continue The (Web design programs) resolver will move on to the
continue The resolver will move on to the next service in the list and attempt resolution using it. The optional (!) character specifies that the status value should be inverted before testing; that is, it means “not.” The available status values on which we can act are: success The requested entry was found without error. The default action for this status is return. notfound There was no error in the lookup, but the target host or network could not be found. The default action for this status is continue. unavail The service queried was unavailable. This could mean that the hosts or networks file was unreadable for the files service or that a name server or NIS server did not respond for the dns or nis services. The default action for this status is continue. tryagain This status means the service is temporarily unavailable. For the files files service, this would usually indicate that the relevant file was locked by some process. For other services, it may mean the server was temporarily unable to accept connections. The default action for this status is continue. A simple example of how you might use this mechanism is shown in Example 6.3. Example 6.3: Sample nsswitch.conf File Using an Action Statement # /etc/nsswitch.conf # # Example configuration of GNU Name Service Switch functionality. # Information about this file is available in the `libc6-doc’ package. hosts: dns [!UNAVAIL=return] files networks: files This example attempts host resolution using the Domain Name Service system. If the return status is anything other than unavailable, the resolver returns whatever it has found. If, and only if, the DNS lookup attempt returns an unavailable status, the resolver attempts to use the local /etc/hosts. This means that we should use the hosts file only if our name server is unavailable for some reason. Configuring Name Server Lookups Using resolv.conf When configuring the resolver library to use the BIND name service for host lookups, you also have to tell it which name servers to use. There is a separate file for this called resolv.conf. If this file does not exist or is empty, the resolver assumes the name server is on your local host. To run a name server on your local host, you have to set it up separately, as will be explained in the following section. If you are on a local network and have the opportunity to use an existing name server, this should always be preferred. If you use a dialup IP connection to the Internet, you would normally specify the name server of your service provider in the resolv.conf file. The most important option in resolv.conf is name server, which gives the IP address of a name server to use. If you specify several name servers by giving the name server option several times, they are tried in the order given. You should therefore put the most reliable server first. The current implementation allows you to have up to three name server statements in resolv.conf. If no name server option is given, the resolver attempts to connect to the name server on the local host. Two other options, domain and search, let you use shortcut names for hosts in your local domain. Usually, when just telnetting to another host in your local domain, you don’t want to type in the fully qualified hostname, but use a name like gauss on the command line and have the resolver tack on the mathematics.groucho.edu part.