diff --git a/external/CMakeLists.txt b/external/CMakeLists.txt
index 360c5a207..d4091a854 100644
--- a/external/CMakeLists.txt
+++ b/external/CMakeLists.txt
@@ -76,4 +76,5 @@ target_include_directories(cpr PUBLIC cpr/include)
 target_compile_definitions(cpr PUBLIC CPR_CURL_NOSIGNAL)
 add_library(cpr::cpr ALIAS cpr)
 
+add_definitions(-D_GNU_SOURCE)
 add_subdirectory(ngtcp2 EXCLUDE_FROM_ALL)