diff --git a/compat.h b/compat.h
index 554700550..1f755d7e5 100644
--- a/compat.h
+++ b/compat.h
@@ -48,6 +48,14 @@
 #  define MAXHOSTNAMELEN	64
 #endif
 
+/*
+ * 4.2BSD lacks FD_* macros (we only use FD_SET and FD_ZERO)
+ */
+#ifndef FD_SETSIZE
+#define FD_SET(fd, fds)		((fds) -> fds_bits[0] |= (1 << (fd)))
+#define FD_ZERO(fds)		((fds) -> fds_bits[0] = 0)
+#endif /* !FD_SETSIZE */
+
 /*
  * Posix versions for those without...
  */