Another query might be: +-----------------------------------------+ Header | OPCODE=CQUERYM, ID=410 | +-----------------------------------------+ Question | QTYPE=A, QCLASS=IN, QNAME=B | +-----------------------------------------+ Answer | <empty> | +-----------------------------------------+ Authority | <empty> | +-----------------------------------------+ Additional | ARPA NULL IN | +-----------------------------------------+ This query is similar to the previous one, but specifies a target of ARPA rather than ISI.ARPA. It also allows multiple matches. In this case the same name server might return: +-----------------------------------------+ Header | OPCODE=RESPONSE, ID=410 | +-----------------------------------------+ Question | QTYPE=A, QCLASS=IN, QNAME=B | +-----------------------------------------+ Answer | B.ISI.ARPA A IN 10.3.0.52 | | - | | B.BBN.ARPA A IN 10.0.0.49 | | - | | B.BBNCC.ARPA A IN 220.127.116.11 | +-----------------------------------------+ Authority | <empty> | +-----------------------------------------+ Additional | ARPA NULL IN | +-----------------------------------------+ This response contains three answers, B.ISI.ARPA, B.BBN.ARPA, and B.BBNCC.ARPA.
Recursive Name Service Recursive service is an optional feature of name servers. When a name server receives a query regarding a part of the name space which is not in one of the name server's zones, the standard response is a message that refers the requestor to another name server. By iterating on these referrals, the requestor eventually is directed to a name server that has the required information. Name servers may also implement recursive service. In this type of service, a name server either answers immediately based on local zone information, or pursues the query for the requestor and returns the eventual result back to the original requestor. A name server that supports recursive service sets the Recursion Available (RA) bit in all responses it generates. A requestor asks for recursive service by setting the Recursion Desired (RD) bit in queries. In some situations where recursive service is the only path to the desired information (see below), the name server may go recursive even if RD is zero. If a query requests recursion (RD set), but the name server does not support recursion, and the query needs recursive service for an answer, the name server returns a "Not Implemented" (NI) error code. If the query can be answered without recursion since the name server is authoritative for the query, it ignores the RD bit. Because of the difficulty in selecting appropriate timeouts and error handling, recursive service is best suited to virtual circuits, although it is allowed for datagrams. Recursive service is valuable in several special situations: In a system of small personal computers clustered around one or more large hosts supporting name servers, the recursive approach minimizes the amount of code in the resolvers in the personal computers. Such a design moves complexity out of the resolver into the name server, and may be appropriate for such systems. Name servers on the boundaries of different networks may wish to offer recursive service to create connectivity between different networks. Such name servers may wish to provide recursive service regardless of the setting of RD. Name servers that translate between domain name service and some other name service may wish to adopt the recursive style. Implicit recursion may be valuable here as well.
These concepts are still under development.
Header section format +-----------------------------------------------+ | | | ***** WARNING ***** | | | | The following format is preliminary and is | | included for purposes of explanation only. In | | particular, the size and position of the | | OPCODE, RCODE fields and the number and | | meaning of the single bit fields are subject | | to change. | | | +-----------------------------------------------+ The header contains the following fields: 1 1 1 1 1 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | ID | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |QR| Opcode |AA|TC|RD|RA| | RCODE | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | QDCOUNT | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | ANCOUNT | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | NSCOUNT | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | ARCOUNT | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ where: ID - A 16 bit identifier assigned by the program that generates any kind of query. This identifier is copied into all replies and can be used by the requestor to relate replies to outstanding questions. QR - A one bit field that specifies whether this message is a query (0), or a response (1). OPCODE - A four bit field that specifies kind of query in this message. This value is set by the originator of a query and copied into the response. The values are: 0 a standard query (QUERY)
1 an inverse query (IQUERY) 2 an completion query allowing multiple answers (CQUERYM) 2 an completion query requesting a single answer (CQUERYU) 4-15 reserved for future use AA - Authoritative Answer - this bit is valid in responses, and specifies that the responding name server is an authority for the domain name in the corresponding query. TC - TrunCation - specifies that this message was truncated due to length greater than 512 characters. This bit is valid in datagram messages but not in messages sent over virtual circuits. RD - Recursion Desired - this bit may be set in a query and is copied into the response. If RD is set, it directs the name server to pursue the query recursively. Recursive query support is optional. RA - Recursion Available - this be is set or cleared in a response, and denotes whether recursive query support is available in the name server. RCODE - Response code - this 4 bit field is set as part of responses. The values have the following interpretation: 0 No error condition 1 Format error - The name server was unable to interpret the query. 2 Server failure - The name server was unable to process this query due to a problem with the name server. 3 Name Error - Meaningful only for responses from an authoritative name server, this code signifies that the domain name referenced in the query does not exist.
