diff --git a/common/include/villas/hash_table.h b/common/include/villas/hash_table.h
index 9b97066f8..2d8479a92 100644
--- a/common/include/villas/hash_table.h
+++ b/common/include/villas/hash_table.h
@@ -32,7 +32,7 @@ extern "C" {
 #endif
 
 struct hash_table_entry {
-	void *key;
+	const void *key;
 	void *data;
 
 	struct hash_table_entry *next;
@@ -62,20 +62,20 @@ int hash_table_destroy(struct hash_table *ht, dtor_cb_t dtor, bool release);
 /** Insert a new key/value pair into the hash table.
  *
  */
-int hash_table_insert(struct hash_table *ht, void *key, void *data);
+int hash_table_insert(struct hash_table *ht, const void *key, void *data);
 
 /** Delete a key from the hash table.
  *
  *
  */
-int hash_table_delete(struct hash_table *ht, void *key);
+int hash_table_delete(struct hash_table *ht, const void *key);
 
 /** Perform a lookup in the hash table.
  *
  * @retval != NULL The value for the given key.
  * @retval NULL The given key is not stored in the hash table.
  */
-void * hash_table_lookup(struct hash_table *ht, void *key);
+void * hash_table_lookup(struct hash_table *ht, const void *key);
 
 /** Dump the contents of the hash table in a human readable format to stdout. */
 void hash_table_dump(struct hash_table *ht);
diff --git a/common/lib/hash_table.c b/common/lib/hash_table.c
index d38aa2f15..4fb5da14b 100644
--- a/common/lib/hash_table.c
+++ b/common/lib/hash_table.c
@@ -25,7 +25,7 @@
 #include <villas/utils.h>
 #include <villas/hash_table.h>
 
-static int hash_table_hash(struct hash_table *ht, void *key)
+static int hash_table_hash(struct hash_table *ht, const void *key)
 {
 	uintptr_t ptr = (uintptr_t) key;
 
@@ -89,7 +89,7 @@ int hash_table_destroy(struct hash_table *ht, dtor_cb_t dtor, bool release)
 	return 0;
 }
 
-int hash_table_insert(struct hash_table *ht, void *key, void *data)
+int hash_table_insert(struct hash_table *ht, const void *key, void *data)
 {
 	int ret, ikey = hash_table_hash(ht, key);
 	struct hash_table_entry *hte, *cur;
@@ -130,7 +130,7 @@ int hash_table_insert(struct hash_table *ht, void *key, void *data)
 
 #include <stdio.h>
 
-int hash_table_delete(struct hash_table *ht, void *key)
+int hash_table_delete(struct hash_table *ht, const void *key)
 {
 	int ret, ikey = hash_table_hash(ht, key);
 	struct hash_table_entry *cur, *prev;
@@ -163,7 +163,7 @@ int hash_table_delete(struct hash_table *ht, void *key)
 	return ret;
 }
 
-void * hash_table_lookup(struct hash_table *ht, void *key)
+void * hash_table_lookup(struct hash_table *ht, const void *key)
 {
 	int ikey = hash_table_hash(ht, key);
 	struct hash_table_entry *hte;