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Multi Tenancy Networking Protections in XenServer
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=================================================
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The purpose of the vif_rules script is to allow multi-tenancy on a XenServer
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host. In a multi-tenant cloud environment a host machine needs to be able to
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enforce network isolation amongst guest instances, at both layer two and layer
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three. The rules prevent guests from taking and using unauthorized IP addresses,
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sniffing other guests traffic, and prevents ARP poisoning attacks. This current
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revision only supports IPv4, but will support IPv6 in the future.
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If the kernel doesn't support these, you will need to obtain the Source RPMS for
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the proper version of XenServer to recompile the dom0 kernel.
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XenServer Requirements (32-bit dom0)
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====================================
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- arptables 32-bit rpm
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XenServer Environment Specific Notes
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====================================
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- XenServer 5.5 U1 based on the 2.6.18 kernel didn't include physdev module
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support. Support for this had to be recompiled into the kernel.
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- XenServer 5.6 based on the 2.6.27 kernel didn't include physdev, ebtables, or
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- XenServer 5.6 FP1 didn't include physdev, ebtables, or arptables but they do
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have a Cloud Supplemental pack available to partners which swaps out the
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kernels for kernels that support the networking rules.
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iptables, ebtables, and arptables drop rules are applied to all forward chains
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on the host. These are applied at boot time with an init script. They ensure
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all forwarded packets are dropped by default. Allow rules are then applied to
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the instances to ensure they have permission to talk on the internet.
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Any time an underprivileged domain or domU is started or stopped, it gets a
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unique domain id (dom_id). This dom_id is utilized in a number of places, one
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of which is it's assigned to the virtual interface (vif). The vifs are attached
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to the bridge that is attached to the physical network. For instance, if you
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had a public bridge attached to eth0 and your domain id was 5, your vif would be
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The networking rules are applied to the VIF directly so they apply at the lowest
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level of the networking stack. Because the VIF changes along with the domain id
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on any start, stop, or reboot of the instance, the rules need to be removed and
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re-added any time that occurs.
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Because the dom_id can change often, the vif_rules script is hooked into the
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/etc/xensource/scripts/vif script that gets called anytime an instance is
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started, or stopped, which includes pauses and resumes.
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Examples of the rules ran for the host on boot:
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iptables -P FORWARD DROP
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iptables -A FORWARD -m physdev --physdev-in eth0 -j ACCEPT
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ebtables -P FORWARD DROP
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ebtables -A FORWARD -o eth0 -j ACCEPT
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arptables -P FORWARD DROP
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arptables -A FORWARD --opcode Request --in-interface eth0 -j ACCEPT
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arptables -A FORWARD --opcode Reply --in-interface eth0 -j ACCEPT
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Examples of the rules that are ran per instance state change:
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iptables -A FORWARD -m physdev --physdev-in vif1.0 -s 10.1.135.22/32 -j ACCEPT
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arptables -A FORWARD --opcode Request --in-interface "vif1.0" \
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--source-ip 10.1.135.22 -j ACCEPT
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arptables -A FORWARD --opcode Reply --in-interface "vif1.0" \
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--source-ip 10.1.135.22 --source-mac 9e:6e:cc:19:7f:fe -j ACCEPT
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ebtables -A FORWARD -p 0806 -o vif1.0 --arp-ip-dst 10.1.135.22 -j ACCEPT
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ebtables -A FORWARD -p 0800 -o vif1.0 --ip-dst 10.1.135.22 -j ACCEPT
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ebtables -I FORWARD 1 -s ! 9e:6e:cc:19:7f:fe -i vif1.0 -j DROP
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Typically when you see a vif, it'll look like vif<domain id>.<network bridge>.
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vif2.1 for example would be domain 2 on the second interface.
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The vif_rules.py script needs to pull information about the IPs and MAC
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addresses assigned to the instance. The current implementation assumes that
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information is put into the VM Record into the xenstore-data key in a JSON
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string. The vif_rules.py script reads out of the JSON string to determine the
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IPs, and MAC addresses to protect.
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An example format is given below:
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# xe vm-param-get uuid=<uuid> param-name=xenstore-data
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vm-data/networking/4040fa7292e4:
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"ips": [{"netmask":"255.255.255.0",
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"ip":"173.200.100.10"}],
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"mac":"40:40:fa:72:92:e4",
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"gateway":"173.200.100.1",
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"dns":["72.3.128.240","72.3.128.241"]};
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vm-data/networking/40402321c9b8:
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"ips":[{"netmask":"255.255.224.0",
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"ip":"10.177.10.10"}],
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"routes":[{"route":"10.176.0.0",
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"netmask":"255.248.0.0",
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"gateway":"10.177.10.1"},
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{"route":"10.191.192.0",
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"netmask":"255.255.192.0",
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"gateway":"10.177.10.1"}],
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"mac":"40:40:23:21:c9:b8"}
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The key is used for two purposes. One, the vif_rules.py script will read from
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it to apply the rules needed after parsing the JSON. The second is that because
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it's put into the xenstore-data field, the xenstore will be populated with this
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data on boot. This allows a guest agent the ability to read out data about the
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instance and apply configurations as needed.
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- Copy host-rules into /etc/init.d/ and make sure to chmod +x host-rules.
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- Run 'chkconfig host-rules on' to add the init script to start up.
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- Copy vif_rules.py into /etc/xensource/scripts
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- Patch /etc/xensource/scripts/vif using the supplied patch file. It may vary
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for different versions of XenServer but it should be pretty self explanatory.
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It calls the vif_rules.py script on domain creation and tear down.
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- Run '/etc/init.d/host-rules start' to start up the host based rules.
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- The instance rules will then fire on creation of the VM as long as the correct
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- You can check to see if the rules are in place with: iptables --list,
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arptables --list, or ebtables --list
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plugins/xenserver/doc/networking.rst
