Pull request 249: 5631-lookup-panic

Merge in GO/dnsproxy from 5631-lookup-panic to master

Updates AdguardTeam/AdGuardHome#5631.

Squashed commit of the following:

commit 8835572
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Fri Mar 24 20:11:30 2023 +0500

    proxy: imp doc

commit db08d96
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Fri Mar 24 20:08:11 2023 +0500

    proxy: fix lookup slicing

proxy/lookup.go

@@ -4,6 +4,7 @@ import (
 	"net"
 
 	"github.com/AdguardTeam/dnsproxy/proxyutil"
+	"github.com/AdguardTeam/golibs/errors"
 
 	"github.com/miekg/dns"
 )
@@ -31,13 +32,19 @@ func (p *Proxy) lookupIPAddr(host string, qtype uint16, ch chan *lookupResult) {
 	ch <- &lookupResult{d.Res, err}
 }
 
+// ErrEmptyHost is returned by LookupIPAddr when the host is empty and can't be
+// resolved.
+const ErrEmptyHost = errors.Error("host is empty")
+
 // LookupIPAddr resolves the specified host IP addresses
 // It sends two DNS queries (A and AAAA) in parallel and returns both results
 func (p *Proxy) LookupIPAddr(host string) ([]net.IPAddr, error) {
-	if host[:1] != "." {
-		host += "."
+	if host == "" {
+		return nil, ErrEmptyHost
 	}
 
+	host = dns.Fqdn(host)
+
 	ch := make(chan *lookupResult)
 	go p.lookupIPAddr(host, dns.TypeA, ch)
 	go p.lookupIPAddr(host, dns.TypeAAAA, ch)