txthinking/brook · error

This is ipv4

Error message

This is ipv4

What it means

Resolve6 resolves a hostname/IP to an IPv6 address. Before doing anything it parses the input; if the input is already an IP with a non-nil To4() (an IPv4 or IPv4-mapped address), it refuses with "This is ipv4" because the AAAA lookup path only handles IPv6. Pure IPv6 literals are returned directly; hostnames go through a DNS AAAA query.

Source

Thrown at resolve.go:12

package brook

import (
	"errors"
	"net"

	"github.com/miekg/dns"
)

func Resolve6(host string) (string, error) {
	if net.ParseIP(host).To4() != nil {
		return "", errors.New("This is ipv4")
	}
	if net.ParseIP(host).To16() != nil {
		return host, nil
	}
	m := &dns.Msg{}
	m.SetQuestion(host+".", dns.TypeAAAA)
	r, err := dns.Exchange(m, "[2001:4860:4860::8888]:53")
	if err != nil {
		return "", err
	}
	for _, v := range r.Answer {
		if t, ok := v.(*dns.AAAA); ok {
			return t.AAAA.String(), nil
		}
	}
	return "", errors.New("Can not find IP")
}

View on GitHub (pinned to 5cd13ef3b1)

Solutions

1. Check the input with net.ParseIP before calling: if ip.To4() != nil use Resolve4 (or the IPv4 path) instead of Resolve6.
2. If the goal is dual-stack resolution, call the generic resolver and let DNS pick A or AAAA records based on connectivity.
3. Fix the configuration or environment so the host field for this code path is a hostname or IPv6 address only.

Example fix

// before
addr, err := Resolve6("8.8.8.8") // error: This is ipv4

// after
ip := net.ParseIP(host)
if ip != nil && ip.To4() != nil {
    addr, err = Resolve4(host)
} else {
    addr, err = Resolve6(host)
}

func isIPv4Literal(host string) bool {
    ip := net.ParseIP(host)
    return ip != nil && ip.To4() != nil
}
// call Resolve4 when isIPv4Literal(host), else Resolve6

func isIPv6(host string) bool {
    ip := net.ParseIP(host)
    return ip != nil && ip.To4() == nil
}

addr, err := Resolve6(host)
if err != nil && strings.Contains(err.Error(), "ipv4") {
    addr, err = Resolve4(host)
}

When it happens

Trigger: Calling Resolve6 with a dotted-quad IPv4 string like "8.8.8.8" or an IPv4-mapped form like "::ffff:8.8.8.8" — net.ParseIP succeeds and To4() is non-nil, so the function returns the error instead of resolving.

Common situations: Config files or URL hosts containing IPv4 addresses passed to an IPv6-only resolution path; code that assumed Resolve6 would fall back to IPv4; environment variables whose host field switches between IPv4 and IPv6 depending on deployment.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

• Invalid prefer
• no question
• no question
• Can not find IP
• invalid type

AI-assisted analysis of txthinking/brook@5cd13ef3b1 (2026-09-06). Data as JSON: /api/errors/2348d081d19346cc. Report an issue: GitHub.