diff --git a/src/tcpdchk.c b/src/tcpdchk.c
index f9841703..6e9a65ad 100644
--- a/src/tcpdchk.c
+++ b/src/tcpdchk.c
@@ -36,6 +36,8 @@
 #endif
 
 #if HAVE_NETDB_H
+/* _GNU_SOURCE: for musl's netdb.h to expose gethostbyaddr */
+#define _GNU_SOURCE
 #include <netdb.h>
 #endif