This guide walks you through setting up domains for your multi-tenant application. There are two main approaches:

Wildcard Domains (*.acme.com)

If you want to offer subdomains like tenant.acme.com, you'll need to:

1. Configure wildcard domains for subdomains like *.acme.com

Custom Domains (tenant.com)

If you want to allow tenants to use their own domains, you'll need to:

1. Enable custom domain support for your tenants
2. Add domains programmatically using the Vercel SDK
3. Verify domain ownership with TXT records
4. Configure domain redirects and handle SEO
5. Clean up domains when tenants leave

You can implement either approach or both depending on your needs.

Wildcard Domain Setup

Step 1: Configure Wildcard Domains

If you plan on offering subdomains like *.acme.com, add a wildcard domain to your Vercel project. This requires using Vercel's nameservers so that Vercel can manage the DNS challenges necessary for generating wildcard SSL certificates.

1. Point your domain to Vercel's nameservers (ns1.vercel-dns.com and ns2.vercel-dns.com).
2. In your Vercel project settings, add the apex domain (e.g., acme.com).
3. Add a wildcard domain: .acme.com.

Now, any tenant.acme.com you create—whether it's tenant1.acme.com or docs.tenant1.acme.com—automatically resolves to your Vercel deployment. Vercel issues individual certificates for each subdomain on the fly.

Custom Domain Setup

The following steps guide you through setting up custom domain support for your tenants. This allows them to use their own domains (e.g., customacmesite.com) instead of subdomains.

Step 1: Enable Custom Domains

You can also give tenants the option to bring their own domain. In that case, you'll want your code to:

1. Provision and assign the tenant's domain to your Vercel project.
2. Verify the domain (to ensure the tenant truly owns it).
3. Automatically generate an SSL certificate.

Step 2: Add Domains Programmatically

You can add a new domain through the Vercel SDK. For example:

import { VercelCore as Vercel } from "@vercel/sdk/core.js";
import { projectsAddProjectDomain } from "@vercel/sdk/funcs/projectsAddProjectDomain.js";

const vercel = new Vercel({
  bearerToken: process.env.VERCEL_TOKEN,
});

// The 'idOrName' is your project name in Vercel, for example: 'multi-tenant-app'
await projectsAddProjectDomain(vercel, {
  idOrName: "my-multi-tenant-app",
  teamId: "team_1234",
  requestBody: {
    // The tenant's custom domain
    name: "customacmesite.com",
  },
});

Once the domain is added, Vercel attempts to issue an SSL certificate automatically.

Step 3: Verify Domain Ownership

If the domain is already in use on Vercel, the user needs to set a TXT record to prove ownership of it.

You can check the verification status and trigger manual verification:

import { VercelCore as Vercel } from "@vercel/sdk/core.js";
import { projectsGetProjectDomain } from "@vercel/sdk/funcs/projectsGetProjectDomain.js";
import { projectsVerifyProjectDomain } from "@vercel/sdk/funcs/projectsVerifyProjectDomain.js";

const vercel = new Vercel({
  bearerToken: process.env.VERCEL_TOKEN,
});

const domain = "customacmesite.com";

const [domainResponse, verifyResponse] = await Promise.all([
  projectsGetProjectDomain(vercel, {
    idOrName: "my-multi-tenant-app",
    teamId: "team_1234",
    domain,
  }),
  projectsVerifyProjectDomain(vercel, {
    idOrName: "my-multi-tenant-app",
    teamId: "team_1234",
    domain,
  }),
]);

const { value: result } = verifyResponse;

if (!result?.verified) {
  console.log(`Domain verification required for ${domain}.`);
  // You can prompt the tenant to add a TXT record or switch nameservers.
}

Step 4: Configure Redirects

Redirecting between apex and "www"

Some tenants might want www.customacmesite.com to redirect automatically to their apex domain customacmesite.com, or the other way around.

1. Add both customacmesite.com and www.customacmesite.com to your Vercel project.
2. Configure a redirect for www.customacmesite.com to the apex domain by setting redirect: customacmesite.com through the API or your Vercel dashboard.

This ensures a consistent user experience and prevents issues with duplicate content.

Avoiding duplicate content across subdomains

If you offer both tenant.acme.com and customacmesite.com for the same tenant, you may want to redirect the subdomain to the custom domain (or vice versa) to avoid search engine duplicate content. Alternatively, set a canonical URL in your HTML <head> to indicate which domain is the "official" one.

Step 5: Clean Up Domains

If a tenant cancels or no longer needs their custom domain, you can remove it from your Vercel account using the SDK:

import { VercelCore as Vercel } from "@vercel/sdk/core.js";
import { projectsRemoveProjectDomain } from "@vercel/sdk/funcs/projectsRemoveProjectDomain.js";
import { domainsDeleteDomain } from "@vercel/sdk/funcs/domainsDeleteDomain.js";

const vercel = new Vercel({
  bearerToken: process.env.VERCEL_TOKEN,
});

await Promise.all([
  projectsRemoveProjectDomain(vercel, {
    idOrName: "my-multi-tenant-app",
    teamId: "team_1234",
    domain: "customacmesite.com",
  }),
  domainsDeleteDomain(vercel, {
    domain: "customacmesite.com",
  }),
]);

The first call disassociates the domain from your project, and the second removes it from your account entirely.