bootstrap DNS resolver feature

resolver.go

@@ -0,0 +1,82 @@
+package main
+
+import (
+	"github.com/AdguardTeam/dnsproxy/upstream"
+	"github.com/miekg/dns"
+	"time"
+)
+
+type Resolver struct {
+	upstream upstream.Upstream
+}
+
+const DOT = 0x2e
+
+func NewResolver(address string, timeout time.Duration) (*Resolver, error) {
+	opts := upstream.Options{Timeout: timeout}
+	u, err := upstream.AddressToUpstream(address, opts)
+	if err != nil {
+		return nil, err
+	}
+	return &Resolver{upstream: u}, nil
+}
+
+func (r *Resolver) ResolveA(domain string) []string {
+	res := make([]string, 0)
+	if len(domain) == 0 {
+		return res
+	}
+	if domain[len(domain)-1] != DOT {
+		domain = domain + "."
+	}
+	req := dns.Msg{}
+	req.Id = dns.Id()
+	req.RecursionDesired = true
+	req.Question = []dns.Question{
+		{Name: domain, Qtype: dns.TypeA, Qclass: dns.ClassINET},
+	}
+	reply, err := r.upstream.Exchange(&req)
+	if err != nil {
+		return res
+	}
+	for _, rr := range reply.Answer {
+		if a, ok := rr.(*dns.A); ok {
+			res = append(res, a.A.String())
+		}
+	}
+	return res
+}
+
+func (r *Resolver) ResolveAAAA(domain string) []string {
+	res := make([]string, 0)
+	if len(domain) == 0 {
+		return res
+	}
+	if domain[len(domain)-1] != DOT {
+		domain = domain + "."
+	}
+	req := dns.Msg{}
+	req.Id = dns.Id()
+	req.RecursionDesired = true
+	req.Question = []dns.Question{
+		{Name: domain, Qtype: dns.TypeAAAA, Qclass: dns.ClassINET},
+	}
+	reply, err := r.upstream.Exchange(&req)
+	if err != nil {
+		return res
+	}
+	for _, rr := range reply.Answer {
+		if a, ok := rr.(*dns.AAAA); ok {
+			res = append(res, a.AAAA.String())
+		}
+	}
+	return res
+}
+
+func (r *Resolver) Resolve(domain string) []string {
+	res := r.ResolveA(domain)
+	if len(res) == 0 {
+		res = r.ResolveAAAA(domain)
+	}
+	return res
+}