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• EC2: is an instance's public DNS stable? Can I rely on it not changing?

  - by Aseem Kishore

  I'm new to Amazon EC2. I've launched my first instance, and am using it as a web server. I see that it has a public DNS (a public URL), e.g.: ec2-123-45-6-789.compute-1.amazonaws.com I can successfully go to this server in my browser, hit it via cURL, etc. I want to use this web server for a back-end service in an app I'm building, so I placed this URL in my app's config, and it works great. But when I manually stop and re-started my instance, I see that the public DNS changes! I've read that this happens when you explicitly stop and re-start, but doesn't happen if you just "reboot". I don't plan on explicitly stopping and re-starting this server ever, but my question is: will this public DNS ever change on its own for any reason? E.g. if the machine abnormally crashes, or whatever. In other words, is it safe to ship an app that's wired to this URL? Thanks!

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• What is the relationship between recursion functions and memory stack?

  - by Eslam

  is there's a direct relationship between recursive functions and the memory stack, for more explanation consider that code: public static int triangle(int n) { System.out.println(“Entering: n = ” + n); if (n == 1) { System.out.println(“Returning 1”); return 1; } else { int temp = n + triangle(n - 1); System.out.println(“Returning“ + temp); return temp; } }? in this example where will the values 2,3,4,5 be stored until the function returns ? note that they will be returned in LIFO(LastInFirstOut) is these a special case of recursion that deals with the memory stack or they always goes together?

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• Can anyone help with java? Finding common nodes from two linked lists using recursion

  - by Dan

  I have to write a method that returns a linked list with all the nodes that are common to two linked lists using recursion, without loops. For example, first list is 2 - 5 - 7 - 10 second list is 2 - 4 - 8 - 10 the list that would be returned is 2 - 10 I am getting nowhere with this.. What I have been think of was to check each value of the first list with each value of the second list recursively but the second list would then be cut by one node everytime and I cannot compare the next value in the first list with the the second list. I hope this makes sense... Can anyone help?

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• How to flush DNS cache in Windows Mobile programmatically?

  - by Bounded

  Hello, My windows mobile application (written in C# with the compact framework) needs to know if a particular machine is active or not. To achieve this goal, I thought to use a ping mechanism. I tried to use the Ping class implemented in the opennetcf framework (the System.Net.NetworkInformation.Ping class for the .NET Framework is not part of the compact framework). Because I give to the Ping.Send function a host name, it first tries to resolve this host name and to retrieve an IP address. But i observe the following problem : If the first dns resolution fails (because the network is down at this moment), and if the application tries immediately again to send the ping, it fails too, even if the network is note down anymore. I check with a famous network protocol analyzer and i saw that only the requests concerning the first dns resolution are sent. The requests concerning the dns resolution of the second ping are not sent. Why is the second dns request not sent ? Is there any dns cache mechanism on such Windows Mobile devices ? If yes, can this mechanism beeing flushed programmatically ? EDIT : I gave up finding a solution to this DNS flush. I chose to ping an IP adress instead of a name machine. The problem of pinging an hard coded adress IP is that we have to be 100% sure that this IP will not change. The gateway IP can be used because it's always reachable (if it does not, it means the network is down).

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• What is the difference between Anycast and GeoDNS / GeoIP wrt HA?

