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+# YARN DNS Server
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+## Introduction
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+
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+The YARN DNS Server provides a standard DNS interface to the information posted into the YARN Registry by deployed applications. The DNS service serves the following functions:
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+
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+1. **Exposing existing service discovery information via DNS** - Information provided in
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+the current YARN service registry’s records will be converted into DNS entries, thus
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+allowing users to discover information about YARN applications using standard DNS
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+client mechanisms (for e.g. a DNS SRV Record specifying the hostname and port
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+number for services).
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+2. **Enabling Container to IP mappings** - Enables discovery of the IPs of containers via
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+standard DNS lookups. Given the availability of the records via DNS, container
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+namebased communication will be facilitated (e.g. ‘curl
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+http://myContainer.myDomain.com/endpoint’).
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+
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+## Service Properties
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+
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+The existing YARN Service Registry is leveraged as the source of information for the DNS Service.
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+
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+The following core functions are supported by the DNS Server:
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+
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+###Functional properties
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+
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+1. Supports creation of DNS records for endpoints of the deployed YARN applications
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+2. Record names remain unchanged during restart of containers and/or applications
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+3. Supports reverse lookups (name based on IP).
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+4. Supports security using the standards defined by The Domain Name System Security
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+Extensions (DNSSEC)
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+5. Highly available
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+6. Scalable - The service provides the responsiveness (e.g. low latency) required to
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+respond to DNS queries (timeouts yield attempts to invoke other configured name
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+servers).
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+
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+###Deployment properties
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+
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+1. Supports integration with existing DNS assets (e.g. a corporate DNS server) by acting as
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+a DNS server for a Hadoop cluster zone/domain. The server is not intended to act as a
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+primary DNS server and does not forward requests to other servers.
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+2. The DNS Server exposes a port that can receive both TCP and UDP requests per
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+DNS standards. The default port for DNS protocols is in a restricted, administrative port
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+range (53), so the port is configurable for deployments in which the service may
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+not be managed via an administrative account.
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+
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+##DNS Record Name Structure
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+
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+The DNS names of generated records are composed from the following elements (labels). Note that these elements must be compatible with DNS conventions (see “Preferred Name Syntax” in RFC 1035):
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+
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+* **domain** - the name of the cluster DNS domain. This name is provided as a
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+configuration property. In addition, it is this name that is configured at a parent DNS
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+server as the zone name for the defined yDNS zone (the zone for which the parent DNS
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+server will forward requests to yDNS). E.g. yarncluster.com
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+* **username** - the name of the application deployer. This name is the simple shortname (for
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+e.g. the primary component of the Kerberos principal) associated with the user launching
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+the application. As the username is one of the elements of DNS names, it is expected
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+that this also confirms DNS name conventions (RFC 1035 linked above) so special translation is performed for names with special characters like hyphens and spaces.
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+* **application name** - the name of the deployed YARN application. This name is inferred
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+from the YARN registry path to the application's node. Application name, rather thn application id, was chosen as a way of making it easy for users to refer to human-readable DNS
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+names. This obviously mandates certain uniqueness properties on application names.
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+* **container id** - the YARN assigned ID to a container (e.g.
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+container_e3741_1454001598828_01_000004)
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+* **component name** - the name assigned to the deployed component (for e.g. a master
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+component). A component is a distributed element of an application or service that is
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+launched in a YARN container (e.g. an HBase master). One can imagine multiple
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+components within an application. A component name is not yet a first class concept in
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+YARN, but is a very useful one that we are introducing here for the sake of yDNS
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+entries. Many frameworks like MapReduce, Slider already have component names
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+(though, as mentioned, they are not yet supported in YARN in a first class fashion).
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+* **api** - the api designation for the exposed endpoint
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+
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+###Notes about DNS Names
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+
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+* In most instances, the DNS names can be easily distinguished by the number of
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+elements/labels that compose the name. The cluster’s domain name is always the last
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+element. After that element is parsed out, reading from right to left, the first element
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+maps to the application user and so on. Wherever it is not easily distinguishable, naming conventions are used to disambiguate the name using a prefix such as
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+“container” or suffix such as “api”. For example, an endpoint published as a
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+management endpoint will be referenced with the name *management-api.griduser.yarncluster.com*.
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+* Unique application name (per user) is not currently supported/guaranteed by YARN, but
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+it is supported by frameworks such as Apache Slider. The yDNS service currently
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+leverages the last element of the ZK path entry for the application as an
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+application name. These application names have to be unique for a given user.
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+
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+##DNS Server Functionality
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+
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+The primary functions of the DNS service are illustrated in the following diagram:
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+
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+
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+
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+
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+###DNS record creation
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+The following figure illustrates at slightly greater detail the DNS record creation and registration sequence (NOTE: service record updates would follow a similar sequence of steps,
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+distinguished only by the different event type):
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+
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+
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+
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+###DNS record removal
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+Similarly, record removal follows a similar sequence
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+
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+
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+
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+(NOTE: The DNS Zone requires a record as an argument for the deletion method, thus
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+requiring similar parsing logic to identify the specific records that should be removed).
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+
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+###DNS Service initialization
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+* The DNS service initializes both UDP and TCP listeners on a configured port. As
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+noted above, the default port of 53 is in a restricted range that is only accessible to an
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+account with administrative privileges.
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+* Subsequently, the DNS service listens for inbound DNS requests. Those requests are
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+standard DNS requests from users or other DNS servers (for example, DNS servers that have the
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+YARN DNS service configured as a forwarder).
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+
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+## Configuration
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+The YARN DNS server reads its configuration properties from the yarnsite.xml file. The following are the DNS associated configuration properties:
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+
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+| Name | Description |
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+| ------------ | ------------- |
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+| hadoop.registry.dns.enabled | The DNS functionality is enabled for the cluster. Default is false. |
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+| hadoop.registry.dns.domain-name | The domain name for Hadoop cluster associated records. |
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+| hadoop.registry.dns.bind-address | Address associated with the network interface to which the DNS listener should bind. |
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+| hadoop.registry.dns.bind-port | The port number for the DNS listener. The default port is 53. However, since that port falls in a administratoronly range, typical deployments may need to specify an alternate port. |
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+| hadoop.registry.dns.dnssec.enabled | Indicates whether the DNSSEC support is enabled. Default is false. |
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+| hadoop.registry.dns.public-key | The base64 representation of the server’s public key. Leveraged for creating the DNSKEY Record provided for DNSSEC client requests. |
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+| hadoop.registry.dns.private-key-file | The path to the standard DNSSEC private key file. Must only be readable by the DNS launching identity. See [dnssec-keygen](https://ftp.isc.org/isc/bind/cur/9.9/doc/arm/man.dnssec-keygen.html) documentation. |
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+| hadoop.registry.dns-ttl | The default TTL value to associate with DNS records. The default value is set to 1 (a value of 0 has undefined behavior). A typical value should be approximate to the time it takes YARN to restart a failed container. |
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+| hadoop.registry.dns.zone-subnet | An indicator of the IP range associated with the cluster containers. The setting is utilized for the generation of the reverse zone name. |
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+| hadoop.registry.dns.zone-mask | The network mask associated with the zone IP range. If specified, it is utilized to ascertain the IP range possible and come up with an appropriate reverse zone name. |
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+| hadoop.registry.dns.zones-dir | A directory containing zone configuration files to read during zone initialization. This directory can contain zone master files named *zone-name.zone*. See [here](http://www.zytrax.com/books/dns/ch6/mydomain.html) for zone master file documentation.|
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Gour Saha
