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• Understanding Backtracking in C++

  - by nikhil

  I have a good basic understanding of the fundamentals of C++, I also have an understanding of how recursion works too. I came across certain problems like the classic eight queens problem and solving a Sudoku with Backtracking. I realize that I'm quite lost when it comes to this, I can't seem to be able to get my mind around the concept of going back in the recursion stack and starting again in order to solve the problem. It seems easy with a pen and paper but when it comes to writing code for this, I'm confused on how to begin attacking these problems. It would be helpful if there were a tutorial aimed at beginners to backtracking or if there were a good book where this was covered. If somebody can shed light on this topic or give me some links to decent references, I'd be really grateful. And yes I do know that it would be easier in functional languages but I'd like to understand the implementation in imperative languages too.

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• Recursion in F#

  - by MarkPearl

  Things are slowly coming together – I was able to look at a bit of F# code and intuitively know what it was going to do (yay)… So today I saw a blog post by Bob Palmer on Fibonacci numbers in F# which inspired me to look at bit into recursion. First the C# example… class Program { public static void CountDown(int n) { switch (n) { case 0: Console.WriteLine("End of Count"); break; default: Console.WriteLine(n); CountDown(n-1); break; } } static void Main(string[] args) { CountDown(10); Console.ReadLine(); } }   In F#, the equivalent would look something like this… open System let rec CountDown n = match n with | 0 -> Console.WriteLine("End of Count"); | n -> Console.WriteLine(n); CountDown (n-1); CountDown 10 Console.ReadLine()   Pretty simple stuff. With F# you when making recursive calls you need to explicitly declare that the function is recursive with the “rec” keyword. Otherwise the code is pretty easy to read and self explanatory.

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• What are basic programs like, recursion, Fibonacci, small trick programs?

  - by Mike

  This question may seem daft (I'm a new to 'programming' and should probably stop if this is the type of question I'm required to ask)... What are: "basic programs like, recursion, fibonacci, factorial, string manipulation, small trick programs"? I've recently read Coding Horror - the non programmer and followed the links to Kegel and How to get hired. Then I delved through some similar questions here (hence the block quote) and I realised that as a fully fledged non-programmer I probably wouldn't know if I knew recursion (or any of the others) because I wouldn't know what it looked like, or why it was used, and what the results would look like after it was used. I suppose I'm trying to get a picture of "the basics". What the principles are and why we learn them - where they'll be used and what result/s your looking for. If they'll be used as an interview question during my first interview sometime in 2020 I would like to look less ignorant than those 199 out of 200 who just don't know the how, or the why, of programming. As always...I'll get my coat. Thanks Mike

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• How do I obtain a valid DNS resolution given just an IP address?

  - by Dee Newcum

  Is there a publicly-available DNS server somewhere that will respond to requests like: 74_125_225_50.anyip.com And will return 74.125.225.50 for the above request? That is, every single possible IP address can queried by name instead of number. http://ipq.co/ is close to what I'm looking for, but it requires you to first register an IP address before you can query its DNS name. I want a service that does a straightforward mapping from domain name to IP address. Why do I want to do this? I have a program that we use at work that requires a DNS lookup, but I need to be able to give it bare IP addresses. (long story... it's a server that I don't control, so I can't work around it using /etc/hosts)

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• How to set the hostname according to the DNS name on Ubuntu 9.10?

  - by tangens

  Motivation I have to manage a lot of virtual machines that I create by copying a template (VmWare image). Problem Now I have the problem that in the template the file /etc/hostname contains a given name that I want to change for each copy of the template. Facts The network interface is configured by DHCP. DNS entries exist. The system is a Ubuntu 9.10 server. Question I wonder if I can configure the template so that on startup it sets its hostname according to its DNS name. I could create an init script that parses the IP address, makes a DNS lookup and sets the hostname accordingly. But is there an easier way?

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• How do I set up DNS for an intranet web site?