  - by Riyad

  Based on the Wikipedia description of Anycast, it includes both the distribution of a domain-name-to-many-IP-mapping across many DNS servers as well as replying to clients with the most geographically close (or fastest) server. In the context of a globally distributed, highly available site like google.com (or any CDN service with many global edge locations) this sounds like the two key features one would need. DNS services like Amazon's Route53, EasyDNS and DNSMadeEasy all advertise themselves as Anycast-enabled networks. Therefore my assumption is that each of these DNS services transparently offer me those two killer features: multi-IP-to-domain mapping AND routing clients to the closest node. However, each of these services seem to separate out these two functionalities, referring to the 2nd one (routing clients to closest node) as "GeoDNS", "GeoIP" or "Global Traffic Director" and charge extra for the service. If a core tenant of an Anycast-capable system is to already do this, why is this functionality being earmarked as this extra feature? What is this "GeoDNS" feature doing that a standard Anycast DNS service won't do (according to the definition of Anycast from Wikipedia -- I understand what is being advertised, just not why it isn't implied already). I get extra-confused when a DNS service like Route53 that doesn't support this nebulous "GeoDNS" feature lists functionality like: Fast – Using a global anycast network of DNS servers around the world, Route 53 is designed to automatically route your users to the optimal location depending on network conditions. As a result, the service offers low query latency for your end users, as well as low update latency for your DNS record management needs. ... which sounds exactly like what GeoDNS is intended to do, but geographically directing clients is something they explicitly don't support it yet. Ultimately I am looking for the two following features from a DNS provider: Map multiple IP addresses to a single domain name (like google.com, amazon.com, etc. does) Utilize a DNS service that will respond to client requests for that domain with the IP address of the nearest server to the requestee. As mentioned, it seems like this is all part of an "Anycast" DNS service (all of which these services are), but the features and marketing I see from them suggest otherwise, making me think I need to learn a bit more about how DNS works before making a deployment choice. Thanks in advance for any clarifications.

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• Bind9 not doing anything with forwarded query responses?

  - by Rykaro

  I have a Bind DNS server that is the local production DNS server and a Windows 2008 R2 domain controller which provides DNS for a lab environment with the domain xyz.lab. I've configured the Bind DNS to forward DNS requests for the domain xyz.lab to the Windows DNS server with this config: zone "xyz.lab" { type forward; forward only; forwarders { x.x.x.x; }; }; zone "x.x.x.in-addr.arpa" { type forward; forward only; forwarders { x.x.x.x; }; }; And Bind options are (the all_internal acl includes the subnets of both the production and lab networks as well as the loopback of the bind server): allow-query { all_internal; }; allow-recursion { all_internal; }; allow-transfer { none; }; notify no; minimal-responses yes; version "unknown"; Unfortunately, when I do an nslookup or dig on the bind server for a host on the lab domain, the request times out. The logs on the Windows 2008 DNS server show it receiving the query and responding to it and a network packet trace shows the query responses arriving at the Bind DNS server. The servers reside on the same switch with a router providing connectivity between the layer 3 subnets (production and lab are on different subnets) and there is a round trip time of between 3ms and 5ms on pings between the two servers, so I don't think there is an issue with latency causing a timeout of the query. In summary a query-response arrives back at the Bind server and the nslookup/dig times-out. Why does the Bind DNS not seem to be doing anything with the query responses when it receives them?

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• Redirect as a backup trick? w/o modifying DNS?

  - by acidzombie24

  I specifically looked up how to do something like this ( Can you set a backup ip for your server in DNS? ) and the answer basically was you can't. If i say specify 2 ip addresses could i somehow use a HTTP response header to ignore it temporary (say 5mins) and go to the other IP address? Or maybe i can play dead however i'm unsure how to play dead using nginx. I then would like to be available after my box notice the other box is down and be some kind of readonly server. I'm sure something like this has been implemented i am just wondering how i might implement it with 2 boxes. I'm sure it isn't very difficult? How might i redirect traffic from a backup box to my main server without modifying the DNS?

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• How to resolve IPs in DNS based on the subnet of the requesting client?

  - by Nohsib

  Is it possible to configure Bind9 or other DNS to resolve the domain name of a machine into different IPs based on the subnet of the requesting client? e.g. Say the same service is running on 2 different application servers at different geographical points and based on the incoming request to resolve the domain name, the name server provides the IP of the application server based on the requesting client's IP, so the service could be offered by servers that are geographically closer to the client. In short, something like a CDN but just the IP resolution part based on the client's subnet. Is this configurable in any DNS?

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• Use Dynamic DNS to access Java servlet, still need port forward ?

  - by Frank

  If I use Dynamic DNS such as the free service at https://www.dyndns.com, do I still need to set up static IP and do port forward ? I have a DSL, most likely with dynamic IP address, and I run a Java servlet to get Paypal IPN messages on my notebook, in order for the messages to reach my notebook, I : [1] set up static IP and [2] did port forwarding. But I found each time the PC re-starts, it has a different external IP, so I was suggested to [3] get Dynamic DNS service like the free one mentioned above, but now I'm a bit confused, if I have step [3], do I still need to do [1] and [2], isn't step [3] supposed to do [1] and [2] for me ? But since I've already done [1],[2], now I wonder if they would cause trouble for step [3], do I need to undo them ? Or do I need all of them together ?

