Hello, I came across an interesting problem in my home lab a few weeks ago as I'm prepping for my RHCE exam using Michael Jang study guide. I've been at this for days now, and I still can't wrap my head around how two or more virtual networks in default NAT configuration are even allowed to communicate with each other despite what the libvirt documentation said. Here's the excerpt I'm referring to in the wiki link here: http://wiki.libvirt.org/page/Networking#Forwarding_Incoming_Connections: > By default, guests that are connected via a virtual network with <forward > mode='nat'/> can make any outgoing network connection they like. Incoming > connections are allowed from the host, and from other guests connected to > the same libvirt network, but all other incoming connections are blocked by > iptables rules. Also here's another assertion from 'The virtual network driver' section in http://libvirt.org/firewall.html: > type=nat > > Allow inbound related to an established connection. Allow outbound, but > only from our expected subnet. Allow traffic between guests. Deny all other > inbound. Deny all other outbound. I have three virtual networks with the following configs: ----------------------------------------------------------------------------- <network connections='1'> <name>default</name> <uuid>9c6796be-d54e-42bc-bcbe-2e4feee7154a</uuid> <forward mode='nat'> <nat> <port start='1024' end='65535'/> </nat> </forward> <bridge name='virbr0' stp='on' delay='0'/> <mac address='52:54:00:5a:5d:0e'/> <ip address='192.168.122.1' netmask='255.255.255.0'> <dhcp> <range start='192.168.122.2' end='192.168.122.254'/> </dhcp> </ip> </network> <network connections='1'> <name>outsider</name> <uuid>247e380a-8795-466a-b94a-5be2d05267bb</uuid> <forward mode='nat'> <nat> <port start='1024' end='65535'/> </nat> </forward> <bridge name='virbr1' stp='on' delay='0'/> <mac address='52:54:00:7f:a1:fb'/> <domain name='outsider'/> <ip address='192.168.100.1' netmask='255.255.255.0'> <dhcp> <range start='192.168.100.2' end='192.168.100.254'/> </dhcp> </ip> </network> <network connections='1'> <name>besider</name> <uuid>cc714cce-dbba-452d-b2bf-d36084dcb723</uuid> <forward mode='nat'> <nat> <port start='1024' end='65535'/> </nat> </forward> <bridge name='virbr2' stp='on' delay='0'/> <mac address='52:54:00:59:67:7f'/> <domain name='besider'/> <ip address='192.168.110.1' netmask='255.255.255.0'> <dhcp> <range start='192.168.110.2' end='192.168.110.254'/> </dhcp> </ip> </network> ---------------------------------------------------------------------------- Here is the output of the 'FORWARD' iptables chain rules on my host (still using firewall-cmd): ------------------------------------------------------------------------ Chain FORWARD (policy ACCEPT 0 packets, 0 bytes) pkts bytes target prot opt in out source destination 8967 14M ACCEPT all -- * virbr2 0.0.0.0/0 192.168.110.0/24 ctstate RELATED,ESTABLISHED 5262 279K ACCEPT all -- virbr2 * 192.168.110.0/24 0.0.0.0/0 0 0 ACCEPT all -- virbr2 virbr2 0.0.0.0/0 0.0.0.0/0 70 5832 REJECT all -- * virbr2 0.0.0.0/0 0.0.0.0/0 reject-with icmp-port-unreachable 0 0 REJECT all -- virbr2 * 0.0.0.0/0 0.0.0.0/0 reject-with icmp-port-unreachable 8510 13M ACCEPT all -- * virbr0 0.0.0.0/0 192.168.122.0/24 ctstate RELATED,ESTABLISHED 5177 275K ACCEPT all -- virbr0 * 192.168.122.0/24 0.0.0.0/0 0 0 ACCEPT all -- virbr0 virbr0 0.0.0.0/0 0.0.0.0/0 61 5100 REJECT all -- * virbr0 0.0.0.0/0 0.0.0.0/0 reject-with icmp-port-unreachable 0 0 REJECT all -- virbr0 * 0.0.0.0/0 0.0.0.0/0 reject-with icmp-port-unreachable 8612 13M ACCEPT all -- * virbr1 0.0.0.0/0 192.168.100.0/24 ctstate RELATED,ESTABLISHED 5172 273K ACCEPT all -- virbr1 * 192.168.100.0/24 0.0.0.0/0 