We could see multiple ip addresses for a single host like facebook, google etc.
My question is why a host maintains so many ip addresses ? what is the purpose for it? Does it wont overlap with any other Public addresses.
The purpose is to distribuite the work load on different datacenter on different geographical areas. They use a single domain name (like facebook.com) to hide a big network of datacenter ready to reply to requests. The real host to which you will be connected is decided dynamically based on your position and the load of the different datacenter.
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At the moment there is a nginx-balancer (Centos 7, a virtual machine with a white address) proxying to a large number of backend Apache servers. It is necessary to implement a failover cluster of two balancers on nginx. Fault tolerance is trially implemented using a virtual ip address (keepalived is used). Tell me what you can read about the pair nginx-balancer or how it can be implemented: all requests coming to them on the same virtual ip-address are evenly distributed between the two of them, but if one of them fails, the remaining one takes everything on itself?
At the moment, it turns out that there are two identical balancers and the benefit of the second is only in insurance. In the moments of full work of the main (master), the second (backup) is uselessly idle.
What you are describing is active-active HA.. you can find something on google for nginx+ but by briefly looking at it I don't really see it as true active/active = there is not just one virtual (floating) IP.. instead active/active is achieved by using two floating IPs (two VRRP groups - one VIP address active on each nginx) and then using round-robin DNS A record containing both addresses.
As far as I know keepalived is using VRRP protocol which in some implementations can provide 'true' active/active.. anyway I'm not sure keepalived supports this. Based on informatin I'm able to lookup it's not possible.
Is it possible to ping mininet ip? I found mininet's ip starts with 10.0.2.15 . I can ping from mininet to others. However, I failed to ping other place to mininet. How can I setup this?
10.0.0.0/8, which is 10.0.0.0 - 10.255.255.255 are IP addresses used only locally, they are not accessed from the internet (other networks). Here is some info from IANA:
These addresses are in use by many millions of independently operated networks, which might be as small as a single computer connected to a home gateway, and are automatically configured in hundreds of millions of devices. They are only intended for use within a private context and traffic that needs to cross the Internet will need to use a different, unique address.
These addresses can be used by anyone without any need to coordinate with IANA or an Internet registry. The traffic from these addresses does not come from ICANN or IANA. We are not the source of activity you may see on logs or in e-mail records.
For example, I've a server hosted at my home with 2 NICs for redundancy obviously.
NIC1 has been assigned with the public IP 103.204.82.22 from ISP1
NIC2 has been assigned with the public IP 144.110.12.64 from ISP2
I can access the server with both IP as usual.
Now, I have a domain acme.com. I've created a subdomain server.acme.com. I want to point server.acme.com to both the IPs so that in case one ISP fails to provide connectivity my server still remains online with the other one.
I've already tried with A and CNAME records. But it isn't working. It's working with A record if I use only one IP for the subdomain.
Can anyone tell me what and how can I point both the IPs to the single subdomain?
Thanks in advance
What you are describing is called DNS round robin, but that won't give you your expected outcome.
Anything you do with DNS if one ISP connection is down, traffic will still go there.
You may have your terminology mixed up a little to start with.
in this case, I suspect you really mean that server.acme.com is a host record, rather than a subdomain. (A subdomain would mean that the server address would be at servername.server.acme.com)
If you create an A record, and put both IP addresses in, and keep the TTL (time to live) short, then when a client wants to contact your machine it will randomly pick one of the addresses. If that address is unavailable, it will move on to the next. If that address stops working, it will keep trying it for the 'TTL' time.
Presuming that the IP addresses don't change, which would be a different problem altogether, then this provide basic load balancing and failover to both connections.
Amazon provide a more advanced type of DNS, that will actively monitor your connections and only provide responses that are live. - https://docs.aws.amazon.com/Route53/latest/DeveloperGuide/routing-policy.html
Google has multiple servers at multiple locations. When I search Google in my web browser, how does the DNS map this name to the corresponding IP address? Google has multiple servers in multiple locations with separate IPs. Is a load balancer used first?
A couple of different approaches are used:
Geographic DNS
When a request comes in for a domain name, the DNS server looks at the IP address making the request and returns an IP address of a nearby server.
Some complicated extensions are required to deal with large shared caching DNS servers (like ISP nameservers), but that's the general idea.
Anycast DNS
Anycast is a weird routing trick where a single IP range can be advertised by multiple ASes. This will cause requests to an IP address in that range to be routed to whichever server is closest.
If a DNS server is hosted on an anycast IP, different instances of that server can be configured to return different IPs. This can be used as a computationally easier alternative to geographic DNS.
Anycast HTTP
If anycast can be used to route DNS to the closest server, why not just go to the next step and use it to route HTTP as well?
(It turns out there's a reason why you usually don't want to do this: Routing changes can break a HTTP connection. This doesn't affect DNS as it's usually used over UDP. Cloudflare does it anyway, though, and it usually works fine… YMMV.)
In large scale reverse proxy server is usually used for this purpose and it can do various tasks including load balancing as well. To the client it appears that you connect only to one server while reverse proxy hides servers behind it.
In small scale you can do similar things just with DNS settings mapping different domain names to different IP addresses. See this article
I was curious as to why one client site on a shared server was performing very poorly and I wanted to know if there was a way to find out how many other sites were being hosted on the same server. I found this reverse IP lookup site:
http://reverseip.domaintools.com/
that claims the client's site IP is also being used by 3000+ other sites. I did a quick survey of other clients' sites and this is more than twice the next closest, most being in the 800 - 1500 range.
Does this mean that there are 3000+ sites being hosted on one server, or could there still be multiple servers sharing an IP? Basically I want to know if this is the main likely reason the site is slow.
On public internet, sharing the same IP address does not mean sharing the same physical server. Here are the ways of sharing an IP, and yet processing on different physical server:
Most often, the public IP addresses are the interfaced by a Load Balancer, or a Reverse Proxy, or a Gateway. This device then routes (technically proxies) the connection to one of the physical servers running behind them. All these are within the firewall/network/data_cente of the "serving" organization.
Unless designed (or ill-designed) to reveal information about the internal IP addresses, there is no way to figure out the IP address of physical device that actually processed the request.
Anycast allows you to have the same IP address being available at different geographical locations. Look at Google's DNS servers (IP address 8.8.8.8). Such services are anycasted, to serve from the nearest geo-location.
This is also true from server's perspective. A server does not necessarily know the "original" IP address from where the request initiated. Most often, we are proxied, and/or NAT'ed by routers and other devices at our home and offices. After all, there are only so many public IP addresses available (at least IPv4), and we cannot have one public IP address for each device :) .
Closing statement: The server and the client only know the ingress/egress points of each other's network. Beyond that, they have no idea of the internal IP addresses of the physical devices.
Yes, it can very well mean that. It is very common, and is the only way companies selling you hosting for pennies can even approach turning a profit.
It is done with virtual hosting support in the web server. This relies on DNS and the browser / client providing the referrer URL to the server as part of the HTTP request. The HTTP server then knows who the client thinks he is requesting a URI from, and maps the request to that site tree. Those trees often sit on the same disk, though the sites may be jailed or virtualized.
I've seen numbers higher than 3000, for example.
If you want better, you have to move to a higher quality provider, and/or obtain your own IP addresses.