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V3/DNS SOA serial auto-incrementation

Overview

Short overview of the problem set and any back ground material or references one would need to understand the details.

Each DNS zone has serial number, i.e. version number of the zone. Nice explanation can be found e.g. in Zytrax book about DNS, chapter 8.

Quotations from Zytrax DNS book:

sn = serial number

This value MUST increment when any resource record in the zone file is updated.

A slave (Secondary) DNS server will read the master DNS SOA record periodically, either on expiry of refresh (defined below) or when it receives a NOTIFY and compares arithmetically its current value of sn with that received from the master DNS. If the sn value from the master is arithmetically HIGHER than that currently stored by the slave then a zone transfer (AXFR/IXFR) is initiated. If the value of sn from the master DNS SOA is the same or LOWER then no zone transfer is initiated.

Note: the arithmetic used by the serial number is defined in RFC 1982.


Structure of DNS sub-tree in LDAP

Please get familiar with the way how DNS data are stored in LDAP.


Use Cases

Correct serial number is a requirement for functional zone tranfers. Corrent zone transfers from IPA to non-IPA DNS server are impossible if zone serial number is not incremented after each change.


Design in IPA 3.0

Design was discussed on freeipa-devel mailing list: At the beginning (in e-mail thread from April 2012) various options were discussed. Maintaining single global SOA serial for all servers was found as hard problem. Attributes like modifyTimestamp can't be used because they can go back in time (because of replication).

In May 2012 different approach was proposed: Maintain SOA serial separately for each IPA server and do not synchronize serials between IPA servers at all.

This design is significantly simpler and was implemented in IPA 3.0.

Advantages

  • It is relatively simple to implement.
  • No DS plugin is required, all the logic is part of bind-dyndb-ldap plugin for BIND.

Disadvantages

Slave DNS servers cannot fall-back to other masters, because of SOA serial inconsistency.

Very basic implementation

  1. Do not replicate idnsSOASerial attribute between IPA servers: Use nsDS5ReplicatedAttributeList attribute to exclude idnsSOASerial from replication.
  2. Use persistent search to watch for incoming changes.
  3. After each change increment "local" SOA serial number (and write it to LDAP - to survive DNS server restart).
  4. Always increment stored SOA serial number after DNS server start. Data in LDAP could be changed when bind-dyndb-ldap didn't listen on persistent search.

Implementation was done in the scope of IPA ticket #2554 and bind-dyndb-ldap ticket #67.

Problems found after implementation to IPA 3.0

Problem described in following text appears only in environments with at least one IPA replica.

IPA command stores newly added zone to one chosen LDAP server, including some initial value of idnsSOAserial attribute. The new zone is replicated to other LDAP servers, except idnsSOAserial attribute. This behaviour creates an inconsistent state, where mandatory attribute idnsSOAserial is not present in some LDAP databases.

IPA tools and bind-dyndb-ldap are not ready for state where mandatory idnsSOAserial attribute is missing.

According to conversation on 389-users-list 389 DS is not able to replicate data during ADD operations and don't replicate data during MOD operations. Rich Megginson proposed another solution:

So idnsSOAserial is a "local only" attribute that lives in a "global" entry.

I agree that this is an attribute that should be managed internally by the directory server, like the entryusn attribute.

Feature Managment

Major configuration options and enablement

SOA serial is managed internally by bind-dyndb-ldap plugin. New boolean option serial_autoincrement was added to /etc/named.conf. Value yes enables this feature.

There is no command for enabling/disabling this feature.

WebUI

WebUI doesn't manage this option.

CLI

New option for ipa-server-install:

   --no-serial-autoincrement
                       Do not enable SOA serial autoincrement

This feature is enabled by default. Persistent search is required for SOA serial auto-increment feature, so --no-persistent-search can't be used without --no-serial-autoincrement.

ipa dnszone commands are unchanged:

  • It is possible to show SOA serial value. Shown value is read from single LDAP server. This value can be different on other LDAP servers.
  • Any modification via ipa dnszone-mod affects only single LDAP server (i.e. server which was selected by IPA CLI for particular operation).

Replication

Zone attribute idnsSOASerial is not replicated between IPA servers. idnsSOASerial is added to the nsDS5ReplicatedAttributeList attribute inside each replication agreement.

Each write to idnsSOAserial can potentially trigger same problem as described in IPA ticket #2534.

