Книга: Linux Network Administrator Guide, Second Edition

Configuring Name Server Lookups Using resolv.conf

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.

This is just the domain statement's purpose. It lets you specify a default domain name to be appended when DNS fails to look up a hostname. For instance, when given the name gauss, the resolver fails to find gauss. in DNS, because there is no such top-level domain. When given mathematics.groucho.edu as a default domain, the resolver repeats the query for gauss with the default domain appended, this time succeeding.

That's just fine, you may think, but as soon you get out of the Math department's domain, you're back to those fully qualified domain names. Of course, you would also want to have shorthands like quark.physics for hosts in the Physics department's domain.

This is when the search list comes in. A search list can be specified using the search option, which is a generalization of the domain statement. Where the latter gives a single default domain, the former specifies a whole list of them, each to be tried in turn until a lookup succeeds. This list must be separated by blanks or tabs.

The search and domain statements are mutually exclusive and may not appear more than once. If neither option is given, the resolver will try to guess the default domain from the local hostname using the getdomainname(2) system call. If the local hostname doesn't have a domain part, the default domain will be assumed to be the root domain.

If you decide to put a search statement into resolv.conf, you should be careful about what domains you add to this list. Resolver libraries prior to BIND 4.9 used to construct a default search list from the domain name when no search list was given. This default list was made up of the default domain itself, plus all of its parent domains up to the root. This caused some problems because DNS requests wound up at name servers that were never meant to see them.

Assume you're at the Virtual Brewery and want to log in to foot.groucho.edu. By a slip of your fingers, you mistype foot as foo, which doesn't exist. GMU's name server will therefore tell you that it knows no such host. With the old-style search list, the resolver would now go on trying the name with vbrew.com and com appended. The latter is problematic because groucho.edu.com might actually be a valid domain name. Their name server might then even find foo in their domain, pointing you to one of their hosts - which clearly was not intended.

For some applications, these bogus host lookups can be a security problem. Therefore, you should usually limit the domains on your search list to your local organization, or something comparable. At the Mathematics department of Groucho Marx University, the search list would commonly be set to maths.groucho.edu and groucho.edu.

If default domains sound confusing to you, consider this sample resolv.conf file for the Virtual Brewery:

# /etc/resolv.conf # Our domain domain vbrew.com # # We use vlager as central name server: name server 172.16.1.1

When resolving the name vale, the resolver looks up vale and, failing this, vale.vbrew.com.

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