-## Allow ping from safe/noloop to untrusted networks.
-run iptables -A FORWARD -j ACCEPT \
- -p icmp ! -f --icmp-type echo-request \
- -m mark --mark $to_untrusted/$MASK_TO
-run iptables -A FORWARD -j ACCEPT \
- -p icmp ! -f --icmp-type echo-reply \
- -m mark --mark $from_untrusted/$MASK_FROM \
- -m state --state ESTABLISHED
-
-## Allow SSH from safe/noloop to untrusted networks.
-run iptables -A FORWARD -j ACCEPT \
- -p tcp ! -f --destination-port $port_ssh \
- -m mark --mark $to_untrusted/$MASK_TO
-run iptables -A FORWARD -j ACCEPT \
- -p tcp ! -f --source-port $port_ssh \
- -m mark --mark $from_untrusted/$MASK_FROM \
- -m state --state ESTABLISHED
+case $forward in
+ 1)
+
+ ## Only allow these packets if they're not fragmented. (Don't trust safe
+ ## hosts's fragment reassembly to be robust against malicious fragments.)
+ ## There's a hideous bug in iptables 1.4.11.1 which botches the meaning
+ ## of `! -f', so we do the negation using early return from a subchain.
+ clearchain fwd-spec-nofrag
+ run iptables -A fwd-spec-nofrag -j RETURN --fragment
+ run ip6tables -A fwd-spec-nofrag -j RETURN \
+ -m ipv6header --soft --header frag
+ run ip46tables -A FORWARD -j fwd-spec-nofrag
+
+ ## Allow ping from safe/noloop to untrusted networks.
+ run iptables -A fwd-spec-nofrag -j ACCEPT \
+ -p icmp --icmp-type echo-request \
+ -m mark --mark $to_untrusted/$MASK_TO
+ run iptables -A fwd-spec-nofrag -j ACCEPT \
+ -p icmp --icmp-type echo-reply \
+ -m mark --mark $from_untrusted/$MASK_FROM \
+ -m state --state ESTABLISHED
+ run ip6tables -A fwd-spec-nofrag -j ACCEPT \
+ -p icmpv6 --icmpv6-type echo-request \
+ -m mark --mark $to_untrusted/$MASK_TO
+ run ip6tables -A fwd-spec-nofrag -j ACCEPT \
+ -p icmpv6 --icmpv6-type echo-reply \
+ -m mark --mark $from_untrusted/$MASK_FROM \
+ -m state --state ESTABLISHED
+
+ ## Allow SSH from safe/noloop to untrusted networks.
+ run iptables -A fwd-spec-nofrag -j ACCEPT \
+ -p tcp --destination-port $port_ssh \
+ -m mark --mark $to_untrusted/$MASK_TO
+ run iptables -A fwd-spec-nofrag -j ACCEPT \
+ -p tcp --source-port $port_ssh \
+ -m mark --mark $from_untrusted/$MASK_FROM \
+ -m state --state ESTABLISHED
+ run ip6tables -A fwd-spec-nofrag -j ACCEPT \
+ -p tcp --destination-port $port_ssh \
+ -m mark --mark $to_untrusted/$MASK_TO
+ run ip6tables -A fwd-spec-nofrag -j ACCEPT \
+ -p tcp --source-port $port_ssh \
+ -m mark --mark $from_untrusted/$MASK_FROM \
+ -m state --state ESTABLISHED
+
+ ;;
+esac