diff --git a/common/constants.h b/common/constants.h
index 3fcf1f1..6b1bf6b 100644
--- a/common/constants.h
+++ b/common/constants.h
@@ -163,6 +163,8 @@
 static constexpr const auto& kPayloadPropertyMetadataHash = "METADATA_HASH";
 // The Authorization: HTTP header to be sent when downloading the payload.
 static constexpr const auto& kPayloadPropertyAuthorization = "AUTHORIZATION";
+// HTTP headers extra entries in the format of key1:val1 key2:val2 key3:val3
+static constexpr const auto& kPayloadPropertyHTTPExtras = "HTTP_EXTRAS";
 // The User-Agent HTTP header to be sent when downloading the payload.
 static constexpr const auto& kPayloadPropertyUserAgent = "USER_AGENT";
 // Set "POWERWASH=1" to powerwash (factory data reset) the device after