4 Not Implemented - The name server does not support the requested kind of query. 5 Refused - The name server refuses to perform the specified operation for policy reasons. For example, a name server may not wish to provide the information to the particular requestor, or a name server may not wish to perform a particular operation (e.g. zone transfer) for particular data. 6-15 Reserved for future use. QDCOUNT - an unsigned 16 bit integer specifying the number of entries in the question section. ANCOUNT - an unsigned 16 bit integer specifying the number of resource records in the answer section. NSCOUNT - an unsigned 16 bit integer specifying the number of name server resource records in the authority records section. ARCOUNT - an unsigned 16 bit integer specifying the number of resource records in the additional records section.
Question section format The question section is used in all kinds of queries other than inverse queries. In responses to inverse queries, this section may contain multiple entries; for all other responses it contains a single entry. Each entry has the following format: 1 1 1 1 1 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | | / QNAME / / / +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | QTYPE | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | QCLASS | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ where: QNAME - a variable number of octets that specify a domain name. This field uses the compressed domain name format described in the next section of this memo. This field can be used to derive a text string for the domain name. Note that this field may be an odd number of octets; no padding is used. QTYPE - a two octet code which specifies the type of the query. The values for this field include all codes valid for a TYPE field, together with some more general codes which can match more than one type of RR. For example, QTYPE might be A and only match type A RRs, or might be MAILA, which matches MF and MD type RRs. The values for this field are listed in Appendix 2. QCLASS - a two octet code that specifies the class of the query. For example, the QCLASS field is IN for the ARPA Internet, CS for the CSNET, etc. The numerical values are defined in Appendix 2.
Resource record format The answer, authority, and additional sections all share the same format: a variable number of resource records, where the number of records is specified in the corresponding count field in the header. Each resource record has the following format: 1 1 1 1 1 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | | / / / NAME / | | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | TYPE | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | CLASS | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | TTL | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | RDLENGTH | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--| / RDATA / / / +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ where: NAME - a compressed domain name to which this resource record pertains. TYPE - two octets containing one of the RR type codes defined in Appendix 2. This field specifies the meaning of the data in the RDATA field. CLASS - two octets which specify the class of the data in the RDATA field. TTL - a 16 bit unsigned integer that specifies the time interval (in seconds) that the resource record may be cached before it should be discarded. Zero values are interpreted to mean that the RR can only be used for the transaction in progress, and should not be cached. For example, SOA records are always distributed with a zero TTL to prohibit caching. Zero values can also be used for extremely volatile data.
