diff --git a/tool/ports/mk/common.inc b/tool/ports/mk/common.inc
index 1c80f09e5..b4e420d85 100644
--- a/tool/ports/mk/common.inc
+++ b/tool/ports/mk/common.inc
@@ -4,6 +4,9 @@
 # \date   2014-05-27
 #
 
+# compound directory where all 3rd-party source codes are installed
+CONTRIB_DIR ?= $(GENODE_DIR)/contrib
+
 SHELL   := bash
 VERBOSE ?= @
 ECHO    := echo -e
diff --git a/tool/ports/mk/front_end.inc b/tool/ports/mk/front_end.inc
index eb5795ad2..a0bb5f807 100644
--- a/tool/ports/mk/front_end.inc
+++ b/tool/ports/mk/front_end.inc
@@ -17,9 +17,6 @@ $(MAKECMDGOALS) default:
 ARGUMENT  := $(MAKECMDGOALS)
 PORT_NAME := $(firstword $(ARGUMENT))
 
-# compound directory where all 3rd-party source codes are installed
-CONTRIB_DIR ?= $(GENODE_DIR)/contrib
-
 # list of all repositories located at '<genode-dir>/repos/'
 REPOSITORIES ?= $(shell find $(GENODE_DIR)/repos -follow -mindepth 1 -maxdepth 1 -type d)
 
@@ -45,18 +42,6 @@ _REL_HASH_FILE := $(notdir $(REP_DIR))/ports/$(notdir $(PORT))
 # directory where to install the port
 PORT_DIR := $(CONTRIB_DIR)/$(PORT_NAME)-$(HASH)
 
-#
-# Protect the GENODE_DIR Git repository by creating an
-# empty Git repository in CONTRIB_DIR. This is to catch
-# Git tools called by port recipes from falling through
-# the parent directories of PORT_DIR and affecting other
-# Git repositories.
-#
-$(PORT_DIR): $(CONTRIB_DIR)/.git
-$(CONTRIB_DIR)/.git:
-	$(VERBOSE)mkdir -p $(CONTRIB_DIR)
-	$(VERBOSE)git init --quiet $(CONTRIB_DIR)
-
 # path to hash file generated during installation
 PORT_HASH_FILE := $(PORT_DIR)/$(PORT_NAME).hash
 
diff --git a/tool/ports/prepare_port b/tool/ports/prepare_port
index b390d8039..d95f49d9b 100755
--- a/tool/ports/prepare_port
+++ b/tool/ports/prepare_port
@@ -12,6 +12,8 @@ endif
 
 export GENODE_DIR := $(realpath $(dir $(MAKEFILE_LIST))/../..)
 
+include $(GENODE_DIR)/tool/ports/mk/common.inc
+
 usage:
 	@echo
 	@echo "Tool for preparing and updating ports"
@@ -29,5 +31,17 @@ TARGETS = $(sort $(MAKECMDGOALS))
 
 .PHONY: $(TARGETS)
 
-$(TARGETS):
+#
+# Protect the GENODE_DIR Git repository by creating an
+# empty Git repository in CONTRIB_DIR. This is to catch
+# Git tools called by port recipes from falling through
+# the parent directories of PORT_DIR and affecting other
+# Git repositories.
+#
+$(PORT_DIR): $(CONTRIB_DIR)/.git/HEAD
+$(CONTRIB_DIR)/.git/HEAD:
+	@$(VERBOSE)mkdir -p $(CONTRIB_DIR)
+	@$(VERBOSE)git init --quiet $(CONTRIB_DIR)
+
+$(TARGETS): $(CONTRIB_DIR)/.git/HEAD
 	@$(MAKE) -f $(GENODE_DIR)/tool/ports/mk/prepare_single_port.mk $@