How do I fix these iptables?

Question

I am trying to setup my networks firewall, and am having trouble because i keep getting kicked out of SSH because of mistakes. Here's what I currently have:

# Custom Rules
iptables -F
iptables -X
iptables -P FORWARD DROP
iptables -N mc
iptables -A mc --src 123.456.789.1 -j ACCEPT
iptables -A mc --src 123.456.789.2 -j ACCEPT
iptables -A mc --src 50.50.50.50 -j ACCEPT
iptables -A mc -i lo -j ACCEPT
iptables -A mc -m state --state RELATED,ESTABLISHED -j ACCEPT
iptables -A mc -m state --state INVALID -j DROP
iptables -A mc -j DROP

# Server Rules (Main Server)
iptables -A INPUT -p tcp --match multiport --dports 64000:64321 -j ACCEPT
iptables -A INPUT -p tcp -m tcp --sport 53 --dport 1024:65535 -m state --state ESTABLISHED -j ACCEPT
iptables -A INPUT -p udp -m udp --sport 53 --dport 1024:65535 -m state --state ESTABLISHED -j ACCEPT
iptables -A INPUT -i eth0 -p icmp --source proxy.ovh.net -j ACCEPT
iptables -A INPUT -i eth0 -p icmp --source proxy.p19.ovh.net -j ACCEPT
iptables -A INPUT -i eth0 -p icmp --source proxy.rbx.ovh.net -j ACCEPT
iptables -A INPUT -i eth0 -p icmp --source proxy.rbx2.ovh.net -j ACCEPT
iptables -A INPUT -i eth0 -p icmp --source ping.ovh.net -j ACCEPT
iptables -A INPUT -i eth0 -p tcp --dport 22 --source cache.ovh.net -j ACCEPT
iptables -A INPUT -p tcp --dport 3306 -j mc
iptables -A INPUT -p tcp -m multiport ! --dports 20,21,22,80,443,9987,20117,25565 -j mc
iptables -A INPUT -j DROP

Basically:

• Allow access to ports 64000:64321
• Allow access on TCP (from sport 53) to ports 1024:65535
• Allow access on UDP (from sport 53) to ports 1024:65535
• Allow access from ICMP ping requests, from the sources listed above
• Allow access to port 3306 using the mc rules
• Filter access from ports 20,21,22,80,443,9987,20117,25565 through mc rules
• Deny all other input

Answer 1

First of all "Filter access from ports 20,21,22,80,443,9987,20117,25565 through mc rules" with this rule:
iptables -A INPUT -p tcp -m multiport ! --dports 20,21,22,80,443,9987,20117,25565 -j mc
will not work because of exclamation mark. Your rule works for every port except 20,21,22,80,443,9987,20117,25565. If you want "filter access from ports 20,21,22,80,443,9987,20117,25565 through mc rules" you should use:
iptables -A INPUT -p tcp -m multiport --dports 20,21,22,80,443,9987,20117,25565 -j mc

For now in your configuration you allow ssh access only for one source addresses cache.ovh.net - in your main INPUT rules. For connections to other ports you have rule:
iptables -A mc -m state --state RELATED,ESTABLISHED -j ACCEPT that allows related and established connections to work. I think connections with source addresses from you mc custom rules should work fine, but for your main INPUT rule you should add something like this:
iptables -A INPUT -i eth0 -p tcp --dport 22 --source cache.ovh.net -m state --state RELATED,ESTABLISHED -j ACCEPT
right after this line:
iptables -A INPUT -i eth0 -p tcp --dport 22 --source cache.ovh.net -j ACCEPT

Also you should check DNS settings. Does ping.ovh.net resolves to the right IP address?