Computer Networks 4th Ed Andrew S. Tanenbaum [Electronic resources] نسخه متنی

7.1 DNSThe Domain Name System

Although programs theoretically could refer to hosts, mailboxes, and other resources by their network (e.g., IP) addresses, these addresses are hard for people to remember. Also, sending e-mail to tana@128.111.24.41 means that if Tana's ISP or organization moves the mail server to a different machine with a different IP address, her e-mail address has to change. Consequently, ASCII names were introduced to decouple machine names from machine addresses. In this way, Tana's address might be something like tana@art.ucsb.edu. Nevertheless, the network itself understands only numerical addresses, so some mechanism is required to convert the ASCII strings to network addresses. In the following sections we will study how this mapping is accomplished in the Internet.

Way back in the ARPANET, there was simply a file, hosts.txt, that listed all the hosts and their IP addresses. Every night, all the hosts would fetch it from the site at which it was maintained. For a network of a few hundred large timesharing machines, this approach worked reasonably well.

However, when thousands of minicomputers and PCs were connected to the net, everyone realized that this approach could not continue to work forever. For one thing, the size of the file would become too large. However, even more important, host name conflicts would occur constantly unless names were centrally managed, something unthinkable in a huge international network due to the load and latency. To solve these problems, DNS (the Domain Name System) was invented.

The essence of DNS is the invention of a hierarchical, domain-based naming scheme and a distributed database system for implementing this naming scheme. It is primarily used for mapping host names and e-mail destinations to IP addresses but can also be used for other purposes. DNS is defined in RFCs 1034 and 1035.

Very briefly, the way DNS is used is as follows. To map a name onto an IP address, an application program calls a library procedure called the resolver, passing it the name as a parameter. We saw an example of a resolver, gethostbyname, in Fig. 6-6. The resolver sends a UDP packet to a local DNS server, which then looks up the name and returns the IP address to the resolver, which then returns it to the caller. Armed with the IP address, the program can then establish a TCP connection with the destination or send it UDP packets.

7.1.1 The DNS Name Space

Managing a large and constantly changing set of names is a nontrivial problem. In the postal system, name management is done by requiring letters to specify (implicitly or explicitly) the country, state or province, city, and street address of the addressee. By using this kind of hierarchical addressing, there is no confusion between the Marvin Anderson on Main St. in White Plains, N.Y. and the Marvin Anderson on Main St. in Austin, Texas. DNS works the same way.

Conceptually, the Internet is divided into over 200 top-level domains, where each domain covers many hosts. Each domain is partitioned into subdomains, and these are further partitioned, and so on. All these domains can be represented by a tree, as shown in Fig. 7-1. The leaves of the tree represent domains that have no subdomains (but do contain machines, of course). A leaf domain may contain a single host, or it may represent a company and contain thousands of hosts.

Figure 7-1. A portion of the Internet domain name space.

The top-level domains come in two flavors: generic and countries. The original generic domains were com (commercial), edu (educational institutions), gov (the U.S. Federal Government), int (certain international organizations), mil (the U.S. armed forces), net (network providers), and org (nonprofit organizations). The country domains include one entry for every country, as defined in ISO 3166.

In November 2000, ICANN approved four new, general-purpose, top-level domains, namely, biz (businesses), info (information), name (people's names), and pro (professions, such as doctors and lawyers). In addition, three more specialized top-level domains were introduced at the request of certain industries. These are aero (aerospace industry), coop (co-operatives), and museum (museums). Other top-level domains will be added in the future.

As an aside, as the Internet becomes more commercial, it also becomes more contentious. Take pro, for example. It was intended for certified professionals. But who is a professional? And certified by whom? Doctors and lawyers clearly are professionals. But what about freelance photographers, piano teachers, magicians, plumbers, barbers, exterminators, tattoo artists, mercenaries, and prostitutes? Are these occupations professional and thus eligible for pro domains? And if so, who certifies the individual practitioners?

