[SR-Users] Kamailio not using system resolver for hostname resolution (DMQ)?
Daniel-Constantin Mierla
miconda at gmail.com
Wed May 20 20:56:26 CEST 2020
Hello,
having use_dns_cache=no is not that bad if you have a local dns cache
application pointed by /etc/resolv.conf, provided that you do not need
srv based load balancing.
Not being the developer of dns resolving part in kamailio and the dns
cache, I investigated a bit and practically the /etc/hosts is the
pre-dns way of associating domains to ip addresses, without ttl and
other attributes specific to dns records. That seems the reason the libc
dns related functions do not use it implicitly, thus also not reflected
inside kamailio dns caching.
Now, you can add records to kamailio dns cache via rpc commands, which
can be done also from config using jsonrpc_exec() from jsonrpcs module
(e.g., using event_route[htable:mod-init] before sip traffic is
handled). The command should be liked:
kamctl rpc dns.add_a domain ttl ipaddr flags
The flags can be 1 to add it as a bad record (blacklisted domain) or 2
to add it as permanent record (do not expire). You can use dns.add_aaaa
for ipv6.
Actually I am now thinking to add the possibility to add to dns cache
via modparam of ipops module, to make it easier overall.
Cheers,
Daniel
On 20.05.20 19:35, George Diamantopoulos wrote:
> Hello all,
>
> Thank you for your enlightening answers. I ended up creating DNS
> entries anyway, so it's all good now. use_dns_cache=no sounded a
> little scary :-).
>
> BR.
> George
>
> On Tue, 19 May 2020 at 19:44, Daniel-Constantin Mierla
> <miconda at gmail.com <mailto:miconda at gmail.com>> wrote:
>
> A quick follow up as I found quite interesting that nowadays *BSDs
> allow
> specifying order of lookup in resolv.conf to include the hosts
> file, but
> that doesn't seem to be in Linux:
>
> * https://man.openbsd.org/resolv.conf.5#lookup
>
> Cheers,
> Daniel
>
>
> On 19.05.20 17:48, Daniel-Constantin Mierla wrote:
> > 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>
> <http://dmq-ng.services.domain.com>
> >>> 172.30.154.189 dmq-ng.services.domain.com
> <http://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>
> >>> <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>
> >>> <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 <mailto: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 <mailto:sr-users at lists.kamailio.org>
> >> https://lists.kamailio.org/cgi-bin/mailman/listinfo/sr-users
> > --
> > Daniel-Constantin Mierla -- www.asipto.com <http://www.asipto.com>
> > www.twitter.com/miconda <http://www.twitter.com/miconda> --
> www.linkedin.com/in/miconda <http://www.linkedin.com/in/miconda>
> > Funding: https://www.paypal.me/dcmierla
> >
> --
> Daniel-Constantin Mierla -- www.asipto.com <http://www.asipto.com>
> www.twitter.com/miconda <http://www.twitter.com/miconda> --
> www.linkedin.com/in/miconda <http://www.linkedin.com/in/miconda>
> Funding: https://www.paypal.me/dcmierla
>
>
> _______________________________________________
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>
>
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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
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