I'm interested in having a second web host run a copy of my website, such that if my first host goes down, the traffic routes to the second host. Is this possible?
My guess would be to add additional nameservers beyond the first two.
I also suspect it's doable with no-ip.com, but I'm not clear on how that works, and if they would require me to leave my first host entirely?
See if your DNS provider will let you do round robbin DNS.
Basically, DNS queries will return more than one IP for your site. Try nslookup google.com to see how it might look.
There are loads of other ways to do geographical load balancing and failover (most are expensive though).
DNS Made Easy provides this service, which is called DNS Failover. For others looking:
http://www.dnsmadeeasy.com/s0306/price/dns.html
Related
It is said that the recursive DNS refers to its cache first before performing recursive search on the authoritative DNS. So, I wanted to know how is the DNS caching done. Is it automatic or if no, what happens? How is the DNS record cached?
Unless you are running a DNS server, then the caching is being done by your client or and LDNS. Your system runs something called a resolver (set of libraries in linux, DNS client service on windows) whose job it is to take names and turn them into IP address, hopefully following the TTL of the returned records. Additionally, browsers and other applications may also add their own level of caching, often not adhering to the TTL returned for the record.
Additionally, your machine will point to another server, either locally or your ISP's, that is your LDNS (local DNS). This is a configuration that's required for proper functioning because it must be specified by IP address, either manually entered or obtained via a mechanism like DHCP.
This is a "Blocked Port 80" related question, but maybe something a bit unique. I've yet to find a good answer. It's more academic than anything as I know running a production server at your house is a ridiculous idea.
I'm running a development server (LAMP) at my home but my ISP blocks port 80. The DNS for my domain is set up to "URL Redirect" to my IP and port number. My router is port-forwarding to my server, and I have Apache set up so it's listening on port 8081.
The issue is that when you access the domain, the URL in the browser is resolved from my domain name to the IP and port number, and is displayed as such. For example, you type "www.banana.com" into the browser, the site is displayed but now the URL is shown as "12.23.456.11:8081".
Is there any way to fix this so that the domain name does not become IP and port number?
Can you use Apache proxy functionality somehow?
Could you use mod_rewrite to change the IP and port number back to the domain name?
Thanks in advance!
This question has three parts. First the issue of the domain: in order to substitute a domain name in place of an IP address you need some name server that can map your desired name to an address. This is at the host level and not the port level so a domain name will encompass all ports you might host from it. If you are using your home Internet connection (which I suspect you are since you talk about a blocked port) then you need to take into account that from time to time your public IP address can change. Your options are to pay for (or request) a static IP from your ISP or use a dynamic DNS service that can rapidly update their records as your IP address changes.
As for your port number. Mod_rewrite only handles the path part of a URL, for using different ports internally you want mod_proxy. The Apache web sever with mod_proxy would be configured to listen on the public port you want (that I assume is port 80) then mod_proxy would take incoming requests and send them to another web server on a different port (or even different server). To the outside user this happens invisibly. The problem is if your ISP wont let you host your site on port 80 then it logically won't let you proxy from port 80. To get around this would be a little harder. Personally I would look at a virtualized server from people like Rackspace or Linode. You would get (for relatively little money) a fully configurable server on the open Internet with no restrictions on port usage and a static IP. Even better if you mess something up you can just virtually delete your server and start over with a fresh OS image.
Finally the clean URLs your question title suggests. It's possible this wasn't part of your actual question but just in case, mod_rewrite is a smart module that can let you map clean URLs like /cars/Toyota/1997 and turn them into more ugly requests like /cars.php?make=Toyota&year=1997. Clean URLs not only look better they make it easier to reorganize web code behind the scenes as your web site evolves.
One last thing, and its amazing to me that this question has gone so long without even a comment about this but, this question is really not a good fit for StackOverflow. Possibly ServerFault.com. Good luck! :)
On our router we have the primary DNS set to a local IP, which is running Windows Server 2008 and the built in DNS server. We use this to resolve domains to local servers, if the domain is not founds locally we have forwarders set up to query external name servers.
The secondary DNS on the router is set to our ISP's primary DNS, incase the local DNS server is down.
The mac clients in our office pick up the DNS servers correctly from the router but it seems very random as to what DNS server it uses. For example, a local site would load up but some of the images would not. If I hard coded my DNS address to be the local DNS server everything would work fine.
So my question is, when would a mac client use the secondary DNS server? I though it'd only use it if the primary DNS was unavailable?
Thanks!