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• What kind of DNS and IIS configuration is needed to allow multiple domains to point to a multi-tenant web application?

  - by holiveira

  I'm developping a multi-tenant web application in ASP.NET MVC and it will provide my users the ability to have a custom subdomain pointing to their account page (like user.myapp.com). I already have it working by using a wildcard DNS entry and a code to query the database to load the user data based on the domain. I'm planning to offer the possibility of using custom domains, allowing the users to buy their own domains and use it instead of the subdomains that will be provided by default. I currently use DNSMadeEasy to host the DNS for the application main domain. I just don't know what kind of settings I must make to allow this feature to work, since the users will have domains hosted in several companies. Will I have to create my own nameservers and provide it to my users? What other things I must consider to implement it efficiently?

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• Failed reverse DNS and SPF only when using Thunderbird!

  - by TruMan1

  I have a reverse DNS and SPF records correctly setup for my mail server. Sending webmail from it works perfect. The problem is when Thunderbird sends out emails, it is using the client's IP address for the hostname. I have SMTP authentication and specified my mail server's as the outgoing SMTP. Mail is being sent, but it is not "signing" the email with the mail server's IP address.. it is using the client's. Is there any way to fix this? This is the spam error I get when sending from Thunderbird: Spam: Reverse DNS Lookup, SPF_SoftFail

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• How to configure DNS so that www.example.com goes to one server, *.example.com to another

  - by fishwebby

  I'm trying to set up my domain as follows, but I'm not actually sure if it's possible. I have a domain where I would like the base and www addresses to go to my static site, but others to go to my application server. For example: My domain is registered with Dreamhost, and my application is on a VPS at Webbynode. I've set up the domain in Dreamhost to use Webbynode's nameservers: ns1.dnswebby.com ns2.dnswebby.com ns3.dnswebby.com And in Webbynode I've set up a wildcard A record to point to the IP address of my VPS: * 1.2.3.4 A and this works nicely, if I go to app.example.com it resolves to my application server at Webbynode. However, what I'd like to do is have example.com and www.example.com go to my static site, hosted back at Dreamhost, whilst still having any other domain go to my app. What I've done to try and achieve this is set up these DNS "NS" entries at Webbynode, trying to get Dreamhost to resolve these domain names: (empty) ns1.dreamhost.com NS (empty) ns2.dreamhost.com NS (empty) ns3.dreamhost.com NS www ns1.dreamhost.com NS www ns2.dreamhost.com NS www ns3.dreamhost.com NS (I don't have a fixed IP address at Dreamhost so I can't just set up simple A records). However this doesn't work... does anyone have any idea if this is possible and if so how it could be done? Update: I've got this working now, as above for the domain (i.e. registered with Dreamhost, but using Webbynode's nameservers). To delegate the DNS for www.example.com to Dreamhost, I've got the following DNS entries set up: www.example.com. ns1.dreamhost.com. NS www.example.com. ns2.dreamhost.com. NS www.example.com. ns3.dreamhost.com. NS (note the full stops at the end) And to get example.com to resolve to my static site, I set up CNAME record: example.com. www.example.com. CNAME So now, example.com and www.example.com go to my static site on Dreamhost, and if they change the IP address of my shared hosting it won't affect me, and all other subdomains go to my application server. This seems to work nicely, but if anyone knows a better way to do it I'd be happy to hear it. Thanks to all who replied.

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• How is my ISP DNS hijacking AFTER all these precautions?

  - by user973917

  In IE9 when I search for anything my ISP hijacks google search and I get this result. To add complication to this I've already changed the default DNS servers (OpenDNS) months ago. This only happens in IE9; even after machine reboots and cache flushing. I even have my router (dd-wrt) intercepting all requests for DNS and I am still getting this result. I have all extensions disabled and there are no toolbars. This is IE9 from M$. This is not malware; it happens all machines with IE installed (even IE7/8).

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• How is DNS related to Active Directory and what are some common configurations that I should be aware of?

