Kamailio and DNS Overview --------------------------- The DNS subsystem in Kamailio can either directly use libresolv and a combination of the locally configured DNS server, /etc/hosts and the local Network Information Service (NIS/YP a.s.o) or cache the query results (both positive and negative) and look first in its internal cache. When its internal DNS cache is enabled, Kamailio can also use DNS failover: if one destination resolves to multiple addresses Kamailio can try all of them until it finds one to which it can successfully send the packet or it exhausts all of them. Kamailio (The tm module to be more precise) uses the DNS failover also when the destination host doesn't send any reply to a forwarded invite within the SIP timeout interval (whose value can be configured using the tm fr_timer parameter). When SRV based load balancing is enabled Kamailio can even do DNS based load balancing (see RFC2782 and the dns_srv_lb option below). This is using the weight value in the DNS SRV record. DNS Cache and Failover Drawbacks -------------------------------- Using the DNS cache and the DNS failover has also some drawbacks: 1. Only the locally configured DNS server (usually in /etc/resolv.conf) is used for the requests (/etc/hosts and the local Network Information Service are ignored). Workaround: disable the DNS cache (use_dns_cache=off or compile without -DUSE_DNS_CACHE). 2. The DNS cache uses extra memory Workaround: disable the DNS cache. 3. The DNS failover introduces a very small performance penalty Workaround: disable the DNS failover (use_dns_failover=off). 4. The DNS failover increases the memory usage (the internal structures used to represent the transaction are bigger when the DNS failover support is compiled). Workaround: compile without DNS failover support (-DUSE_DNS_FAILOVER). Turning it off from the config file is not enough in this case (the extra memory will still be used). On the other hand using the DNS cache saves lots of DNS queries and makes DNS based failover and DNS based load balancing possible. If the destination blocklist is enabled, Kamailio can do failover even if forwarding in stateless mode. In the ideal case with the DNS cache enabled Kamailio will do only one query for a NAPTR (if enabled) or SRV lookup and then it will use the results for the record's TTL (for example if all the resulting records have 1 minute TTL, the server won't make another query for this domain for 1 minute). Even negative answers will be cached. Without the DNS cache, each NAPTR or SRV lookup will result in at least 2 queries. These queries will happen every time, for each message (even if all of them go to the same domain). DNS Resolver Options -------------------- The DNS resolver options control how Kamailio will interact with the external DNS servers. These options (with the dns_try_ipv6 exception) are passed to libresolv and are used each time a DNS request is made. The default values are system specific and generally depend on the /etc/resolv.conf content. For servers doing a lot of DNS requests it is highly recommended to change the default values in the Kamailio config file (even if using Kamailio's internal DNS cache). dns_try_ipv6 = on | off - if on and Kamailio listens on at least one ipv6 socket, ipv6 (AAAA) lookups will be performed if the ipv4 (A) lookups fail. If off only ipv4 (A) lookups will be used. Default: on if Kamailio is compiled with ipv6 support. dns_try_naptr = on | off - if on Kamailio will first try a NAPTR lookup for destinations that don't have the protocol or port specified and are not simple ip addresses (as described in RFC 3263). This will introduce a slight performance penalty and will probably cause extra DNS lookups. For example a lookup for a non-existing domain will produce one extra query: NAPTR(domain), SRV(_sip._udp.domain) and A/AAAA(domain). If the result of a query contains several NAPTR records, Kamailio will select among them according to the RFC2915 and Kamailio preference towards a specific protocol (see dns_udp_pref, dns_tcp_pref and dns_tls_pref below). For an RFC3263 compliant configuration (choose the remote side preferred protocol if supported), set dns_udp_pref, dns_tcp_pref and dns_tls_pref to the same value (>=0), e.g. 0. Default: off dns_udp_pref = number - udp protocol preference when doing NAPTR lookups. This option works together with dns_tcp_pref, dns_tls_pref and dns_sctp_pref. If all this options have the same positive value and more NAPTR records are available, Kamailio will select the NAPTR record preferred