ipchainsの設定をしてみました。この設定はどうでしょうか？

# ipchains -nL
Chain input (policy ACCEPT):
target prot opt source destination ports
fw all ------ 0.0.0.0/0 0.0.0.0/0 n/a
Chain forward (policy DENY):
target prot opt source destination ports
MASQ all ------ 192.168.1.0/24 0.0.0.0/0 n/a
Chain output (policy ACCEPT):
Chain fw (1 references):
target prot opt source destination ports
ACCEPT tcp ------ 0.0.0.0/0 0.0.0.0/0 * -> 25
ACCEPT tcp ------ 192.168.1.0/24 0.0.0.0/0 * -> 0:1023
ACCEPT udp ------ 192.168.1.0/24 0.0.0.0/0 * -> 0:1023
DENY tcp ------ 0.0.0.0/0 0.0.0.0/0 * -> 0:1023
DENY udp ------ 0.0.0.0/0 0.0.0.0/0 * -> 0:1023