1- After load balancing, I have dropped packets, so what is the best way to keep each session used by the same line it started, for example, I opened Facebook and keep the user on the same session so that Facebook does not drop the packets and make a new request for IP change
The key here is connection marking. When mangling the initial packet of each connection using the load distribution rules, set a connection mark identifying the WAN to be used instead of routing mark. Then, translate these connection marks to routing marks for every packet (including the first ones). So the load distribution will work per connection, not per packet.
And be sure to prevent any connection-marked connection from being matched by an
action=fasttrack-connection
rule.
2- if I have another WAN lets say on ether 4, how can I make DST Addresses go only from ether 4, example a client wants to watch videos on Youtube and want him to take youtube from specific WAN only and that is ether 4
When setting connection marks, use
to assign a specific connection mark which you later translate into routing mark for a routing table using ether4 as WAN before your load distributing rules. This way, whatever you put to that
will be accessed via ether4. Address list can contain domain names and RouterOS expands them to IP addresses and keeps them up to date, tracking the time to live of the DNS responses.