by the remote side (according to RFC2915). If the values are positive but different, Kamailio will select the NAPTR record whose protocol it prefers the most (the protocol with the highest dns_<proto>_pref number). If there are several NAPTR records with the same preferred protocol, Kamailio will select among them based on their order and preference (see RFC2915). To completely disable selecting a specific protocol, use a negative number. For example dns_tcp_pref=-1 will completely disable selection of tcp NAPTR records, even if this will result in the NAPTR lookup failure. Note: if a protocol is disabled in Kamailio (e.g. tls_disable=1) the corresponding NAPTR records selection will be also disabled, irrespective of the dns_<proto>_preference value. Default: dns_udp_pref=30, dns_tcp_pref=20, dns_tls_pref=10 and dns_sctp_pref=20. (prefer udp, but if no udp NAPTR record found or no SRV-resolvable udp NAPTR record found use tcp or sctp records and if this fails too use tls) dns_tcp_pref = number (see dns_udp_pref above) dns_tls_pref = number (see dns_udp_pref above) dns_sctp_pref = number (see dns_udp_pref above) dns_retr_time = time - time in s before retrying a DNS request. Default: system specific, depends also on the/etc/resolv.conf content (usually 5 s). dns_retr_no = no. - number of DNS retransmissions before giving up. Default: see above (usually 4) dns_servers_no = no. - how many DNS servers from the ones defined in /etc/resolv.conf will be used. Default: all of them. dns_use_search_list= yes/no - if no, the search list in /etc/resolv.conf will be ignored (=> fewer lookups => gives up faster). Default: yes. HINT: even if you don't have a search list defined, setting this option to "no" will still be "faster", because an empty search list is in fact search "" (so even if the search list is empty/missing there will still be 2 DNS queries, eg. foo+'.' and foo+""+'.') dns_search_full_match = yes/no - controls the check of the name part which is found in the answer expanding the searched name before the answer is treated as correct and "link" (fake CNAME record) between the short name (query) and long name (answer) is created which is then stored in dns_cache and reused for next queries. If set to no - no additional check is done. If set to yes - the additional part is checked against the search list. The maximum time a DNS request can take (before failing) is: (dns_retr_time*dns_retr_no) * (search_list_domains) If dns_try_ipv6 is yes, mutliply it again by 2. The option combination that produces the "fastest" DNS resolver config (the "faster" in the sense that it gives up the quickest) is: dns_try_ipv6=no dns_retr_time=1 dns_retr_no=1 dns_servers_no=1 dns_use_search_list=no The recommended DNS configuration is to have a "close" DNS caching recursive server configured in /etc/resolv.conf, set the DNS resolver options in Kamailio's config as in the above example and enable the DNS cache (in Kamailio). Pay particular attention to dns_servers_no and dns_use_search_list. It's a good idea to make sure you don't need / use the search list or more then one DNS server (to avoid unnecessary extra lookups). DNS Resolver Compile Options ---------------------------- USE_NAPTR - if defined the naptr lookup support will be compiled in. NAPTR support still has to be enabled from Kamailio's config file (it's off by default). DNS Cache and Failover Config Variables --------------------------------------- use_dns_cache = on | off - if off the DNS cache won't be used (all dns lookups will result into a DNS request). When on all the DNS request results will be cached. WARNING: when enabled /etc/hosts will be completely bypassed, all the dns request will go directly to the system configured (resolv.conf) dns server. Default: on. use_dns_failover = on |off - if on and sending a request fails (due to not being allowed from an onsend_route, send failure, blocklisted destination or, when using tm, invite timeout), and the destination resolves to multiple ip addresses and/or multiple SRV records, the send will be re-tried using the next ip/record. In tm's case a new branch will be created for each new send attempt. Default: off. Depends on use_dns_cache being on. If tm is used along with DNS failover is recommended to also turn on dst_blocklist. dns_srv_lb = on | off or dns_srv_loadbalancing = on | off - if on instead of doing simple DNS failover (like above), Kamailio will load balance requests to different srv records of the same priority based on the SRV records weights (like described in RFC2782). For