Thanks to John Cummings.

src/cmd/upas/send/authorize.c

@@ -0,0 +1,29 @@
+#include "common.h"
+#include "send.h"
+
+/*
+ *  Run a command to authorize or refuse entry.  Return status 0 means
+ *  authorize, -1 means refuse.
+ */
+void
+authorize(dest *dp)
+{
+	process *pp;
+	String *errstr;
+
+	dp->authorized = 1;
+	pp = proc_start(s_to_c(dp->repl1), (stream *)0, (stream *)0, outstream(), 1, 0);
+	if (pp == 0){
+		dp->status = d_noforward;
+		return;
+	}
+	errstr = s_new();
+	while(s_read_line(pp->std[2]->fp, errstr))
+		;
+	if ((dp->pstat = proc_wait(pp)) != 0) {
+		dp->repl2 = errstr;
+		dp->status = d_noforward;
+	} else
+		s_free(errstr);
+	proc_free(pp);
+}