  - by BradyKelly

  I have a web site running on our local server, 'OBAMA'. In my hosts file on that server, I map engenxt.local to 127.0.0.1, so typing engenxt.local on that machine itself brings up the website. I would like other machines to access the site via domain name 'engenxt'. On a Windows Server 2008 R2 Standard server, where would I begin, where should I go, and where should I end? I know precious little about DNS, or rather precious nothing, but am a quick study, besides, that is why I'm asking how to do this here. Episode 2 When I run DNS as Ryan suggests, it says 'the server OBAMA is unavailable. Do I want to add it?' It adds it, and tells me to choose to 'Configure a DNS server' from the actions menu, but nearly all the actions in that menu are disabled.

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• Where did this incorrect cached DNS lookup come from?

  - by Stephen Jennings

  Somehow, I've been having a chronic issue where my computer will get an invalid DNS lookup in its cache for either of the two Exchange servers I use from Mail.app. My workplace runs one of the Exchange servers and I run the other (they are totally unrelated, hosted by different companies, etc.). The problem manifests as a certificate domain error. When it happens, I can run nslookup mail.mydomain.com and I see the incorrect IP address (usually owned by either Apple or Akamai), but if I run nslookup mail.mydomain.com 8.8.8.8, I get the correct address. My real quest is to find out why this keeps happening, and to do that, I'd like to know which server is supplying me this bad DNS entry. Is there a way to check my DNS cache to see where this bad lookup came from?

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• Should I use my ISP's DNS, or Google's 8.8.8.8?

  - by Torben Gundtofte-Bruun

  It seems like a good idea to use Google's public DNS 8.8.8.8 and 8.8.4.4 because it's really fast -- much faster than my own ISP's DNS! -- and probably more reliable, too. That seems like a ridiculously quick win for me, and much easier to remember. Assuming we're not all "tin foil hat" about Google, why shouldn't everybody use Google DNS? Note: I've seen this question, but I don't want a comparison to OpenDNS. This is about everyday use by everyday people in their homes. Update: I seem to have put my hand in a wasps' nest of privacy concerns. I appreciate the issue, but I was expecting a more technology-oriented discussion...

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• Server 2008 Active Directory DNS Entries Deleted. Dcdiag unable to contact local AD controller.

  - by Jim Smith

  I've never seen anything like this. At a potential customer site, I noticed that the PC's were all unable to locate the domain controller and netlogon was failing. I fixed the DNS entries on the client PC's so the AD server was DNS server and tried to rejoin the domain. The PC was unable to locate the domain controller. On the server, I checked the DNS settings and while there is the high level AD folder, every single entry related to Active Directory appears to have been completely deleted. There are no backups from what I can tell and this has been happening for 6 months at least. Does anyone have any recommendations for repairing this? Thanks.

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• Why does my DNS change (and break) at regular intervals?

  - by Peter Kelly

  I have a laptop running Windows 7. Up until recently, everything was fine. We have multiple devices in my house connecting to the one access point with no problems. No issues with ISP. Now my Windows 7 PC DNS settings change every minute or so. Before the problem occurs if I do an ipconfig /all I have two DNS settings (primary/secondary) and everything is fine. After a short period of time this change to a sole DNS, 10.0.0.1. Webpages no longer resolve. If I do an ipconfig /renew, this fixes the problem. I have tried uninstalling various programs I thought might be related but the problem persists. Any ideas of potential causes?

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• How to secure Firefox traffic (+DNS) through SOCKS proxy under Ubuntu 10.04?

  - by Maarx

  I'm using Ubuntu 10.04, and starting a SOCKS proxy with 'ssh -D', and setting Ubuntu to use it with "System - Preferences - Network Proxy". Firefox uses the proxy, and the proxy's IP appears when I visit a site like http://www.whatismyip.com/. My question is, is Firefox resolving DNS requests through this proxy? Is my web-browsing truly secure? (That is, until I exit the other end of the proxy. I know it's insecure after that.) (And I've verified the keys, I'm not being man-in-the-middled) (And--screw it. You know what I mean. Is it resolving DNS requests through the proxy?) I don't know how I would go about verifying such a thing for myself. Using additional hardware such as another debugging proxy is not an option. If Firefox isn't resolving my DNS requests through the SOCKS proxy, how do I go about fixing it?

