From c569e7755ace1e95a2130ae08fdaf43800ff9617 Mon Sep 17 00:00:00 2001
From: tedu <tedu@openbsd.org>
Date: Tue, 4 Jul 2017 00:30:45 +0000
Subject: [PATCH] properly adjust the ttl of replies instead of freezing them
 in time

---
 usr.sbin/rebound/rebound.c | 61 ++++++++++++++++++++++++++++++++++++--
 1 file changed, 59 insertions(+), 2 deletions(-)

diff --git a/usr.sbin/rebound/rebound.c b/usr.sbin/rebound/rebound.c
index 03c93332472..40b4e4a9b04 100644
--- a/usr.sbin/rebound/rebound.c
+++ b/usr.sbin/rebound/rebound.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: rebound.c,v 1.86 2017/07/03 16:36:48 tedu Exp $ */
+/* $OpenBSD: rebound.c,v 1.87 2017/07/04 00:30:45 tedu Exp $ */
 /*
  * Copyright (c) 2015 Ted Unangst <tedu@openbsd.org>
  *
@@ -82,6 +82,7 @@ struct dnscache {
 	struct dnspacket *resp;
 	size_t resplen;
 	struct timespec ts;
+	struct timespec basetime;
 };
 static TAILQ_HEAD(, dnscache) cachefifo;
 static RB_HEAD(cachetree, dnscache) cachetree;
@@ -201,6 +202,61 @@ freecacheent(struct dnscache *ent)
 	free(ent);
 }
 
+static int
+adjustttl(struct dnscache *ent)
+{
+	struct dnspacket *resp = ent->resp;
+	char *p = (char *)resp;
+	u_int rlen = ent->resplen;
+	u_int used = 0;
+	uint32_t ttl, cnt, i;
+	uint16_t len;
+	time_t diff;
+
+	diff = now.tv_sec - ent->basetime.tv_sec;
+	if (diff <= 0)
+		return 0;
+
+	/* checks are redundant; checked when cacheent is created */
+	/* skip past packet header */
+	used += sizeof(struct dnspacket);
+	if (used >= rlen)
+		return -1;
+	if (ntohs(resp->qdcount) != 1)
+		return -1;
+	/* skip past query name, type, and class */
+	used += strnlen(p + used, rlen - used);
+	used += 2;
+	used += 2;
+	cnt = ntohs(resp->ancount);
+	for (i = 0; i < cnt; i++) {
+		if (used >= rlen)
+			return -1;
+		/* skip past answer name, type, and class */
+		used += strnlen(p + used, rlen - used);
+		used += 2;
+		used += 2;
+		if (used + 4 >= rlen)
+			return -1;
+		memcpy(&ttl, p + used, 4);
+		ttl = ntohl(ttl);
+		/* expired */
+		if (diff >= ttl)
+			return -1;
+		ttl -= diff;
+		ttl = ntohl(ttl);
+		memcpy(p + used, &ttl, 4);
+		used += 4;
+		if (used + 2 >= rlen)
+			return -1;
+		memcpy(&len, p + used, 2);
+		used += 2;
+		used += ntohs(len);
+	}
+	ent->basetime.tv_sec += diff;
+	return 0;
+}
+
 static struct dnscache *
 cachelookup(struct dnspacket *dnsreq, size_t reqlen)
 {
@@ -224,7 +280,7 @@ cachelookup(struct dnspacket *dnsreq, size_t reqlen)
 	key.req = dnsreq;
 	hit = RB_FIND(cachetree, &cachetree, &key);
 	if (hit) {
-		if (timespeccmp(&hit->ts, &now, <=)) {
+		if (adjustttl(hit) != 0) {
 			freecacheent(hit);
 			hit = NULL;
 		} else
@@ -446,6 +502,7 @@ sendreply(struct request *req)
 			return;
 		ent->ts = now;
 		ent->ts.tv_sec += MINIMUM(ttl, 300);
+		ent->basetime = now;
 		ent->resp = malloc(r);
 		if (!ent->resp) {
 			RB_REMOVE(cachetree, &cachetree, ent);
-- 
2.20.1