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libvirt & network filtering iptables/ebtables
[libvirt] FYI: a short guide to libvirt & network filtering iptables/ebtables use
Firewall / network filtering in libvirt =======================================There are three pieces of libvirt functionality which do networkfiltering of some type. At a high level they are: - The virtual network driver. This provides a isolated bridge device (ie no physical NICs enslaved). Guest TAP devices are attached to this bridge. Guests can talk to each other & the host, and optionally the wider world. - The QEMU driver MAC filtering This provides a generic filtering of MAC addresses to prevent the guest spoofing its MAC address. This is mostly obsoleted by the next item, so won‘t be discussed further. - The network filter driver This provides fully configurable, arbitrary network filtering of traffic on guest NICs. Generic rulesets are defined at the host level to control traffic in some manner. Rules sets are then associated with individual NICs of a guest. While not as expressive as directly using iptables/ebtables, this can still do nearly everything you would want to on a guest NIC filter.The virtual network driver==========================The typical configuration for guests is to use bridging of thephysical NIC on the host to connect the guest directly to the LAN.In RHEL6 there is also the possibility of using macvtap/sr-iovand VEPA connectivity. None of this stuff plays nicely with wirelessNICs, since they will typically silently drop any traffic with aMAC address that doesn‘t match that of the physical NIC.Thus the virtual network driver in libvirt was invented. This takesthe form of an isolated bridge device (ie one with no physical NICsenslaved). The TAP devices associated with the guest NICs are attachedto the bridge device. This immediately allows guests on a single hostto talk to each other and to the host OS (modulo host IPtables rules).libvirt then uses iptables to control what further connectivity isavailable. There are three configurations possible for a virtualnetwork at time of writing - isolated: all off-node traffic is completely blocked - nat: outbound traffic to the LAN is allowed, but MASQUERADED - forward: outbound traffic to the LAN is allowedThe latter ‘forward‘ case requires the virtual network be on aseparate sub-net from the main LAN, and that the LAN admin hasconfigured routing for this subnet. In the future we intend toadd support for IP subnetting and/or proxy-arp. This allows forthe virtual network to use the same subnet as the main LAN &should avoid need for the LAN admin to configure special routing.Libvirt will optionally also provide DHCP services to the virtualnetwork using DNSMASQ. In all cases, we need to allow DNS/DHCPqueries to the host OS. Since we can‘t predict whether the hostfirewall setup is already allowing this, we insert 4 rules intothe head of the INPUT chain target prot opt in out source destination ACCEPT udp -- virbr0 * 0.0.0.0/0 0.0.0.0/0 udp dpt:53 ACCEPT tcp -- virbr0 * 0.0.0.0/0 0.0.0.0/0 tcp dpt:53 ACCEPT udp -- virbr0 * 0.0.0.0/0 0.0.0.0/0 udp dpt:67 ACCEPT tcp -- virbr0 * 0.0.0.0/0 0.0.0.0/0 tcp dpt:67Note we have restricted our rules to just the bridge associatedwith the virutal network, to avoid opening undesirable holes inthe host firewall wrt the LAN/WAN.The next rules depend on the type of connectivity allowed, and goin the main FORWARD chain:type=isolated-------------Allow traffic between guests. Deny inbound. Deny outbound. target prot opt in out source destination ACCEPT all -- virbr1 virbr1 0.0.0.0/0 0.0.0.0/0 REJECT