0 0 ACCEPT all -- virbr1 virbr1 0.0.0.0/0 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 0 0 REJECT all -- virbr1 * 0.0.0.0/0 0.0.0.0/0 reject-with icmp-port-unreachable 0 0 ACCEPT all -- * * 0.0.0.0/0 0.0.0.0/0 ctstate RELATED,ESTABLISHED 0 0 ACCEPT all -- lo * 0.0.0.0/0 0.0.0.0/0 0 0 FORWARD_direct all -- * * 0.0.0.0/0 0.0.0.0/0 0 0 FORWARD_IN_ZONES_SOURCE all -- * * 0.0.0.0/0 0.0.0.0/0 0 0 FORWARD_IN_ZONES all -- * * 0.0.0.0/0 0.0.0.0/0 0 0 FORWARD_OUT_ZONES_SOURCE all -- * * 0.0.0.0/0 0.0.0.0/0 0 0 FORWARD_OUT_ZONES all -- * * 0.0.0.0/0 0.0.0.0/0 0 0 DROP all -- * * 0.0.0.0/0 0.0.0.0/0 ctstate INVALID 0 0 REJECT all -- * * 0.0.0.0/0 0.0.0.0/0 reject-with icmp-host-prohibited -------------------------------------------------------------------------- I have a VM in each network: nest1.example.com (virbr0) - 192.168.122.50 nest2.example.org (virbr1) - 192.168.100.100 nest3.example.net (virbr2) - 192.168.110.25 I'm quite aware the above iptables rules were added by libvirt, but I'm still managing the firewall primarily through the *firewall-cmd* command. >From what I gathered... ---------------------------------------------------------- nest3 can ping nest1 and nest2. Nest3 ping nest1 and made a hit here: 8967 14M ACCEPT all -- * virbr2 0.0.0.0/0 192.168.110.0/24 ctstate RELATED,ESTABLISHED 5262 279K ACCEPT all -- virbr2 * 192.168.110.0/24 0.0.0.0/0 Nest3 ping nest2 and made a hit here: 8967 14M ACCEPT all -- * virbr2 0.0.0.0/0 192.168.110.0/24 ctstate RELATED,ESTABLISHED 5262 279K ACCEPT all -- virbr2 * 192.168.110.0/24 0.0.0.0/0 ---------------------------------------------------------- ---------------------------------------------------------- Nest1 can ping nest2, but cannot ping nest3. Nest1 ping nest2 and made a hit here: 8510 13M ACCEPT all -- * virbr0 0.0.0.0/0 192.168.122.0/24 ctstate RELATED,ESTABLISHED 5177 275K ACCEPT all -- virbr0 * 192.168.122.0/24 0.0.0.0/0 Nest1 ping nest3 and made a hit here: 70 5832 REJECT all -- * virbr2 0.0.0.0/0 0.0.0.0/0 reject-with icmp-port-unreachable ---------------------------------------------------------- ---------------------------------------------------------- Nest2 cannot ping nest1 and nest3. Nest2 ping nest1 and made a hit here: 61 5100 REJECT all -- * virbr0 0.0.0.0/0 0.0.0.0/0 reject-with icmp-port-unreachable Nest2 ping test3 and made a hit here: 70 5832 REJECT all -- * virbr2 0.0.0.0/0 0.0.0.0/0 reject-with icmp-port-unreachable ---------------------------------------------------------- >From my observation, I see that the order of the virtual networks in the iptables FORWARD chain makes a difference. Each chunk associated with the virtual network in the chain, consisting of five lines, is exactly as described in the aforementioned link (http://libvirt.org/firewall.html). The virtual network in the top chunk of the chain can communicate with virtually all networks as opposed to the network in the last chunk that is consistent with the intention of the original design. I'm using CentOS 7.3 with libvirt 2.0. I even tried reproducing this with CentOS 6.9 as I thought it was possible the firewalld may have influenced the change, but I was still getting similar result. Right now, I'm not certain if this is already a reported known bug, but I'm highly convinced this configuration is unofficially unsupported for quite a while. Can someone confirm this? This is my very first mailing list submission ever, and I apologize in advance if I couldn't figure out how to conveniently search up a similar discussion as this one in the archive. If this is in fact near impossible to implement in accordance to the intended design, then I'd like to have this confirmation publicly included in the docs.