The general idea of a secondary DNS server was that in case the primary DNS server doesn't reply (e.g. it is offline, unreachable, restarting, etc.), the system can fall back to a secondary one, so it won't be unable to resolve DNS names during that time. Doesn't reply means "no reply at all", it will not ask the secondary when the primary one said that a name is unknown. Answering that a name is unknown is a reply.
The problem here is that DNS uses UDP and UDP is connectionless. So if a DNS server is offline, the system won't notice that other by not receiving a reply from it. As an UDP packet may as well get lost and the round-trip time (RTT) is unknown, it will have to resend the request a couple of times, every time waiting for several seconds, before it finally gets to the conclusion that this server is dead. This means it can take up to an entire minute and above to resolve a DNS name if the first DNS server dies.
As that seems unacceptable, different operating system developed different strategies to handle this in a better way. As both DNS servers are supposed to deliver the same result for the same domain (if not, your setup is actually flawed as the secondary should be a 1-to-1 replacement for the primary one), it shouldn't matter which one is being used. Some systems may send a request to the primary one but if no reply comes back within a few seconds, they don't resend to it but first try the secondary one (then they resend to the primary one and so on). Some may also query both at once, make the faster one win and then keep using that one for a while (until they start another race to see if it is still the faster one). Some may also prefer the primary one but do some kind of load balancing and switch to the secondary one if more than a certain amount of queries are currently pending on the primary one. Some will just alternate between them as a poor man's load balancing. All of this is actually allowed.
In your case, though, I'm afraid something is wrong with your primary server as by default, macOS will only use the primary one. If it constantly falls back to the secondary one, it may consider the primary one to be too slow. Every time that happens, the secondary server becomes the primary one, see this older knowlebase article. This cnet article explained how this can be disabled but I'm not sure this is still possible in current systems. I wasn't able to find any reference on this but IIRC from the very back of my head, Apple once mentioned on a WWDC that they are now more aggressive at DNS querying and may even try to contact multiple DNS servers at once with the fastest one winning in some cases but I might be wrong on this (maybe this was iOS only or so).
I googled this article which explains newer MacOS DNS search order. And this one which explains how to tweak it to obtain results that you desire.
Though the general idea is that it was never intended (in any OS) that first server is the one used and the second one is a backup. ( Even on windows, if first server for some reason doesn't answers very quickly, the second one will be queried.) It's wiser to regard server query order as unspecified.
To help users, I would like my code to discover Oracle databases on the LAN. I thought to do this by first detecting all hosts, then checking each host to see if it is listening on Oracle's default port.
Any ideas how to go about this? Preferably in Java, but any language or algorithm would do.
Are you using DHCP? If so, your DHCP server has a list of the leases it has passed out. That should do you for a list of hosts on the LAN. Then try opening a connection to the Oracle port on each of those hosts and see if it accepts the connection.
It should be pretty simple to implement as a shell script with half a dozen lines or so. Java seems like overkill for something like this. Loop through the leases file, grab the IP from each lease, and telnet to the Oracle port; if it connects, disconnect and print the IP to standard out.
If you want to stay platform-independant, and unless you have access to some kind of database that lists the hosts, the only way to get a list is to try each IP address in the local network - might as well try to connect to the Oracle port on each of them.
There are lots of problems with this approach:
Will only search through the local network, which may only be a small part of the LAN (in case of large companies with lots of subnets)
Can take a long time (you definitely want to reduce the timeout for the connection attempts, but if someone has configured his LAN as a class A network, it will still take forever)
Can trigger all kinds of alerts, such as desktop users' personal firewalls, and intrusion detection systems - because you're doing exactly the same thing someone trying to exploit a security hole in Oracle servers would do
As brazzy points out, scanning for hosts is likely to cause problems, especially if there is a bug in your scanner.
A better approach may be to get the owners of the databases to register them somewhere, for example in a local DNS service (or does Oracle have zeroconf support?), or simply on some intranet webpage or wiki.
You better register the SID names/addresses to some server with a fixed address(maybe with a simple web service), and then query the list from there. Another approach is the bruteforce one (explained by #brazzy) by scanning one or more subnets, but this isn't really a good thing to do.
In case you are looking for a tool Loo#Lan can do this for you. Unfortunately there's no source available...
All of these smart answers are the reasons why many companies do not use the default port. Using a different port for each database is entirely possible, you know.
Consider the following setup:
A windows PC with a LAN interface and a WiFi interface (the standard for any new laptop). Each of the interfaces might be connected or disconnected from a network. I need a way to determine which one of the adapters is the one connected to the internet - specifically, in case they are both connected to different networks, one with connection to the internet and one without.
My current solution involves using IPHelper's "GetBestInterface" function and supplying it with the IP address "0.0.0.0".
