diff --git a/micropython/extmod/modtrezormsg/modtrezormsg-unix.h b/micropython/extmod/modtrezormsg/modtrezormsg-unix.h
index eacae226..f00499ae 100644
--- a/micropython/extmod/modtrezormsg/modtrezormsg-unix.h
+++ b/micropython/extmod/modtrezormsg/modtrezormsg-unix.h
@@ -15,6 +15,7 @@
 #include "../../trezorhal/usb.h"
 #include "../../trezorhal/touch.h"
 
+#define TREZOR_UDP_IFACE 0
 #define TREZOR_UDP_PORT 21324
 
 static int sock;
@@ -74,6 +75,9 @@ int usb_vcp_add(const usb_vcp_info_t *info) {
 }
 
 int usb_hid_can_read(uint8_t iface_num) {
+    if (iface_num != TREZOR_UDP_IFACE) {
+        return 0;
+    }
     struct pollfd fds[] = {
         { sock, POLLIN, 0 },
     };
@@ -86,6 +90,9 @@ int usb_hid_can_read(uint8_t iface_num) {
 }
 
 int usb_hid_can_write(uint8_t iface_num) {
+    if (iface_num != TREZOR_UDP_IFACE) {
+        return 0;
+    }
     struct pollfd fds[] = {
         { sock, POLLOUT, 0 },
     };
@@ -98,6 +105,9 @@ int usb_hid_can_write(uint8_t iface_num) {
 }
 
 int usb_hid_read(uint8_t iface_num, uint8_t *buf, uint32_t len) {
+    if (iface_num != TREZOR_UDP_IFACE) {
+        return 0;
+    }
     struct sockaddr_in si;
     socklen_t sl = sizeof(si);
     ssize_t r = recvfrom(sock, buf, len, MSG_DONTWAIT, (struct sockaddr *)&si, &sl);
@@ -116,6 +126,9 @@ int usb_hid_read(uint8_t iface_num, uint8_t *buf, uint32_t len) {
 }
 
 int usb_hid_write(uint8_t iface_num, const uint8_t *buf, uint32_t len) {
+    if (iface_num != TREZOR_UDP_IFACE) {
+        return 0;
+    }
     ssize_t r = len;
     if (slen > 0) {
         r = sendto(sock, buf, len, MSG_DONTWAIT, (const struct sockaddr *)&si_other, slen);