Fix: Operators had to specify DNS resolution

The DNS resolver defaulted to `resolv.conf`, which is not the default on all supported systems.

This adds an option to detect if systemd-resolved is available, and else defaults to `/etc/resolv.conf`.

.github/workflows/test-on-droplet-ubuntu-22.04.yml

@@ -65,7 +65,6 @@ jobs:
           scp packaging/target/aleph-vm.ubuntu-22.04.deb root@${DROPLET_IPV4}:/opt
           ssh root@${DROPLET_IPV4} DEBIAN_FRONTEND=noninteractive "apt install -y /opt/aleph-vm.ubuntu-22.04.deb"
           ssh root@${DROPLET_IPV4} "echo ALEPH_VM_SUPERVISOR_HOST=0.0.0.0 >> /etc/aleph-vm/supervisor.env"
-          ssh root@${DROPLET_IPV4} "echo ALEPH_VM_DNS_RESOLUTION=resolvectl >> /etc/aleph-vm/supervisor.env"
           ssh root@${DROPLET_IPV4} "echo ALEPH_VM_ALLOCATION_TOKEN_HASH=9f86d081884c7d659a2feaa0c55ad015a3bf4f1b2b0b822cd15d6c15b0f00a08 >> /etc/aleph-vm/supervisor.env"
           ssh root@${DROPLET_IPV4} "systemctl restart aleph-vm-supervisor"
 

doc/INSTALL-Debian-12.md

@@ -56,19 +56,19 @@ ALEPH_VM_DOMAIN_NAME=vm.example.org
 
 #### Network configuration
 
+The network configuration is detected automatically.
+
 The default network interface is detected automatically from the IP routes. 
-On some systems, this is not the desired configuration and you will want to configure the default interface
-by adding:
+You can configure the default interface manually instead by adding:
 ```
 ALEPH_VM_NETWORK_INTERFACE=enp0s1
 ```
 (don't forget to replace `enp0s1` with the name of your default network interface).
 
-Debian 12 by default uses `/etc/resolv.conf` for DNS resolution. The VM Supervisor uses this by default.
-If your system uses [systemd-resolved](https://manpages.debian.org/bullseye/systemd/systemd-resolved.8.en.html)
-instead, uncomment and add the following setting:
+You can configure the DNS resolver manually by using one of the following options:
 ```
-#ALEPH_VM_DNS_RESOLUTION=resolvctl
+ALEPH_VM_DNS_RESOLUTION=resolvectl
+ALEPH_VM_DNS_RESOLUTION=resolv.conf
 ```
 
 > 💡 You can instead specify the DNS resolvers used by the VMs using `ALEPH_VM_DNS_NAMESERVERS=["1.2.3.4", "5.6.7.8"]`.

doc/INSTALL-Ubuntu-22.04.md

@@ -56,22 +56,24 @@ ALEPH_VM_DOMAIN_NAME=vm.example.org
 
 #### Network configuration
 
-Ubuntu 22.04 by default uses [systemd-resolved](https://manpages.ubuntu.com/manpages/jammy/man8/systemd-resolved.service.8.html)
-for DNS resolution. The following setting configures the VM Supervisor to use it instead of reading the default `/etc/resolv.conf`.
-```
-ALEPH_VM_DNS_RESOLUTION=resolvectl
-```
-
-> 💡 You can instead specify the DNS resolvers used by the VMs using `ALEPH_VM_DNS_NAMESERVERS=["1.2.3.4", "5.6.7.8"]`.
+The network configuration is detected automatically.
 
 The default network interface is detected automatically from the IP routes. 
-On some systems, this is not the desired configuration and you will want to configure the default interface
-by adding:
+You can configure the default interface manually instead by adding:
 ```
 ALEPH_VM_NETWORK_INTERFACE=enp0s1
 ```
 (don't forget to replace `enp0s1` with the name of your default network interface).
 