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• What settings need to be changed to allow EC2 instances to use Amazon's Route 53 for DNS?

  - by ks78

  I have a number of Amazon EC2 instances, all running Ubuntu, which I'd like to configure to use Amazon's Route 53. I setup a script, following Shlomo Swidler's article, but ran into script-related issues, which were answered here. Now, I have the script working, but my instances are still not able to access Route 53's DNS. By this I mean, they are not able to resolve hostnames to IP addresses. My instances are currently configured with the DNS server IP address Amazon pushes out to them by default, does that need to be changed when using Route 53? I'm also IP-restricting my instances using the Security Groups. Could that be the problem? Is there a certain IP address or port I should open to allow communication with Route 53? It seems that DNS requests should be originating from my instances so the Security Groups shouldn't be an issue, but I've been wrong before. If anyone has any ideas, I'd really appreciate it.

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• Windows 7: "localhost name resolution is handled within DNS itself". Why?

  - by Portman

  After 18 years of hosts files on Windows, I was surprised to see this in Windows 7 build 7100: # localhost name resolution is handled within DNS itself. # 127.0.0.1 localhost # ::1 localhost Does anyone know why this change was introduced? I'm sure there has to be some kind reasoning. And, perhaps more relevantly, are there any other important DNS-related changes in Windows 7? It scares me a little bit to think that something as fundamental as localhost name resolution has changed... makes me think there are other subtle but important changes to the DNS stack in Win7.

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• How are my DNS entries safe in a shared hosting environment?

  - by Jake

  I'm trying to understand how DNS works in a shared hosting environment. I went to my registrar and set my name servers to my host's ns1.foo.com and ns2.foo.com. I'm using a cloud hosting provider who has a web portal where I can set my DNS entries. However I am confused by the lack of security. when I entered in the entries for my domain there was never any step to prove that I actually own that domain. What is to stop somebody else on the same hosting service (a nasty neighbor) from writing over my DNS entries and pointing my traffic to their server instead?

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• NT4 server generate too much weird DNS queries, How can i see the source PID?

  - by Hanan N.

  I have a NT4 server that in the last two weeks started to generate too many weird DNS queries to the DNS server is set to use. I have got warnings from the IPS system that it has blocked the responses from the DNS server back to the NT4 server. The queries it generate doesn't relate to any computer in the network, it is like 120624100088.xxxxxxx.net where xxx is the internal network, the numbers are just random at each query. I have done some research on how to get the PID that is generating the queries, and i found that only Process Monitor could give me that information, but since it is NT4 system Process Monitor doesn't work on it. It is a production server and i am just can't stop services as i want. I would like to get your advice on how can i get the PID that is generating these queries? Thanks.

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• how to create stub DNS zone for emulating my customer production environment ?

  - by Albert Widjaja

  Hi All, Is it possible to emulate my customer production environment inside my AD domain by just creating the same domain inside my primary DNS server ? Can I created mycustomer.com DNS zone (STUB) just for the sake of listing few database servers and application servers and then for the other DNS records eg. MX, NS and the other refer to the REAL MX record entry so that my Exchange Server email flow is unaffected to mycustomer.com ? because if I just create A record in my current domain for some of the servers, the FQDN is not exactly what I want. Thanks.

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• How to setup dhcp3-server to advertise the DNS server the server itself has got from DHCP?

  - by Ivan

  The Ubuntu 10.04 server has eth0 Internet interface configured by means of an ISP's DHCP. At the same time the server has static eth0 LAN interface to which it provides masquerading (NAT) and LAN-internal DHCP service (dhcp3-server). As far as I've understood the manual, I had to hardcode DNS servers to advertise through LAN DHCP with option domain-name-servers in dhcpd.conf. But what if the ISP changes his DNS server IP silently (we use a SOHO-class ISP, so this won't surprise me much)? Can I configure dhcpd to advertise the DNS server the server uses itself, the one gotten by its DHCP client mechanism?

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• Testing DNS configuration of domain by using hosts file?