RDLENGTH- an unsigned 16 bit integer that specifies the length in octets of the RDATA field. RDATA - a variable length string of octets that describes the resource. The format of this information varies according to the TYPE and CLASS of the resource record. For example, the if the TYPE is A and the CLASS is IN, the RDATA field is a 4 octet ARPA Internet address. Formats for particular resource records are shown in Appendicies 2 and 3. Domain name representation and compression Domain names messages are expressed in terms of a sequence of labels. Each label is represented as a one octet length field followed by that number of octets. Since every domain name ends with the null label of the root, a compressed domain name is terminated by a length byte of zero. The high order two bits of the length field must be zero, and the remaining six bits of the length field limit the label to 63 octets or less. To simplify implementations, the total length of label octets and label length octets that make up a domain name is restricted to 255 octets or less. Since the trailing root label and its dot are not printed, printed domain names are 254 octets or less. Although labels can contain any 8 bit values in octets that make up a label, it is strongly recommended that labels follow the syntax described in Appendix 1 of this memo, which is compatible with existing host naming conventions. Name servers and resolvers must compare labels in a case-insensitive manner, i.e. A=a, and hence all character strings must be ASCII with zero parity. Non-alphabetic codes must match exactly. Whenever possible, name servers and resolvers must preserve all 8 bits of domain names they process. When a name server is given data for the same name under two different case usages, this preservation is not always possible. For example, if a name server is given data for ISI.ARPA and isi.arpa, it should create a single node, not two, and hence will preserve a single casing of the label. Systems with case sensitivity should take special precautions to insure that the domain data for the system is created with consistent case. In order to reduce the amount of space used by repetitive domain names, the sequence of octets that defines a domain name may be terminated by a pointer to the length octet of a previously specified label string. The label string that the pointer
specifies is appended to the already specified label string. Exact duplication of a previous label string can be done with a single pointer. Multiple levels are allowed. Pointers can only be used in positions in the message where the format is not class specific. If this were not the case, a name server that was handling a RR for another class could make erroneous copies of RRs. As yet, there are no such cases, but they may occur in future RDATA formats. If a domain name is contained in a part of the message subject to a length field (such as the RDATA section of an RR), and compression is used, the length of the compressed name is used in the length calculation, rather than the length of the expanded name. Pointers are represented as a two octet field in which the high order 2 bits are ones, and the low order 14 bits specify an offset from the start of the message. The 01 and 10 values of the high order bits are reserved for future use and should not be used. Programs are free to avoid using pointers in datagrams they generate, although this will reduce datagram capacity. However all programs are required to understand arriving messages that contain pointers. For example, a datagram might need to use the domain names F.ISI.ARPA, FOO.F.ISI.ARPA, ARPA, and the root. Ignoring the other fields of the message, these domain names might be represented as:
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 20 | 1 | F | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 22 | 3 | I | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 24 | S | I | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 26 | 4 | A | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 28 | R | P | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 30 | A | 0 | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 40 | 3 | F | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 42 | O | O | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 44 | 1 1| 20 | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 64 | 1 1| 26 | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 92 | 0 | | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ The domain name for F.ISI.ARPA is shown at offset 20. The domain name FOO.F.ISI.ARPA is shown at offset 40; this definition uses a pointer to concatenate a label for FOO to the previously defined F.ISI.ARPA. The domain name ARPA is defined at offset 64 using a pointer to the ARPA component of the name F.ISI.ARPA at 20; note that this reference relies on ARPA being the last label in the string at 20. The root domain name is defined by a single octet of zeros at 92; the root domain name has no labels. Organization of the Shared database While name server implementations are free to use any internal data structures they choose, the suggested structure consists of several separate trees. Each tree has structure corresponding to the domain name space, with RRs attached to nodes and leaves. Each zone of authoritative data has a separate tree, and one tree holds all non-authoritative data. All of the trees corresponding to zones are managed identically, but the non-authoritative or cache tree has different management procedures.
Data stored in the database can be kept in whatever form is convenient for the name server, so long as it can be transformed back into the format needed for messages. In particular, the database will probably use structure in place of expanded domain names, and will also convert many of the time intervals used in the domain systems to absolute local times. Each tree corresponding to a zone has complete information for a "pruned" subtree of the domain space. The top node of a zone has a SOA record that marks the start of the zone. The bottom edge of the zone is delimited by nodes containing NS records signifying delegation of authority to other zones, or by leaves of the domain tree. When a name server contains abutting zones, one tree will have a bottom node containing a NS record, and the other tree will begin with a tree location containing a SOA record. Note that there is one special case that requires consideration when a name server is implemented. A node that contains a SOA RR denoting a start of zone will also have NS records that identify the name servers that are expected to have a copy of the zone. Thus a name server will usually find itself (and possibly other redundant name servers) referred to in NS records occupying the same position in the tree as SOA records. The solution to this problem is to never interpret a NS record as delimiting a zone started by a SOA at the same point in the tree. (The sample programs in this memo deal with this problem by processing SOA records only after NS records have been processed.) Zones may also overlap a particular part of the name space when they are of different classes. Other than the abutting and separate class cases, trees are always expected to be disjoint. Overlapping zones are regarded as a non-fatal error. The scheme described in this memo avoids the overlap issue by maintaining separate trees; other designs must take the appropriate measures to defend against possible overlap. Non-authoritative data is maintained in a separate tree. This tree is unlike the zone trees in that it may have "holes". Each RR in the cache tree has its own TTL that is separately managed. The data in this tree is never used if authoritative data is available from a zone tree; this avoids potential problems due to cached data that conflicts with authoritative data. The shared database will also contain data structures to support the processing of inverse queries and completion queries if the local system supports these optional features. Although many schemes are possible, this memo describes a scheme that is based on tables of pointers that invert the database according to key.
