resolved: switch cache option to a tri-state option (systemd#5552).

Change the resolved.conf Cache option to a tri-state "no, no-negative, yes" values.

If a lookup returns SERVFAIL systemd-resolved will cache the result for 30s (See 201d995),
however, there are several use cases on which this condition is not acceptable (See systemd#5552 comments)
and the only workaround would be to disable cache entirely or flush it , which isn't optimal.

This change adds the 'no-negative' option when set it avoids putting in cache
negative answers but still works the same heuristics for positive answers.

Signed-off-by: Jorge Niedbalski <jnr@metaklass.org>

src/shared/resolve-util.c

@@ -41,3 +41,12 @@ bool dns_server_address_valid(int family, const union in_addr_union *sa) {
 
         return true;
 }
+
+DEFINE_CONFIG_PARSE_ENUM(config_parse_dns_cache_mode, dns_cache_mode, DnsCacheMode, "Failed to parse DNS cache mode setting")
+
+static const char* const dns_cache_mode_table[_DNS_CACHE_MODE_MAX] = {
+        [DNS_CACHE_MODE_YES] = "yes",
+        [DNS_CACHE_MODE_NO] = "no",
+        [DNS_CACHE_MODE_NO_NEGATIVE] = "no-negative",
+};
+DEFINE_STRING_TABLE_LOOKUP_WITH_BOOLEAN(dns_cache_mode, DnsCacheMode, DNS_CACHE_MODE_YES);

src/shared/resolve-util.h

@@ -8,6 +8,16 @@
 /* 127.0.0.53 in native endian */
 #define INADDR_DNS_STUB ((in_addr_t) 0x7f000035U)
 
+typedef enum DnsCacheMode DnsCacheMode;
+
+enum DnsCacheMode {
+        DNS_CACHE_MODE_NO,
+        DNS_CACHE_MODE_YES,
+        DNS_CACHE_MODE_NO_NEGATIVE,
+        _DNS_CACHE_MODE_MAX,
+        _DNS_CACHE_MODE_INVALID = 1
+};
+
 typedef enum ResolveSupport ResolveSupport;
 typedef enum DnssecMode DnssecMode;
 typedef enum DnsOverTlsMode DnsOverTlsMode;
@@ -56,6 +66,7 @@ enum DnsOverTlsMode {
 CONFIG_PARSER_PROTOTYPE(config_parse_resolve_support);
 CONFIG_PARSER_PROTOTYPE(config_parse_dnssec_mode);
 CONFIG_PARSER_PROTOTYPE(config_parse_dns_over_tls_mode);
+CONFIG_PARSER_PROTOTYPE(config_parse_dns_cache_mode);
 
 const char* resolve_support_to_string(ResolveSupport p) _const_;
 ResolveSupport resolve_support_from_string(const char *s) _pure_;
@@ -67,3 +78,6 @@ const char* dns_over_tls_mode_to_string(DnsOverTlsMode p) _const_;
 DnsOverTlsMode dns_over_tls_mode_from_string(const char *s) _pure_;
 
 bool dns_server_address_valid(int family, const union in_addr_union *sa);
+
+const char* dns_cache_mode_to_string(DnsCacheMode p) _const_;
+DnsCacheMode dns_cache_mode_from_string(const char *s) _pure_;