Thanks to John Cummings.

src/cmd/upas/smtp/rmtdns.c

@@ -0,0 +1,58 @@
+#include	"common.h"
+#include	<ndb.h>
+
+int
+rmtdns(char *net, char *path)
+{
+
+	int fd, n, r;
+	char *domain, *cp, buf[1024];
+
+	if(net == 0 || path == 0)
+		return 0;
+
+	domain = strdup(path);
+	cp = strchr(domain, '!');
+	if(cp){
+		*cp = 0;
+		n = cp-domain;
+	} else
+		n = strlen(domain);
+
+	if(*domain == '[' && domain[n-1] == ']'){	/* accept [nnn.nnn.nnn.nnn] */
+		domain[n-1] = 0;
+		r = strcmp(ipattr(domain+1), "ip");
+		domain[n-1] = ']';
+	} else
+		r = strcmp(ipattr(domain), "ip");	/* accept nnn.nnn.nnn.nnn */
+
+	if(r == 0){
+		free(domain);
+		return 0;
+	}
+
+	snprint(buf, sizeof(buf), "%s/dns", net);
+
+	fd = open(buf, ORDWR);			/* look up all others */
+	if(fd < 0){				/* dns screw up - can't check */
+		free(domain);
+		return 0;
+	}
+
+	n = snprint(buf, sizeof(buf), "%s all", domain);
+	free(domain);
+	seek(fd, 0, 0);
+	n = write(fd, buf, n);
+	close(fd);
+	if(n < 0){
+		rerrstr(buf, sizeof(buf));
+		if (strcmp(buf, "dns: name does not exist") == 0)
+			return -1;
+	}
+	return 0;
+}
+
+/*
+void main(int, char *argv[]){ print("return = %d\n", rmtdns("/net.alt/tcp/109", argv[1]));}
+
+*/