  - by Alex Blundell

  I'm currently migrating a website to another server, and want to test the DNS configuration (more specifically, email mx records) before moving the domain over. I've configured the DNS on the new server to have mx entries for Google Apps in the same way that it's configured on the old server. The domain is controlled by nameservers on the old server at the moment, so the change would simply be updating the nameservers to the new servers. (What I'm getting at is DNS is controlled at the server level, not registrar level). Since the website has quite a number of users, I want to make sure the configuration is right before flicking the switch. For this, can I add an entry to the hosts file of my local computer to point the domain to the new server? I've done this, and the web server works, but would this also test the email mx records on the new server?

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• How do I setup a local DNS server on Mac OS Lion?

  - by Peter Kovacs

  I had some serious lag to resolve website address and sometimes things simply wouldn't load (pages kept loading for 5+ minutes without even a timeout error). So I had setup a local dns server/cache using BIND on Leopard and Snow Leopard. Now that I have Lion, i have the same problem, but the instructions no longer apply to Lion and I can't find a way to do it. Has anyone attempted to do this? Are there viable alternatives for DNS servers on OS X 10.7? For those who are wondering I already tried several external DNS server. Only my computer has this issue on the network.

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• Windows XP clients do not update server 2008 DNS forward lookup zone.

  - by whatsisname

  I have a Cisco 5505 working as a DHCP server, and a server 2008 DNS server running an AD domain. I am having problems with all XP computers not updating the forward lookup zone. The reverse lookup zone updates are working. Windows vista and 7 computers update just fine. Additionally the DNS server accepts both secure and non-secure updates. When people are connected through the Cisco's VPN, they cannot resolve to any machines that have reverse lookup zones, but they can resolve entries in the forward lookup zone. I have tried ipconfig /registerdns, but the forward lookup zone entries for the XP clients are not being populated. How can I get the XP Dynamic DNS client to make the updates, or what can I do to debug what's going on? Thanks

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• What are the pros of switching DNS names with a database server hardware upgrade?

  - by wilbbe01

  When we upgrade to new hardware at work we usually increment a number in the DNS name. For example. We have a server called database-2, that is slated to become database-3 in the coming days. I haven't been able to find a good reason why this is good behavior. To me the work of trying to catch all end user machines, as well as all servers dependent on the database server is far riskier than simply moving the database and ip/name with it to the new hardware. A little over a year ago we spent several months of requests coming in, as infrequent users began using software that needed to be updated to point to a new DNS name. I am struggling to find answers as to why this is a good practice. So the question. Why is using DNS names as a "server hardware version identifier" a good idea? What am I overlooking? Thanks much.

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• Two group of name server records, where to put them?

  - by sazary

  I've registered my domain by a registrar that has very poor DNS management tools. I need to point from my registrar to another third-party DNS manager, and then from there point to the name servers of my host, along with some other DNS records (such as SPF records). What I've done now is this: I've given the address of the name servers of my third-party DNS manager to the DNS manager of my registrar, and then I've given the address of the name servers of my host to the third-party DNS manager, along with some SPF and MX records. Is this work correct? Or should I add the NS address of my host to my registrar DNS manager too? The problem is that my domain doesn't resolve to my host, and I see some strange records in some DNS servers around the world that I have not set!

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• "The zone can be scavenged after" keeps incrementing

  - by kce

  What are you trying to do? I'm trying to enable DNS scavenging on a DNS zone that has about a hundred stale DNS records. What have you tried in order to make it happen? I setup DNS Scavenging per everyone's favorite TechNet Blog post: Don't be afraid of DNS Scavenging. Just be patient. I first disabled scavenging on all of our domain controllers: DNSCmd . /ZoneResetScavengeServers contoso.com 192.168.1.1 192.168.1.2 I then enabled automatic scavenging on the DNS zone: I then enabled DNS scavenging on one of the domain controllers: I then found a few records that I expected to get delete with timstamps from a few years ago and ensured that that the Delete this record when it becomes stale and that time stamp was actually set: Finally I reloaded the zone and waited 14 days (the sum of the Refresh + No-Refresh periods). What results did you expect? I expected to see a 2501 Event in the DNS server logs noting the deletion of a bunch of DNS records. What actually happened? Nothing happened. The Zone Aging/Scavenging Properties showed that the zone could be scavenged after 6/12/2014 10:00:00 AM last week. No 2501/2502 events were recorded. All of the records with "aged" time stamps are still present. The date at which the zone can be scavenged after incremented another seven days to ?6/?18/?2014 10:00:00 AM. As I understand it until that date stays at least 14 days in the past nothing will ever even be eligible for scavenging let alone actually be scavenged. The only 2501 events recorded in the event logs are ones that I have triggered by right clicking and selecting "Scavenge Stale Resource Records". They note that scavenging will try to run again in 168 hours which was this morning. I have DNS scavenging enabled for a few months and have waited patiently for something to happen. I have reloaded the zone multiple times (which resets this timestamp). What am I missing here?

