wondering if anyone can throw an advice or deeper knowledge on a network issue im facing since my server got updated to 24.04 last week from 22.04, in short i have two NICs, a backup 1gb enp2s0 in the mobo that allows it to receive the WoL signal from the NAS if a power outage drained the UPS battery, the second enp3s0 NIC is the 10gb that is for network traffic, the update crashed my network, i tried debugging with chatgpt which may very much likely introduced additional issues summarized version of the issues and fix attempts from the chat below the netplans
Previously working netplan on v22
enp3s0:
dhcp4: true
dhcp6: false
mtu: 9000
dhcp4-overrides:
route-metric: 100
optional: true
lan0:
dhcp4: true
dhcp6: false
mtu: 1500
dhcp4-overrides:
route-metric: 300
optional: true
last attempt netplan config:
ethernets:
enp3s0:
dhcp4: no
addresses:
gateway4: 192.168.1.1
mtu: 1500
optional: true
enp2s0:
dhcp4: no
addresses:
optional: true
--skip if not interested in the summary--
- Ubuntu Server (recent LTS, likely 24.04)
- Netplan + systemd-networkd
- Dual NIC setup:
- enp3s0 → 10G (primary)
- enp2s0 → 1G (fallback)
- Same subnet:
- 192.168.1.135 (10G)
- 192.168.1.136 (1G)
- Gateway: 192.168.1.1
CURRENT STATE
IP addresses assigned correctly:
ip addr
- enp3s0 → 192.168.1.135/24
- enp2s0 → 192.168.1.136/24
Routing table:
ip route
Output:
192.168.1.0/24 dev enp3s0 scope link
192.168.1.0/24 dev enp2s0 scope link
NO default route exists, failed to add manually or via netplan
Connectivity behavior
ping 192.168.1.1
→ Network is unreachable
ARP works:
ip neigh show dev enp3s0
192.168.1.1 lladdr XX:XX:XX:XX:XX:XX REACHABLE
systemd-networkd status (blurry as no network to screenshot) pic attached
networkctl status enp3s0
Key output:
State: routable (failed)
Could not set route: Nexthop has invalid gateway. Network is unreachable
Netplan behavior
sudo netplan generate
sudo netplan apply
- Very slow execution
- Warning:
gateway4 has been deprecated
- Still fails to apply default route either as default via 192.168.1.1 or as gateway4
COMMANDS EXECUTED (multiple attempts)
Interface reset
ip addr flush dev enp3s0
ip link set enp3s0 down/up
ip addr add 192.168.1.135/24 dev enp3s0
Route attempts
ip route add default via 192.168.1.1 dev enp3s0
→ Nexthop has invalid gateway
ip route add default via 192.168.1.1 dev enp3s0 onlink
→ accepted, but no connectivity
Subnet route attempts
ip route add 192.168.1.0/24 dev enp3s0 scope link
→ sometimes “File exists”
Full routing reset
ip route flush table main
ip addr flush dev enp3s0
NIC isolation test
ip link set enp2s0 down with no luck to get the 3s0 to work
ARP verification
ip neigh show dev enp3s0
→ gateway reachable
ADDITIONAL OBSERVATIONS
- networkd fails specifically when applying gateway
- ARP resolution works → NOT physical/network issue
- Both NICs in same subnet (possible routing ambiguity)
- After route flush → system unable to recover default route
- Manual routing also fails to restore connectivity
- Netplan apply is unusually slow
KEY SYMPTOMS
- ❌ Cannot add default route (invalid gateway)
- ❌ “Network is unreachable” even with valid IP
- ❌ systemd-networkd reports interface as routable (failed)
- ✅ ARP to gateway works
- ❌ Neither external/internal connectivity (ping 1.1.1.1 fails just the same as pinging the gateway 192.168.1.1)
- ❌ SSH unreachable
- ❌ cloudflared tunnel fails
WHAT WAS WORKING BEFORE
- Same hardware
- Same subnet config
- Same dual NIC design
- Static routing worked for several months