all -- * virbr1 0.0.0.0/0 0.0.0.0/0 reject-with icmp-port-unreachable REJECT all -- virbr1 * 0.0.0.0/0 0.0.0.0/0 reject-with icmp-port-unreachabletype=nat--------Allow inbound related to an established connection. Allowoutbound, but only from our expected subnet. Allow trafficbetween guests. Deny all other inbound. Deny all other outbound. target prot opt in out source destination ACCEPT all -- * virbr0 0.0.0.0/0 192.168.122.0/24 state RELATED,ESTABLISHED ACCEPT all -- virbr0 * 192.168.122.0/24 0.0.0.0/0 ACCEPT all -- virbr0 virbr0 0.0.0.0/0 0.0.0.0/0 REJECT all -- * virbr0 0.0.0.0/0 0.0.0.0/0 reject-with icmp-port-unreachable REJECT all -- virbr0 * 0.0.0.0/0 0.0.0.0/0 reject-with icmp-port-unreachabletype=routed-----------Allow inbound, but only to our expected subnet. Allowoutbound, but only from our expected subnet. Allow trafficbetween guests. Deny all other inbound. Deny all other outbound. target prot opt in out source destination ACCEPT all -- * virbr2 0.0.0.0/0 192.168.124.0/24 ACCEPT all -- virbr2 * 192.168.124.0/24 0.0.0.0/0 ACCEPT all -- virbr2 virbr2 0.0.0.0/0 0.0.0.0/0 REJECT all -- * virbr2 0.0.0.0/0 0.0.0.0/0 reject-with icmp-port-unreachable REJECT all -- virbr2 * 0.0.0.0/0 0.0.0.0/0 reject-with icmp-port-unreachableFinally, with type=nat, there is also an entry in the POSTROUTINGchain to apply masquerading target prot opt in out source destination MASQUERADE all -- * * 192.168.122.0/24 !192.168.122.0/24The network filter driver=========================This driver provides a fully configurable network filtering capabilitythat leverages ebtables, iptables and ip6tables. This was written bythe libvirt guys at IBM and although its XML schema is defined by libvirt,the conceptual model is closely aligned with the DMTF CIM schema for network filtering http://www.dmtf.org/standards/cim/cim_schema_v2230/CIM_Network.pdfThe filters are managed in libvirt as a top level, standalone object.This allows the filters to then be referenced by any libvirt objectthat requires their functionality, instead tieing them only to useby guest NICs. In the current implementation, filters can be associatedwith individual guest NICs via the libvirt domain XML format. In thefuture we might allow filters to be associated with the virtual networkobjects. Further we‘re expecting to define a new ‘virtual switch‘ objectto remove the complexity of configuring bridge/sriov/vepa networkingmodes. This make also end up making use of network filters.There are a new set of virsh commands for managing network filters virsh nwfilter-define define or update a network filter from an XML file virsh nwfilter-undefine undefine a network filter virsh nwfilter-dumpxml network filter information in XML virsh nwfilter-list list network filters virsh nwfilter-edit edit XML configuration for a network filterThere are equivalently named C APIs for each of these commands.As with all objects libvirt manages, network filters are configuredusing an XML format. At a high level the format looks like this: <filter name=‘no-spamming‘ chain=‘XXXX‘> <uuid>d217f2d7-5a04-0e01-8b98-ec2743436b74</uuid> <rule ...