SVCB DNS Record

What the SVCB record does

In ServiceMode a record names a target host and attaches SvcParams describing how to reach it: which ALPN protocols the endpoint speaks, which port, and address hints that let a connection start before the A and AAAA answers arrive.

AliasMode is the other half and the more interesting one. A record with priority 0 and no parameters redirects resolution to another name, much like a CNAME, except it does not claim the whole name. That is the property CNAME could never have, and it is why SVCB can sit at a zone apex alongside SOA, NS, and MX. Proprietary ALIAS types now have a standard equivalent.

Zone-file and wire format

The RDATA is SvcPriority TargetName SvcParams: a 16-bit priority, a domain name, then zero or more key=value pairs, which the wire format stores in strictly increasing key order. Priority 0 means AliasMode and forbids parameters; any nonzero value is ServiceMode, lower preferred. A ServiceMode TargetName of . means the owner name itself, which is how an apex record points at its own addresses. Non-HTTP schemes prefix the owner name _port._scheme.

; owner                 pri  target              SvcParams
; --- AliasMode: priority 0, no parameters, legal at the apex ---
example.com.       3600 IN SVCB 0    svc.provider.net.

; --- ServiceMode: priority > 0, target "." means "this same name" ---
_8443._foo.example.com. 3600 IN SVCB 1 svc.example.com. (
        alpn="h2"           ; protocols the endpoint speaks
        port="8443"         ; non-default port
        ipv4hint="203.0.113.10"
        ipv6hint="2001:db8::10" )

; lower SvcPriority is tried first; 2 here is a fallback endpoint
_8443._foo.example.com. 3600 IN SVCB 2 svc2.example.com. alpn="h2"

Common uses

Treat SVCB as the mechanism and its per-scheme forms as the products. If you serve HTTP you publish HTTPS records, not SVCB ones, and browsers never query type 64. SVCB itself is what a new protocol adopts for the same benefit: a port and an ALPN set in the answer that names the host, plus apex redirection via AliasMode. The parameter registry is extensible, so a protocol defines its own keys instead of minting a record type and waiting for resolver support.

Troubleshooting

First verify which mode you actually published. Priority 0 with parameters attached is invalid and will be ignored; a nonzero priority with no useful target and no parameters achieves nothing. Providers differ in how they accept the parameter syntax, and quoting mistakes produce a record that parses but omits a key.

Hints are advisory. If clients reach the wrong address, check the target’s real A and AAAA records, not the ipv4hint: a conforming client resolves the target anyway, so a bad hint only buys a slow first attempt.

# SVCB is type 64; ask for it by name
dig _8443._foo.example.com SVCB +noall +answer

# older dig builds print the unknown-type format instead
dig example.com TYPE64 +short

# an AliasMode record should send you on to the target's own records
dig svc.provider.net SVCB +short

The defining RFC

SVCB is type 64, defined in RFC 9460 alongside HTTPS. The two modes are §2.4.2 and §2.4.3, presentation format is §2.1, and the initial parameters — mandatory, alpn, no-default-alpn, port, ipv4hint, ipv6hint, ech — are in §7. New keys go in the IANA Service Parameter Keys registry. Underscore prefixes follow RFC 8552. Resolve one with the DNS lookup tool.

Reference and tooling

Every record type and its assigned numeric value is listed in theIANA DNS Parameters registry. To query a live zone from the browser, use theDNS lookup tool. If the record you are chasing is TLS-related, the TLS inspector andCertificate Transparency lookup are usually the next two stops.

SVCB record

Zone-file example