diff --git a/clock.c b/clock.c
index e5f104e..1668383 100644
--- a/clock.c
+++ b/clock.c
@@ -47,6 +47,10 @@
 #define N_CLOCK_PFD (N_POLLFD + 1) /* one extra per port, for the fault timer */
 #define POW2_41 ((double)(1ULL << 41))
 
+struct interface {
+	STAILQ_ENTRY(interface) list;
+};
+
 struct port {
 	LIST_ENTRY(port) list;
 };
diff --git a/config.c b/config.c
index e033842..f20c5f7 100644
--- a/config.c
+++ b/config.c
@@ -32,6 +32,10 @@
 #include "print.h"
 #include "util.h"
 
+struct interface {
+	STAILQ_ENTRY(interface) list;
+};
+
 enum config_section {
 	GLOBAL_SECTION,
 	UC_MTAB_SECTION,
diff --git a/interface.c b/interface.c
index 63ed7e4..7cd5b41 100644
--- a/interface.c
+++ b/interface.c
@@ -7,6 +7,13 @@
 #include <stdlib.h>
 #include "interface.h"
 
+struct interface {
+	STAILQ_ENTRY(interface) list;
+	char name[MAX_IFNAME_SIZE + 1];
+	char ts_label[MAX_IFNAME_SIZE + 1];
+	struct sk_ts_info ts_info;
+};
+
 struct interface *interface_create(const char *name)
 {
 	struct interface *iface;
diff --git a/interface.h b/interface.h
index b61f4d6..6cc50ac 100644
--- a/interface.h
+++ b/interface.h
@@ -17,13 +17,8 @@
 #error if_namesize larger than expected.
 #endif
 
-/** Defines a network interface, with PTP options. */
-struct interface {
-	STAILQ_ENTRY(interface) list;
-	char name[MAX_IFNAME_SIZE + 1];
-	char ts_label[MAX_IFNAME_SIZE + 1];
-	struct sk_ts_info ts_info;
-};
+/** Opaque type */
+struct interface;
 
 /**
  * Creates an instance of an interface.
diff --git a/nsm.c b/nsm.c
index 1292c6b..5aa925b 100644
--- a/nsm.c
+++ b/nsm.c
@@ -35,6 +35,10 @@
 #define IFMT		"\n\t\t"
 #define NSM_NFD		3
 
+struct interface {
+	STAILQ_ENTRY(interface) list;
+};
+
 struct nsm {
 	struct config		*cfg;
 	struct fdarray		fda;