chore: clarify network naming in network-setup.md

Make it more clear that the `iface_id` is only used internally
in Firecracker.
Move information about the order of network interface creation
and network interface names in the guest from FAQ into
network-setup.md.

Signed-off-by: Egor Lazarchuk <[email protected]>

Author: ShadowCurse

FAQ.md

@@ -178,22 +178,6 @@ them according to your needs.
 The relatively high FD usage is expected and correct. Firecracker heavily relies
 on event file descriptors to drive device emulation.
 
-### How does network interface numbering work?
-
-There is no relation between the numbering of the `/network-interface` API calls
-and the number of the network interface in the guest. Rather, it is usually the
-order of network interface creation that determines the number in the guest (but
-this depends on the distribution).
-
-For example, when you create two network interfaces by calling
-`/network-interfaces/1` and then `/network-interfaces/0`, it may result in this
-mapping:
-
-```console
-/network-interfaces/1 -> eth0
-/network-interfaces/0 -> eth1
-```
-
 ### How can I gracefully reboot the guest? How can I gracefully poweroff the guest?
 
 Firecracker does not virtualize guest power management, therefore operations

docs/network-setup.md

@@ -107,9 +107,6 @@ sudo iptables-nft -A FORWARD -i tap0 -o eth0 -j ACCEPT
 
 ## Setting Up Firecracker
 
-Before starting the guest, configure the network interface using Firecracker's
-API:
-
 **Note:** If you use the rootfs from the
 [getting started guide](getting-started.md), you need to use a specific `MAC`
 address like `06:00:AC:10:00:02`. In this `MAC` address, the last 4 bytes
@@ -118,13 +115,27 @@ it is `172.16.0.2`. Otherwise, you can skip the `guest_mac` field for network
 configuration. This way, the guest will generate a random MAC address on
 startup.
 
+**Note:** The `iface_id` used during VM configuration is internal to Firecracker
+and only used for management purposes. The name of the network interface in the
+guest is determined by the guest itself. In this example we assume the guest
+will name the network interface `eth0`.
+
+> [!NOTE]
+> Firecracker cannot guarantee that the network interfaces in the guest will be
+> initialized in the guest in the same order as API calls used to set them up.
+> At the same time most kernels/distributions do initialize devices in the API 
+> defined order.
+
+Before starting the guest, configure the network interface using Firecracker's
+API:
+
 ```bash
 curl --unix-socket /tmp/firecracker.socket -i \
-  -X PUT 'http://localhost/network-interfaces/eth0' \
+  -X PUT 'http://localhost/network-interfaces/my_network0' \
   -H 'Accept: application/json' \
   -H 'Content-Type: application/json' \
   -d '{
-      "iface_id": "eth0",
+      "iface_id": "my_network0",
       "guest_mac": "06:00:AC:10:00:02",
       "host_dev_name": "tap0"
     }'
@@ -136,7 +147,7 @@ configuration file like this:
 ```json
 "network-interfaces": [
   {
-    "iface_id": "eth0",
+    "iface_id": "my_network0",
     "guest_mac": "06:00:AC:10:00:02",
     "host_dev_name": "tap0"
   }
@@ -148,9 +159,6 @@ Alternatively, if you are using firectl, add
 
 ## In The Guest
 
-Once you have booted the guest, it will have its networking interface with the
-name specified by `iface_id` in the Firecracker configuration.
-
 You'll now need to assign the guest its IP, activate the guest's networking
 interface and set up the `tap` IP as the guest's gateway address, so that
 packets are routed through the `tap` device, where they are then picked up by