DNS (Domain Name System)
DNS is used for the name resolution. In modern networking, we cannot remember IP address of every website access it and also IP address is dynamic as it may change time to time. To overcome this, DNS comes into role where it assigns a name to a IP so we can use it access instead of IP address. There are 5 name servers which is used to connect domain named with IP.
- Caching/Recursive name server
- Root name server
- TLD name server
- Authoritative name server
Caching name server is also a Recursive name server but it looks for the previous records rather than performing the entire process. Root name server receives the request in a zone and forward TLD(Top level domain) to the TLD name server. There are 13 main Root name server are present at the time. TLD name server maintains the domain names for the common domain extension such as .com or .in. The Authoritative name server will respond to Recursive name server with the information that is given by the organization like IP. Thus by following this process, a Fully Qualified Domain Name (FQDN) will be formed.
DNS at most time uses UPD. This is because, TCP uses SYN and ACK which may take more time and less efficient.
Resource Records
There are many resource records in which we discuss very few important one’s.
A record – It is used to point the IP address for the given domain. For example, 134.233.10.3 will be stored in A record for http://www.google.com
AAAA record – It is similar to type A record, but type A is assigned to IPv4 address where AAAA record is assigned to IPv6 address
CNAME (Canonical name) – For example, sometimes user may search for google.com instead of http://www.google.com . In such cases, CNAME is used. CNAME is used to map the domain into another domain.
DHCP
DHCP is mostly used for assigning IP addresses for the nodes in a network. There are few types of allocation in which DHCP use to allocate IP address.
- Dynamic Allocation – In this method, a set of IP addresses is kept aside and when a client request for a IP, the DHCP server will allocated for it.
- Automatic Allocation – This method is similar to dynamic allocation except it will keep a track of IP address which is assigned to client. So next time, the DHCP server will mostly try to allocate the same IP for the client.
- Fixed Allocation – The client request IP to DHCP server with a specific MAC address. If not found, then it perform dynamic or automatic allocation.
DHCP Discovery Process
Initially, a client may not connected to the internet. DHCP server will helps assign a IP to client. There are four main processes in assigning a IP.
DHCPDISCOVER – The client will send a broadcast message to all the nodes with a destination port 67 and a source port 68. The DHCP server will receive message which present in the local server.
DHCPOFFER – The DHCP server will send a offer message message with a IP to the client at 0.0.0.0:68
DHCPREQUEST – The client will accept the request and send the IP request message to the DHCP server.
DHCPACK – Finally, the DHCP server will assign a IP to the client.
NAT (Network Address Translation)
NAT is used for hiding the IP address of the client and the server. Thus when a client send a packet to server, the packet is sent to NAT and the NAT changes the IP address of the client to its IP address and then it forwards it to the server. From server to client, the process again takes place. This process is also called IP masquerading.
Port Preservation – In this method, the port number chosen by the client is same as the router.
Port Forwarding – Lets take a web server that has IP with 10.1.1.0 with port 300. When client want to access it, he/she need not to know the IP address of the server rather they have to know about the external IP of the router. So when a client sends traffic to the IP of router with the port 300, the router will automatically direct traffic to the web server with the same port.
VPN and Proxies
VPN (Virtual Private Network)
VPN are used to provide access to the network, in which the user may not in the particular network. Lets take an employee of the company who is on vacation, but he needs to access the company network. The VPN acts like a tunnel and the employee can access all the private network from his location without actually present.
Proxy
Proxies act as a intermediate between client and server. They are similar to firewall and is used to filter the users based on location.
Reverse Proxy
A organization may contain many server but in outside, it looks like they are a single server. Reverse proxies are used to direct traffic to different servers.
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