+You can configure the DNS resolver manually by using one of the following options:
+```
+ALEPH_VM_DNS_RESOLUTION=resolvectl
+ALEPH_VM_DNS_RESOLUTION=resolv.conf
+```
+
+> 💡 You can instead specify the DNS resolvers used by the VMs using `ALEPH_VM_DNS_NAMESERVERS=["1.2.3.4", "5.6.7.8"]`.
+
+
 #### Volumes and partitions
 
 Two directories are used to store data from the network:

vm_supervisor/conf.py

@@ -5,7 +5,7 @@
 from enum import Enum
 from os.path import abspath, exists, isdir, isfile, join
 from pathlib import Path
-from subprocess import check_output
+from subprocess import CalledProcessError, check_output
 from typing import Any, Dict, Iterable, List, Literal, NewType, Optional, Union
 
 from pydantic import BaseSettings, Field
@@ -19,6 +19,7 @@
 
 
 class DnsResolver(str, Enum):
+    detect = "detect"  # Detect the resolver used by the system
     resolv_conf = "resolv.conf"  # Simply copy from /etc/resolv.conf
     resolvectl = "resolvectl"  # Systemd-resolved, common on Ubuntu
 
@@ -79,6 +80,30 @@ def get_default_interface() -> Optional[str]:
     return None
 
 
+def obtain_dns_ips(dns_resolver: DnsResolver, network_interface: str) -> List[str]:
+    # The match syntax is not yet available as of Python 3.9
+    # match dns_resolver:
+    if dns_resolver == DnsResolver.detect:
+        # Use a try-except approach since resolvectl can be present but disabled and raise the following
+        # "Failed to get global data: Unit dbus-org.freedesktop.resolve1.service not found."
+        try:
+            return list(resolvectl_dns_servers_ipv4(interface=network_interface))
+        except (FileNotFoundError, CalledProcessError):
+            if Path("/etc/resolv.conf").exists():
+                return list(etc_resolv_conf_dns_servers())
+            else:
+                raise FileNotFoundError("No DNS resolver found")
+
+    elif dns_resolver == DnsResolver.resolv_conf:
+        return list(etc_resolv_conf_dns_servers())
+
+    elif dns_resolver == DnsResolver.resolvectl:
+        return list(resolvectl_dns_servers_ipv4(interface=network_interface))
+
+    else:
+        assert "No DNS resolve defined, this should never happen."
+
+
 class Settings(BaseSettings):
     SUPERVISOR_HOST = "127.0.0.1"
     SUPERVISOR_PORT: int = 4020
@@ -134,7 +159,7 @@ class Settings(BaseSettings):
         description="Use the Neighbor Discovery Protocol Proxy to respond to Router Solicitation for instances on IPv6",
     )
 
-    DNS_RESOLUTION: Optional[DnsResolver] = DnsResolver.resolv_conf
+    DNS_RESOLUTION: Optional[DnsResolver] = DnsResolver.detect
     DNS_NAMESERVERS: Optional[List[str]] = None
 
     FIRECRACKER_PATH = Path("/opt/firecracker/firecracker")
@@ -286,15 +311,10 @@ def setup(self):
             self.NETWORK_INTERFACE = get_default_interface()
 
         if self.DNS_NAMESERVERS is None and self.DNS_RESOLUTION:
-            if self.DNS_RESOLUTION == DnsResolver.resolv_conf:
-                self.DNS_NAMESERVERS = list(etc_resolv_conf_dns_servers())
-
-            elif self.DNS_RESOLUTION == DnsResolver.resolvectl:
-                self.DNS_NAMESERVERS = list(
-                    resolvectl_dns_servers_ipv4(interface=self.NETWORK_INTERFACE)
-                )
-            else:
-                assert "This should never happen"
+            self.DNS_NAMESERVERS = obtain_dns_ips(
+                dns_resolver=self.DNS_RESOLUTION,
+                network_interface=self.NETWORK_INTERFACE,
+            )
 
     def display(self) -> str:
         attributes: Dict[str, Any] = {}