In general, getting a second-level domain, such as name-of-company.com, is easy. It merely requires going to a registrar for the corresponding top-level domain (com in this case) to check if the desired name is available and not somebody else's trademark. If there are no problems, the requester pays a small annual fee and gets the name. By now, virtually every common (English) word has been taken in the com domain. Try household articles, animals, plants, body parts, etc. Nearly all are taken.

Each domain is named by the path upward from it to the (unnamed) root. The components are separated by periods (pronounced ''dot''). Thus, the engineering department at Sun Microsystems might be eng.sun.com., rather than a UNIX-style name such as /com/sun/eng. Notice that this hierarchical naming means that eng.sun.com. does not conflict with a potential use of eng in eng.yale.edu., which might be used by the Yale English department.

Domain names can be either absolute or relative. An absolute domain name always ends with a period (e.g., eng.sun.com.), whereas a relative one does not. Relative names have to be interpreted in some context to uniquely determine their true meaning. In both cases, a named domain refers to a specific node in the tree and all the nodes under it.

Domain names are case insensitive, so edu, Edu, and EDU mean the same thing. Component names can be up to 63 characters long, and full path names must not exceed 255 characters.

In principle, domains can be inserted into the tree in two different ways. For example, cs.yale.edu could equally well be listed under the us country domain as cs.yale.ct.us. In practice, however, most organizations in the United States are under a generic domain, and most outside the United States are under the domain of their country. There is no rule against registering under two top-level domains, but few organizations except multinationals do it (e.g., sony.com and sony.nl).

Each domain controls how it allocates the domains under it. For example, Japan has domains ac.jp and co.jp that mirror edu and com. The Netherlands does not make this distinction and puts all organizations directly under nl. Thus, all three of the following are university computer science departments:

cs.yale.edu (Yale University, in the United States)

cs.vu.nl (Vrije Universiteit, in The Netherlands)

cs.keio.ac.jp (Keio University, in Japan)

To create a new domain, permission is required of the domain in which it will be included. For example, if a VLSI group is started at Yale and wants to be known as vlsi.cs.yale.edu, it has to get permission from whoever manages cs.yale.edu. Similarly, if a new university is chartered, say, the University of Northern South Dakota, it must ask the manager of the edu domain to assign it unsd.edu. In this way, name conflicts are avoided and each domain can keep track of all its subdomains. Once a new domain has been created and registered, it can create subdomains, such as cs.unsd.edu, without getting permission from anybody higher up the tree.

Naming follows organizational boundaries, not physical networks. For example, if the computer science and electrical engineering departments are located in the same building and share the same LAN, they can nevertheless have distinct domains. Similarly, even if computer science is split over Babbage Hall and Turing Hall, the hosts in both buildings will normally belong to the same domain.

7.1.2 Resource Records

Every domain, whether it is a single host or a top-level domain, can have a set of resource records associated with it. For a single host, the most common resource record is just its IP address, but many other kinds of resource records also exist. When a resolver gives a domain name to DNS, what it gets back are the resource records associated with that name. Thus, the primary function of DNS is to map domain names onto resource records.

A resource record is a five-tuple. Although they are encoded in binary for efficiency, in most expositions, resource records are presented as ASCII text, one line per resource record. The format we will use is as follows:

Domain_name  Time_to_live  Class  Type  Value 

The Domain_name tells the domain to which this record applies. Normally, many records exist for each domain and each copy of the database holds information about multiple domains. This field is thus the primary search key used to satisfy queries. The order of the records in the database is not significant.

The Time_to_live field gives an indication of how stable the record is. Information that is highly stable is assigned a large value, such as 86400 (the number of seconds in 1 day). Information that is highly volatile is assigned a small value, such as 60 (1 minute). We will come back to this point later when we have discussed caching.

The third field of every resource record is the Class. For Internet information, it is always IN. For non-Internet information, other codes can be used, but in practice, these are rarely seen.

The Type field tells what kind of record this is. The most important types are listed in Fig. 7-2.

Figure 7-2. . The principal DNS resource record types for IPv4.

An SOA record provides the name of the primary source of information about the name server's zone (described below), the e-mail address of its administrator, a unique serial number, and various flags and timeouts.

