txthinking/brook · error
Invalid prefer
Error message
Invalid prefer
What it means
NewDialWithDNS validates the prefer parameter, which selects whether A (IPv4) or AAAA (IPv6) DNS records are queried. Any value other than the exact strings "A" or "AAAA" is rejected with "Invalid prefer" before a client is constructed.
Source
Thrown at plugins/dialwithdns/dialwithdns.go:35
import (
"errors"
"net"
"strconv"
"strings"
"github.com/txthinking/brook"
"github.com/txthinking/socks5"
)
type DialWithDNS struct {
DNSClient *brook.DNSClient
DOHClient *brook.DOHClient
Prefer string
}
func NewDialWithDNS(dns, prefer string) (*DialWithDNS, error) {
if prefer != "A" && prefer != "AAAA" {
return nil, errors.New("Invalid prefer")
}
if !strings.HasPrefix(dns, "https://") {
return &DialWithDNS{DNSClient: &brook.DNSClient{Server: dns}, Prefer: prefer}, nil
}
dc, err := brook.NewDOHClient(dns)
if err != nil {
return nil, err
}
return &DialWithDNS{DOHClient: dc, Prefer: prefer}, nil
}
func (p *DialWithDNS) IP(domain string) (net.IP, error) {
if p.Prefer == "A" {
if p.DNSClient != nil {
ip, err := p.DNSClient.A(domain)
if err != nil {
return nil, err
}View on GitHub (pinned to 5cd13ef3b1)
Solutions
- Pass exactly "A" for IPv4 preference or "AAAA" for IPv6 preference.
- Normalize/validate the config value before calling: uppercase it and map synonyms ("ipv4"->"A", "ipv6"->"AAAA").
- Guard against empty values by defaulting to "A" when the config field is unset.
Example fix
// before
p, err := NewDialWithDNS(dns, "ipv4")
// after
prefer := "A"
if strings.EqualFold(cfg.Prefer, "AAAA") || strings.EqualFold(cfg.Prefer, "ipv6") {
prefer = "AAAA"
}
p, err := NewDialWithDNS(dns, prefer) Defensive patterns
Strategy: validation
Validate before calling
func validPrefer(p string) bool { return p == "A" || p == "AAAA" }
if !validPrefer(cfg.Prefer) { return fmt.Errorf("prefer must be \"A\" or \"AAAA\", got %q", cfg.Prefer) } Try / catch
p, err := NewDialWithDNS(dns, prefer)
if err != nil && strings.Contains(err.Error(), "Invalid prefer") {
prefer = "A" // safe default
p, err = NewDialWithDNS(dns, prefer)
} Prevention
- Validate the prefer value at config-load time, before constructing plugins.
- Normalize synonyms (ipv4/ipv6) to "A"/"AAAA" upstream.
- Use constants instead of raw strings when calling NewDialWithDNS.
- Remember the check is case-sensitive: "a" is invalid.
When it happens
Trigger: Calling NewDialWithDNS(dns, prefer) with prefer values like "a", "ipv4", "A/AAAA", "", or any other non-exact string; wiring the plugin with a mistyped config flag.
Common situations: Case-sensitivity mistakes ("a" instead of "A"); passing descriptive labels like "ipv6"; empty prefer read from an unset config value.
Understand the failure class
Background: Invalid enum value errors: "Unknown type", "Invalid scope", "must be one of" — when a string is not on the library's allowed list — this error's family across 23 libraries.
Related errors
AI-assisted analysis of txthinking/brook@5cd13ef3b1 (2026-09-06).
Data as JSON: /api/errors/e6e221cbc9fa7686.
Report an issue: GitHub.