- Every 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:
- 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.
- 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. 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.
- 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.1.
- For an example of the kind of information one might find in the DNS database of a domain, see Fig2. This figure depicts part of a (semihypothetical) database for the cs.vu.nl domain shown in Fig.of DNS Namespace. The database contains seven types of resource records.
Figure2. A portion of a possible DNS database for cs.vu.nl
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- The first noncomment line of Fig.2 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.
- 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 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).
- 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 Fig1.
Figure1. The principal DNS resource record types for IPv4.
- 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 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).
- 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 Fig1.
Figure1. 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.
- 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. 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.
- 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.1.
- For an example of the kind of information one might find in the DNS database of a domain, see Fig2. This figure depicts part of a (semihypothetical) database for the cs.vu.nl domain shown in Fig.of DNS Namespace. The database contains seven types of resource records.
Figure2. A portion of a possible DNS database for cs.vu.nl
- The first noncomment line of Fig.2 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.
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