Each kind of retrieval has a separate set of tables, with one table per zone. When a zone is updated, these tables must also be updated. The contents of these tables are discussed in the "Inverse query processing" and "Completion query processing" sections of this memo. The database implementation described here includes two locks that are used to control concurrent access and modification of the database by name server query processing, name server maintenance operations, and resolver access: The first lock ("main lock") controls access to all of the trees. Multiple concurrent reads are allowed, but write access can only be acquired by a single process. Read and write access are mutually exclusive. Resolvers and name server processes that answer queries acquire this lock in read mode, and unlock upon completion of the current message. This lock is acquired in write mode by a name server maintenance process when it is about to change data in the shared database. The actual update procedures are described under "NAME SERVER MAINTENANCE" but are designed to be brief. The second lock ("cache queue lock") controls access to the cache queue. This queue is used by a resolver that wishes to add information to the cache tree. The resolver acquires this lock, then places the RRs to be cached into the queue. The name server maintenance procedure periodically acquires this lock and adds the queue information to the cache. The rationale for this procedure is that it allows the resolver to operate with read-only access to the shared database, and allows the update process to batch cache additions and the associated costs for inversion calculations. The name server maintenance procedure must take appropriate precautions to avoid problems with data already in the cache, inversions, etc. This organization solves several difficulties: When searching the domain space for the answer to a query, a name server can restrict its search for authoritative data to that tree that matches the most labels on the right side of the domain name of interest. Since updates to a zone must be atomic with respect to searches, maintenance operations can simply acquire the main lock, insert a new copy of a particular zone without disturbing other zones, and then release the storage used by the old copy. Assuming a central table pointing to valid zone trees, this operation can be a simple pointer swap.
TTL management of zones can be performed using the SOA record for the zone. This avoids potential difficulties if individual RRs in a zone could be timed out separately. This issue is discussed further in the maintenance section. Query processing The following algorithm outlines processing that takes place at a name server when a query arrives: 1. Search the list of zones to find zones which have the same class as the QCLASS field in the query and have a top domain name that matches the right end of the QNAME field. If there are none, go to step 2. If there are more than one, pick the zone that has the longest match and go to step 3. 2. Since the zone search failed, the only possible RRs are contained in the non-authoritative tree. Search the cache tree for the NS record that has the same class as the QCLASS field and the largest right end match for domain name. Add the NS record or records to the authority section of the response. If the cache tree has RRs that are pertinent to the question (domain names match, classes agree, not timed-out, and the type field is relevant to the QTYPE), copy these RRs into the answer section of the response. The name server may also search the cache queue. Go to step 4. 3. Since this zone is the best match, the zone in which QNAME resides is either this zone or a zone to which this zone will directly or indirectly delegate authority. Search down the tree looking for a NS RR or the node specified by QNAME. If the node exists and has no NS record, copy the relevant RRs to the answer section of the response and go to step 4. If a NS RR is found, either matching a part or all of QNAME, then QNAME is in a delegated zone outside of this zone. If so, copy the NS record or records into the authority section of the response, and search the remainder of the zone for an A type record corresponding to the NS reference. If the A record is found, add it to the additional section. Go to step 2. If the node is not found and a NS is not found, there is no such name; set the Name error bit in the response and exit. 4. When this step is reached, the answer and authority sections are complete. What remains is to complete the additional section. This procedure is only possible if the name server