Do you have any other solutions you might suggest to this problem?
Following some of the answers, let me elaborate:
I need this because I have a product that has to choose which adapter to bind to. I have no way of controlling the setup of the network or the host where the product will run and so I need a solution that is as robust as possible, with as few assumptions as possible.
I need to do this in code, since this is part of a product.
#Chris Upchurch: This makes me dependent on google.com being up (usually not a problem) and on any personal firewall that might be installed to allow pinging.
#Till: Like Steve Moon said, relying on the adapter's address is kind of risky because you make a lot of assumptions on the internal network setup.
#Steve Moon: Looking at the routing table sounds like a good idea, but instead of applying the routing logic myself, I am trying to use "GetBestInterface" as described above. I believe what it should do is exactly what you outlined in your answer, but I am not really sure. The reason I'm reluctant to implement my own "routing logic" is that there's a better chance that I'll get it wrong than if I use a library/API written and tested by more "hard-core" network people.
Technically, there is no "connected to the Internet". The real question is, which interface is routeable to a desired address. Right now, you're querying for the "default route" - the one that applies if no specific route to destination exists. But, you're ignoring any specific routes.
Fortunately, for 99.9% of home users, that'll do the trick. They're not likely to have much of a routing table, and GetBestInterface will automatically prefer wired over wireless - so you should be good. Throw in an override option for the .1% of cases you screw up, and call it a day.
But, for corporate use, you should be using GetBestInterface for a specific destination - otherwise, you'll have issues if someone is on the same LAN as your destination (which means you should take the "internal" interface, not the "external") or has a specific route to your destination (my internal network could peer with your destination's network, for instance).
Then again, I'm not sure what you plan to do with this adapter "connected to the Internet", so it might not be a big deal.
Apparently, in Vista there are new interfaces that enable querying for internet connectivity and more. Take a look at the NLM Interfaces and specifically at INetworkConnection - you can specifically query if the network connection has internet connectivity using the GetConnectivity method.
See also: Network Awareness on Windows Vista
Unfortunately, this is only available on Vista, so for XP I'd have to keep my original heuristic.
I'd look at the routing table. Whichever NIC has an 0.0.0.0 route AND is enabled AND has the lowest metric, is the nic that's currently sending packets to the internet.
So in my case, the top one is the 'internet nic'.
IPv4 Route Table
===========================================================================
Active Routes:
Network Destination Netmask Gateway Interface Metric
0.0.0.0 0.0.0.0 10.0.0.10 10.0.0.51 20
0.0.0.0 0.0.0.0 10.0.0.10 10.0.0.50 25
(much other stuff deleted)
Another alternative is to ping or GetBestInterface 4.2.2.2 - this is an old and venerable DNS server, currently held by GTEI; formerly by Sprint if I remember right.
Start > Run > cmd.exe (this works in XP and Vista): ipconfig /all
This displays all info about the interfaces in your computer. The "public" facing interface should have a public IP address. For starters, it should not be 192.168.x.x or 10.x.x.x :)
running traceroute to some public site will show you. Of course, there may be more than one interface that would get you there.
Look at the routing table? Generally, unless you're routing between the networks in windows (which is possible, but unusual for a client computer these days) the interface that holds the default route is going to have the Internet connection.
Your question didn't detail why or what you're doing this with so I can't provide any specifics. The command line tool "route" may be of some help, but there are probably libraries for whatever programming language you're using to look at the routing table.
You can't rely on the IP address of the interface (e.g., assuming an RFC-1918 address [192.168.0.0/16, 172.16.0.0/12, 10.0.0.0/8] isn't the internet) since most sites have some kind of NATed firewall or proxy setup and the "internet" interface is really on a "private" lan that gets you out to the Internet.
UPDATE: Based on your further information, it sounds like you have a decent solution. I'm not so sure about the choice of 0.0.0.0 since that's a boundary case for IP address -- might be OK on your particular mix of platform/language. Sounds (from the API description) like you could just specify an address, so why not some address known to be on the Internet, e.g. the IP address of your web site, or something more random like 65.66.67.68? Just make sure not to pick one of the rfc-1918 addresses, or the localhost range (127.0.0.0/8), or multicast, any other reserved range, and any address that resolves to a .mil or .gov (while it doesn't sound like getbestinterface sends any traffic, it would suck to find out by having the feds break your door down... :)
Looking at the network point of view, either could be routing to the "internet" at any time. If things like spanning tree protocol are enabled on a switch then you may find that what may have been the routing card to begin with may not be anymore.
Ping google.com though each NIC.