diff --git a/adsbus/rand.c b/adsbus/rand.c
index c916224..c6a79e1 100644
--- a/adsbus/rand.c
+++ b/adsbus/rand.c
@@ -1,16 +1,24 @@
+#include <stdio.h>
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <fcntl.h>
 #include <assert.h>
+#include <sys/uio.h>
 #include <unistd.h>
+#include <string.h>
+
+#include "common.h"
 
 #include "rand.h"
 
+struct buf rand_buf = BUF_INIT;
 static int rand_fd;
 
 void rand_init() {
 	rand_fd = open("/dev/urandom", O_RDONLY);
 	assert(rand_fd >= 0);
+	assert(read(rand_fd, buf_at(&rand_buf, 0), BUF_LEN_MAX) == BUF_LEN_MAX);
+	rand_buf.length = BUF_LEN_MAX;
 }
 
 void rand_cleanup() {
@@ -18,5 +26,24 @@ void rand_cleanup() {
 }
 
 void rand_fill(void *value, size_t size) {
-	assert(read(rand_fd, value, size) == size);
+	if (size <= rand_buf.length) {
+		memcpy(value, buf_at(&rand_buf, 0), size);
+		buf_consume(&rand_buf, size);
+		return;
+	}
+
+	struct iovec iov[2] = {
+		{
+			.iov_base = rand_buf.buf,
+			.iov_len = rand_buf.start,
+		},
+		{
+			.iov_base = value,
+			.iov_len = size,
+		},
+	};
+
+	assert(readv(rand_fd, iov, 2) == rand_buf.start + size);
+	rand_buf.start = 0;
+	rand_buf.length = BUF_LEN_MAX;
 }