How does alias mode (priority 0) work?

With priority 0, the SVCB record works as an alias to the target. No parameters are specified, the client simply follows the indicated target.

What's the difference between SVCB and HTTPS?

HTTPS is the web-dedicated variant of SVCB (port 443 by default). SVCB is generic and used for any service type (IMAP, SMTP, etc.).

What do alpn and port parameters mean?

alpn announces supported protocols (h2, h3, imap...). port indicates the service port if different from standard.

What are ipv4hint and ipv6hint for?

These are indicative IP addresses to speed up connection. They don't replace A and AAAA records which remain necessary.

Where should I use an SVCB record?

On dedicated service names like _imap._tcp or _smtp._tcp when the protocol provides for it, or at the apex in alias mode to avoid a CNAME.

SVCB Record Lookup (Service Binding)

Describe your services and their connection parameters

Configure service discovery via DNS. Verify that your SVCB records correctly announce protocols and connection parameters.

Host

Type

Resolver

Iterative trace

In iterative trace mode, the resolver is ignored.

Propagation test

Query multiple public resolvers to compare answers.

Service description

Announce how to connect to a service: supported protocols, port, and fallback addresses.

Multi-resolver

Compare responses from Google, Cloudflare, and Quad9 to detect propagation issues.

Alias mode

Use priority 0 to create an alias to a target without using a CNAME.

Advanced parameters

Publish alpn, port, ipv4hint, ipv6hint, and ech to guide clients to the right configuration.

Free and unlimited

Test as many domains as needed. No signup required.

How to use the DNS lookup engine options effectively

What is the iterative trace?

The trace performs resolution step by step. The resolver first queries the root servers, then the TLD (.com, .fr, .eu), and then the authoritative servers of the target zone. At each step, the page shows the queried server, the answer, the RCODE, and the latency.

1. Root
Discovery of the TLD servers for the requested name.
2. TLD
Reference to the zone's NS (delegation).
3. Authoritative
Final answer (or error) with TTL and latency.

What is it for?

Compare answers across resolvers and regions
Detect a hot cache, an overly long TTL, or an incomplete delegation
Explain a latency difference or an unexpected RCODE

Tip: keep the trace disabled for quick checks; enable it when investigating or preparing a ticket/post‑mortem.

What is the classic trace?

The classic trace queries only the selected resolver (UDP or DoH) and displays the answer as it is perceived from that network vantage point. You get the RCODE, the response sections, and the latency for the client → resolver leg.

1. Chosen resolver
Uses the preset or custom configuration to run the query exactly like your service would.
2. Protocol preserved
Respects the selected transport (UDP, TCP, or DoH) so you reproduce the real behaviour.
3. Detailed answer
Shows the question, answer, and authority/additional sections when present, together with TTL and useful metadata.

Why use it?

Check the view of a specific resolver before suspecting delegation issues
Confirm cached values and the impact of a TTL or a flush
Document a resolution exactly as a client or microservice sees it

Tip: keep the iterative trace option turned off when auditing a given resolver; enable it afterwards to compare with the root → TLD → authoritative path.

How does the propagation test work?

The test queries a set of public resolvers (Google, Cloudflare, Quad9, OpenDNS, ISPs…) in parallel and groups the answers by content and RCODE. You instantly see who already picked up the update.

1. Multi-point resolvers
Enables the propagation presets to question several actors spread around the world.
2. Automatic comparison
Groups identical answers and highlights divergences or resolver-specific errors.
3. Actionable summary
Provides a clear recap, the resolver list, their latencies, and each group's status.

When to use it?

Track how a DNS change propagates worldwide
Spot stale caches and decide on a targeted flush
Share a propagation snapshot in a ticket or post-mortem

Tip: while the propagation test is active, the resolver selector is frozen. Disable the mode to return to single-resolver diagnostics.

What is an SVCB record?

An SVCB (Service Binding) record describes how to reach a service. It can designate a target and publish parameters like protocol, port, and fallback addresses.

SVCB record structure:

Field	Description	Example
Priority	0 = alias, 1+ = service	`1`
Target	Service hostname	`mail.provider.com.`
alpn	Supported protocols	`imap`
port	Service port	`993`
ipv4hint	Indicative IPv4 addresses	`203.0.113.10`

Operating modes

Service mode (priority > 0)

Announces connection parameters:

_imap._tcp.captaindns.com.  3600  IN  SVCB  1 mail.provider.com. alpn=imap port=993

Alias mode (priority = 0)

Redirects to another target without parameters:

apex.captaindns.com.  3600  IN  SVCB  0 cdn.provider.com.

Common parameters

Parameter	Description	Example
alpn	Supported protocols	`imap`, `h2`, `h3`
port	Service port	`993`, `443`
ipv4hint	Indicative IPv4 addresses	`203.0.113.10`
ipv6hint	Indicative IPv6 addresses	`2001:db8::1`
ech	ECH configuration	(base64 encoded)

Use cases

IMAP service

_imap._tcp.captaindns.com.  3600  IN  SVCB  1 mail.provider.com. alpn=imap port=993

Apex alias

captaindns.com.  3600  IN  SVCB  0 cdn.provider.com.

Application service

_api._tcp.captaindns.com.  300  IN  SVCB  1 . alpn=h2 port=8443

Important rules

Compatibility

Rule	Explanation
Keep A/AAAA	Legacy clients don't use SVCB
Valid target	Target must have A/AAAA records
Consistent priority	0 = alias only, > 0 = service

Best practices

Practice	Why
Prefer HTTPS for web	Dedicated variant with adapted semantics
Short TTL initially	Makes corrections easier
Document parameters	Traceability of modifications

Command line verification

Linux/Mac

dig SVCB _imap._tcp.captaindns.com

Windows

nslookup -type=svcb _imap._tcp.captaindns.com

Tool	Purpose
HTTPS Record Lookup	Check web records
A Record Lookup	Check IPv4 addresses
DNS Propagation Check	Check worldwide propagation

Useful resources

RFC 9460 - SVCB and HTTPS Records