The most important record type is the A (Address) record. It holds a 32-bit IP address for some host. Every Internet host must have at least one IP address so that other machines can communicate with it. Some hosts have two or more network connections, in which case they will have one type A resource record per network connection (and thus per IP address). DNS can be configured to cycle through these, returning the first record on the first request, the second record on the second request, and so on.

The next most important record type is the MX record. It specifies the name of the host prepared to accept e-mail for the specified domain. It is used because not every machine is prepared to accept e-mail. If someone wants to send e-mail to, for example, bill@microsoft.com, the sending host needs to find a mail server at microsoft.com that is willing to accept e-mail. The MX record can provide this information.

The NS records specify name servers. For example, every DNS database normally has an NS record for each of the top-level domains, so, for example, e-mail can be sent to distant parts of the naming tree. We will come back to this point later.

CNAME records allow aliases to be created. For example, a person familiar with Internet naming in general and wanting to send a message to someone whose login name is paul in the computer science department at M.I.T. might guess that paul@cs.mit.edu will work. Actually, this address will not work, because the domain for M.I.T.'s computer science department is lcs.mit.edu. However, as a service to people who do not know this, M.I.T. could create a CNAME entry to point people and programs in the right direction. An entry like this one might do the job:

cs.mit.edu  86400  IN  CNAME  lcs.mit.edu 

Like CNAME, PTR points to another name. However, unlike CNAME, which is really just a macro definition, PTR is a regular DNS datatype whose interpretation depends on the context in which it is found. In practice, it is nearly always used to associate a name with an IP address to allow lookups of the IP address and return the name of the corresponding machine. These are called reverse lookups.

HINFO records allow people to find out what kind of machine and operating system a domain corresponds to. Finally, TXT records allow domains to identify themselves in arbitrary ways. Both of these record types are for user convenience. Neither is required, so programs cannot count on getting them (and probably cannot deal with them if they do get them).

Finally, we have the Value field. This field can be a number, a domain name, or an ASCII string. The semantics depend on the record type. A short description of the Value fields for each of the principal record types is given in Fig. 7-2.

For an example of the kind of information one might find in the DNS database of a domain, see Fig. 7-3. This figure depicts part of a (semihypothetical) database for the cs.vu.nl domain shown in Fig. 7-1. The database contains seven types of resource records.

Figure 7-3. A portion of a possible DNS database for cs.vu.nl

The first noncomment line of Fig. 7-3 gives some basic information about the domain, which will not concern us further. The next two lines give textual information about where the domain is located. Then come two entries giving the first and second places to try to deliver e-mail sent to person@cs.vu.nl. The zephyr (a specific machine) should be tried first. If that fails, the top should be tried as the next choice.

After the blank line, added for readability, come lines telling that the flits is a Sun workstation running UNIX and giving both of its IP addresses. Then three choices are given for handling e-mail sent to flits.cs.vu.nl. First choice is naturally the flits itself, but if it is down, the zephyr and top are the second and third choices. Next comes an alias, www.cs.vu.nl, so that this address can be used without designating a specific machine. Creating this alias allows cs.vu.nl to change its World Wide Web server without invalidating the address people use to get to it. A similar argument holds for ftp.cs.vu.nl.

The next four lines contain a typical entry for a workstation, in this case, rowboat.cs.vu.nl. The information provided contains the IP address, the primary and secondary mail drops, and information about the machine. Then comes an entry for a non-UNIX system that is not capable of receiving mail itself, followed by an entry for a laser printer that is connected to the Internet.

What are not shown (and are not in this file) are the IP addresses used to look up the top-level domains. These are needed to look up distant hosts, but since they are not part of the cs.vu.nl domain, they are not in this file. They are supplied by the root servers, whose IP addresses are present in a system configuration file and loaded into the DNS cache when the DNS server is booted. There are about a dozen root servers spread around the world, and each one knows the IP addresses of all the top-level domain servers. Thus, if a machine knows the IP address of at least one root server, it can look up any DNS name.

