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V3/Recover DNA Ranges


Overview

IPA uses the 389-ds Distributed Numeric Assignment (DNA) plugin to automatically manage POSIX uid/gid assignment. This plugin manages the range of available IDs cross all masters. https://access.redhat.com/knowledge/docs/en-US/Red_Hat_Directory_Server/8.1/html/Configuration_and_Command_Reference/dna-attributes.html

When a master is deleted then its range is not recovered, potentially leaving a huge hole in the number of available entries. For example, the first master gets half the range. The second gets the other half. If either of these is deleted, or replaced, then half the available range is gone as well.

A master does not receive a range until the DNA plugin is first used, so it is perfectly possible for a master to have no defined range.

Use Cases

A master has run out of IDs

The simplest case is that a master has exhaused its range and its request for more was not fulfilled.

The admin is going to need to evaluate the current range usage and re-adjust as necessary. This may involve manually splitting an existing range or extending past the initial 200k entries we configure.

3-way IPA MMR, delete one, lose half the range

Install initial server with --idstart=100000 --idmax=102000 giving a total range of 2k users (and groups). Install two additional masters from this master (so A->B and A->C) and create a user on B and C.

Start with two masters:

master A:

  • dnaNextValue: 100000
  • dnaMaxValue: 101000

master B:

  • dnaNextValue: 101002
  • dnaMaxValue: 102000

Note that master A original had the full range. After master B is added the max went to 101000).

master C:

  • dnaNextValue: 100502
  • dnaMaxValue: 101000

master A dnaMaxValue goes to 100500.

If master B is deleted we lose half the range which will quickly lead to the previous use case.

Dead master, lose the range

A master has died for some reason and it will be deleted. We perhaps can't know the range, or can't connect to the server to detect the range. In this case the range will be lost and a tool will be required to manually recover the range.

Design

This solution will consist of two parts:

  1. Attempt to recover any lost ranges into the "on-deck" range of another master
  2. Provide a tool to manage the current and on-base ranges of masters

The on-deck range is used internally by DNA when a range transfer takes place. If a replica hits the configured low-water mark (the "dnaThreshold" attribute), it will request a range transfer from another replica. It might still have some active range values left to use, so it puts the transferred range on-deck and swaps it to the active range once the active range is fully exhausted. When you are trying to choose a server to reassign the deleted replica range to, you should make sure it doesn't already have another range on-deck.

The on-deck range is defined by the "dnaNextRange" attribute, which takes a value in the form "<start>-<end>" ("500-1000" would be an example).

The tool will be an extension to ipa-replica-manage to list and modify the on-deck range. I don't believe we want to allow modifying the active range itself.

Additional ACI Permissions

We will need to write a new ACI to allow admins to write a subset of DNA attributes, including dnaNextRange, dnaNextValue and dnaMaxValue. This will look like:

dn: cn=Posix IDs,cn=Distributed Numeric Assignment Plugin,cn=plugins,cn=config
changetype: modify
add: aci
aci: (targetattr=dnaNextRange || dnaNextValue || dnaMaxValue)(version 3.0;acl "permission:Modify DNA Range";allow (write) groupdn = "ldap:///cn=Modify DNA Range,cn=permissions,cn=pbac,$SUFFIX";)

A new permission will need to be added and included in the privilege 'Replication Administrators'.

Setting the on-deck range

In the del operation of ipa-replica-manage we will query the values of dnaNextValue and dnaMaxValue from the server to be removed. We will then walk the rest of the masters. The first one with no dnaNextRange value will have it added in the form of dnaNextValue-dnaMaxValue.

If no servers have an empty dnaNextRange we will WARN about the range of values that will be lost and inform the user of the command to manage it.

We will NOT try to modify the current DNA range configuration.

Determining if a replica has a range

Identifying the on-deck range is easy, use dnaNextRange.

To see if a range has been assigned, the replication agreement is by default pre-configured with an exhausted range with dnaNextValue = 1101 and the dnaMaxValue = 1100.

