Simple iptables chain question

Question

Hi,

I'm writing a bash script to add simple firewalling for Xen.

Here's the actual firewall configuration :

Chain INPUT (policy ACCEPT)
target     prot opt source               destination
RH-Firewall-1-INPUT  all  --  anywhere             anywhere

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination
RH-Firewall-1-INPUT  all  --  anywhere             anywhere

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination

Chain RH-Firewall-1-INPUT (2 references)
target     prot opt source               destination
ACCEPT     all  --  anywhere             anywhere
ACCEPT     icmp --  anywhere             anywhere            icmp any
ACCEPT     esp  --  anywhere             anywhere
ACCEPT     ah   --  anywhere             anywhere
ACCEPT     udp  --  anywhere             224.0.0.251         udp dpt:mdns
ACCEPT     udp  --  anywhere             anywhere            udp dpt:ipp
ACCEPT     tcp  --  anywhere             anywhere            tcp dpt:ipp
ACCEPT     all  --  anywhere             anywhere            state RELATED,ESTABLISHED
ACCEPT     udp  --  anywhere             anywhere            state NEW udp dpt:ha-cluster
ACCEPT     tcp  --  anywhere             anywhere            state NEW tcp dpt:ssh
ACCEPT     tcp  --  anywhere             anywhere            state NEW tcp dpt:http
ACCEPT     tcp  --  anywhere             anywhere            state NEW tcp dpt:https
REJECT     all  --  anywhere             anywhere            reject-with icmp-host-prohibited

I'd like to add a new chain for each of my virtual machines (each of them has a virtual interface called vif1.0, vif2.0, etc). Output interface (bridge) is xenbr0.

Here's what I do (for example to block ping 'in'to domU1, vif1.0) :

iptables -N domUFirewall
iptables -I FORWARD -j domUFirewall
iptables -I INPUT -j domUFirewall
iptables -A domUFirewall -i vif1.0 -p icmp -j DROP

But .. it doesn't work, i'm still able to ping in/out the domU.

Must be something really 'dumb' but I can't find out what's wrong.

Any clues ?

Thx

Answer 1

Since you're using XEN with bridged networking, packets are being intercepted at a level before ordinary iptables commands can influence them. Thus, you'll probably need to use the ebtables command to influence packet routing in the way that you want to.

• ebtables/iptables interaction on a Linux-based bridge
• ebtables(8) - Linux man page
• Xen Wiki * XenNetworking

Original answer left below that will work for other configurations, but not for XEN with bridged networking.

I am going to pretend for the sake of example that the IP address of vif1.0 is 192.168.1.100.

I would redo the logic to not check the input device, but to instead check by IP Address. At the input chain, the packet is coming from (say) device eth0, not from vif1.0. Thus, this rule:

iptables -I INPUT -i vif1.0 -j domUFirewall

that I previously proposed will never match any packets. However, if you do the following, it should do what you want:

iptables -I INPUT -d 192.168.1.100 -j domUFirewall

where in this case the chain domUFirewall is set up by:

iptables -N domUFirewall
iptables -F domUFirewall
iptables -A domUFirewall -p icmp -j DROP

If a given chain is for a single device, then you want to make this check before jumping into the chain, on a rule with the "-j chainName" action. Then, in the chain itself, you never have to check for the device or IP Address.

Second, I would always flush (empty) the chain in your script, just in case you're re-running the script. Note that when you rerun the script, you may get complaints on the -N line. That's OK.

There are other ways you could do this, but to give a different example, I would need to know specifically how your VM is set up -- bridged networking? NAT? Etc. But the example I gave here should work in any of these modes.

Here are some useful links for the future:

• Quick HOWTO, Ch14: Linux Firewalls Using iptables
• Sandbox a VMware Virtual Machine With iptables