diff --git a/target/linux/ramips/image/mt7621.mk b/target/linux/ramips/image/mt7621.mk
index 33fcc805d98..1c6cd237d30 100644
--- a/target/linux/ramips/image/mt7621.mk
+++ b/target/linux/ramips/image/mt7621.mk
@@ -99,13 +99,15 @@ endef
 
 define Build/znet-header
 	$(eval version=$(word 1,$(1)))
+	$(eval magic=$(if $(word 2,$(1)),$(word 2,$(1)),ZNET))
+	$(eval hdrlen=$(if $(word 3,$(1)),$(word 3,$(1)),0x30))
 	( \
 		data_size_crc="$$(dd if=$@ 2>/dev/null | gzip -c | \
 			tail -c 8 | od -An -N4 -tx4 --endian big | tr -d ' \n')"; \
 		payload_len="$$(dd if=$@ bs=4 count=1 skip=1 2>/dev/null | od -An -tdI --endian big | tr -d ' \n')"; \
 		payload_size_crc="$$(dd if=$@ ibs=1 count=$$payload_len 2>/dev/null | gzip -c | \
 			tail -c 8 | od -An -N4 -tx4 --endian big | tr -d ' \n')"; \
-		echo -ne "\x5A\x4E\x45\x54" | dd bs=4 count=1 conv=sync 2>/dev/null; \
+		echo -ne "$(magic)" | dd bs=4 count=1 conv=sync 2>/dev/null; \
 		echo -ne "$$(printf '%08x' $$(stat -c%s $@) | fold -s2 | xargs -I {} echo \\x{} | tac | tr -d '\n')" | \
 			dd bs=4 count=1 conv=sync 2>/dev/null; \
 		echo -ne "$$(echo $$data_size_crc | sed 's/../\\x&/g')" | \
@@ -114,7 +116,7 @@ define Build/znet-header
 			dd bs=4 count=1 conv=sync 2>/dev/null; \
 		echo -ne "\x12\x34\x56\x78" | dd bs=4 count=1 conv=sync 2>/dev/null; \
 		echo -ne "$(version)" | dd bs=28 count=1 conv=sync 2>/dev/null; \
-		dd if=/dev/zero bs=262096 count=1 conv=sync 2>/dev/null | tr "\000" "\377"; \
+		dd if=/dev/zero bs=$$((0x40000 - $(hdrlen))) count=1 conv=sync 2>/dev/null | tr "\000" "\377"; \
 		cat $@; \
 	) > $@.new
 	mv $@.new $@