diff --git a/Config.mk b/Config.mk
index 7f3d8c7d75..9b39d6c033 100644
--- a/Config.mk
+++ b/Config.mk
@@ -213,15 +213,15 @@ EMBEDDED_EXTRA_CFLAGS += -fno-exceptions
 XSM_ENABLE ?= n
 FLASK_ENABLE ?= $(XSM_ENABLE)
 
-XEN_EXTFILES_URL=http://xenbits.xen.org/xen-extfiles
+XEN_EXTFILES_URL ?= http://xenbits.xen.org/xen-extfiles
 # All the files at that location were downloaded from elsewhere on
 # the internet.  The original download URL is preserved as a comment
 # near the place in the Xen Makefiles where the file is used.
 
 ifeq ($(GIT_HTTP),y)
-QEMU_REMOTE=http://xenbits.xen.org/git-http/qemu-xen-unstable.git
+QEMU_REMOTE ?= http://xenbits.xen.org/git-http/qemu-xen-unstable.git
 else
-QEMU_REMOTE=git://xenbits.xen.org/qemu-xen-unstable.git
+QEMU_REMOTE ?= git://xenbits.xen.org/qemu-xen-unstable.git
 endif
 
 ifeq ($(GIT_HTTP),y)
diff --git a/tools/firmware/etherboot/Makefile b/tools/firmware/etherboot/Makefile
index 15561fc0a8..a0578d27c4 100644
--- a/tools/firmware/etherboot/Makefile
+++ b/tools/firmware/etherboot/Makefile
@@ -5,14 +5,14 @@ include $(XEN_ROOT)/tools/Rules.mk
 include Config
 
 ifeq ($(GIT_HTTP),y)
-IPXE_GIT_URL := http://git.ipxe.org/ipxe.git
+IPXE_GIT_URL ?= http://git.ipxe.org/ipxe.git
 else
-IPXE_GIT_URL := git://git.ipxe.org/ipxe.git
+IPXE_GIT_URL ?= git://git.ipxe.org/ipxe.git
 endif
 
 IPXE_GIT_TAG := 9a93db3f0947484e30e753bbd61a10b17336e20e
 
-IPXE_TARBALL_URL := $(XEN_EXTFILES_URL)/ipxe-git-$(IPXE_GIT_TAG).tar.gz
+IPXE_TARBALL_URL ?= $(XEN_EXTFILES_URL)/ipxe-git-$(IPXE_GIT_TAG).tar.gz
 
 D=ipxe
 T=ipxe.tar.gz