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resolved: cache SERVFAIL responses for 30s

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Some domains (such as us.ynuf.alipay.com) almost appear as if they actively
want to sabotage our DNSSEC work. Specifically, they unconditionally
return SERVFAIL on SOA lookups and always only after a 1s delay (at
least). This is pretty bad for our validation logic, as we use SOA
lookups to distuingish zones from non-terminal names. Moreover, SERVFAIL
is an error that is typically returned if we send requests a server
doesn't grok, and thus is reason for us to downgrade our protocol and
try again. In case of these zones this means we'll accept the SERVFAIL
response only after a full iterative downgrade to our lowest feature
level: TCP. In combination with the 1s delays this has the effect of
making us hit our transaction timeout way to easily.

As first attempt to improve the situation: let's start caching SERVFAIL
responses in our cache, after the full downgrade for a short period of
time.

Conceptually this is exposed as "weird rcode" caching, but for now we
only consider SERVFAIL a "weird rcode" worthy of caching. Later on we
might want to add more.
This commit is contained in:
Lennart Poettering 2017-02-15 18:28:23 +01:00
parent dc349f5f7a
commit 201d99584e
1 changed files with 93 additions and 30 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

123
src/resolve/resolved-dns-cache.c
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@ -34,6 +34,10 @@
/* We never keep any item longer than 2h in our cache */
#define CACHE_TTL_MAX_USEC (2 * USEC_PER_HOUR)
/* How long to cache strange rcodes, i.e. rcodes != SUCCESS and != NXDOMAIN (specifically: that's only SERVFAIL for
* now) */
#define CACHE_TTL_STRANGE_RCODE_USEC (30 * USEC_PER_SEC)
typedef enum DnsCacheItemType DnsCacheItemType;
typedef struct DnsCacheItem DnsCacheItem;
@ -41,12 +45,14 @@ enum DnsCacheItemType {
DNS_CACHE_POSITIVE,
DNS_CACHE_NODATA,
DNS_CACHE_NXDOMAIN,
DNS_CACHE_RCODE, /* "strange" RCODE (effective only SERVFAIL for now) */
};
struct DnsCacheItem {
DnsCacheItemType type;
DnsResourceKey *key;
DnsResourceRecord *rr;
int rcode;
usec_t until;
bool authenticated:1;
@ -60,6 +66,27 @@ struct DnsCacheItem {
LIST_FIELDS(DnsCacheItem, by_key);
};
static const char *dns_cache_item_type_to_string(DnsCacheItem *item) {
assert(item);
switch (item->type) {
case DNS_CACHE_POSITIVE:
return "POSITIVE";
case DNS_CACHE_NODATA:
return "NODATA";
case DNS_CACHE_NXDOMAIN:
return "NXDOMAIN";
case DNS_CACHE_RCODE:
return dns_rcode_to_string(item->rcode);
}
return NULL;
}
static void dns_cache_item_free(DnsCacheItem *i) {
if (!i)
return;
@ -484,7 +511,6 @@ static int dns_cache_put_negative(
assert(c);
assert(key);
assert(soa);
assert(owner_address);
/* Never cache pseudo RR keys. DNS_TYPE_ANY is particularly
@ -495,13 +521,17 @@ static int dns_cache_put_negative(
if (dns_type_is_pseudo(key->type))
return 0;
if (nsec_ttl <= 0 || soa->soa.minimum <= 0 || soa->ttl <= 0) {
log_debug("Not caching negative entry with zero SOA/NSEC/NSEC3 TTL: %s",
dns_resource_key_to_string(key, key_str, sizeof key_str));
return 0;
}
if (IN_SET(rcode, DNS_RCODE_SUCCESS, DNS_RCODE_NXDOMAIN)) {
if (!soa)
return 0;
if (!IN_SET(rcode, DNS_RCODE_SUCCESS, DNS_RCODE_NXDOMAIN))
/* For negative replies, check if we have a TTL of a SOA */
if (nsec_ttl <= 0 || soa->soa.minimum <= 0 || soa->ttl <= 0) {
log_debug("Not caching negative entry with zero SOA/NSEC/NSEC3 TTL: %s",
dns_resource_key_to_string(key, key_str, sizeof key_str));
return 0;
}
} else if (rcode != DNS_RCODE_SERVFAIL)
return 0;
r = dns_cache_init(c);
@ -514,12 +544,17 @@ static int dns_cache_put_negative(
if (!i)
return -ENOMEM;
i->type = rcode == DNS_RCODE_SUCCESS ? DNS_CACHE_NODATA : DNS_CACHE_NXDOMAIN;
i->until = calculate_until(soa, nsec_ttl, timestamp, true);
i->type =
rcode == DNS_RCODE_SUCCESS ? DNS_CACHE_NODATA :
rcode == DNS_RCODE_NXDOMAIN ? DNS_CACHE_NXDOMAIN : DNS_CACHE_RCODE;
i->until =
i->type == DNS_CACHE_RCODE ? timestamp + CACHE_TTL_STRANGE_RCODE_USEC :
calculate_until(soa, nsec_ttl, timestamp, true);
i->authenticated = authenticated;
i->owner_family = owner_family;
i->owner_address = *owner_address;
i->prioq_idx = PRIOQ_IDX_NULL;
i->rcode = rcode;