> .... </rule> <filterref filter=‘XXXX‘/> </filter>Every filter has a name and UUID which serve as unique identifiers.A filter can have zero-or-more <rule> elements which are used toactually define network controls. Filters can be arranged into aDAG, so zero-or-more <filterref/> elements are also allowed. Cyclesin the graph are not allowed.The <rule> element is where all the interesting stuff happens. Ithas three attributes, an action, a traffic direction and an optionalpriority. eg <rule action=‘drop‘ direction=‘out‘ priority=‘500‘>Within the rule there are a wide variety of elements allowed, whichdo protocol specific matching. Supported protocols currently include‘mac‘, ‘arp‘, ‘rarp‘, ‘ip‘, ‘ipv6‘, ‘tcp/ip‘, ‘icmp/ip‘, ‘igmp/ip‘,‘udp/ip‘, ‘udplite/ip‘ ‘esp/ip‘, ‘ah/ip‘, ‘sctp/ip‘, ‘tcp/ipv6‘,‘icmp/ipv6‘, ‘igmp/ipv6‘, ‘udp/ipv6‘, ‘udplite/ipv6‘, ‘esp/ipv6‘,‘ah/ipv6‘, ‘sctp/ipv6‘. Each protocol defines what is valid insidethe <rule> element, the general pattern though is <protocol match=‘yes|no‘ attribute1=‘value1‘ attribute2=‘value2‘/>so, eg a TCP protocol, matching ports 0-1023 would be expressedas: <tcp match=‘yes‘ srcportstart=‘0‘ srcportend=‘1023‘/>Attributes can included references to variables defined by theobject using the rule. So the guest XML format allows each NICto have a MAC address and IP address defined. These are madeavailable to filters via the variables $IP and $MAC.So to define a filter the prevents IP address spoofing we cansimply match on source IP address != $IP <filter name=‘no-ip-spoofing‘ chain=‘ipv4‘> <rule action=‘drop‘ direction=‘out‘> <ip match=‘no‘ srcipaddr=‘$IP‘ /> </rule> </filter>I‘m not going to go into details on all the other protocolmatches you can do, because it‘ll take far too much space.You can read about the options here http://libvirt.org/formatnwfilter.html#nwfelemsRulesProtoOut of the box in RHEL6/Fedora rawhide, libvirt ships with aset of default useful rules # virsh nwfilter-list UUID Name ---------------------------------------------------------------- 15b1ab2b-b1ac-1be2-ed49-2042caba4abb allow-arp 6c51a466-8d14-6d11-46b0-68b1a883d00f allow-dhcp 7517ad6c-bd90-37c8-26c9-4eabcb69848d allow-dhcp-server 3d38b406-7cf0-8335-f5ff-4b9add35f288 allow-incoming-ipv4 5ff06320-9228-2899-3db0-e32554933415 allow-ipv4 db0b1767-d62b-269b-ea96-0cc8b451144e clean-traffic f88f1932-debf-4aa1-9fbe-f10d3aa4bc95 no-arp-spoofing 772f112d-52e4-700c-0250-e178a3d91a7a no-ip-multicast 7ee20370-8106-765d-f7ff-8a60d5aaf30b no-ip-spoofing d5d3c490-c2eb-68b1-24fc-3ee362fc8af3 no-mac-broadcast fb57c546-76dc-a372-513f-e8179011b48a no-mac-spoofing dba10ea7-446d-76de-346f-335bd99c1d05 no-other-l2-traffic f5c78134-9da4-0c60-a9f0-fb37bc21ac1f no-other-rarp-traffic 7637e405-4ccf-42ac-5b41-14f8d03d8cf3 qemu-announce-self 9aed52e7-f0f3-343e-fe5c-7dcb27b594e5 qemu-announce-self-rarpMost of these are just building blocks. The interesting one hereis ‘clean-traffic‘. This pulls together all the building blocksinto one filter that you can then associate with a guest NIC.This stops the most common bad things a guest might try, IPspoofing, arp spoofing and MAC spoofing. To look at the rules forany of these just do virsh nwfilter-dumpxml FILTERNAME|UUIDThey are all stored in /etc/libvirt/nwfilter, but don‘t editfiles there directly. Use ‘virsh nwfilter-define‘ to updatethem. This ensures the guests have their iptables/ebtablesrules recreated.To associate the clean-trafffic filter with a guest, edit theguest XML config and change the <interface> element to includea <filterref> and also specify the whitelisted <ip addres/> theguest is allowed to use <interface type=‘bridge‘> <mac address=‘52:54:00:56:44:32‘/> <source bridge=‘br1‘/> <ip address=‘10.33.8.131‘/> <target dev=‘vnet0‘/> <model type=‘virtio‘/> <filterref filter=‘clean-traffic‘/> </interface>If no <ip address> is included, the network filter driver willactivate its ‘learning mode‘. This uses libpcap to snoop onnetwork traffic the guest sends and attempts to identify thefirst IP address it uses. It then locks traffic to this address.Obviously this isn‘t entirely secure, but it does offer someprotection against the guest being