[bug#77153,v2,3/3] doc: cookbook: Custom NAT-based libvirt networks.
Commit Message
* doc/guix-cookbook.texi (Virtual Machines): [Custom NAT-based network
for libvirt]: New section.
Change-Id: Ice79c5dc8183ec694ac8b846a5ec88cb98cac9ff
---
doc/guix-cookbook.texi | 124 +++++++++++++++++++++++++++++++++++++++++
1 file changed, 124 insertions(+)
@@ -3751,6 +3751,7 @@ Virtual Machines
@menu
* Network bridge for QEMU::
* Routed network for libvirt::
+* Custom NAT-based network for libvirt::
@end menu
@node Network bridge for QEMU
@@ -3975,6 +3976,129 @@ Routed network for libvirt
should work from within your VM; you can e.g.@: run @samp{ping gnu.org}
to verify that it functions correctly.
+@node Custom NAT-based network for libvirt
+@section Custom NAT-based network for libvirt
+
+As mentioned in the preceding section (@pxref{Routed network for libvirt}),
+libvirt allows virtual networks to be defined via XML files and managed
+by the @command{virsh} command. The details of the creation and removal
+of virtual network switches are handled by libvirt, so the user does not
+have to deal with them.
+
+However, libvirt's handling of virtual network switches can sometimes
+clash with more complex networking setups. In particular, the iptables
+rules inserted by libvirt for switches operating in the NAT mode can
+clash with existing iptables/nftables rules, leading to insecure or
+broken packet filtering.
+
+In such cases, the only solution is to manually set up a virtual network
+switch. This section will provide instructions on how to do so using
+Guix System services.
+
+This section is based on
+@url{https://jamielinux.com/docs/libvirt-networking-handbook/custom-nat-based-network.html,
+the corresponding section from the (unofficial) libvirt Networking
+Handbook}. It should be noted that at the time of writing (March 2025),
+this resource had not been updated since 2015, and is therefore somewhat
+outdated. In particular, the creation of a `dummy interface' is no
+longer necessary.
+
+@subsection Creating the virtual network bridge
+
+The @code{static-networking-service-type} can be used to create a
+virtual network bridge and assign an IP address to it:
+
+@example lisp
+(service static-networking-service-type
+ (list (static-networking
+ ;; The default provision is 'networking; if you're using any
+ ;; other service with this provision, such as
+ ;; `network-manager-service-type`, then you need to change the
+ ;; default.
+ (provision '(static-networking))
+ (links
+ (list (network-link
+ (name "virbr0")
+ (type 'bridge)
+ (arguments '((stp_state . 1))))))
+ (addresses
+ (list (network-address
+ (device "virbr0")
+ (value "192.168.10.1/24")))))))
+@end example
+
+@subsection Running dnsmasq for the virtual network bridge
+
+The @code{dnsmasq-service-type} can be used to provide DNS and DHCP for
+guests connected to this virtual network switch:
+
+@example lisp
+(service dnsmasq-service-type
+ (dnsmasq-configuration
+ ;; You can have multiple instances of `dnsmasq-service-type` as long
+ ;; as each one has a different provision.
+ (provision '(dnsmasq-virbr0))
+ (extra-options (list
+ ;; Only bind to the virtual bridge. This
+ ;; avoids conflicts with other running
+ ;; dnsmasq instances.
+ "--except-interface=lo"
+ "--interface=virbr0"
+ "--bind-dynamic"
+ ;; IPv4 addresses to offer to VMs. This
+ ;; should match the chosen subnet.
+ "--dhcp-range=192.168.10.2,192.168.10.254"))))
+@end example
+
+@subsection Configuring NAT for the virtual network switch
+
+If you intend to use the virtual network switch in NAT mode, you will
+need to use nftables (or iptables) rules to set up IP masquerading. The
+following example shows how to use @code{nftables-service-type} to do
+this:
+
+@example lisp
+(service nftables-service-type
+ (nftables-configuration
+ (ruleset
+ (plain-file "nftables.conf"
+ "\
+table inet filter @{
+
+ chain input @{
+ type filter hook input priority filter; policy drop;
+ # Add your existing packet filtering rules here...
+ iifname virbr0 udp dport 67 counter accept comment \"allow dhcp on virbr0\"
+ iifname virbr0 meta l4proto @{tcp, udp@} th dport 53 accept \\
+ comment \"allow dns on virbr0\"
+ @}
+
+ chain forward @{
+ type filter hook forward priority filter; policy drop;
+ # Add your existing forwarding rules here...
+ iifname virbr0 accept comment \"allow outbound traffic from virbr0\"
+ oifname virbr0 ct state @{established, related @} accept \\
+ comment \"allow established traffic to virbr0\"
+ @}
+
+@}
+
+table inet nat @{
+ chain postrouting @{
+ type nat hook postrouting priority srcnat; policy accept;
+ # Add your existing nat rules here...
+ iifname virbr0 ip daddr @{ 224.0.0.0/24, 255.255.255.255/32 @} return \\
+ comment \"don't masquerade to reserved address blocks\"
+ iifname virbr0 oifname != virbr0 masquerade \\
+ comment \"masquerade all outgoing traffic from VMs\"
+ @}
+@}
+"))))
+@end example
+
+Ensure that you have IPv4 forwarding enabled (you can use
+@code{sysctl-service-type} for this).
+
@c *********************************************************************
@node Advanced package management
@chapter Advanced package management