I have a firewall mechanism based on iptables/ip6tables that allows its users to block or accept traffic from hosts, by adding rules at the top of iptables/ip6tables.
I would like all traffic generated by localhost to be allowed no matter the state of iptables/ip6tables.
Is there a way to do that?
No, iptables controls the flow. You cannot possibly always allow localhost no matter the state of iptables. First set a rule
iptables -I INPUT -i lo0 -j ACCEPT Then don’t allow users to override or add rules on top before this rule. I would suggest not allowing users to fiddle with iptables directly at all, and instead give them a script which they can invoke with predefined functions/parameters that does specific allowed tasks for them.
If you allow users to modify iptables, one could add a rule on top like the one I posted with -j DROP and that’s that. That’s what iptables is for. There is no mechanism to “circumvent iptables for a specific set of parameters”, it’s the job of iptables to handle this.
Yeah, there are ways to bypass iptables/ip6tables rules for localhost traffic. You can configure the rules to allow all traffic to and from the loopback interface (lo).
For IPv4, allow all traffic on the loopback interface:
sudo iptables -A INPUT -i lo -j ACCEPT sudo iptables -A OUTPUT -o lo -j ACCEPT Make sure that localhost traffic is allowed:
sudo iptables -A INPUT -s 127.0.0.0/8 -j ACCEPT sudo iptables -A OUTPUT -d 127.0.0.0/8 -j ACCEPT For IPv6, allow all traffic on the loopback interface:
sudo ip6tables -A INPUT -i lo -j ACCEPT sudo ip6tables -A OUTPUT -o lo -j ACCEPT Make sure that localhost traffic is allowed:
sudo ip6tables -A INPUT -s ::1 -j ACCEPT sudo ip6tables -A OUTPUT -d ::1 -j ACCEPT Save the rules
Debian/Ubuntu:
sudo sh -c "iptables-save > /etc/iptables/rules.v4" sudo sh -c "ip6tables-save > /etc/iptables/rules.v6" Redhat:
sudo sh -c "iptables-save > /etc/sysconfig/iptables" sudo sh -c "ip6tables-save > /etc/sysconfig/ip6tables" Good luck!
--append(-A) option for appending new rules, and not allow users to use the--insert(-I) option, for inserting new rules to the top of rules chain. When doing so, you use--insertoption only for superuser access to firewall rules and create rules for allowing traffic on localhost. You need to make sure that users have no sudo/root access and that users can only edit a restricted part of iptables configuration, which another superuser program/script then assembles with your superuser-iptables configuration and applies it.