LINODE: TTL does not support "Default" (0)

[str4d]

Describe the bug
I set up a dnsconfig.js matching the records for a domain I manage through Linode. All of the records in Linode have their TTL column set to "Default". I do not configure an explicit TTL in dnsconfig.js for any record, leaving it at the dnscontrol default.

When I run dnscontrol preview, every record is marked as a modification such as:

#1: MODIFY A example.com: (1.2.3.4 ttl=0) -> (1.2.3.4 ttl=300), Linode ID: NNN: {"type":"A","name":"","target":"1.2.3.4","ttl_sec":300}

To Reproduce

1. Configure a domain name to use Linode for DNS records.
2. Add a DNS A record to Linode with some label and IP, and leave the TTL as "Default".
3. Write a dnsconfig.js with the same A record label and IP, and no configured TTL.
4. dnscontrol preview

Expected behavior
No modifications should be proposed.

Versions

• DNSControl version: 4.1.1 (Homebrew)
• DNS Provider: LINODE

Additional context
The LINODE provider documentation has a caveat that the TTLs must be from a specific list. The file that it links to does not exist in the Linode source tree anymore. When I searched for an equivalent list, I found this one that explicitly includes the option { label: 'Default', value: 0 }.

https://github.com/linode/manager/blob/07be31a25399c8583a55a3727b8a38e0a6e25cd7/packages/manager/src/features/Domains/DomainRecordDrawer.tsx#L320-L336

[str4d]

The dnscontrol source code has the permalink for the file referenced in the docs:

dnscontrol/providers/linode/linodeProvider.go

Lines 126 to 133 in d00804b

	func (api *linodeProvider) GetZoneRecordsCorrections(dc *models.DomainConfig, existingRecords models.Records) ([]*models.Correction, error) {
	// Linode doesn't allow selecting an arbitrary TTL, only a set of predefined values
	// We need to make sure we don't change it every time if it is as close as it's going to get
	// By experimentation, Linode always rounds up. 300 -> 300, 301 -> 3600.
	// https://github.com/linode/manager/blob/edd99dc4e1be5ab8190f243c3dbf8b830716255e/src/domains/components/SelectDNSSeconds.js#L19
	for _, record := range dc.Records {
	record.TTL = fixTTL(record.TTL)
	}

From that file, it appears that Linode did support a ttl_sec value of 0 back then, which it mapped to ONE_DAY: https://github.com/linode/manager/blob/edd99dc4e1be5ab8190f243c3dbf8b830716255e/src/domains/components/SelectDNSSeconds.js#L51

[str4d]

Linode's own documentation confirms that the default TTL is still one day.

[TomOnTime]

CC @koesie10

Interesting bug!

That raises an interesting question. Should allowedTTLValues (

dnscontrol/providers/linode/linodeProvider.go

Line 29 in d00804b

var allowedTTLValues = []uint32{

) be changed to include 0 or should the fixTTL function turn 0 into 86400? (I think the first option)

[str4d]

I personally agree that 0 should be allowed as a setting, because that is what the Linode API is returning when "Default" is set via their UI. If fixTTL turned 0 into 86400, that would still result in a MODIFY (it would be a functional no-op, but not a technical one).

The question though is whether dnscontrol has a good way to represent "Default" in its own UX. Do any other providers have a similar concept? How do they handle a TTL of 0?

[koesie10]

I agree that 0 should be added to the allowedTTLValues. I believe this should work and does not require any further modifications.

I have created a PR for this, which also updates the docs. I'm just finding conflicting information about what 0 actually means:

• The DNS manager documentation says the default is 1 day
• The API documentation says "A value of 0 is equivalent to the default value of 1209600", which would be 14 days

I've taken the value from the API documentation since that's what the provider is using, but I'm not sure if that actually matches with what Linode does.

[str4d]

@koesie10 I think you were looking at the wrong part of that documentation.

data	array of objects
expire_sec	integerThe amount of time in seconds that may pass before this Domain is no longer authoritative.Valid values are 0, 300, 3600, 7200, 14400, 28800, 57600, 86400, 172800, 345600, 604800, 1209600, and 2419200.Any other value is rounded up to the nearest valid value.A value of 0 is equivalent to the default value of 1209600.
--	--
ttl_sec	integer“Time to Live” - the amount of time in seconds that this Domain’s records may be cached by resolvers or other domain servers.Valid values are 0, 300, 3600, 7200, 14400, 28800, 57600, 86400, 172800, 345600, 604800, 1209600, and 2419200.Any other value is rounded up to the nearest valid value.A value of 0 is equivalent to the default value of 86400.
--	--

AFAICT the default is still 1 day for ttl_sec, not 14 days.

[str4d]

Fixed the comment as part of #2444.