7.1.3 Name Servers

In theory at least, a single name server could contain the entire DNS database and respond to all queries about it. In practice, this server would be so overloaded as to be useless. Furthermore, if it ever went down, the entire Internet would be crippled.

To avoid the problems associated with having only a single source of information, the DNS name space is divided into nonoverlapping zones. One possible way to divide the name space of Fig. 7-1 is shown in Fig. 7-4. Each zone contains some part of the tree and also contains name servers holding the information about that zone. Normally, a zone will have one primary name server, which gets its information from a file on its disk, and one or more secondary name servers, which get their information from the primary name server. To improve reliability, some servers for a zone can be located outside the zone.

Figure 7-4. Part of the DNS name space showing the division into zones.

Where the zone boundaries are placed within a zone is up to that zone's administrator. This decision is made in large part based on how many name servers are desired, and where. For example, in Fig. 7-4, Yale has a server for yale.edu that handles eng.yale.edu but not cs.yale.edu, which is a separate zone with its own name servers. Such a decision might be made when a department such as English does not wish to run its own name server, but a department such as computer science does. Consequently, cs.yale.edu is a separate zone but eng.yale.edu is not.

When a resolver has a query about a domain name, it passes the query to one of the local name servers. If the domain being sought falls under the jurisdiction of the name server, such as ai.cs.yale.edu falling under cs.yale.edu, it returns the authoritative resource records. An authoritative record is one that comes from the authority that manages the record and is thus always correct. Authoritative records are in contrast to cached records, which may be out of date.

If, however, the domain is remote and no information about the requested domain is available locally, the name server sends a query message to the top-level name server for the domain requested. To make this process clearer, consider the example of Fig. 7-5. Here, a resolver on flits.cs.vu.nl wants to know the IP address of the host linda.cs.yale.edu. In step 1, it sends a query to the local name server, cs.vu.nl. This query contains the domain name sought, the type (A) and the class (IN).

Figure 7-5. How a resolver looks up a remote name in eight steps.

Let us suppose the local name server has never had a query for this domain before and knows nothing about it. It may ask a few other nearby name servers, but if none of them know, it sends a UDP packet to the server for edu given in its database (see Fig. 7-5), edu-server.net. It is unlikely that this server knows the address of linda.cs.yale.edu, and probably does not know cs.yale.edu either, but it must know all of its own children, so it forwards the request to the name server for yale.edu (step 3). In turn, this one forwards the request to cs.yale.edu (step 4), which must have the authoritative resource records. Since each request is from a client to a server, the resource record requested works its way back in steps 5 through 8.

Once these records get back to the cs.vu.nl name server, they will be entered into a cache there, in case they are needed later. However, this information is not authoritative, since changes made at cs.yale.edu will not be propagated to all the caches in the world that may know about it. For this reason, cache entries should not live too long. This is the reason that the Time_to_live field is included in each resource record. It tells remote name servers how long to cache records. If a certain machine has had the same IP address for years, it may be safe to cache that information for 1 day. For more volatile information, it might be safer to purge the records after a few seconds or a minute.

It is worth mentioning that the query method described here is known as a recursive query, since each server that does not have the requested information goes and finds it somewhere, then reports back. An alternative form is also possible. In this form, when a query cannot be satisfied locally, the query fails, but the name of the next server along the line to try is returned. Some servers do not implement recursive queries and always return the name of the next server to try.

It is also worth pointing out that when a DNS client fails to get a response before its timer goes off, it normally will try another server next time. The assumption here is that the server is probably down, rather than that the request or reply got lost.

While DNS is extremely important to the correct functioning of the Internet, all it really does is map symbolic names for machines onto their IP addresses. It does not help locate people, resources, services, or objects in general. For locating these things, another directory service has been defined, called LDAP (Lightweight Directory Access Protocol). It is a simplified version of the OSI X.500 directory service and is described in RFC 2251. It organizes information as a tree and allows searches on different components. It can be regarded as a ''white pages'' telephone book. We will not discuss it further in this book, but for more information see (Weltman and Dahbura, 2000).