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DNS unable to resolve host

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Alexander514
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Joined: Tue Mar 06, 2018 4:34 pm

DNS unable to resolve host

Sun Jun 09, 2019 1:12 pm

Hello,

At least once a day MIkroTik stops resolving anything via first DNS server (192.168.10.1 in this example)
Code: Select all
[myuser@maplite] /ip dns cache> :resolve gateway.lan
failure: dns name does not exist
[myuser@maplite] /ip dns cache> :resolve desktop.lan
failure: dns name does not exist

DNS servers:
Code: Select all
[myuser@maplite] /ip dns cache> /ip dns print
                      servers: 192.168.10.1,1.1.1.1
              dynamic-servers:
        allow-remote-requests: yes
          max-udp-packet-size: 4096
         query-server-timeout: 2s
          query-total-timeout: 10s
       max-concurrent-queries: 100
  max-concurrent-tcp-sessions: 20
                   cache-size: 2048KiB
                cache-max-ttl: 1w
                   cache-used: 26KiB
[myuser@maplite] /ip dns cache>

DNS cache doesn't contain any *.lan records except one static:
Code: Select all
[myuser@maplite] /ip dns cache> print
Flags: S - static
 #   NAME                                                        ADDRESS                                                                                       TTL
 0 S map.lan                                                     192.168.88.1                                                                                  1d
 2   mobile-gtalk.l.google.com                                   64.233.165.188                                                                                6s

But all starts working after specifying nameserver manually:
Code: Select all
[myuser@maplite] /ip dns cache> :put [:resolve desktop.lan server=192.168.10.1]
192.168.10.100
[myuser@maplite] /ip dns cache> :put [:resolve desktop.lan]
192.168.10.100
[myuser@maplite] /ip dns cache> :put [:resolve gateway.lan server=192.168.10.1]
192.168.10.1
[myuser@maplite] /ip dns cache> :put [:resolve gateway.lan]
192.168.10.1

Is it a bug in DNS resolver implementation?
How can I fix this?



Router info:
Code: Select all
[myuser@maplite] /ip dns cache> /system routerboard print
       routerboard: yes
        board-name: mAP lite
             model: RouterBOARD mAP L-2nD
          revision: r2
     serial-number: 7E3308A24D88
     firmware-type: qca9531L
  factory-firmware: 3.41
  current-firmware: 3.41
  upgrade-firmware: 6.44.3

[myuser@maplite] /ip dns cache> /system package print
Flags: X - disabled
 #   NAME                                                                  VERSION                                                                  SCHEDULED
 0   openflow                                                              6.44.3
 1   system                                                                6.44.3
 2 X ipv6                                                                  6.44.3
 3   wireless                                                              6.44.3
 4   hotspot                                                               6.44.3
 5   dhcp                                                                  6.44.3
 6   mpls                                                                  6.44.3
 7   routing                                                               6.44.3
 8   ppp                                                                   6.44.3
 9   security                                                              6.44.3
10   advanced-tools                                                        6.44.3
11   ntp                                                                   6.44.3
[myuser@maplite] /ip dns cache>
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mkx
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Re: DNS unable to resolve host

Sun Jun 09, 2019 2:06 pm

It seems that the way how multiple DNS servers, set up in /ip dns, are utilized in ROS, is to use one until it fails then switch over to another one and use that one until it fails, etc. So use of multiple DNS servers is fine as long they all resolve whatever needed.

In your case, the second DNS server doesn't resolve "your" .lan TLD ... so if your application depends on proper resolving of .lan hostnames, you should remove any other DNS srrver from /ip dns configuration.
BR,
Metod
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sindy
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Re: DNS unable to resolve host

Sun Jun 09, 2019 2:25 pm

You haven't provided much context, but I suppose 192.168.10.1 to be a local one (another Mikrotik?) which can resolve *.lan while 1.1.1.1 is an external one which is obviously unable to resolve *.lan.

The local resolver does use the configured downstream servers as backup of each other, but in a different way than you probably expect - it asks one of them, and if it provides any answer, including an empty one, it uses that answer, caches it if it contains any useful information, and forwards it upstream, i.e. to the querying external client or internal application. Only if no response comes, it tries the other server and if that one answers, the resolver keeps using it also for the subsequent queries, until it eventually doesn't answer so there is a reason to try with yet another one (which may be the one used previously if you have just two).

