Commit 8fe18c05 authored by Ondřej Surý's avatar Ondřej Surý Committed by Michał Kępień
Browse files

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.
parent 5f07ee0d
......@@ -160,7 +160,7 @@ 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 */
......
......@@ -4759,8 +4759,11 @@ 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);
......
......@@ -3350,9 +3350,9 @@ 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
......
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