[libvirt] [PATCH 4/5] network: regain guest network connectivity after firewalld switch to nftables

Tue Jan 15 16:39:45 UTC 2019

On Wed, Jan 09, 2019 at 09:57:36PM -0500, Laine Stump wrote:
> From: Laine Stump <laine at redhat.com>
> 
> In the past (when both libvirt and firewalld used iptables), if either
> libvirt's rules *OR* firewalld's rules accepted a packet, it would be
> accepted. This was because libvirt and firewalld rules were processed
> by the same kernel hook.
> 
> But now firewalld can use nftables for its backend, while libvirt's
> firewall rules are still using iptables; iptables rules are still
> processed, but at a different time during packet processing
> (i.e. during a different hook) than the firewalld nftables rules. The
> result is that a packet must be accepted by *BOTH* the libvirt
> iptables rules *AND* the firewalld nftable rules in order to be
> accepted.
> 
> This causes pain because
> 
> 1) libvirt always adds rules to permit DNS and DHCP (and sometimes
> TFTP) from guests to the local host. But libvirt's bridges are in
> firewalld's "default" zone (which is usually the zone called
> "public"). The public zone allows ssh, but doesn't allow DNS, DHCP, or
> TFTP. So even though libvirt's rules allow the DHCP and DNS traffic,
> the firewalld rules dont, thus guests connected to libvirt's bridges
> can't acquire an IP address from DHCP, nor can they make DNS queries
> to the DNS server libvirt has setup on the host.
> 
> 2) firewalld's higher level "rich rules" don't yet have the ability to
> configure the acceptance of forwarded traffic (traffic that is going
> somewhere beyond the host), so any traffic that needs to be forwarded
> is rejected by the public zone's default "reject" policy (which
> rejects all traffic in the zone not specifically allowed by the rules
> in the zone, whether that traffic is destined to be forwarded or
> locally received by the host).
> 
> libvirt can't send "direct" nftables rules (firewalld only supports
> that for iptables), so we can't solve this problem by just sending
> explicit nftables rules instead of explicit iptables rules (which, if
> it could be done, would place libvirt's rules in the same hook as
> firewalld's native rules, and thus eliminate the need for packets to
> be accepted by both libvirt's and firewalld's own rules).
> 
> However, we can take advantage of a quirk in firewalld zones that have
> a default policy of "accept" (meaning any packet that doesn't match a
> specific rule in the zone will be *accepted*) - this default accept will
> also accept forwarded traffic (not just traffic destined for the host).
> 
> Putting each network's bridge in a new zone called "libvirt" which has
> a default policy of accept will allow the forwarded traffic to pass,
> but the same default accept policy that fixes forwarded traffic also
> causes *all* traffic from guest to host to be accepted. To solve this
> new problem, we can take advantage of a new feature in firewalld
> (currently slated for firewalld-0.7.0) - priorities for rich rules -
> to add a low priority rule that rejects all local traffic (but leaves
> alone all forwarded traffic).

Ok, so we depend on newer firewalld. Any older version using nftables
will still be broken.

I think it would be quite desirable if we queried the firewalld version
and backend and reported a fatal error to the user if they try to
start a virtual network when we know it will be broken.

You can get version from /org/fedoraproject/FirewallD1

    org.freedesktop.Properties.Get("org.fedoraproject.FirewallD1", "version")

And backend from /org/fedoraproject/FirewallD1/config

    org.freedesktop.Properties.Get("org.fedoraproject.FirewallD1.config", "FirewallBackend")

> diff --git a/src/network/bridge_driver_linux.c b/src/network/bridge_driver_linux.c
> index dd08222653..a32f19bcf0 100644
> --- a/src/network/bridge_driver_linux.c
> +++ b/src/network/bridge_driver_linux.c
> @@ -27,6 +27,7 @@
>  #include "virstring.h"
>  #include "virlog.h"
>  #include "virfirewall.h"
> +#include "virfirewalld.h"
>  
>  #define VIR_FROM_THIS VIR_FROM_NONE
>  
> @@ -638,6 +639,14 @@ int networkAddFirewallRules(virNetworkDefPtr def)
>      virFirewallPtr fw = NULL;
>      int ret = -1;
>  
> +
> +    /* if firewalld is active, try to set the default "libvirt" zone,
> +     * but ignore failure, since the version of firewalld on the host
> +     * may have failed to load the libvirt zone
> +    */

Ah, I was wondering when you checked the version. This is the place
where I think we ought to raise an error

> +    if (virFirewallDStatus() >= 0)
> +       ignore_value(virFirewallDInterfaceSetZone(def->bridge, "libvirt"));
> +
>      fw = virFirewallNew();
>  
>      virFirewallStartTransaction(fw, 0);
> diff --git a/src/network/libvirt.zone b/src/network/libvirt.zone
> new file mode 100644
> index 0000000000..1750ba2f06
> --- /dev/null
> +++ b/src/network/libvirt.zone
> @@ -0,0 +1,14 @@
> +<?xml version="1.0" encoding="utf-8"?>
> +<zone target="ACCEPT">
> +  <short>libvirt</short>
> +  <description>The default policy of "ACCEPT" allows all packets to/from interfaces in the zone to be forwarded, while the (*low priority*) reject rule blocks any traffic destined for the host, except those services explicitly listed (that list can be modified as required by the local admin). This zone is intended to be used only by libvirt virtual networks - libvirt will add the bridge devices for all new virtual networks to this zone by default.</description>

Line wrapping :-)

> +
> +<rule priority='127'>
> +  <reject/>
> +</rule>
> +<service name='dhcp'/>
> +<service name='dhcpv6'/>
> +<service name='dns'/>
> +<service name='ssh'/>
> +<service name='tftp'/>
> +</zone>

I wonder if we should have an configure time check so that we do
*not* install this file if built agains an old firewalld version.
By unconditionally installing it we're probably causing firewalld
to emit an error message in syslog which some sysadmin is almost
certainly to report a bug about

Regards,
Daniel
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