Why doesn't my iptables entries block pinging a Xen virtual machine? [closed]
Question
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
Solution
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: