Experimente essas regras:
iptables -I INPUT -p 47 -m state --state ESTABLISHED -j ACCEPT
iptables -I OUTPUT -p 47 -m state --state NEW,ESTABLISHED -j ACCEPT
iptables -I INPUT -p tcp --sport 1723 -m state --state ESTABLISHED -j ACCEPT
iptables -I OUTPUT -p tcp --dport 1723 -m state --state NEW,ESTABLISHED -j ACCEPT