a destination which has different priorities for all its SRV records, this option will be equivalent with simple DNS failover. Note: this option requires having DNS failover enabled (see use_dns_failover above). Default: off. dns_try_ipv6 = on | off - shared with the resolver (see resolver description). dns_try_naptr = on | off - shared with the resolver (see resolver description). dns_udp_pref = number - shared with the resolver (see resolver description). dns_tcp_pref = number - shared with the resolver (see resolver description). dns_tls_pref = number - shared with the resolver (see resolver description). dns_cache_flags = DNS cache specific resolver flags, used for overriding the default behaviour (low level). Possible values: 1 - ipv4 only: only DNS A requests are performed, even if Kamailio listens also on ipv6 addresses. 2 - ipv6 only: only DNS AAAA requests are performed. Ignored if dns_try_ipv6 is off or Kamailio doesn't listen on any ipv6 address. 4 - prefer ipv6: try first to resolve a host name to an ipv6 address (DNS AAAA request) and only if this fails try an ipv4 address (DNS A request). By default the ipv4 addresses are preferred. Default: 0 dns_cache_negative_ttl = time to live for negative results ("not found") in seconds. Use 0 to disable. Default: 60 s. dns_cache_min_ttl = minimum accepted time to live for a record, in seconds. If a record has a lower ttl, its value will be discarded and dns_cache_min_ttl will be used instead. Default: 0 dns_cache_max_ttl = maximum accepted time to live for a record, in seconds. If a record has a higher ttl, its value will be discarded and dns_cache_max_ttl will be used instead. Default: MAXINT dns_cache_mem = maximum memory used for the DNS cache in Kb. Default: 500 Kb dns_cache_gc_interval = how often (in s) the DNS cache will be garbage collected. Default: 120 s. dns_cache_del_nonexp = yes | no or dns_cache_delete_nonexpired = yes | no - allow deletion of non-expired records from the cache when there is no more space left for new ones. The last-recently used entries are deleted first. Default: no dns_cache_init = on | off - if off, the DNS cache is not initialized at startup and cannot be enabled runtime, that saves some memory. Default: on DNS Cache Compile Options ------------------------- USE_DNS_CACHE - if defined the DNS cache support will be compiled in (default). If not needed/wanted the dns_cache can be disabled from the Kamailio's config file. The only advantages for not compiling the DNS cache support is a slight decrease of the executable size and an extremely small performance increase (1 less comparison per DNS request). USE_DNS_FAILOVER - if defined the DNS failover support will be compiled in. (default). Compiling the DNS failover support has a few disadvantages, see the "Drawbacks" section. DNS_SRV_LB - if defined (default) support for load balancing using SRV records weights (as described in RFC2782) will be compiled in. Note however that it still must be enabled from the Kamailio config, it's disabled by default (see the dns_srv_lb config option). USE_NAPTR - (shared with the resolver) if defined NAPTR support will be compiled in (default). Note that even if compiled, NAPTR support must be enabled also from the Kamailio config (see the dns_try_naptr option). NAPTR_CACHE_ALL_ARS - if defined all the additional records in a NAPTR answer will be cached. Normally Kamailio would cache only "related" records (records that are directly referred), but for answers with lots of A/AAAA records it might happen that not all of the SRV records will fit in the AR section. In this case, without this compile option Kamailio will not cache the un-referred A/AAAA records. BY default this option is disabled. CACHE_RELEVANT_RECS_ONLY - if defined (default), records in the AR section of an answer will be cached only if they are "related" to the query. For example if the query is for a SRV record, A & AAAA records in the AR section will be cached only if there are SRV records pointing to them. This avoids adding possible garbage to the cache. If this option is not defined (experimental), everything in the AR section will be added to the cache. Note: To remove a compile options, edit Kamailio's Makefile.defs and remove it from DEFS list. To add a compile options add it to the make command line, e.g.: make proper; make all extra_defs=-DUSE_DNS_FAILOVER or for a permanent solution, edit Makefile.defs and add it to DEFS (don't foget to prefix it with -D). Some options require editing dns_cache.c or resolve.[ch] (just grep after them).