pdns/docs/powerdns-technical.sgml

   1 <!doctype linuxdoc system>
   2
   3 <article>
   4
   5 <!-- Title information -->
   6 <title>PowerDNS technical overview</title>
   7 <author>PowerDNS BV (bert hubert &lt;bert@trilab.com&gt;) &nl;
   8 Trilab BV</author>
   9 <date>v1.1 $Date: 2002/11/27 15:18:35 $</date>
  10 <abstract>
  11 This document contains a technical description of PowerDNS.
  12 </abstract>
  13 <toc>
  14 <sect>PowerDNS is a next generation authoritative nameserver
  15 <p>
  16 DNS is among the most mission-critical parts of the internet. While in
  17 essence very simple, current implementations are complicated applications
  18 with source code often spanning dozens of megabytes.
  19
  20 The growth of the number of domains means that there is a growing need for a
  21 lean and mean nameserver that is capable of serving millions of users with
  22 millions of domains.
  23
  24 The operation of PowerDNS consists of three different parts:
  25 <itemize>
  26 <item>Internet Interface
  27 <item>Logical Engine
  28 <item>Query Backend
  29 </itemize>
  30
  31 The Internet Interface receives a question, and hands it to the Logical
  32 Engine. This Logical Engine then splits up the question into the
  33 sub-queries, which are handed to the Query Backend, which in turn sends
  34 queries to any number of data sources. The answer is then transferred back by
  35 the Logical Engine to the Internet Interface, which sends out the packet
  36 containing the requested data.
  37
  38 <descrip>
  39 <tag>The Internet Interface</tag>
  40 PowerDNS supports receiving queries over UDP and TCP. When a question is
  41 received, relevant parts of the packet containing the question are compared
  42 to queries received earlier.
  43
  44 <tag>The Logical Engine</tag>
  45 A DNS query cannot be translated directly into a backend query. A question
  46 might be 'What is the IP Address of www.site.com'. In order to answer this
  47 question, some separate steps need to be performed:
  48
  49 <itemize>
  50 <item>Is this nameserver Authoritative for this domain, or any of its parent
  51 domains?
  52 <item>Do we have a Canonical Name for www.site.com?
  53 <item>Does www.site.com exist?
  54 <item>Do we have an IP address for it?
  55 <item>If we don't, do we know who does?
  56 <item>Possibly send IP addresses for the nameservers who do know
  57 </itemize>
  58
  59 This algorithm is described at length in RFC 1034.
  60
  61 <tag>The Query Backend</tag>
  62 A real life nameserver may have many data sources. Several customer
  63 databases might exist, as well as standard Zone files. The Query Backend
  64 fields questions to any number of backends, in a prescribed order. This
  65 allows for maximum flexibility.
  66 </descrip>
  67
  68 <sect>Simplicity brings reliability
  69 <p>
  70
  71 By building on top of existing databases, PowerDNS is as trustworthy as your
  72 favorite database. Data storage and retrieval is a well solved problem. A
  73 nameserver should not reinvent it. We support almost all industry standard
  74 databases and also do custom backend development to graft PowerDNS on an
  75 existing database or schema.
  76
  77 Due to the completely from scratch implementation without an existing
  78 installed base to appease, PowerDNS has remained very lean and mean.
  79
  80 Monitoring is at the root of reliability, so the PowerDNS runtime can be
  81 queried by external scripts. This enables the operator to be informed of any
  82 problems at an early stage. Some sample scripts for the popular MRTG program
  83 are supplied.
  84
  85 <sect>Incredible performance
  86 <p>
  87 Because of the many steps of the algorithm prescribed by RFC 1034, just
  88 hooking on a database to a nameserver is not a recipe for great performance.
  89 Steps need to be taken to streamline the process.
  90
  91 PowerDNS does so in two ways:
  92 <descrip>
  93 <tag>Reordering the steps of the algorithm</tag>
  94 It is often possible to skip some of the steps in the algorithm initially,
  95 and only perform the other steps when it is really needed, which is often
  96 not the case.
