diff --git a/lib/tls/openssl/openssl-client.c b/lib/tls/openssl/openssl-client.c
index 985a63f3a..b98b9758a 100644
--- a/lib/tls/openssl/openssl-client.c
+++ b/lib/tls/openssl/openssl-client.c
@@ -21,6 +21,12 @@
  * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
  * IN THE SOFTWARE.
  */
+
+#include "lws_config.h"
+#ifdef LWS_HAVE_X509_VERIFY_PARAM_set1_host
+/* Before glibc 2.10, strnlen required _GNU_SOURCE */
+#define _GNU_SOURCE
+#endif
 #include <string.h>
 
 #include "private-lib-core.h"