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Re: [libvirt-users] UDP broadcasts vs. nat Masquerading issue

On 6/28/19 10:23 AM, Nikolai Zhubr wrote:
Hi all,

I'm observing an issue that as soon as libvirt starts, UPD broadcasts going through physical network (and unrelated to any virtualization) get broken. Specifically, windows neighbourhood browsing through samba's nmbd starts suffering badly (Samba is running on this same box).

At the moment I'm running a quite outdated version 1.2.9 of libvirt, but other than this issue, it does its job pretty well, so I'd first consider some patching/backporting rather than totally replacing it with a new one. Anyway, I first need to better understand what is going on and what is wrong with it.
This could also be related somewhat to
but I suppose it is not exactly that thing.

I've already figured the source of trouble is anyway related to these rules added:


*None* of those rules were added by libvirt (unless your build of libvirt, in addition to being ancient, has also been heavily hacked by a third party with downstream-only patches, although I can't imagine how the rules you show could possibly be a useful addition).

Detailed documentation of what iptables rules are added by libvirt can be found here:


(particularly in the "virtual network" section).

The masquerade rules added by libvirt are based on the IP address of the NATed subnet, e.g.:

A LIBVIRT_PRT -s ! -d -p tcp \
  -j MASQUERADE --to-ports 1024-65535
-A LIBVIRT_PRT -s ! -d -p udp \
   -j MASQUERADE --to-ports 1024-65535
-A LIBVIRT_PRT -s ! -d \

(this is from a less ancient version of libvirt that puts the rules on its own private chain, but in the version you are using the rule would be the same, except that "LIBVIRT_PRT" would be replaced with "POSTROUTING")

You can verify my "counter-claim" by running "virsh net-destroy" for all of your libvirt networks, and seeing that the offending rules haven't been removed.

In short, you need to look elsewhere for the culprit.

Here, virbr2_nic and vnet0 are used by libvirt for arranging network configurations for VMs, ok. However, br0 is a main interface of this host with primary ip address, with enp0s25 being a physical nic of this host, and it is used for all sorts of regular (unrelated to virtualization) communications. Also, br0 is used for attaching bridged (as opposed to NATed) VMs managed by libvirt.

Clearly, libvirt somehow chooses to set up masquerading for literally all existing network interfaces here (except lo),

It's clear that the rules are there. It's not clear that they were added by libvirt.

but I can't see a real reason for the first two rules in the list above. Furthermore, they corrupt UDP broadcats coming from outside and reaching this host (through enp0s25/br0) such that source address gets replaced by this hosts primary address (as per masquerading). I've verified this by arranging a hand-crafted UDP listener and printing the respective source addresses as seen by normal userspace.

Now I've discovered that I can "eliminate" the problem by either:

1. Removing "-A POSTROUTING -o br0 -j MASQUERADE" (manually)
2. Inserting "-A POSTROUTING -s -d -j ACCEPT"
(Of course correcting rules by hand is not a solution, just a test)

So question is, how the correct rules should ideally look like? And, is this issue known/fixed in most current libvirt?

Except for putting the libvirt-added rules in their own private chains (appearing in libvirt 5.1.0, released on Feb 1, 2019), the iptables rules added by libvirt to support its virtual networks didn't materially change in > 10 years. Your email is the first time I've ever seen such rules attributed to libvirt so, as I said above, I think you need to take a deeper dive into your host system's config.

Good luck!

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