diff --git a/mfshell/commands/host.c b/mfshell/commands/host.c
index dabf80e..15e3630 100644
--- a/mfshell/commands/host.c
+++ b/mfshell/commands/host.c
@@ -18,6 +18,7 @@
  */
 
 #define _POSIX_C_SOURCE 200809L // for strdup and getline
+#define _GNU_SOURCE             // for strdup on old systems
 
 #include <stddef.h>
 #include <stdio.h>
diff --git a/mfshell/commands/lcd.c b/mfshell/commands/lcd.c
index a1cad9d..1d3acf0 100644
--- a/mfshell/commands/lcd.c
+++ b/mfshell/commands/lcd.c
@@ -18,6 +18,7 @@
  */
 
 #define _POSIX_C_SOURCE 200809L // for strdup
+#define _GNU_SOURCE             // for strdup on old systems
 
 #include <stdio.h>
 #include <stdlib.h>