Name resolution
When name resolution is known processes which make it possible to translate names of computers or services in the computer editable, usually numeric addresses.
Applications
Name resolution is used in many areas. A well-known application is the translation of host names or domain names to network addresses. In addition, many network services names are assigned. The service name "www " resolves, for example, to TCP port "80". Protocol name such as " TCP" on their part have an associated number, for example "17" for " UDP".
On Unix, each user or group is assigned an identification number ( UID or GID) between 0 and 65535. Again, the user logs in with his name, the verification of access rights is, however, based on the numeric UID and GID of the groups to which the user belongs. Programs like "ls" and "ps ", however, convert the numeric IDs to names again to produce a well- readable output for the user.
Method
There are several methods that can be used to resolve the name. Often, several methods are used that are queried in a certain order, until one was able to provide a valid response.
The name resolution, for example, on a Windows computer in an SMB network (hybrid node) occurs in this order:
Local configuration files
The simplest method of name resolution represent local files that contain a corresponding assignment. In text files - such as in a table - a name associated with one or more addresses or numbers.
Use local configuration files, if a central organization of the data is required or if the data changes very rarely. Since these files are available locally, the resolution will work even if the network is completely or partially failed. Therefore, very important data are often (also) registered in local files.
Examples (UNIX):
- / etc / passwd Local user database. Here the administrator user ("root" ) and system users of services are ( for example, the user under whose account the web server is working ) on this day. If there is no central user management instead, here also " normal" users are registered.
- / etc / protocols From the IANA assigned protocol names and numbers. This data changes very rarely.
- / etc / hosts file ( Windows:% SystemRoot % \ system32 \ drivers \ etc \ hosts ) Local table for host names. Here is defined as the alias "localhost". A web server could enter the name of the database server to also still be able to communicate with the database when the network-based name resolution fails.
Figurative comparison: The address book in a mobile phone is comparable to a local configuration file: The stored phone numbers can be looked up without asking another person. The information is also available when the telephone information is unavailable due to a technical error. For the address book is not maintained automatically: Any new or changed phone number will automatically appear in the address book.
Broadcast (broadcast )
About broadcast onto the directly connected network is trying to make the name identified. This approach is for example used by ARP in Ethernet networks to find out the MAC address to an IPv4 address.
Figurative comparison: It calls for open office in which you are in, the name of the partner with whom you want to call and ask for his phone number. If you get this, you can call the desired partner. Due to the principle only people can be reached, who are in the same room.
Special services
The computer asks for the desired information in a particular network service. Depending on the application, other services used. The central data management changes in one place can be carried out, requesting computer must not be adjusted.
Examples:
Figurative comparison: One knows ( only ) the telephone number of the telephone directory. This calls you to find out the phone number of a third person. New telephone numbers only need to be notified to the information, but not every phone owner. If the respondent is having technical difficulties, you can, however, make calls if you have an alternative way to make the phone number identified.
Local Cache
The computer looks in its local cache to see if he has resolved this name before. If so, is in the cache, the required address / number, he can reuse. This caching is especially used in conjunction with name services to reduce the number of requests and accelerate the resolution.
Figurative comparison: You want to call someone with whom you have recently phoned before. So just press the redial because the phone has last numbers saved yet.
Order
The order is not standardized but depends on different factors.
- On the operating system: So, for example, prioritized Windows XP and Windows 2000, the DNS name resolution and Windows NT NetBIOS name resolution.
- Of the protocols used IP or NetBIOS over TCP / IP: If only the IP protocol is used, the name can be resolved only with the DNS name resolution to an IP address. If NetBIOS over TCP / IP, NetBIOS and DNS name resolution can be used.
- Configuration files and options: For UNIX -like systems, the order by entries in the / etc file is / set nsswitch.conf (/ etc / netsvc.conf AIX ). The Samba and Samba -TNG server know the " name resolve order ". In Windows you can since Windows NT 3.5, the order under the registry key HKEY_LOCAL_MACHINE \ SYSTEM \ CurrentControlSet \ services \ Tcpip \ Service Provider set (small number means high priority). Standard on Windows 7 is 1 local name, 2nd hosts file, DNS 3 and 4 NetBIOS.
Special shapes
- Host header (HTTP server)
With this method, a web server can be configured to different sites name have the same IP address. To distinguish between the Web server uses the host header contained in the HTTP request. The various websites referred to in this case as a ( name-based ) virtual hosts.
Figurative comparison: It calls a telephone number, which have been determined in one of the ways described above. Which reporting ( under this number ) interlocutor said, who exactly do you want to talk.