trojaned once up & running.In the future we intend to enhance the learning mode so that itlooks for DHCPOFFERS from a trusted DHCP server and only allowsthe offered IP address to be used.Now, how is all this implemented... The network filter driveruses a combination of ebtables, iptables and ip6tables, dependingon which protocols are referenced in a filter. The out of the box‘clean-traffic‘ filter rules only require use of ebtables. If youwant to do matching at tcp/udp/etc protocols (eg to add a newfilter ‘no-email-spamming‘ to block port 25), then iptables willalso be used.The driver attempts to keep its rules separate from those thatthe host admin might already have configured. So the first thingit does with ebtables, is to add two hooks in POSTROUTING &PREROUTING chains, to redirect traffic to custom chains. Thesehooks match on the TAP device name of the guest NIC, so theyshould not interact badly with any administrator defined rules Bridge chain: PREROUTING, entries: 1, policy: ACCEPT -i vnet0 -j libvirt-I-vnet0 Bridge chain: POSTROUTING, entries: 1, policy: ACCEPT -o vnet0 -j libvirt-O-vnet0To keep things managable & easy to follow, the driver will thencreate further sub-chains for each protocol then it needs to matchagainst: Bridge chain: libvirt-I-vnet0, entries: 5, policy: ACCEPT -p IPv4 -j I-vnet0-ipv4 -p ARP -j I-vnet0-arp -p 0x8035 -j I-vnet0-rarp -p 0x835 -j ACCEPT -j DROP Bridge chain: libvirt-O-vnet0, entries: 4, policy: ACCEPT -p IPv4 -j O-vnet0-ipv4 -p ARP -j O-vnet0-arp -p 0x8035 -j O-vnet0-rarp -j DROPFinally, come the actual implementation of the filters. Theseexample is showing the ‘clean-traffic‘ filter implementation.I‘m not going to explain what this is doing now :-) Bridge chain: I-vnet0-ipv4, entries: 2, policy: ACCEPT -s ! 52:54:0:56:44:32 -j DROP -p IPv4 --ip-src ! 10.33.8.131 -j DROP Bridge chain: O-vnet0-ipv4, entries: 1, policy: ACCEPT -j ACCEPT Bridge chain: I-vnet0-arp, entries: 6, policy: ACCEPT -s ! 52:54:0:56:44:32 -j DROP -p ARP --arp-mac-src ! 52:54:0:56:44:32 -j DROP -p ARP --arp-ip-src ! 10.33.8.131 -j DROP -p ARP --arp-op Request -j ACCEPT -p ARP --arp-op Reply -j ACCEPT -j DROP Bridge chain: O-vnet0-arp, entries: 5, policy: ACCEPT -p ARP --arp-op Reply --arp-mac-dst ! 52:54:0:56:44:32 -j DROP -p ARP --arp-ip-dst ! 10.33.8.131 -j DROP -p ARP --arp-op Request -j ACCEPT -p ARP --arp-op Reply -j ACCEPT -j DROP Bridge chain: I-vnet0-rarp, entries: 2, policy: ACCEPT -p 0x8035 -s 52:54:0:56:44:32 -d Broadcast --arp-op Request_Reverse --arp-ip-src 0.0.0.0 --arp-ip-dst 0.0.0.0 --arp-mac-src 52:54:0:56:44:32 --arp-mac-dst 52:54:0:56:44:32 -j ACCEPT -j DROP Bridge chain: O-vnet0-rarp, entries: 2, policy: ACCEPT -p 0x8035 -d Broadcast --arp-op Request_Reverse --arp-ip-src 0.0.0.0 --arp-ip-dst 0.0.0.0 --arp-mac-src 52:54:0:56:44:32 --arp-mac-dst 52:54:0:56:44:32 -j ACCEPT -j DROP NB, we would have liked to include the prefix ‘libvirt-‘ in allof our chain names, but unfortunately the kernel limits namesto a very short maximum length. So only the first two customchains can include that prefix. The others just include theTAP device name + protocol name.If I define a new filter ‘no-spamming‘ and then add this to the‘clean-traffic‘ filter, I can illustrate how iptables usage works. # cat > /root/spamming.xml <<EOF <filter name=‘no-spamming‘ chain=‘root‘> <uuid>d217f2d7-5a04-0e01-8b98-ec2743436b74</uuid> <rule action=‘drop‘ direction=‘out‘ priority=‘500‘> <tcp dstportstart=‘25‘ dstportend=‘25‘/> </rule> </filter> EOF # virsh nwfilter-define /root/spamming.xml # virsh nwfilter-edit clean-traffic ...add <filterref filter=‘no-spamming‘/>All active guests