[SR-Users] Kamailio not using system resolver for hostname resolution (DMQ)?

[Daniel-Constantin Mierla]

Hello,

worth to clarify that Kamailio doesn't implement a dns resolver from
scratch, it still uses the functions from libc. It only does caching and
try to use cashing records first.

By default, the following dns functions are used internally when the
cache doesn't have the record already -- they are set in the dns_func
structure:

struct dns_func_t dns_func = {
    res_init,
    res_search,
    gethostbyname,
#ifdef HAVE_GETHOSTBYNAME2
    gethostbyname2
#else
    NULL
#endif
};

So practically by leveraging res_init()/res_search() is relying on
/etc/resolv.conf to do DNS queries, which is not using the /etc/hosts
(iirc). For more see:

  * https://linux.die.net/man/3/res_search

Cheers,
Daniel

On 19.05.20 17:23, Alex Balashov wrote:
> You are correct that Kamailio does not use the libc resolver /
> libresolv, but rather its own resolver, which ignores /etc/hosts.
>
> -- Alex
>
> On 5/19/20 10:42 AM, George Diamantopoulos wrote:
>> Hello all,
>>
>> I've come across this today when trying to setup DMQ replication:
>>
>> root at sbcpub0-stage-lhe0-cn1:/root# systemctl restart kamailio
>> root at sbcpub0-stage-lhe0-cn1:/root# kamcmd dmq.list_nodes
>> {
>>          host: 172.30.154.189
>>          port: 5090
>>          resolved_ip: 172.30.154.189
>>          status: active
>>          last_notification: 0
>>          local: 1
>> }
>> root at sbcpub0-stage-lhe0-cn1:/root# getent hosts
>> dmq-ng.services.domain.com <http://dmq-ng.services.domain.com>
>> 172.30.154.189 dmq-ng.services.domain.com
>> <http://dmq-ng.services.domain.com>
>> 172.17.130.13 dmq-ng.services.domain.com
>> <http://dmq-ng.services.domain.com>
>>
>> root at sbcpub0-stage-lhe0-cn1:/root# grep dmq-ng /etc/kamailio/*
>> /etc/kamailio/kamailio-module-params.cfg:modparam("dmq",
>> "notification_address", "sip:dmq-ng.services.domain.com:5090
>> <http://dmq-ng.services.domain.com:5090>")
>>
>> In another case, where the resolving IPs for the DMQ service hostname
>> were listed as A records in DNS, this worked as expected.
>> In this case, the second host of the DMQ group is not listed in
>> dmq.list_nodes, despite its being set in /etc/hosts, as shown by the
>> getent command.
>>
>> Doesn't kamailio use nss for hostname resolution? Am I missing
>> something else? Thanks!
>>
>> BR,
>> George
>>
>>
>> _______________________________________________
>> Kamailio (SER) - Users Mailing List
>> sr-users at lists.kamailio.org
>> https://lists.kamailio.org/cgi-bin/mailman/listinfo/sr-users
>>
>
> -- 
> Alex Balashov | Principal | Evariste Systems LLC
>
> Tel: +1-706-510-6800 / +1-800-250-5920 (toll-free)
> Web: http://www.evaristesys.com/, http://www.csrpswitch.com/
>
> _______________________________________________
> Kamailio (SER) - Users Mailing List
> sr-users at lists.kamailio.org
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-- 
Daniel-Constantin Mierla -- www.asipto.com
www.twitter.com/miconda -- www.linkedin.com/in/miconda
Funding: https://www.paypal.me/dcmierla