  97 <tag>The PacketCache</tag>
  98 The PacketCache is quite revolutionary in that it caches entire query
  99 packets for short amounts of time. This PacketCache is consulted before
 100 running the RFC 1034 Algorithm in the Logical Engine. In production, it has
 101 been confirmed that even a 1 minute cache can achieve a 80% hitrate and
 102 thereby prevent 4 out of 5 database queries from ever happening.
 103 </descrip>
 104
 105 Benchmarking has shown that PowerDNS should be able to reach in the order of
 106 20.000 queries/second on a reasonably fast database. When using direct
 107 tables like those supported by Berkeley DB, 50.000 should be achievable.
 108
 109 The use of POSIX Threads also allows PowerDNS to use a large number of
 110 processors efficiently on architectures that support it.
 111 <sect>Complete security
 112 <p>
 113 PowerDNS is written in highly portable C++ using the ISO Standard C++
 114 Library (STL). This Library comes with dynamic string classes which all but
 115 erase the possibility of the much feared buffer overflows that have been
 116 hitting other nameservers.
 117
 118 The very modular design of PowerDNS also makes for strict internal
 119 interfaces which can prevent any undesired action from having deleterious
 120 effects.
 121
 122 Due to the use of modern tools and libraries, PowerDNS consists of only 7000
 123 lines of source. This is very well auditable in a reasonable period of time
 124 and can be regarded as a trusted computing base.
 125
 126 Because the database is most often external, it is highly useful to grant
 127 PowerDNS read-only access to that database. Even a successful compromise can
 128 than not easily be exploited, because the database refuses to accept updates
 129 from the nameserver.
 130
 131 <sect>Special Functions
 132 <p>
 133 Besides doing lookups in well known databases such as Oracle, Microsoft SQL
 134 Server, MySQL, PostgreSQL and Sybase, there are special purpose backends
 135 available.
 136
 137 <sect1>Very Large Zone support
 138 <p>
 139 For customers with very large zones and a lots of secondaries, a special
 140 module has been developed to meet the following goals:
 141 <itemize>
 142 <item>Absolute 100% robustness
 143 <item>Incremental updates
 144 <item>Near realtime updates
 145 <item>Idempotent update packets
 146 <item>Instantaneous zone reloading
 147 </itemize>
 148
 149 In short, this means that updates are broadcast from a central point. These
 150 updates can be broadcast as many times as desired because there is no harm
 151 in applying them more than once. Each of these updates is applied within
 152 seconds.
 153
 154 It does not use a relational database but instead relies on any of the
 155 well known table engines that are available, with a strong slant towards
 156 Berkeley DB.
 157 <sect1>Global Redirection
 158 <p>
 159 With the aid of a comprehensive map of IP addresses, it is possible to do
 160 smart routing of customers to servers geographically near them. While not
 161 providing pin-point accuracy, it is broadly effective and very fast.
 162
 163 <sect1>DNS based loadbalancing and failover
 164 <p>
 165 Because of the efficiency of PowerDNS, it is feasible to use very low TTLs
 166 on answers. This in turn makes it possible to perform DNS based
 167 loadbalancing and failover. This can be very robust because the DNS
 168 infrastructure itself is redundant, whereas regular loadbalancing agents are
 169 themselves a single point of failure.
 170
 171 <sect1>CORBA backend
 172 <p>
 173 By allowing questions to be asked over this industry standard protocol,
 174 it becomes trivially easy to integrate PowerDNS with existing middleware
 175 applications and/or customer databases.
 176
 177 <sect>Standards compliance
 178 <p>
 179 PowerDNS is committed to being fully standards compliant. Current supported
 180 standards include RFC 1034, RFC 1035 and RFC 2181.
 181
 182 <sect>Availability
 183 <p>
 184 PowerDNS is available immediately for use. It comes with completely
 185 documented source and a license that allows the end-user to improve and or
 186 change the source.
 187
 188 Licensing is possible on a per-CPU or on a per-Domain basis.
 189
 190 PowerDNS is a fully supported product with different levels of support
 191 available.
 192 </article>
 193