yahoo messenger uses 5000,5010 and 5050
To block AOL IM and ICQ:
# iptables -A FORWARD –dport 5190 -j DROP
# iptables -A FORWARD -d login.oscar.aol.com -j DROP
To block MSN messenger:
# iptables -A FORWARD -p TCP –dport 1863 -j DROP
# iptables -A FORWARD -d 64.4.13.0/24 -j DROP
To block Yahoo messenger:
# iptables -A FORWARD -p TCP –dport 5000:5010 -j REJECT
# iptables -A FORWARD -d cs.yahoo.com -j REJECT
# iptables -A FORWARD -b scsa.yahoo.com -j REJECT
Note that this will only block clients on the computer itself. If you are using the computer as a router with NAT, you will want to use “-t nat -A PREROUTING” instead.
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source:http://linuxpoison.blogspot.com/2008/05/135781758010061.html