  - by MDMarra

  Please note that I do know the answer to this and have provided one below. I see a lot of newer systems administrators that don't understand the contents of my answer, so I hope that all of the beginner AD DNS questions will be closed as a duplicate of this. If you have a minor improvement, feel free to edit my answer. If you can provide a more comprehensive complete answer, feel free to leave one. How is DNS related to Active Directory and what are some common configurations that I should be aware of?

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• how can I find if xyz is valid DNS server?

  - by Radek

  how can I find out (using windows or internet) if xyz (in particular NS1.XYZNAMESERVER.COM) is valid DNS server?

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• Windows Server 2008 R2 Domain Controller DNS Reverse lookup zone needed?

  - by Joost Verdaasdonk

  When I create a new Domain Controller with dcpromo then the wizard will also add a DNS Role to the server because the first domain controller must be the global catalog server for the forest. After the install when I look at the DNS then I see the forward lookup zone for the newly created domain. However no zone is created for the Reverse lookup zone. So my question is: Is this an advisable endresult or not? In other words is it a good idea to add my domain to the reverse lookup zone as well? Just curious to hear how other people use this zone in the domain controller. Thanks

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• How do I either use DNS forwarding or aliasing to display a specific domain?

  - by PeanutsMonkey

  We have a domain abc.com from which a particular page will display the contents from the domain def.com which does not belong to us. Now rather than display def.com in the address bar in the browser, we would like to continue using abc.com. We would like to achieve it without using iframes or screen scraping. Is there a way to achieve what we are after using DNS forwarding or aliasing? If so how? We have a choice of using Microsoft's DNS server or BIND.

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• VPN is working, except for DNS lookups. Firewall (Cisco ASA 5505) issue?

  - by macke

  I've got the following set up: LAN -> DHCP / DNS / VPN server (OSX 10.6) -> Cisco ASA 5505 -> WAN Connecting to the LAN via VPN works fine. I get all the details properly and I can ping any host on the internal network using their IP. However, I can't do any host lookups whatsoever. I've looked through the logs on and found this nugget in the firewall log: 3 Sep 08 2010 10:46:40 305006 10.0.0.197 65371 portmap translation creation failed for udp src inside:myhostname.local/53 dst inside:10.0.0.197/65371 Port 53 is dns services, no? Because of that log entry, I'm thinking that the issue is with the firewall, not the server. Any ideas? Please keep in mind that I have very little knowledge and experience with this kind of firewall and the little experience I do have is with the ASDM GUI console, not the CLI console.

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• Doesn't DNS diversity negatively affect performance? Why/how?

  - by cnst

  If you look at the press releases of various orgs that run the internet, you can see them praise the fact that now they run root server X in city Y, as if that magically makes everyone in city Y get all the relevant resolutions from the local server X, instead of going 200ms across the oceans and lands to other continents for resolutions. Similarly, the zones of some geographical domain names, like .ru, are being mirrored not just within Europe, but also, for example, in Hong Kong, which is no more, no less, but is about 300ms away from central Europe, since the traffic is often crossing the two oceans on each way. Doesn't all of this negatively affect DNS performance? Isn't it more of a liability to have a diverse pool of geodispersed authoritative servers, especially if your target audience is quite geographically concentrated? Perhaps a better question is, are there any DNS resolvers that use something better than the naive round-robin for choosing which authoritative server to contact?

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• How to update debian dns server? New VM with same hostname as old VM

  - by opensourcechris

  We run several linux VM's on our Hyper-V cluster. Our old IT manager configured the dns server to resolve the url 'devlabs.ourdomain.com' to a debian squeeze apache webserver hosted on the hyper v cluster with the hostname: devlabs. We recently created a new Ubuntu vm to replace the original squeeze vm. When we created the new Ubuntu VM we used the same hostname of 'devlabs" to name the new VM. My problem is that now I am only able to access the new Ubuntu VM by using the IP address. How can I update our DNS server to point the url 'devlabs.ourdomain.com' to the new VM?

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• What kind of issues would occur if resolve.conf had no dns servers set?

