diff --git a/include/patty/ax25/if.h b/include/patty/ax25/if.h
index d3c030b..3892c28 100644
--- a/include/patty/ax25/if.h
+++ b/include/patty/ax25/if.h
@@ -41,6 +41,8 @@ typedef struct _patty_ax25_if {
 
     char name[8];
 
+    uint32_t flags;
+
     void *rx_buf,
          *tx_buf;
 
diff --git a/src/if.c b/src/if.c
index dee7f80..fc315c1 100644
--- a/src/if.c
+++ b/src/if.c
@@ -48,6 +48,13 @@ patty_ax25_if *patty_ax25_if_new(int opts, patty_ax25_if_info *info) {
         goto error_invalid_address;
     }
 
+    /*
+     * TODO: Eventually inherit the half/full-duplex flag from the TNC this
+     * interface is bound to
+     */
+    iface->flags = PATTY_AX25_PARAM_DEFAULT_CLASSES
+                 | PATTY_AX25_PARAM_CLASSES_HALF_DUPLEX;
+
     if ((iface->rx_buf = malloc(PATTY_AX25_IF_DEFAULT_MRU)) == NULL) {
         goto error_malloc_rx_buf;
     } else {