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sysadmins to notify before changing ns1.gnu.org's IP address

ns1.gnu.org is a zone master for most fsf, gnu domains, etc, but there are a couple exceptions. we delegate that role to savanah and guix admins for their own subdomains, of which we are slaves. however, they still point to our dns servers as being the authoritative name servers. so that means that they can change their dns records, and we then serve them.

the upshot of this is that if we ever change the ip address of ns1.gnu.org, or if it goes offline, we need to notify the admins responsible for configuring their servers to transfer their zones to the new ip address of our ns1.gnu.org server. this must be coordinated well in advance, to avoid any downtime. see the email addresses in bind.git/named.conf.slaves-ns1.

it may be possible for them to configure their dns server to send to the new and old address before the changeover occurs. also, we may need to convert the new ns1.gnu.org, or whatever replaces it, into a primary rather than secondary name server, because we give those machines different configs regarding zone transfers, etc. see the script in bind.git/oob-push-slaves.sh.