Updates and Upgrades

  • Option serial_autoincrement yes has to be added to /etc/named.conf.
  • Persistent search is required for SOA serial auto-increment feature, so psearch option has to be switched to yes.

Dependencies

  • bind-dyndb-ldap version >= 2.0 is required.

External Impact

(Hopefully) none.


Design in IPA 3.1

Move SOA serial maintenance from bind-dyndb-ldap to (new) 389 DS plugin: IPA ticket #3347.

DS plugin watches cn=dns sub-tree for changes.

Any change in DNS record in this subtree will increment idnsSOAserial attribute in record's parent zone.

Basic idea

if objectClass is idnsZone
    increment idnsSOAserial in the same object
else if objectClass is idnsRecord
    increment idnsSOAserial in object's immediate parent
    e.g. change in idnsName=test, idnsName=example.com, cn=dns will increment idnsSOAserial in object idnsName=example.com, cn=dns
    if parent's objectClass is not idnsZone
         log an error (This should never happen :-))
 else
    do nothing


SOA serial incrementation algorithm

OLDSerial = actual idnsSOAserial value
timestamp = actual UNIX timestamp
if (OLDSerial < timestamp)
    newSerial = timestamp
else
   newSerial = OLDSerial + 1
Write newSerial value to particular idnsSOAserial attribute


Interaction with BIND serial update mechanism

BIND does direct SOA serial update (not trigerred by serial autoincrement feature) after any dynamic update. We have to catch those attempts and ignore them:

  • A plugin can intercept any modify and manipulate it, including suppressing changes to SOA Serial.
  • It should be possible to catch & discard SOA serial modifications inside BIND. This will save some load from LDAP server.


Possible optimization

Increment serial value at most once per second.

Problem: How to solve LDAP server crash?

Problematic scenario:

(numbers represent time in "second.millisecond" format)

1.000 : new_serial = time() + 1
1.100 : record test.example.com. updated
1.100 : zone serial overwritten with new_serial
1.500 : zone transfer started
1.500 : search result for all records and zone serial stored
1.500 : search result is transferred to slaves
1.700 : record test2.example.com. updated
1.700 : zone serial overwritten with new_serial
<no changes from now>

Result: Zones on master and it's slave servers have serial = "new_serial" but the zone content is different (records under test2.example.com. are not equal).


Variant with delayed serial write

When updating zone serial:
if (old serial value < time())
   cancel scheduled serial write (if exists)
   write zone serial = time()
else
   schedule serial bump after 1 second
   (do nothing if bump is scheduled already)
When starting Directory Server:
Bump each serial by one.


Variant with modified search operation

Modify search operation for zone serial to return:

if (serial value == time())
   return (serial value - 1)
else
   return (serial value)
Scenario:
1.000 : new_serial = time()
1.100 : record test.example.com. updated
1.100 : zone serial overwritten with new_serial
1.500 : zone transfer started
1.500 : search result for all records and zone serial stored
1.500 : zone serial in search result is (new_serial - 1)
1.500 : snapshot is transferred to slaves
1.700 : record test2.example.com. updated
1.700 : zone serial overwritten with new_serial
<no changes from now>
2.000 : now the search for serial value returns new_serial, i.e. slaves see value incremented by one from last zone transfer (1.500)
9.000 : serial value is unchanged from last search

Expected result: Zone data can be inconsistent between master and slaves for only one second. Data will be consistent if directory server crashed at 1.701 - new zone transfer can be initiated after server restart.

Requirement: DS plugin have to modify serial value during reads.

Problem: It is hard to intercept and modify search operation.


Implementation

Any additional requirements or changes discovered during the implementation phase.

Feature Management

  • Add new option like serial_remote to /etc/named.conf. This option should be mutually exclusive with serial_autoincrement option from IPA 3.0.
  • Do not create UI for enabling/disabling this feature. We can provide some boolean directly in plugin configuration, but nothing else.

Replication

No change from IPA 3.0.

Updates and Upgrades

Replace serial_autoincrement option in /etc/named.conf with serial_remote option.

Dependencies

New version of bind-dyndb-ldap + the new 389 DS plugin.

External Impact

Hopefully none.

Impact on testing

Zone serial should be incremented after each change. Delay between record change and serial change should be at most 1 second.

In IPA 3.0 there was some delay, but prediction for 3.0 is harder.