immediately have their iptables/ebtables rulesrebuilt.The network filter driver deals with iptables in a very similarway. First it separates out its rules from those the admin mayhave defined, by adding a couple of hooks into the INPUT/FORWARDchains Chain INPUT (policy ACCEPT 13M packets, 21G bytes) target prot opt in out source destination libvirt-host-in all -- * * 0.0.0.0/0 0.0.0.0/0 Chain FORWARD (policy ACCEPT 5532K packets, 3010M bytes) target prot opt in out source destination libvirt-in all -- * * 0.0.0.0/0 0.0.0.0/0 libvirt-out all -- * * 0.0.0.0/0 0.0.0.0/0 libvirt-in-post all -- * * 0.0.0.0/0 0.0.0.0/0These custom chains, then do matching based on the TAP devicename, so they won‘t open holes in the admin defined matches forthe LAN/WAN (if any).Chain libvirt-host-in (1 references) target prot opt in out source destination HI-vnet0 all -- * * 0.0.0.0/0 0.0.0.0/0 [goto] PHYSDEV match --physdev-in vnet0Chain libvirt-in (1 references) target prot opt in out source destination FI-vnet0 all -- * * 0.0.0.0/0 0.0.0.0/0 [goto] PHYSDEV match --physdev-in vnet0Chain libvirt-in-post (1 references) target prot opt in out source destination ACCEPT all -- * * 0.0.0.0/0 0.0.0.0/0 PHYSDEV match --physdev-in vnet0Chain libvirt-out (1 references) target prot opt in out source destination FO-vnet0 all -- * * 0.0.0.0/0 0.0.0.0/0 [goto] PHYSDEV match --physdev-out vnet0Finally, we can see the interesting bit which is the actualimplementation of my filter to block port 25 access:Chain FI-vnet0 (1 references) target prot opt in out source destination DROP tcp -- * * 0.0.0.0/0 0.0.0.0/0 tcp dpt:25Chain FO-vnet0 (1 references) target prot opt in out source destination DROP tcp -- * * 0.0.0.0/0 0.0.0.0/0 tcp spt:25Chain HI-vnet0 (1 references) target prot opt in out source destination DROP tcp -- * * 0.0.0.0/0 0.0.0.0/0 tcp dpt:25One thing in looking at this that you may notice is that if thereare many guests all using the same filters, we will be duplicatingthe iptables rules over & over for each guest. This is merely alimitation of the current rules engine implementation. At the libvirtobject modelling level you can clearly see we‘ve designed the modelso that filter rules are define in one place, and indirectly referencedby guests. Thus it should be possible to change the impl in the futureso that we can share the actual iptables/ebtables rules for eachguest to create a more scalable system. The stuff in current libvirtis more or less the very first working impl we‘ve had of this stuff, so there‘s not been much optimization work yet.Also notice that at the XML level we don‘t expose the fact that weare using iptables or ebtables at all. The rule definition is done interms of network protocols. Thus if we ever find a need, we couldplug in an alternative implementation that calls out to a differentfirewall implementation instead of ebtables/iptables (providing thatimpl was suitably expressive of course)Finally, in terms of problems we have in deployment. The biggestproblem is that if the admin does ‘service iptables restart‘ allour work gets blown away. We‘ve experimented with using lokkitto record our custom rules in a persistent config file, but thatcaused different problem. Admins who were not using lokkit fortheir config found that all their own rules got blown away. Sowe threw away our lokkit code. Instead we document that if yourun ‘service iptables restart‘, you need to send SIGHUP to libvirtto make it recreate its rules.Finally a reminder, that the main documentation we have on thisis online at http://libvirt.org/formatnwfilter.html
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