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Cache using barcode integrating for vs .net control to generate, create code-128b image in vs .net applications. Visual Studio .NET 2003 Response A DNS message that is sent in response to a query has the following structure: Header record Information about this message Query record Repeats the query Answer records Resource records that answer the query Authority records Resource records for servers that have authority for the answers Additional records Additional resource records, such as NS records The dig utility does not consult /etc/nsswitch.conf (page 455) to determine which server to query. The following example uses dig to query a DNS server:.

Introduction to DNS 783 $ dig fedorapr ..


;; ANSWER SECTION: fedoraproject.



16 16. IN IN 209.132.176.

122 66.35.62.

162. 3556 3556. IN IN NS NS 12739 5415. IN IN 64.34.184.

179 209.124.61.

35. Reverse Name Resolution In addition to normal or forward name resolution, DNS provides reverse name resolution, also referred to as inverse mapping or reverse mapping, so that you can look up domain names given an IP address. Because resource records in the forward DNS database are indexed hierarchically by domain name, DNS cannot perform an efficient search by IP address on this database. DNS implements reverse name resolution by means of a special domain named in-addr.

arpa (IPv4) or (IPv6). Resource records in these domains have Name fields that hold IP addresses; the records are indexed hierarchically by IP address.

The Data fields hold the FQDN that corresponds to the IP address. Reverse name resolution can verify that someone is who he says he is or at least is from the domain he says he is from. In general, it allows a server to retrieve and record the domain names of the clients it provides services to.

For example, legitimate mail contains the domain of the sender and the IP address of the sending machine. A mail server can verify the stated domain of a sender by checking the domain associated with the IP address. Reverse name resolution is also used by anonymous FTP servers to verify that a domain specified in an email address used as a password is legitimate.

For example, to determine the domain name that corresponds to an IP address of 209.132.177.

110, a resolver would query DNS for information about the domain named 110.177.132. (Figure 24-5, next page).

The following example uses dig to query DNS for the IP address that corresponds to, which is 209.


The second command line uses the dig utility to query the same IP address, reversed, and appended with

784 24 DNS/BIND: Tracking Domain Names and Addresses com net arpa sobell redhat in-addr fedora Figure 24-5. Reverse name resolution and the domain 110.177.132.

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arpa to display a PTR resource record (page 781). The data portion of the resultant resource record is the domain name from the original query: rhn.redhat.

com.. $ dig rhn.redh .




110. $ dig 110.177. 132. PTR .


177.132.209. IN ;; ANSWER SECTION: 110.

177.132.209. 245 IN .

... Instead of ref ormatting the IP address as in the preceding example, you can use the x option to dig to perform a reverse query:. $ dig -x ... ;; QUESTION SECTION: ; About DNS 785 ;; ANSWER SECTION: 269 IN ... Or you can just use host:. $ host domain name pointer About DNS This section discusses how DNS works and provides resources for additional information on DNS. How DNS Works Application pr ograms do not issue DNS queries directly but rather use the gethostbyname() system call. How the system comes up with the corresponding IP address is transparent to the calling program. The gethostbyname() call examines the hosts line in /etc/nsswitch.

conf file (page 455) to determine which files it should examine and/or which services it should query and in what order to obtain the IP address corresponding to a domain name. When it needs to query DNS, the local system (i.e.

, the DNS client) queries the DNS database by calling the resolver library on the local system. This call returns the required information to the application program..

Prerequisites Install the fo llowing packages: bind bind-utils (provides dig among other utilities) caching-nameserver (RHEL, optional, used to set up a caching-only nameserver; FEDORA includes the contents of this package in the bind package) system-config-bind (optional) bind-chroot (optional, sets up BIND to run in a chroot jail) Run chkconfig to cause named to start when the system enters multiuser mode:.
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