Example of iptables NATΒΆ
If using Red Hat Enterprise Linux (or Fedora), install iptables and save the
rules below as /etc/sysconfig/iptables.
# yum install iptables-services
# service iptables enable
If using Debian, install iptables and save the rules below as
/etc/iptables/rules.v4.
# apt-get install iptables-persistent
# update-rc.d netfilter-persistent enable
In this example:
- The virtual bridge is called
virbr10. - The private subnet chosen is
192.168.100.0/24.- VMs can bind to addresses from
192.168.100.2to192.168.100.254. - VMs see the libvirt server as
192.168.100.1.
- VMs can bind to addresses from
# This format is understood by iptables-restore. See `man iptables-restore`.
*mangle
:PREROUTING ACCEPT [0:0]
:INPUT ACCEPT [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
:POSTROUTING ACCEPT [0:0]
# DHCP packets sent to VMs have no checksum (due to a longstanding bug).
-A POSTROUTING -o virbr10 -p udp -m udp --dport 68 -j CHECKSUM --checksum-fill
COMMIT
*nat
:PREROUTING ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
:POSTROUTING ACCEPT [0:0]
# Do not masquerade to these reserved address blocks.
-A POSTROUTING -s 192.168.100.0/24 -d 224.0.0.0/24 -j RETURN
-A POSTROUTING -s 192.168.100.0/24 -d 255.255.255.255/32 -j RETURN
# Masquerade all packets going from VMs to the LAN/Internet.
-A POSTROUTING -s 192.168.100.0/24 ! -d 192.168.100.0/24 -p tcp -j MASQUERADE --to-ports 1024-65535
-A POSTROUTING -s 192.168.100.0/24 ! -d 192.168.100.0/24 -p udp -j MASQUERADE --to-ports 1024-65535
-A POSTROUTING -s 192.168.100.0/24 ! -d 192.168.100.0/24 -j MASQUERADE
COMMIT
*filter
:INPUT ACCEPT [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
# Allow basic INPUT traffic.
-A INPUT -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
-A INPUT -i lo -j ACCEPT
-A INPUT -p icmp --icmp-type 8 -m conntrack --ctstate NEW -j ACCEPT
# Accept SSH connections.
-A INPUT -p tcp -m tcp --syn -m conntrack --ctstate NEW --dport 22 -j ACCEPT
# Accept DNS (port 53) and DHCP (port 67) packets from VMs.
-A INPUT -i virbr10 -p udp -m udp -m multiport --dports 53,67 -j ACCEPT
-A INPUT -i virbr10 -p tcp -m tcp -m multiport --dports 53,67 -j ACCEPT
# Reject everything else.
-A INPUT -m conntrack --ctstate INVALID -j DROP
-A INPUT -p tcp -m tcp -j REJECT --reject-with tcp-reset
-A INPUT -j REJECT --reject-with icmp-port-unreachable
# Allow established traffic to the private subnet.
-A FORWARD -d 192.168.100.0/24 -o virbr10 -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
# Allow outbound traffic from the private subnet.
-A FORWARD -s 192.168.100.0/24 -i virbr10 -j ACCEPT
# Allow traffic between virtual machines.
-A FORWARD -i virbr10 -o virbr10 -j ACCEPT
# Reject everything else.
-A FORWARD -i virbr10 -j REJECT --reject-with icmp-port-unreachable
-A FORWARD -o virbr10 -j REJECT --reject-with icmp-port-unreachable
COMMIT