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COM is. COM, it will search the Internet to find out who owns www. Detailed working procedures are as follows. They are two essential technology developed for us to use the network or Internet conveniently. An address resolving mechanism. DNS server is responsible for accepting the queries through client and responding back with the results. What is DNS? What is a FQDN? Download now. Aaron January 3, at am. Hello Can you help me provide some information?
Thank you Reply. Michael Grandjean January 6, at am. Zodwa March 7, at pm. How to ensure that files on dhcp and dns servers in the same network are contiguous Reply. Dirk Ahrnke March 9, at am. Mark Baker November 9, at pm. Thanks Mark Reply. Dirk Ahrnke November 12, at pm. Best Regards, Dirk Reply. Functions October 14, at am. The server has two basic functions: Managing IP addresses — The DHCP server controls a range of IP addresses and allocates them to clients, either permanently or for a defined period of time.
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This is where it's defined in what order and where to look for different information, including where to look for hosts. It's better to debug DNS problems using commands dig and nslookup. For example, in order to request information from nameserver 8. Before this, all the examples were made on a local machine. Of course, it's useful for your perception, but it's not that interesting. That's why we will solidify everything we've read using virtual machines and libvirt, and also get acquainted with a couple of terms.
First of all, let's create a VM using virt-install :. A block To look at the detailed information about a certain network, you can use either virsh net-info or virsh net-dumpxml. The second command will return a lot more details, that's why let's use it:. You can read a detailed description of all the possible options of this XML-file in the libvirt documentation. But right now we are interested in two words: bridge and dhcp. All the requests from one VM to another within one network go through this virtual switch.
Libvirt creates one virtual switch for each network, and every switch is recognized as a separate device on host machine:. Creating a network in libvirt is by default equated with creating a virtual switch which all the VMs are connected to, thus creating a local area network, LAN. The virbr0 switch is implemented using Linux Bridge — a technology originally intended exactly for creating virtual local area networks. You can see a list of all the switches executing the brctl show command on the host machine.
Linux Bridge is "slightly" different from a typical hardware L2 switch. During the years of its existence a lot of features were added to it, like traffic filtering and firewall. The most proper way to call it is L3 switch, but here your obedient servant isn't completely sure. Here a block of addresses used for virtual machines in this network is declared. By default, the traffic from VM goes outside through a host-machine.
As an entertainment, you can set a configuration for a traffic to go to one virtual machine through another. When you were reading about CIDR, there was something that might get your attention: even if we divide the network into many blocks, the total amount of IP-addresses isn't going to increase.
Actually, a combination of private and public addresses is always used. Usually, one public address hides a lot of machines, each one having its own private address. This is also true for our VMs. Each one has the private IP-address from the block The host machine, if we continue to use our private laptop at home as it, is hidden behind our Wi-Fi router and also doesn't have a public address.
At first glance, the fact that VMs have an access to Internet seems evident. But the VM only has a private address, which is not accessible outside of the host machine. A public server VM requests to needs to forward a response somewhere, but it just won't be able to find VM's private IP-address because it is private.
It is a mechanism of resolving IP-addresses in network packages. Usually, sender's and receiver's IP-addresses are included in a package. NAT makes it possible to change these addresses dynamically and save the table of changed addresses.
When a package is sent, its source address is replaced with the host machine address. When a response from the target server goes back, the address is changed from the host machine address to the VM address.
It is the router that changes the address. DNAT destination NAT does pretty much the same, but vice versa: this is when you request to some public address which hides private, local addresses. NAT is the default way of VM's communicating with the world.
But libvirt is a flexible thing. For example, you can connect VMs directly to a host's physical interface instead of a virtual switch. Actually, there are a lot of ways to create a network. Libvirt uses iptables for NAT.
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