are no TXT records for the _acme-challenge-subdomain (e.g., by using dig TXT _acme-challenge.CUSTOM_DOMAIN). If there are TXT records, then they are most likely left-over from previous ACME-challenges by your domain provider. Note that these records often do not show up in your domain management dashboard. Try disabling all TLS/SSL-certificate offerings from your domain provider to remove these records.
• Check the ic-domains file by downloading it directly from your canister (e.g., by opening CANISTER_ID.icp0.io/.well-known/ic-domains in your browser or using curl CANISTER_ID.icp0.io/.well-known/ic-domains in your terminal).

Updating a custom domain

In case you want to update the domain to point to a different canister, you first need to update the DNS record of your domain and then notify a boundary node:

• Step 1: Update the TXT entry to contain the new canister ID for the _canister-id-subdomain of your domain (e.g., _canister-id.CUSTOM_DOMAIN).

• Step 2: Notify a boundary node of the change using a PUT request and the ID of your registration (REQUEST_ID).

curl -sLv -X PUT \
    https://icp0.io/registrations/REQUEST_ID

In case you forgot the ID of your registration, you can just submit another registration request for your domain and the boundary node will return the corresponding ID.

Removing a custom domain

In case you want to remove your domain, you just need to remove the DNS records and notify a boundary node:

• Step 1: Remove the TXT entry containing the canister ID for _canister-id-subdomain (e.g., _canister-id.CUSTOM_DOMAIN) and the CNAME entry for the _acme-challenge-subdomain (e.g., _acme-challenge.CUSTOM_DOMAIN).

• Step 2: Notify a boundary node of the removal using a DELETE request.

curl -sLv -X DELETE \
    https://icp0.io/registrations/REQUEST_ID

In case you forgot the ID of your registration, you can just submit another registration request for your domain and the boundary node will return the corresponding ID.

Custom domains using your own infrastructure

• Step 1: Deploy your canister to ICP and note the canister id.

• Step 2: Clone the official ICP repo, checkout the following commit 2304690ca1d241d9ce6e19a427a18de9aac95b82 and navigate to the service worker folder located under ic/typescript/service-worker.

• Step 3: Map your domain to the canister ID by adding your domain-to-canister mapping to hostnameCanisterIdMap in the file service-worker/src/sw/domains/static.ts.

• Step 4: Build the service worker according to the instructions in service-worker/README.md. The output should be:

  • an index.html,
  • a minified .js file, and
  • a .map file.

• Step 5: Host the assets (index.html, .js and .map files) from a server or CDN and point your custom domain name at this server.

• Step 6: Test.

For front-ends that use Internet Identity (II) to authenticate users: the principals provided by II depend on the domain from which the login request was started. So if you authenticate your users through the canister URL and want to switch over to a custom domain, users will not have the same principals anymore. You can prevent this by setting up Alternative Origins.