Hi all ;
i am using a X86 pc with OS 3.6 and L4 , there are two internet modems connected to it .
for time i wasn’t use the MT as DNS server , then i add it as a DNS server .
it work fine without any problems as follow :
i used the DNS server for my X ISP as the first one and then the primary DNS server of my first isp and the primary DNS server for my second isp server and a general DNS server .
it was work fine as i mentioned before but when i changed the configuration to put the DNS server of my first isp as the first one the problems begin occurring .
i changed again the dns servers in the DHCP server as it was before but also i faced the same problem and because of that i stopped using the MT as DNS server .
the photos below showing you my settings to the DNS server in the MT and DHCP output to users regarding the DNS servers .
also you will see what the nslookup command gives when the problem happens where the Yahoo messenger stop and browsing stop and pop3 stop .
i changed the lan of my pc setting regarding the speed & duplex , where it was 100MB full duplex i changed it to auto and i see that the problem had been solved .
during that some of clients start calling me to report that the internet is extremely slow and the browsing is nothing .
so as i said i stopped the MT as DNS server to do another test later and not to make the users upset of loosing the connection .
so any idea about this problem and why it happened to me ?
also how to fix it to re-use the MT as DNS server while it improve the service as i know .
if you need further illustration i will write it .
with best regards .
Hi..
In your DHCP server settings, only give your MT Router’s IP address as your DNS server. You are giving other IP’s also as secondary DNS server.
Check your ISP dns servers for reliability.
I have had exactly the same problem as you.
At first, mudasir already mentioned it, it has no use to put extra dns server IP’s in the dhcp if these are the same as what you set already in the MT dns cache server.
The MT box sends the dns request always only to the first dns IP mentioned as the Primary DNS.
Only if this Primary DNS sends a message back it cannot resolve the MT will send the request to the Secundairy DNS.
But, if the Primary DNS is just not reachable (it is down) the MT box will keep on trying untill it recieves either the dns resolvement or the message it cannot be done.
If the MT box this way cannot resolve the dns request, it sends this message to the station requestioning. But by dhcp you gave this station as second and third option only the same dns IP’s the MT box already failed to use, so only option 4 the very last, is left for the requesting station, and by the time this DNS IP will then be able to supply the request already loads of microseconds have passed. So dns resolving becomes very slow (and thus browsing)
I worked with both given DNS1 and DNS2 IP of my ISP as Primary. But both DNS servers I found to be very unreliable. I even tried ´open dns´ as my Primary and even there I had problems.
Then I started using an DNS Ip from my countries major ISP (Former national Comm firm) which I happen to have on a standby modem and since then (3 months now) never had any problems no more!
I still use the first provider for all my traffic, since it has high bandwidth, while all my dns requests are now routed (and dst-nat’d) through the modem of the low bandwidth ISP. All clients are now very happy, never seen so fast browsing and like said, its working now for about 4,5 weeks without a single interrupt! 
Rudy
Hi Rudy ;
Thanks for your reply , i will do some tests today to check if it will help or not .
here i need to ask you about the quoted statement you wrote and i put above ,
could you please tell us how you did this configuration ? , you say you made a routing and dst-nat etc .
i hope this topic be helpful to others .
with best regards .
Well, I will give you the basic outlay of my network in general, which is ´routed´. How to do routing can be found in the Wiki - manuals - other topics in this forum.
but, like said, my complete network is routed and works therefore with several networks each behind a router.
It´s a sort pyramid like construction. On top my main gateway router.
On one of the interfaces of this router is my LAN. This LAN with its network has several routers connected which in return also have new networks on their LAN side. And so on, I go now already 7 levels deep.
Each of the underlying (or ´deeper´) level network has in its top routers (= gateway for that network) a default route 0.0.0.0/24 for all traffic which the router does not know where to go. All this traffic (with destination 0.0.0.0/24 which means to any existing IP) will be send to the next router which is give here.
thus; default gateway for all traffic with destination 0.0.0.0/24 goes to gateway xxx.xxx.xxx.xxx which is the IP of the next higher router. This is repeated in each router until we reach the main gateway on top of the pyramid.
The main gateway has now one, or more, other interfaces connected to modems (I call these interfaces WAN1 and WAN2) which are connected to the internet. If you have for instance two interfaces connected to two modems connected to the internet you have to put the IP addresses of these modems as gateway for default traffic that is leaving the main router towards the internet. How you actually do this depends on how you want to spread the traffic load on the two lines.
Now the dns:
All my routers are MT, but I think it should work with other manufacturers as well. I opt to use MT only for all my routers so I can concentrate on their ros only and don’t need to learn how other equipments work.
Each router in my network with the pyramid like topology has the dns cache option enabled and has the ´recieve remote requests´ enabled and the underlying (its ´clients´) are set to lookup their dns requests at that router.
The primary dns IP in the dns option of each router is now set to its ´higher´ router IP. Which is in fact mostly the same as the IP of the default gw for that router.
This way all dns request are ultimately (if not resolved, because stored in the cache, by a previous router) handled by the main router on top to the pyramid.
Now this main router normally looks for its dns resolving towards the dns1 IP of either one of the internet connections we have and which are supplied by the ISP. (Actually you make it to ´look´ at the modem dns cache first if the modem has that option and then the modem has to resolve it on the internet by the ISP dns servers)
To have the dns now routed to a specific dns server that is actually not supplied by the ISP that delivers me the two ADSL lines I have a third modem somewhere in my network connected to this alternative ISP.
Since this modem is somewhere in my network (somewhere ´inside my pyramid´) I have to make sure that the dns requests are send to that location.
To arrange this I have to set up the dns of the main router to look at an IP of a ´lower´ router as Primary dns server. This has to be done each router deeper into the network again until we reach the network that has my third modem.
but, since the routers don´t know where to send this traffic to (all the routers don´t know nothing else then that all traffic (0.0.0.0/24) has to be send to the ´top´ router.) we have to tell each router that traffic with has a certain destination IP ´inside´ the network (so an IP that exists inside my ´pyramid´) has to be send to that specific router that has the network connected on one of its interfaces that has that specific IP.
You create thus an route back into the network for traffic that has destination of a specific IP.
Ultimately your dns request reaches this way the third modem and that one has also cache enabled and looks itself his requests up on the ISP provided dns1 IP address.
Actually I don´t think in this way dst-nat is really needed. The ´answer´ on the request will find its way back to the sender through the routing and the arp tables.
But my situation is actually even a bit more complicated so I need dst-nat to make sure that the request package carries the proper IP as ´sender´ so the routers all know where to send back to the return of the request.
So, this will give you an idea on how routing is basically done. The problem is that the bigger the network, the more WAN connections, the way load balance is been arranged and probably even more factors are involved, the more complicated a network, and therefore the routing tables, becomes. And each (W)ISP has another topology with different wishes and different ways of workaround solutions etc. it will make it a bit difficult to give a standard example.
Here come the knowledge of a good network operator at hand. This can be you yourself after a lot of investigating and trying and failures or it can be a highly paid consultant.
Rudy