is host name to ip a one-to-one mapping? how can a host name have multiple ip addresses?
according to Java's getAllByName, it returns a list of ips for a hostName, but its documentation seems to suggest otherwise or at least not explaining the one to many mapping:
The host name can either be a machine name, such as "java.sun.com", or a textual representation of its IP address.
http://download.oracle.com/javase/1.4.2/docs/api/java/net/InetAddress.html#getAllByName%28java.lang.String%29
No, every host name can have multiple DNS A records pointing to different IPs. This is often used for high-level load balancing. You can check that for example on google.com:
$ host google.com
google.com has address 209.85.148.104
google.com has address 209.85.148.105
google.com has address 209.85.148.103
google.com has address 209.85.148.106
google.com has address 209.85.148.147
google.com has address 209.85.148.99
google.com mail is handled by 30 alt2.aspmx.l.google.com.
google.com mail is handled by 20 alt1.aspmx.l.google.com.
google.com mail is handled by 50 alt4.aspmx.l.google.com.
google.com mail is handled by 40 alt3.aspmx.l.google.com.
google.com mail is handled by 10 aspmx.l.google.com.
Actually the IP's for a machine depends upon the number of Network Interface Cards(NIC) which are being used for that box. So if a machine has 3 NIC's installed, there will be 1 hostname for that box but 3 different IP's.
No, a hostname may have several A records and in this case resolvers will return all IP addresses as a list. You can check it on almost every large web site:
;; QUESTION SECTION:
;google.com. IN A
;; ANSWER SECTION:
google.com. 283 IN A 74.125.232.82
google.com. 283 IN A 74.125.232.81
google.com. 283 IN A 74.125.232.83
google.com. 283 IN A 74.125.232.80
google.com. 283 IN A 74.125.232.84
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As far as i know what we get from a dns query is a ip address. So in the end of the day if thats true we are still using ip addresses to connect the server and domains are pretty names for them.
So how does a server know which domain i used to query that ip address?
How does vhosts work an understand that if the domain data is lost during dns query?
The Internet works in layers. Each layer uses different kind of parameters to do its work.
Layer 3 is typically IP aka Internet Protocol. To work it uses IP addresses, each computer has at least one to be able to discuss with another one. And there are two families in fact: version 4 and version 6.
Since multiple services can be on any given computer at some point, you need a layer on top of that, layer 4, that deals with transport. The "predominant" one is TCP aka Transport Control Protocol, but there is also UDP. TCP and UDP uses ports: a 2 bytes integer that encodes for a specific protocol.
For example, HTTP was given port number 80 (completely arbitrary), and HTTPS port 443.
The DNS, which itself uses UDP and TCP (on port 53), allows, among other things, to map a given hostname to a given IP address or multiple IP addresses. This is the typical A and AAAA records. There is also a CNAME record that maps one domain name to another. There also exists a SRV record that maps a service (which is a protocol name + a transport) to a given hostname and port number.
When one computer connects to another, its first step for all the above is to find out which IP address to use to connect to. It can use the DNS for that. Typically it will get only the IP address, but, depending on the protocol (layer above 4), may also get a port (if using SRV records).
The HTTP world does not use SRV records. So a browser just uses the hardcoded 80 or 443 ports, or the port number appearing in the URL.
Then we are at the transport level, let us say TCP.
The connection is done (since now the remote IP address and port are known) and the protocol above TCP, like HTTP, is free to convey any kind of extra data, such as the hostname that the client initially used (as taken from the URL) to find out the IP address.
This is done through the HTTP host header, see RFC 2616
Note that if you do things through TLS (which conceptually sits between TCP and HTTP) there is even something else happening: SNI or Server Name Indication.
When doing the TLS handshake, so before any kind of HTTP headers or content, the client will send the final hostname desired in some specific TLS message. Why? So that the server can find which specific certificate it should answer which as otherwhise it would not be able to know which hostname is requested as this sits in some HTTP header which do not exist until the TLS handshake is finished.
A webserver will be able to see both the SNI content to find out which certificate to send back and then the host header to find out which VirtualHost (in Apache) section is relevant to the query being processed.
If you are not in HTTP world, then it all depends on the protocol used. Older protocols, like FTP, did not plan for "multihoming" at the beginning, a given IP address meant only one hostname and service for example.
I am now just starting to understand Httpcontexts and server side logic. Total beginner.
I have understood how to read the http requests from a client and get the remote IP etc.
I came across the hostname property.
I believe hostname and domain are completely different.
My undersanding is that stackoverflow is the domain name.
Given its widespread reach, stackoverflow can have multiple servers and hence multiple ip addresses.
So is there a way to determine to which server a client is requesting to in asp.net core 2.0?
You are correct that stackoverflow.com is a domain name. A domain name points to an IP address. That's called DNS. stackoverflow.com points to one single IP address not multiple. There's probably a load balancer at that IP address which then points you to an available server, which is internal you can't see that, but that's another story.
If you ping stackoverflow.com you get:
The whole point of DNS is to not use IP address but a domain name.
I'm not sure why you would want to see the IP. Maybe this stackoverflow question can help: Resolve HostName to IP
I'm a little confused about IP addresses.
I know that every web domain has an ip address.
Does the IP address represent the physical machine / host the website files are stored on?
Therefore when DNS lookup is performed, the domain's IP address is returned to the client. The client then uses this ip to contact the server that the web files reside on.
Is my understanding correct?
Many thanks
You are correct.
It is kind of like how some companies may say Dial PIZZAHUT instead of saying, Dial 74992488. PIZZAHUT is easier to remember, but you actually are dialling the number.
You're talking about HTTP protocol. Yes, FQDN (web domain) is resolved to IP address by DNS server. Client will connect to server IP address. Since you're probably using HTTP 1.1, HTTP request will contain also FQDN. This information is used by web server to perform several checks, like SSL certificate validation or Virtualhost management (several domains on a single IP address).
I am checking the internet connection of my computer and do not understand a few points.
The following is a result from $ netstat:
I do not understand why for some items in the list have localhost:xxxxx as their foreign address.
since netstat shows the internet connections, shouldn't this show IP addresses of outside my computer?
netstat per default tries to convert the data it finds into host names. It does so by reverse resolving the IP addresses via DNS.
For example, 127.0.0.1 gets replaced with localhost in this case.
The same happens with the ports, but there it doesn't happen via DNS, but via a file like /etc/services which provides a mapping between port numbers and service names.
You can prevent this with the netstat option -n.
I have a static local IP Address: 10.8.4., and the public IP Address of my machine is: 72.43.135.. when the server(sitting on different network from my workstation) gets a request from my machine, it sees my IP address from
Context.Request.UserHostAddress
and got 10.20.102.*.
why it the server not getting the IP as: 72.43.135.*?
If you define public and local, you will get to know that these terms might refere to the same network under some conditions. This could be a demilitarized zone (DMZ) for example.
What IP the destination server sees, depends on the interface you send the packets through and the routers it crosses.
Is there masquerading (NAT) ? - Is the main question. You can be on totally different networks but the routers might still forward your local IP, now this also depends on the routing table. Can a packet find its way back to your host? Is there a reversed route from the host to your machine?
The destination host is propably having 2 interfaces, 1 with IP 72.43.. one with a 10.8.. maybe it recieves through the 72 but sends back through the 10.8 because it has a different route back. Networking can be real voodoo! Trace your packets, ask your sysadmins..
(not talking about proxies here, they deliver different custom headers with different IPs)