knows the data formats implied by the class of records in the answer and authority sections. Hence this procedure is class dependent. Appendix 3 discusses this procedure for Internet class data. While this algorithm deals with typical queries and databases, several additions are required that will depend on the database supported by the name server: QCLASS=* Special procedures are required when the QCLASS of the query is "*". If the database contains several classes of data, the query processing steps above are performed separately for each CLASS, and the results are merged into a single response. The name error condition is not meaningful for a QCLASS=* query. If the requestor wants this information, it must test each class independently. If the database is limited to data of a particular class, this operation can be performed by simply reseting the authoritative bit in the response, and performing the query as if QCLASS was the class used in the database. * labels in database RRs Some zones will contain default RRs that use * to match in cases where the search fails for a particular domain name. If the database contains these records then a failure must be retried using * in place of one or more labels of the search key. The procedure is to replace labels from the left with "*"s looking for a match until either all labels have been replaced, or a match is found. Note that these records can never be the result of caching, so a name server can omit this processing for zones that don't contain RRs with * in labels, or can omit this processing entirely if * never appears in local authoritative data. Inverse query processing Name servers that support inverse queries can support these operations through exhaustive searches of their databases, but this becomes impractical as the size of the database increases. An alternative approach is to invert the database according to the search key. For name servers that support multiple zones and a large amount of data, the recommended approach is separate inversions for each
zone. When a particular zone is changed during a refresh, only its inversions need to be redone. Support for transfer of this type of inversion may be included in future versions of the domain system, but is not supported in this version. Completion query processing Completion query processing shares many of the same problems in data structure design as are found in inverse queries, but is different due to the expected high rate of use of top level labels (ie., ARPA, CSNET). A name server that wishes to be efficient in its use of memory may well choose to invert only occurrences of ARPA, etc. that are below the top level, and use a search for the rare case that top level labels are used to constrain a completion.
NAME SERVER MAINTENANCE Introduction Name servers perform maintenance operations on their databases to insure that the data they distribute is accurate and timely. The amount and complexity of the maintenance operations that a name server must perform are related to the size, change rate, and complexity of the database that the name server manages. Maintenance operations are fundamentally different for authoritative and non-authoritative data. A name server actively attempts to insure the accuracy and timeliness of authoritative data by refreshing the data from master copies. Non-authoritative data is merely purged when its time-to-live expires; the name server does not attempt to refresh it. Although the refreshing scheme is fairly simple to implement, it is somewhat less powerful than schemes used in other distributed database systems. In particular, an update to the master does not immediately update copies, and should be viewed as gradually percolating though the distributed database. This is adequate for the vast majority of applications. In situations where timliness is critical, the master name server can prohibit caching of copies or assign short timeouts to copies. Conceptual model of maintenance operations The vast majority of information in the domain system is derived from master files scattered among hosts that implement name servers; some name servers will have no master files, other name servers will have one or more master files. Each master file contains the master data for a single zone of authority rather than data for the whole domain name space. The administrator of a particular zone controls that zone by updating its master file. Master files and zone copies from remote servers may include RRs that are outside of the zone of authority when a NS record delegates authority to a domain name that is a descendant of the domain name at which authority is delegated. These forward references are a problem because there is no reasonable method to guarantee that the A type records for the delegatee are available unless they can somehow be attached to the NS records. For example, suppose the ARPA zone delegates authority at MIT.ARPA, and states that the name server is on AI.MIT.ARPA. If a resolver gets the NS record but not the A type record for AI.MIT.ARPA, it might try to ask the MIT name server for the address of AI.MIT.ARPA.
