upstream with actual client ip to upstream

main.go

@@ -7,6 +7,7 @@ import (
 	"html/template"
 	"log"
 	"log/slog"
+	"net"
 	"net/http"
 	"os"
 	"strconv"
@@ -94,9 +95,17 @@ func (resolver *DNSResolver) handleDNSRequest(w dns.ResponseWriter, r *dns.Msg)
 			}
 
 			// Fallback to upstream for all other cases
-			resp, err := resolver.resolveUpstream(r)
+			var clientIP net.IP
+			switch addr := w.RemoteAddr().(type) {
+			case *net.UDPAddr:
+				clientIP = addr.IP
+			case *net.TCPAddr:
+				clientIP = addr.IP
+			}
+
+			resp, err := resolver.resolveUpstream(r, clientIP)
 			if err == nil && resp != nil {
-				log.Printf("[UPSTREAM] Forwarded %s", q.Name)
+				log.Printf("[UPSTREAM] Forwarded %s (ClientIP: %s)", q.Name, clientIP)
 				m.Answer = resp.Answer
 				m.Ns = resp.Ns
 				m.Extra = resp.Extra
@@ -122,13 +131,50 @@ func (resolver *DNSResolver) getRecordFromDB(domain string, recordType string) (
 	return ip, nil
 }
 
-func (resolver *DNSResolver) resolveUpstream(r *dns.Msg) (*dns.Msg, error) {
+func (resolver *DNSResolver) resolveUpstream(r *dns.Msg, clientIP net.IP) (*dns.Msg, error) {
 	rows, err := resolver.db.Query("SELECT address FROM upstreams")
 	if err != nil {
 		return nil, err
 	}
 	defer rows.Close()
 
+	// Create a copy of the message to avoid side effects
+	req := r.Copy()
+
+	// Add EDNS0 Client Subnet option
+	if clientIP != nil {
+		o := req.IsEdns0()
+		if o == nil {
+			req.SetEdns0(4096, false)
+			o = req.IsEdns0()
+		}
+
+		// Filter out existing SUBNET options to ensure we are authoritative about the client IP
+		var newOptions []dns.EDNS0
+		for _, opt := range o.Option {
+			if opt.Option() != dns.EDNS0SUBNET {
+				newOptions = append(newOptions, opt)
+			}
+		}
+
+		e := new(dns.EDNS0_SUBNET)
+		e.Code = dns.EDNS0SUBNET
+		if ip4 := clientIP.To4(); ip4 != nil {
+			e.Family = 1 // IPv4
+			e.SourceNetmask = 32
+			e.SourceScope = 0
+			e.Address = ip4
+		} else {
+			e.Family = 2 // IPv6
+			e.SourceNetmask = 128
+			e.SourceScope = 0
+			e.Address = clientIP
+		}
+
+		newOptions = append(newOptions, e)
+		o.Option = newOptions
+	}
+
 	c := new(dns.Client)
 	c.Net = "udp"
 
@@ -140,7 +186,7 @@ func (resolver *DNSResolver) resolveUpstream(r *dns.Msg) (*dns.Msg, error) {
 			continue
 		}
 
-		resp, _, err := c.Exchange(r, upstreamAddr)
+		resp, _, err := c.Exchange(req, upstreamAddr)
 		if err == nil && resp != nil && resp.Rcode != dns.RcodeServerFailure {
 			return resp, nil
 		}