From: Ondřej Surý <ondrej@sury.org>
Date: Fri, 24 Sep 2021 07:35:11 +0000 (+0200)
Subject: Disable lame-ttl cache
X-Git-Tag: v9.17.20~18^2~5^2~2
X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=011e9418ce9bb25675de6ac8d47536efedeeb312;p=thirdparty%2Fbind9.git

Disable lame-ttl cache

The lame-ttl cache is implemented in ADB as per-server locked
linked-list "indexed" with <qname,qtype>.  This list has to be walked
every time there's a new query or new record added into the lame cache.
Determined attacker can use this to degrade performance of the resolver.

Resolver testing has shown that disabling the lame cache has little
impact on the resolver performance and it's a minimal viable defense
against this kind of attack.
---

diff --git a/bin/named/config.c b/bin/named/config.c
index fa8473db7cb..b6453b814ea 100644
--- a/bin/named/config.c
+++ b/bin/named/config.c
@@ -151,7 +151,7 @@ options {\n\
 	fetches-per-server 0;\n\
 	fetches-per-zone 0;\n\
 	glue-cache yes;\n\
-	lame-ttl 600;\n"
+	lame-ttl 0;\n"
 #ifdef HAVE_LMDB
 			    "	lmdb-mapsize 32M;\n"
 #endif /* ifdef HAVE_LMDB */
diff --git a/bin/named/server.c b/bin/named/server.c
index 638703e8c2b..35ad6a0b7f2 100644
--- a/bin/named/server.c
+++ b/bin/named/server.c
@@ -4806,8 +4806,11 @@ configure_view(dns_view_t *view, dns_viewlist_t *viewlist, cfg_obj_t *config,
 	result = named_config_get(maps, "lame-ttl", &obj);
 	INSIST(result == ISC_R_SUCCESS);
 	lame_ttl = cfg_obj_asduration(obj);
-	if (lame_ttl > 1800) {
-		lame_ttl = 1800;
+	if (lame_ttl > 0) {
+		cfg_obj_log(obj, named_g_lctx, ISC_LOG_WARNING,
+			    "disabling lame cache despite lame-ttl > 0 as it "
+			    "may cause performance issues");
+		lame_ttl = 0;
 	}
 	dns_resolver_setlamettl(view->resolver, lame_ttl);
 
diff --git a/doc/arm/reference.rst b/doc/arm/reference.rst
index 3bc44397453..fea854f3d18 100644
--- a/doc/arm/reference.rst
+++ b/doc/arm/reference.rst
@@ -3358,9 +3358,9 @@ Tuning
 ^^^^^^
 
 ``lame-ttl``
-   This sets the number of seconds to cache a lame server indication. 0
-   disables caching. (This is **NOT** recommended.) The default is
-   ``600`` (10 minutes) and the maximum value is ``1800`` (30 minutes).
+   This is always set to 0. More information is available in the
+   `security advisory for CVE-2021-25219
+   <https://kb.isc.org/docs/cve-2021-25219>`_.
 
 ``servfail-ttl``
    This sets the number of seconds to cache a SERVFAIL response due to DNSSEC