# DNS checks

A DNS check asks a DNS server for a domain's addresses on a schedule, and alerts when the server stops answering or answers wrong.

:::tip[New to Uptime?]
Start with [Get started with Uptime](/docs/uptime/).
:::

## Adding a DNS check

In **Add check**, choose **DNS server doesn't respond** under **Alert us when…**, then fill in:

| Field | Details |
| --- | --- |
| DNS server to monitor | The server's public IP address, IPv4 or IPv6, such as `1.1.1.1` or your own name server's address. |
| Domain to query the DNS server with | The domain to look up, such as `example.com`. Labels may start with `_`, as in `_dmarc.example.com`. |
| Keyword to find in the DNS response | Optional. Usually the address the domain should point to. Leave it empty to accept any address. |

![The Add check dialog set to DNS server doesn't respond, asking 1.1.1.1 for example.com.](../../../assets/screenshots/uptime-dns-form.webp)

The check's card reads `example.com via 1.1.1.1`.

## When a DNS check is down

The check asks the server for the domain's `A` and `AAAA` records together, and is up when either one returns an address. It's down when:

| What happened | The check says |
| --- | --- |
| The server doesn't answer within the timeout | `The DNS server 1.1.1.1 didn't answer in time` |
| The server reports a server failure (`SERVFAIL`) | `The DNS server 1.1.1.1 couldn't look up example.com (server failure)` |
| The server refuses the query (`REFUSED`) | `The DNS server 1.1.1.1 refused to answer for example.com` |
| The server refuses the connection | `The DNS server 1.1.1.1 refused the connection` |
| The server says the domain doesn't exist | `1.1.1.1 answered that example.com doesn't exist` |
| The domain has no address | `1.1.1.1 answered, but example.com has no address (no A or AAAA record)` |
| The keyword isn't in the answer | `1.1.1.1 answered example.com without "93.184.215.14": …` |
| Any other error | `The DNS server 1.1.1.1 didn't answer for example.com (<error code>)` |

When it's up, the result reads `1.1.1.1 answered:` followed by the addresses.

![A DNS check's page: healthy from UAE, Canada, and Germany, with recent results reading 1.1.1.1 answered: and the addresses.](../../../assets/screenshots/uptime-dns-locations.webp)

Cloudflare-hosted zones answer a name that doesn't exist with "no address", not "doesn't exist", so expect the "no address" message for them.

## Limits

- The DNS server must be a public IP address. Private and reserved addresses, such as `10.x.x.x`, `192.168.x.x`, `127.x.x.x`, and `100.64.0.0/10`, are refused with "The DNS server must be a public IP address, like 1.1.1.1".
- Queries go to port 53 only.
- There's no **Internet Protocol (IP) version** setting: the DNS server's address decides whether it's asked over IPv4 or IPv6.
- The request timeout is 5 or 10 seconds, as for ping checks.
- DNS checks can also be created and changed through the REST API. See [Uptime checks in the API](/docs/uptime/checks/#uptime-checks-in-the-api).

**Run Now** on the check's page works for DNS checks. Check frequency, the confirmation period, and [checking from several locations](/docs/uptime/checks/#checking-from-several-locations) work as for other checks. See [Working with checks](/docs/uptime/checks/#settings).