The solution is to allow type A records that are outside of the zone of authority to be copied with the zone. While these records won't be found in a search for the A type record itself, they can be protected by the zone refreshing system, and will be passed back whenever the name server passes back a referral to the corresponding NS record. If a query is received for the A record, the name server will pass back a referral to the name server with the A record in the additional section, rather than answer section. The only exception to the use of master files is a small amount of data stored in boot files. Boot file data is used by name servers to provide enough resource records to allow zones to be imported from foreign servers (e.g. the address of the server), and to establish the name and address of root servers. Boot file records establish the initial contents of the cache tree, and hence can be overridden by later loads of authoritative data. The data in a master file first becomes available to users of the domain name system when it is loaded by the corresponding name server. By definition, data from a master file is authoritative. Other name servers which wish to be authoritative for a particular zone do so by transferring a copy of the zone from the name server which holds the master copy using a virtual circuit. These copies include parameters which specify the conditions under which the data in the copy is authoritative. In the most common case, the conditions specify a refresh interval and policies to be followed when the refresh operation cannot be performed. A name server may acquire multiple zones from different name servers and master files, but the name server must maintain each zone separately from others and from non-authoritative data. When the refresh interval for a particular zone copy expires, the name server holding the copy must consult the name server that holds the master copy. If the data in the zone has not changed, the master name server instructs the copy name server to reset the refresh interval. If the data has changed, the master passes a new copy of the zone and its associated conditions to the copy name server. Following either of these transactions, the copy name server begins a new refresh interval. Copy name servers must also deal with error conditions under which they are unable to communicate with the name server that holds the master copy of a particular zone. The policies that a copy name server uses are determined by other parameters in the conditions distributed with every copy. The conditions include a retry interval and a maximum holding time. When a copy name server is
unable to establish communications with a master or is unable to complete the refresh transaction, it must retry the refresh operation at the rate specified by the retry interval. This retry interval will usually be substantially shorter than the refresh interval. Retries continue until the maximum holding time is reached. At that time the copy name server must assume that its copy of the data for the zone in question is no longer authoritative. Queries must be processed while maintenance operations are in progress because a zone transfer can take a long time. However, to avoid problems caused by access to partial databases, the maintenance operations create new copies of data rather than directly modifying the old copies. When the new copy is complete, the maintenance process locks out queries for a short time using the main lock, and switches pointers to replace the old data with the new. After the pointers are swapped, the maintenance process unlocks the main lock and reclaims the storage used by the old copy. Name server data structures and top level logic The name server must multiplex its attention between multiple activities. For example, a name server should be able to answer queries while it is also performing refresh activities for a particular zone. While it is possible to design a name server that devotes a separate process to each query and refresh activity in progress, the model described in this memo is based on the assumption that there is a single process performing all maintenance operations, and one or more processes devoted to handling queries. The model also assumes the existence of shared memory for several control structures, the domain database, locks, etc. The model name server uses the following files and shared data structures: 1. A configuration file that describes the master and boot files which the name server should load and the zones that the name server should attempt to load from foreign name servers. This file establishes the initial contents of the status table. 2. Domain data files that contain master and boot data to be loaded. 3. A status table that is derived from the configuration file. Each entry in this table describes a source of data. Each entry has a zone number. The zone number is zero for
non-authoritative sources; authoritative sources are assigned separate non-zero numbers. 4. The shared database that holds the domain data. This database is assumed to be organized in some sort of tree structure paralleling the domain name space, with a list of resource records attached to each node and leaf in the tree. The elements of the resource record list need not contain the exact data present in the corresponding output format, but must contain data sufficient to create the output format; for example, these records need not contain the domain name that is associated with the resource because that name can be derived from the tree structure. Each resource record also internal data that the name server uses to organize its data. 5. Inversion data structures that allow the name server to process inverse queries and completion queries. Although many structures could be used, the implementation described in this memo supposes that there is one array for every inversion that the name server can handle. Each array contains a list of pointers to resource records such that the order of the inverted quantities is sorted. 6. The main and cache queue locks 7. The cache queue The maintenance process begins by loading the status table from the configuration file. It then periodically checks each entry, to see if its refresh interval has elapsed. If not, it goes on to the next entry. If so, it performs different operations depending on the entry: If the entry is for zone 0, or the cache tree, the maintenance process checks to see if additions or deletions are required. Additions are acquired from the cache queue using the cache queue lock. Deletions are detected using TTL checks. If any changes are required, the maintenance process recalculates inversion data structures and then alters the cache tree under the protection of the main lock. Whenever the maintenance process modifies the cache tree, it resets the refresh interval to the minimum of the contained TTLs and the desired time interval for cache additions. If the entry is not zone 0, and the entry refers to a local file, the maintenance process checks to see if the file has been modified since its last load. If so the file is reloaded using the procedures specified under "Name server file
