diff --git a/mfshell/main.c b/mfshell/main.c
index 1ff4828..d7d8d6d 100644
--- a/mfshell/main.c
+++ b/mfshell/main.c
@@ -18,6 +18,7 @@
  */
 
 #define _POSIX_C_SOURCE 200809L // for strdup
+#define _GNU_SOURCE             // for strdup on old systems
 
 #include <openssl/ssl.h>
 #include <stddef.h>
diff --git a/mfshell/mfshell.c b/mfshell/mfshell.c
index 038fcbb..1fc8fcb 100644
--- a/mfshell/mfshell.c
+++ b/mfshell/mfshell.c
@@ -23,6 +23,7 @@
 #else
 #define _WITH_GETLINE           // on freebsd for getline
 #endif
+#define _GNU_SOURCE             // for strdup on old systems
 
 #include <stdio.h>
 #include <stdlib.h>
diff --git a/mfshell/options.c b/mfshell/options.c
index 12a9d1e..b2c93b2 100644
--- a/mfshell/options.c
+++ b/mfshell/options.c
@@ -17,6 +17,7 @@
  */
 
 #define _POSIX_C_SOURCE 200809L // for strdup
+#define _GNU_SOURCE             // for strdup on old systems
 
 #include <stdio.h>
 #include <getopt.h>