if (i->type == DNS_CACHE_NXDOMAIN) {
/* NXDOMAIN entries should apply equally to all types, so we use ANY as
@ -543,7 +578,7 @@ static int dns_cache_put_negative(
return r;
log_debug("Added %s cache entry for %s "USEC_FMT"s",
i->type == DNS_CACHE_NODATA ? "NODATA" : "NXDOMAIN",
dns_cache_item_type_to_string(i),
dns_resource_key_to_string(i->key, key_str, sizeof key_str),
(i->until - timestamp) / USEC_PER_SEC);
@ -615,6 +650,7 @@ int dns_cache_put(
const union in_addr_union *owner_address) {
DnsResourceRecord *soa = NULL, *rr;
bool weird_rcode = false;
DnsAnswerFlags flags;
unsigned cache_keys;
int r, ifindex;
@ -624,18 +660,28 @@ int dns_cache_put(
dns_cache_remove_previous(c, key, answer);
/* We only care for positive replies and NXDOMAINs, on all
* other replies we will simply flush the respective entries,
* and that's it */
if (!IN_SET(rcode, DNS_RCODE_SUCCESS, DNS_RCODE_NXDOMAIN))
return 0;
/* We only care for positive replies and NXDOMAINs, on all other replies we will simply flush the respective
* entries, and that's it. (Well, with one further exception: since some DNS zones (akamai!) return SERVFAIL
* consistently for some lookups, and forwarders tend to propagate that we'll cache that too, but only for a
* short time.) */
if (dns_answer_size(answer) <= 0) {
char key_str[DNS_RESOURCE_KEY_STRING_MAX];
if (IN_SET(rcode, DNS_RCODE_SUCCESS, DNS_RCODE_NXDOMAIN)) {
log_debug("Not caching negative entry without a SOA record: %s",
dns_resource_key_to_string(key, key_str, sizeof key_str));
return 0;
if (dns_answer_size(answer) <= 0) {
char key_str[DNS_RESOURCE_KEY_STRING_MAX];
log_debug("Not caching negative entry without a SOA record: %s",
dns_resource_key_to_string(key, key_str, sizeof key_str));
return 0;
}
} else {
/* Only cache SERVFAIL as "weird" rcode for now. We can add more later, should that turn out to be
* beneficial. */
if (rcode != DNS_RCODE_SERVFAIL)
return 0;
weird_rcode = true;
}
cache_keys = dns_answer_size(answer);
@ -690,19 +736,20 @@ int dns_cache_put(
if (r > 0)
return 0;
/* See https://tools.ietf.org/html/rfc2308, which say that a
* matching SOA record in the packet is used to enable
* negative caching. */
/* See https://tools.ietf.org/html/rfc2308, which say that a matching SOA record in the packet is used to
* enable negative caching. We apply one exception though: if we are about to cache a weird rcode we do so
* regardless of a SOA. */
r = dns_answer_find_soa(answer, key, &soa, &flags);
if (r < 0)
goto fail;
if (r == 0)
return 0;
/* Refuse using the SOA data if it is unsigned, but the key is
* signed */
if (authenticated && (flags & DNS_ANSWER_AUTHENTICATED) == 0)
if (r == 0 && !weird_rcode)
return 0;
if (r > 0) {
/* Refuse using the SOA data if it is unsigned, but the key is
* signed */
if (authenticated && (flags & DNS_ANSWER_AUTHENTICATED) == 0)
return 0;
}
r = dns_cache_put_negative(
c,
@ -799,6 +846,7 @@ int dns_cache_lookup(DnsCache *c, DnsResourceKey *key, bool clamp_ttl, int *rcod
DnsCacheItem *j, *first, *nsec = NULL;
bool have_authenticated = false, have_non_authenticated = false;
usec_t current;
int found_rcode = -1;
assert(c);
assert(key);
@ -842,6 +890,8 @@ int dns_cache_lookup(DnsCache *c, DnsResourceKey *key, bool clamp_ttl, int *rcod
n++;
} else if (j->type == DNS_CACHE_NXDOMAIN)
nxdomain = true;
else if (j->type == DNS_CACHE_RCODE)
found_rcode = j->rcode;
if (j->authenticated)
have_authenticated = true;
@ -849,6 +899,19 @@ int dns_cache_lookup(DnsCache *c, DnsResourceKey *key, bool clamp_ttl, int *rcod
have_non_authenticated = true;
}
if (found_rcode >= 0) {
log_debug("RCODE %s cache hit for %s",
dns_rcode_to_string(found_rcode),
dns_resource_key_to_string(key, key_str, sizeof(key_str)));
*ret = NULL;
*rcode = found_rcode;
*authenticated = false;
c->n_hit++;
return 1;
}
if (nsec && !IN_SET(key->type, DNS_TYPE_NSEC, DNS_TYPE_DS)) {
/* Note that we won't derive information for DS RRs from an NSEC, because we only cache NSEC RRs from
* the lower-zone of a zone cut, but the DS RRs are on the upper zone. */
@ -1042,7 +1105,7 @@ void dns_cache_dump(DnsCache *cache, FILE *f) {
fputs(dns_resource_key_to_string(j->key, key_str, sizeof key_str), f);
fputs(" -- ", f);
fputs(j->type == DNS_CACHE_NODATA ? "NODATA" : "NXDOMAIN", f);
fputs(dns_cache_item_type_to_string(j), f);
fputc('\n', f);
}
}