Note too that one can also look in the dnaSharedCfgDN value for the replica, something like:

dnaHostname=dart.example.com+dnaPortNum=389,cn=posix-ids,cn=dna,cn=ipa,cn=etc,dc=example,dc=com

dnaRemainingValues will be 0.

Enhancing ipa-replica-manage

Four new commands will be added, in sets of two: dnarange-show and dnarange-set and dnanextrange-show and dnanextrange-set.

# ipa-replica-manage dnarange-show
masterA.example.com: 100-500
masterB.example.com: 500-1000
masterC.example.com: No range set
# ipa-replica-manage dnarange-show masterA.example.com
 masterA.example.com: 100-500
# ipa-replica-manage dnarange-set masterA.example.com 250-499
# ipa-replica-manage dnanextrange-show
masterA.example.com: 1001-1500
masterB.example.com: No on-deck range set
masterC.example.com: No on-deck range set
 # ipa-replica-manage dnanextrange-show masterA.example.com
 masterA.example.com: 1001-1500
# ipa-replica-manage dnanextrange-set masterB.example.com 1001-5000

A show on no specific host will show them all. A show on a specific host will show only that host.

The list of masters comes from cn=masters.

When any range is set we will need to make sure that it does not overlap with existing DNA ranges AND any existing on-deck ranges.

Setting a range of 0-0 in an on-deck range deletes the range (removes the attribute altogether). We do not allow removing the main DNA range.

NOTE: We will need to be clear that this range has nothing to do with Trust ranges.

That doesn't remove our responsibility to not test for overlaps in the idranges though. We will need to verify that the manual configuration changes do:

  • not overlap with ranges from trusted domains (objectclass=ipatrustedaddomainrange) in cn=ranges,cn=etc,$SUFFIX, or reject the change otherwise.
  • overlap completely with ranges from the local IPA domain (objectclass=ipaDomainIDRange) in cn=ranges,cn=etc,$SUFFIX, or give a warning otherwise which asks the user to add a new suitable idrange."

Codewise the logic could be:

  1. check if the new range is a subset of a local idrange(s), if yes, all is fine
  2. if not, check if it overlaps with an idrange of a trusted domain, if yes, reject
  3. if not, reject and ask to add a new idrange for the local domain

The overall logic for deleting a master and saving the range(s):

  1. Connect to the remote master
  2. If the connection fails, report this and ask to continue (and lose the range(s)). If so, skip the rest and delete the master.
  3. Put the remote master into read-only and force a sync
  4. Retrieve the DNA range and on-deck values (if any)
  5. Check for overlap (just in case)
  6. If overlap, report the overlap and skip the range
  7. Search through list of masters, excluding the one we're deleting to find one without an on-deck
  8. Set first any valid DNA range on the first available master with an on-deck
  9. Search for another available master if the deleted master has an on-deck and set that
  10. Report errors as needed

Hosts that are down

We need specially handling for hosts that are not up. This could be either a temporary or a permanent issue. I think that when modifying a range we need to prompt that an overlap can occur if they continue.

The --force flag will be used to avoid the prompt. The default answer to the Proceed question is No.

Implementation

  • No special handling was needed to deal with hosts that are down because the --force flag is required to get very far at all. A message was added that any range on the host would be lost, but no additional prompts were added.

Feature Managment

UI

It will not have a UI component.

CLI

The ipa-replica-manage tool.

Major configuration options and enablement

None.

Replication

Indirectly. It can affect the available range(s) to a replica. If a replica runs out and not enough values are left then the DNA plugin will give up:

# ipa user-add --first=tim --last=user tuser4
ipa: ERROR: Operations error: Allocation of a new value for range cn=posix ids,cn=distributed numeric assignment plugin,cn=plugins,cn=config failed! Unable to proceed.

Managing ranges can be dangerous. If there are overlapping ranges you run the risk of 2 different masters assigning the same value. This will then cause grief when the entries are replicated.

Updates and Upgrades

No

Dependencies

No

External Impact

No