  - by Stuart Woodward

  I want to create a server for a customer and have that customer finish the configuration for themselves. It was been decided that rather than setting default DNS servers (i.e. something like Google's) that the customer should enter the information by themselves. I assume that the customer is technically competent enough to do this. If however they forget or neglect to set this up they might spend some time trying to figure out what is wrong and eventually contact support. (In this case, I think that setting a default might have been better.) Apart from the obvious inability to resolve hosts, what other issues might they face until they have set valid dns servers in resolve.conf?

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• Is setting an empty/blank DNS A record allowed?

  - by John

  I'm trying to wrap my head around the wonderful world on DNS. I have created a zone file for example.com which contains: @ A 1.2.3.4 * A 1.2.3.4 However I am also setting up my local DNS, local.example.com which I have created a separate zone file for containing the following: machine1 A 192.168.0.1 machine2 A 192.168.0.2 When I dig machine1.local.example.com it returns A record 192.168.0.1, great. Sadly, badmachine.local.example.com returns 1.2.3.4, as does local.example.com. I'm not sure of the best way to prevent this. If I add the following to the local.example.com empty A records are returned for the above 2 examples as is the behaviour I desire: @ A * A Is this allowed? Is this best practice, or am I doing things terribly wrong? I'm using PowerDNS with BIND backend. Thanks for your thoughts!

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• How to setup dns to redirect app.example.com to another ip?

  - by AZ.

  I have a site www.example.com running on a hosting company. Now I want to create a separate web app on my VPS and let it accessible via app.example.com How can I set the DNS to redirect app.example.com to my VPS' ip address? CNAME or A Record? Also, If I want to do a mail server on my VPS too, how to setup the DNS? EDIT: Existing site: www.example.com Location: some hosting company that I don't control. It's running PHP with nginx I guess (or aphache) New site (that I'm working on): app.example.com Location: my VPS, it has an IP address, the VPS is running nodejs. It can run along with nginx but currently it's not. I want the existing website continue working (as customer visit www.example.com) and I want customer to visit app.example.com for some new features. The two websites are NOT on the same server and not using the same IP address.

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• Roughly, what percentage of users will reach changed DNS?

  - by user3722246

  If my main server go offline for some reason for +1hrs, I'm planning to make a DNS change so users will access secondary server. It is not a perfect solution to decrease downtime but it is simple and would work. I'm not sure about its usefulness. So I have a question. If I'm going to make a DNS change to an A record for my domain (changing from one IP to another), what percentage of users are moved over to the new info in 2hrs? (roughly) I know this is a vague question and there are lots of variables but any input is welcomed because I had painful downtime experiences and don't want to experience it again. Thanks

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• Recursion with an Array; can't get the right value to return

  - by Matt

  Recursive Solution: Not working! Explanation: An integer, time, is passed into the function. It's then used to provide an end to the FOR statement (counter<time). The IF section (time == 0) provides a base case where the recursion should terminate, returning 0. The ELSE section is where the recursive call occurs: total is a private variable defined in the header file, elsewhere. It's initialized to 0 in a constructor, elsewhere. The function calls itself, recursively, adding productsAndSales[time-1][0] to total, again, and again, until the base call. Then the total is returned, and printed out later. Well, that's what I hoped for anyway. What I imagined would happen is that I would add up all the values in this one column of the array and the value would get returned, and printed out. Instead if returns 0. If I set the IF section to "return 1", I noticed that it returns powers of 2, for whatever value time is. EG: Time = 3, it returns 2*2 + 1. If time = 5, it returns 2*2*2*2 + 1. I don't understand why it's not returning the value I'm expecting. int CompanySales::calcTotals( int time ) { cout << setw( 4 ); if ( time == 0 ) { return 0; } else { return total += calcTotals( productsAndSales[ time-1 ][ 0 ]); } } Iterative Solution: Working! Explanation: An integer, time, is passed into the function. It's then used to provide an end to the FOR statement (counter<time). The FOR statement cycles through an array, adding all of the values in one column together. The value is then returned (and elsewhere in the program, printed out). Works perfectly. int CompanySales::calcTotals( int time ) { int total = 0; cout << setw( 4 ); for ( int counter = 0; counter < time; counter++ ) { total += productsAndSales[counter][0]; } return total0; }

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