Duplicate Servers

Hal Murray hmurray at megapathdsl.net
Wed Sep 25 11:57:22 UTC 2019

> Internally, my systems use my NTP servers (marked with prefer) and the pool
> to provide additional sources. As is typical, ntpd prefers IPv6 after
> resolving the hostname. 

ntpd scans DNS answers in the order they are returned by getaddrinfo()

> From time to time, the pool will serve me my own servers. I think ntpd
> automatically deduplicates the sources if I get the IPv6 address. However, if
> I get one of my own servers back by IPv4 address, I can end up with a
> duplicate source. 

ntpd skips addresses that are already in use.

Yes, you can use a server twice via IPv4 and IPv6.  And more if it is 

> If I should care, any suggestions on what I should do? Is using my sources
> plus the pool stupid? If not, maybe we could add some sort of "blacklist this
> IP from the pool" feature, which would cover this scenario plus others?

Using local servers and pool servers is reasonable.  It should do the right 
thing without any prefer.

The restrict stuff could be used to implement a blacklist.  The DNS/NTS code 
isn't smart enough to check the restrict list before setting things up as a 
server.  It would be reasonable to implement a specific blacklist configure 
option - maybe cleaner to implement and explain than using restrict.

You can also implement a blacklist by setting up that IP Address with 
noselect.  That will log things in rawstats so you can make pretty graphs 
which might convince you that blacklisting is good or bad.

These are my opinions.  I hate spam.

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