loading". The refresh interval is reset to that specified in the SOA record if the file is a master file. If the entry is for a remote master file, the maintenance process checks for a new version using the procedure described in "Names server remote zone transfer". Name server file loading Master files are kept in text form for ease of editing by system maintainers. These files are not exchanged by name servers; name servers use the standard message format when transferring zones. Organizations that want to have a domain, but do not want to run a name server, can use these files to supply a domain definition to another organization that will run a name server for them. For example, if organization X wants a domain but not a name server, it can find another organization, Y, that has a name server and is willing to provide service for X. Organization X defines domain X via the master file format and ships a copy of the master file to organization Y via mail, FTP, or some other method. A system administrator at Y configures Y's name server to read in X's file and hence support the X domain. X can maintain the master file using a text editor and send new versions to Y for installation. These files have a simple line-oriented format, with one RR per line. Fields are separated by any combination of blanks and tab characters. Tabs are treated the same as spaces; in the following discussion the term "blank" means either a tab or a blank. A line can be either blank (and ignored), a RR, or a $INCLUDE line. If a RR line starts with a domain name, that domain name is used to specify the location in the domain space for the record, i.e. the owner. If a RR line starts with a blank, it is loaded into the location specified by the most recent location specifier. The location specifiers are assumed to be relative to some origin that is provided by the user of a file unless the location specifier contains the root label. This provides a convenient shorthand notation, and can also be used to prevent errors in master files from propagating into other zones. This feature is particularly useful for master files imported from other sites. An include line begins with $INCLUDE, starting at the first line position, and is followed by a local file name and an optional offset modifier. The filename follows the appropriate local conventions. The offset is one or more labels that are added to the offset in use for the file that contained the $INCLUDE. If the offset is omitted, the included file is loaded using the
offset of the file that contained the $INCLUDE command. For example, a file being loaded at offset ARPA might contain the following lines: $INCLUDE <subsys>isi.data ISI $INCLUDE <subsys>addresses.data The first line would be interpreted to direct loading of the file <subsys>isi.data at offset ISI.ARPA. The second line would be interpreted as a request to load data at offset ARPA. Note that $INCLUDE commands do not cause data to be loaded into a different zone or tree; they are simply ways to allow data for a given zone to be organized in separate files. For example, mailbox data might be kept separately from host data using this mechanism. Resource records are entered as a sequence of fields corresponding to the owner name, TTL, CLASS, TYPE and RDATA components. (Note that this order is different from the order used in examples and the order used in the actual RRs; the given order allows easier parsing and defaulting.) The owner name is derived from the location specifier. The TTL field is optional, and is expressed as a decimal number. If omitted TTL defaults to zero. The CLASS field is also optional; if omitted the CLASS defaults to the most recent value of the CLASS field in a previous RR. The RDATA fields depend on the CLASS and TYPE of the RR. In general, the fields that make up RDATA are expressed as decimal numbers or as domain names. Some exceptions exist, and are documented in the RDATA definitions in Appendicies 2 and 3 of this memo. Because CLASS and TYPE fields don't contain any common identifiers, and because CLASS and TYPE fields are never decimal numbers, the parse is always unique. Because these files are text files several special encodings are necessary to allow arbitrary data to be loaded. In particular: . A free standing dot is used to refer to the current domain name. @ A free standing @ is used to denote the current origin.
.. Two free standing dots represent the null domain name of the root. \X where X is any character other than a digit (0-9), is used to quote that character so that its special meaning does not apply. For example, "\." can be used to place a dot character in a label. \DDD where each D is a digit is the octet corresponding to the decimal number described by DDD. The resulting octet is assumed to be text and is not checked for special meaning. ( ) Parentheses are used to group data that crosses a line boundary. In effect, line terminations are not recognized within parentheses. ; Semicolon is used to start a comment; the remainder of the line is ignored. Name server file loading example A name server for F.ISI.ARPA , serving as an authority for the ARPA and ISI.ARPA domains, might use a boot file and two master files. The boot file initializes some non-authoritative data, and would be loaded without an origin: .. 9999999 IN NS B.ISI.ARPA 9999999 CS NS UDEL.CSNET B.ISI.ARPA 9999999 IN A 10.3.0.52 UDEL.CSNET 9999999 CS A 302-555-0000 This file loads non-authoritative data which provides the identities and addresses of root name servers. The first line contains a NS RR which is loaded at the root; the second line starts with a blank, and is loaded at the most recent location specifier, in this case the root; the third and fourth lines load RRs at B.ISI.ARPA and UDEL.CSNET, respectively. The timeouts are set to high values (9999999) to prevent this data from being discarded due to timeout. The first master file loads authoritative data for the ARPA domain. This file is designed to be loaded with an origin of ARPA, which allows the location specifiers to omit the trailing .ARPA labels.