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• Cannot access domain from windows 2003 client

  - by Peuge

  Hey all, First off I am a novice at AD and DNS so please bear with me. This is my current situation: I have one server which is a DC and DNS server (win2k3) - Machine 1. I have another machine which is trying to join this domain - Machine2. This machine is also a win2k3 server. This is what I have done so far: I have setup DNS on the DC and its tcp/ip dns is pointing to itself. On machine2 I have set its dns to point to the dc. The DNS has been setup with a forward lookup zone with the same name as the domain (accdirect.com). I can ping machine1 from the machine2 by its FQDN and ip. I have set up forwarders on the DC for our ISP dns and can browse the internet on both machines. In the DNS mmc on the DC I can see a host (A) has been created for machine2. The problem is I still cannot join the domain. When I try join the domain via my computer - properties then it brings up the username/password box and after I go "ok" it says cannot find domain accdirect.com If I run this from machine2 dcdiag /s:accdirect.com /u:accdirect.com\admin /p: then I get the following: Performing initial setup: ** Warning: could not confirm the identity of this server in the directory versus the names returned by DNS servers. If there are problems accessing this directory server then you may need to check that this server is correctly registered with DNS [accdirect.com] Directory Binding Error 1722: Win32 Error 1722 This may limit some of the tests that can be performed. Done gathering initial info. On the dc all dcdiag and netdiag results pass. If anyone could help me I would really appreciate this! Sorry if any of my terminology is a bit off, I have only been doing this for two days. thanks Peuge

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• DNS failover in a two datacenter scenario

  - by wanson

  I'm trying to implement a low-cost solution for website high availability. I'm looking for the downsides of the following scenario: I have two servers with the same configuration, content, mysql replication (dual-master). They are in different datacenters - let's call them serverA and serverB. Users use serverA - serverB is more like a backup. Now, I want to use DNS failover, to switch users from serverA to serverB when serverA goes down. My idea is that I setup DNS servers (bind/powerdns) on serverA and serverB - let's call them ns1.website.com and ns2.website.com (assuming I own website.com). Then I configure my domain to use them as its nameservers. Both DNS servers will return serverA IP as my website's IP. If serverA goes down I can (either manually or automatically from serverB) change configuration of serverB's DNS, to return IP of serverB as website's IP. Of course the TTL will be low, as it's supposed to be in DNS failovers. I know that it may take some time to switch to serverB (DNS ttl, time to detect serverA failure, serverB DNS reconfiguration etc), and that some small part of users won't use serverB anyway. And I'm OK with that. But what are other downsides of such an approach? An alternative scenario is that ns1.website.com will return serverA IP as website's IP, and ns2.website.com will return serverB IP as website's IP. But AFAIK clients not always use primary nameserver and sometimes would use secondary one. So some small part of users would use serverB instead of serverA which is not quite what I'd like. Can you confirm that DNS clients behave like that and can you tell what percentage of clients would possibly use serverB instead of serverA (statistically)? This one also has the downside that when serverA goes back up, it will be automatically used as website's primary server, which is also a bad situation (cold cache, mysql replication could fail in the meantime etc). So I'm adding it only as a theoretical alternative. I was thinking about using some professional DNS failover companies but they charge for the number of DNS requests and the fees are very high (why?)

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