Update alias.rst

[stasic]

If a zone is operated with different nameserver software than PowerDNS and ALIAS is used it could result in an out-of-sync sencondary.

Short description

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I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)
• checked that this code was merged to master

[coveralls]

Pull Request Test Coverage Report for Build 6492285293

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 47 unchanged lines in 12 files lost coverage.
• Overall coverage decreased (-0.02%) to 56.357%

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Files with Coverage Reduction	New Missed Lines	%
pdns/sstuff.hh	1	60.24%
pdns/dbdnsseckeeper.cc	2	85.61%
pdns/iputils.hh	2	73.66%
pdns/misc.cc	2	61.32%
pdns/channel.hh	3	55.97%
pdns/dnsdistdist/dnsdist-healthchecks.cc	3	55.31%
pdns/stubresolver.cc	3	81.63%
pdns/tcpiohandler.cc	3	66.83%
pdns/test-distributor_hh.cc	3	88.66%
pdns/dnsdist.cc	5	71.05%

Totals
Change from base Build 6486035458:	-0.02%
Covered Lines:	102357
Relevant Lines:	151170

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💛 - Coveralls

[klaus-nicat]

I would add the sentence:
"To keep secondary nameservers up2date with the ALIAS target, you have to periodically retransfer the zone from PowerDNS, to force ALIAS-resolving during AXFR. This can be done either "dumb", for example every minute, or implement some "out of band" logic to retransfer zones only if the ALIAS target actually changed."

[Habbie]

and please look at Klaus' comment, thanks!

[Habbie]

Suggested change

	If secondaries are operated with nameserver software which is not capable of synthesising ALIAS records the zone will probably go out of sync. Because updated ALIAS target records will not result in an increment of the Serial, which means that Secondaries won't pull the Zone.
	If secondaries are operated with name server software which is not capable of synthesizing ALIAS records the zone will probably go out of sync. Because updated ALIAS target records will not result in an increment of the Serial, which means that Secondaries won't pull the Zone.