How to organize Internet reservation for incoming connections?

Question

Computer networks

There is an office. There are two incoming lines from different providers. The kerio control-based gateway provides balancing and redundancy of the Internet channel.
Now there is a task to reserve not only outgoing, but also incoming connections.
The only thing that came to my mind was to make a dns name for two ip addresses and in case of a drop in one channel, switch dns to the remaining channel. But this is clearly not the best idea.
Are there any ways for reasonable money to make a reservation in this case?

Answer 1

No AS - only DNS records to steer. If you had an AS, you could pull up BGP and routes at the third level for such a reservation. True, you will have to agree with providers.
In general, DNS records can also be used to raise the balance, especially if you hang DNS servers on both IPs so that when you query DNS on ip1, ip1 is given, and when you query on ip2, ip2 is given, both records with a lifetime of 15 minutes (well, or how many it makes sense to do it for automatic decay when the channel goes down), in this case, while both channels are working, traffic to your service that you balance / reserve will come somewhere equally from each side, if one falls, then after the expiration of the lifetime of DNS records on clients the traffic will go only through the live channel.
PS: you will have to mark each connection on Kerio so that traffic leaves through the same interface through which the request came, otherwise the service will not be available.

Answer 2

yes, the best option. A DNS server can resolve a name to multiple IP addresses. The customer should can choose any address.
for example

$ nslookup yandex.ru

Server: 10.0.**
Address: 10.0.**#53
Non-authoritative answer:
Name: yandex.ru
Address: 77.88.55.50
Name: yandex.ru
Address: 77.88.55.55
Name: yandex.ru
Address: 5.255.255.55
Name: yandex.ru
Address: 5.255.255.50
Name: yandex.ru
Address: 2a02:6b8:a::a

check with your DNS provider about this service.
actively used in Round_robin_DNS
Another option is
to put a proxy-balancer outside on a stable server, which will distribute the load on your two IP addresses.
you have identical services on both ip-addresses.
in case of disconnection of one connection - its ip-exclude in the balancer.
and the speed of incoming will develop and there will be stability.
minus - payment for an external service and proxy becomes a "single point of disconnection".

Answer 3

If incoming connections are not directly to the gateway, but to some services in the local network (the ports are simply forwarded on the gateway), then such connections can be limited at the exit from the gateway towards the local network.