diff --git a/proxy/proxy.go b/proxy/proxy.go
index 3df4caa..e661a71 100644
--- a/proxy/proxy.go
+++ b/proxy/proxy.go
@@ -314,10 +314,14 @@ func (p *SNIProxy) buildProxyProtocolHeaderFromInfo(proxyInfo *ProxyProtocolInfo
 	if srcIP == nil {
 		return "PROXY UNKNOWN\r\n"
 	}
+	srcIPStr := proxyInfo.SrcIP
 
-	if srcIP.To4() != nil {
-		// Source is IPv4, use TCP4 protocol
+	if srcIP4 := srcIP.To4(); srcIP4 != nil {
+		// Source is IPv4, use TCP4 protocol. Write it in dotted form: an
+		// IPv4-mapped IPv6 source ("::ffff:a.b.c.d", as sent in a TCP6 line
+		// by a dual-stack upstream) is not a valid TCP4 address.
 		protocol = "TCP4"
+		srcIPStr = srcIP4.String()
 		if targetTCP.IP.To4() != nil {
 			// Target is also IPv4, use as-is
 			targetIP = targetTCP.IP.String()
@@ -343,7 +347,7 @@ func (p *SNIProxy) buildProxyProtocolHeaderFromInfo(proxyInfo *ProxyProtocolInfo
 
 	return fmt.Sprintf("PROXY %s %s %s %d %d\r\n",
 		protocol,
-		proxyInfo.SrcIP,
+		srcIPStr,
 		targetIP,
 		proxyInfo.SrcPort,
 		targetTCP.Port)
diff --git a/proxy/proxy_test.go b/proxy/proxy_test.go
index 7e3e6e6..99fb097 100644
--- a/proxy/proxy_test.go
+++ b/proxy/proxy_test.go
@@ -185,3 +185,31 @@ func TestParseProxyProtocolHeaderUnknownPreservesPayload(t *testing.T) {
 		t.Fatalf("Expected payload %q, got %q", payload, got)
 	}
 }
+
+// TestBuildProxyProtocolHeaderFromInfoIPv4MappedSource checks that an IPv4
+// client that a dual-stack upstream (e.g. HAProxy bound to "::") reports as
+// "PROXY TCP6 ::ffff:a.b.c.d ..." is forwarded with a valid TCP4 line.
+// PROXY v1 requires TCP4 addresses in dotted-quad form, and parsers such as
+// go-proxyproto (used by Traefik) reject "TCP4 ::ffff:a.b.c.d".
+func TestBuildProxyProtocolHeaderFromInfoIPv4MappedSource(t *testing.T) {
+	proxy, err := NewSNIProxy(8443, "", "", "127.0.0.1", 443, nil, true, nil)
+	if err != nil {
+		t.Fatalf("Failed to create SNI proxy: %v", err)
+	}
+
+	proxyInfo := &ProxyProtocolInfo{
+		Protocol: "TCP6",
+		SrcIP:    "::ffff:203.0.113.7",
+		DestIP:   "::ffff:10.0.0.1",
+		SrcPort:  51000,
+		DestPort: 443,
+	}
+
+	targetAddr, _ := net.ResolveTCPAddr("tcp", "127.0.0.1:8080")
+	header := proxy.buildProxyProtocolHeaderFromInfo(proxyInfo, targetAddr)
+
+	expected := "PROXY TCP4 203.0.113.7 127.0.0.1 51000 8080\r\n"
+	if header != expected {
+		t.Errorf("Expected header %q, got %q", expected, header)
+	}
+}