@ IN SOA F.ISI.ARPA Action.E.ISI.ARPA ( 20 ; SERIAL 3600 ; REFRESH 600 ; RETRY 3600000; EXPIRE 60) ; MINIMUM NS F.ISI.ARPA ; F.ISI.ARPA is a name server for ARPA NS A.ISI.ARPA ; A.ISI.ARPA is a name server for ARPA MIT NS AI.MIT.ARPA; delegation to MIT name server ISI NS F.ISI.ARPA ; delegation to ISI name server UDEL MD UDEL.ARPA A 10.0.0.96 NBS MD NBS.ARPA A 10.0.0.19 DTI MD DTI.ARPA A 10.0.0.12 AI.MIT A 10.2.0.6 F.ISI A 10.2.0.52 The first group of lines contains the SOA record and its parameters, and identifies name servers for this zone and for delegated zones. The Action.E.ISI.ARPA field is a mailbox specification for the responsible person for the zone, and is the domain name encoding of the mail destination Action@E.ISI.ARPA. The second group specifies data for domain names within this zone. The last group has forward references for name server address resolution for AI.MIT.ARPA and F.ISI.ARPA. This data is not technically within the zone, and will only be used for additional record resolution for NS records used in referrals. However, this data is protected by the zone timeouts in the SOA, so it will persist as long as the NS references persist. The second master file defines the ISI.ARPA environment, and is loaded with an origin of ISI.ARPA: @ IN SOA F.ISI.ARPA Action\.ISI.E.ISI.ARPA ( 20 ; SERIAL 7200 ; REFRESH 600 ; RETRY 3600000; EXPIRE 60) ; MINIMUM NS F.ISI.ARPA ; F.ISI.ARPA is a name server A A 10.1.0.32 MD A.ISI.ARPA MF F.ISI.ARPA B A 10.3.0.52 MD B.ISI.ARPA
MF F.ISI.ARPA F A 10.2.0.52 MD F.ISI.ARPA MF A.ISI.ARPA $INCLUDE <SUBSYS>ISI-MAILBOXES.TXT Where the file <SUBSYS>ISI-MAILBOXES.TXT is: MOE MB F.ISI.ARPA LARRY MB A.ISI.ARPA CURLEY MB B.ISI.ARPA STOOGES MB B.ISI.ARPA MG MOE.ISI.ARPA MG LARRY.ISI.ARPA MG CURLEY.ISI.ARPA Note the use of the \ character in the SOA RR to specify the responsible person mailbox "Action.ISI@E.ISI.ARPA". Name server remote zone transfer When a name server needs to make an initial copy of a zone or test to see if a existing zone copy should be refreshed, it begins by attempting to open a virtual circuit to the foreign name server. If this open attempt fails, and this was an initial load attempt, it schedules a retry and exits. If this was a refresh operation, the name server tests the status table to see if the maximum holding time derived from the SOA EXPIRE field has elapsed. If not, the name server schedules a retry. If the maximum holding time has expired, the name server invalidates the zone in the status table, and scans all resource records tagged with this zone number. For each record it decrements TTL fields by the length of time since the data was last refreshed. If the new TTL value is negative, the record is deleted. If the TTL value is still positive, it moves the RR to the cache tree and schedules a retry. If the open attempt succeeds, the name server sends a query to the foreign name server in which QTYPE=SOA, QCLASS is set according to the status table information from the configuration file, and QNAME is set to the domain name of the zone of interest. The foreign name server will return either a SOA record indicating that it has the zone or an error. If an error is detected, the virtual circuit is closed, and the failure is treated in the same way as if the open attempt failed. If the SOA record is returned and this was a refresh, rather than an initial load of the zone, the name server compares the SERIAL