Thanks, Travis Johnson

_______________________________________________ libvirt-users mailing list libvirt-users(a)redhat.com https://www.redhat.com/mailman/listinfo/libvirt-users

On Tue, Jun 20, 2017 at 10:05:19AM +0200, Martin Kletzander wrote: > On Tue, Jun 20, 2017 at 02:26:59AM -0400, Travis S. Johnson wrote: >> Hello, >> >> I came across an interesting problem in my home lab a few weeks ago >> as I'm >> prepping for my RHCE exam using Michael Jang study guide. I've been >> at this >> for days now, and I still can't wrap my head around how two or more >> virtual >> networks in default NAT configuration are even allowed to communicate >> with >> each other despite what the libvirt documentation said. >> >> >> Here's the excerpt I'm referring to in the wiki link here: >> http://wiki.libvirt.org/page/Networking#Forwarding_Incoming_Connections: >> >>> By default, guests that are connected via a virtual network with >>> <forward >>> mode='nat'/> can make any outgoing network connection they like. >>> Incoming >>> connections are allowed from the host, and from other guests >>> connected to >>> the same libvirt network, but all other incoming connections are >>> blocked by >>> iptables rules. >> I did not read this properly... >> >> Also here's another assertion from 'The virtual network driver' >> section in >> http://libvirt.org/firewall.html: >> >>> type=nat >>> >>> Allow inbound related to an established connection. Allow outbound, but >>> only from our expected subnet. Allow traffic between guests. Deny >>> all other I would expect the 'traffic between guests' to mean only guests on the same network. Also that lines up with what's written above.

>>> inbound. Deny all other outbound. >> [...] > > Thanks for reporting this. It's clearly a bug in libvirt. The rules > are in this order: > > all rules for virbr0 > all rules for virbr1 > all rules for virbr2 > > But what we should do instead is: > > input rules for all networks > local rules for all networks > output rules for all networks > reject rules for all networks

> > The problem is that we do not know how other rules look like. So what > we might need to do is create chains where rules for the first network > are, then only append network rules into those chains. > > Would you mind filing a bug for this issue, so we can properly track it > and don't forget about it? I'll have a look at it in the meantime, but > don't promise anything since I'm not that familiar with that part of the > codebase. > Actually, now that I'm thinking about it, the problem is not that we disallow the communication from virbr1 to virbr0, but the problem is that we allow the connection from virbr0 to everywhere.

Maybe the solution would be: for each network, insert the rules on top of the forward chain and for each started network, explicitly reject that one. So that after one network starts it would look like this (I'm writing this from memory just to illustrate the idea, not actually looking up how stuff looks): ACCEPT any virbr0 RELATED,ESTABLISHED ACCEPT virbr0 virbr0 ACCEPT virbr0 any src:192.168.122.0/24 REJECT any virbr0 REJECT virbr0 any And after second network is started, we'd have: # This one is new: REJECT virbr1 virbr0

# These ones would be normally at the end, IIRC: ACCEPT any virbr1 RELATED,ESTABLISHED ACCEPT virbr1 virbr1 ACCEPT virbr1 any src:192.168.122.0/24 REJECT any virbr1 REJECT virbr1 any # These are left as they were: ACCEPT any virbr0 RELATED,ESTABLISHED ACCEPT virbr0 virbr0 ACCEPT virbr0 any src:192.168.122.0/24 REJECT any virbr0 REJECT virbr0 any Cc'ing Laine so that he can weigh in as he has way more knowledge of this part of the code =)