Now once you indicate the 192.168.10.1 as a server to be used, its response is cached, but it does not make the resolver start using that server for subsequent queries, it remembers the last one it chose on its own until it stops responding.

To date you cannot tell the embedded resolver to use a particular downstream server for a particular domain suffix. Only ugly workarounds allowing that are possible, which involve layer7-protocol firewall rules and some in-depth knowledge of the DNS protocol packet format and of Mikrotik's handling of packet data by means of regular expressions.
Instead of writing novels, post /export hide-sensitive. Use find&replace in your favourite text editor to systematically replace all occurrences of each public IP address potentially identifying you by a distinctive pattern such as my.public.ip.1.
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Alexander514
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Re: DNS unable to resolve host

Sun Jun 09, 2019 2:51 pm

Thank you for reply.

Hm, ok. But what if someone uses several networks with their own DNS servers and wants to resolve custom domains for appropriate network?
For example:
Network 1: 172.17.1.0/24, DNS server 172.16.1.1, TLD: .lan
Network 2: 172.17.2.0/24, DNS server 172.16.2.1, TLD: .local
Network 3: 172.17.3.0/24, DNS server 172.16.3.1, TLD: .corp

Also I did not find any settings to "reset failed status" for DNS server and start using first one again. During my troubleshooting I have found that MikroTik stops using first DNS after one failed attempt and then switches to another. In my case first server is behind VPN tunnel and sometimes there are delays which can cause such issues, but I still want to resolve ".lan" domains via this network (or at least, get resolve error when VPN is down).

Specifying only first server in DNS settings is not a good idea for me, because when VPN becomes unreachable, DNS resolve stops working for any domains.

And I'm wondering why MikroTik does not try to use another server when gets failed resolve message from one of DNS servers in this list.
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sindy
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Re: DNS unable to resolve host

Sun Jun 09, 2019 3:05 pm

As said above, there is no ready-made way for that, only workarounds. This thread has it all, look mainly at @Sob's posts. If you would like to redirect Mikrotik's own DNS queries instead of redirecting those received from external clients, it takes an additional layer of headache.
Instead of writing novels, post /export hide-sensitive. Use find&replace in your favourite text editor to systematically replace all occurrences of each public IP address potentially identifying you by a distinctive pattern such as my.public.ip.1.
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Alexander514
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Re: DNS unable to resolve host

Thu Jun 13, 2019 12:08 am

The issue has been resolved by adding following rule:
Code: Select all
/ip firewall layer7-protocol
add name=".lan dns regexp" regexp=".lan.\\x01"

/ip firewall nat
add action=dst-nat chain=dstnat dst-port=53 in-interface-list=LAN layer7-protocol=".lan dns regexp" protocol=udp to-addresses=192.168.10.1
Thank you for your help!
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Sob
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Re: DNS unable to resolve host

Thu Jun 13, 2019 4:31 am

Use regexp="\\x03lan.\\x01" instead (3 = length of "lan"). Just the "." alone means any character, so if there would be TLD .<something>lan, your regexp would match too.
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Alexander514
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Re: DNS unable to resolve host

Thu Jun 13, 2019 5:04 pm

Oops, my mistake.
Added \\x03 to regexp.
Thanks!

---
As I understood from comment
viewtopic.php?t=133767#p658776

MikroTik ignores all null bytes in any packets, so when I write l7 filter regexp for any type of data, I should just skip any sequence of 00's.
If it's wrong, please correct me.
Code: Select all
0000   04 74 65 73 74 03 6c 61 6e 00 00 01 00 01         .test.lan.....
                       ^           ^ ^
                 "lan" lenght      Null bytes, ignored by ROS


Anyway, the issue has been resolved. Thank you!
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Sob
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Re: DNS unable to resolve host

Fri Jun 14, 2019 1:24 am

It's actually:
Code: Select all
0000   04 74 65 73 74 03 6c 61 6e 00 00 01 00 01         .test.lan.....
                                  ^^ null byte
                                     ^^ ^^ record type (16 bits)
                                           ^^ ^^ class (16 bits)
So a foolproof way (from two posts later in same referenced thread) should be:
Code: Select all
\0x03lan...?.?$
Which means ".lan" followed by 2-4 non-zero bytes at the end of packet. In practice, class is always 1 and record types over 255 are only few and nothing you'll see in regular network.
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