[libvirt-users] guest A from virbr0 can talk to guest B in virbr1 but not vice versa
Travis S. Johnson
johnsontravis21 at gmail.com
Tue Jun 20 06:26:59 UTC 2017
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
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