virtbr0 unable to Access the Internet

virtbr unable connecting to internet

Description

Saat melakukan instalasi Ubuntu Server 25.10 menggunakan QEMU/KVM di host Arch Linux, Virtual Machine (VM) berhasil mendapatkan alamat IP melalui DHCP dari bridge virbr0, namun gagal mengakses internet (error Network Unreachable atau IGN pada tahap pemilihan mirror).

setelah trouble shouting ternyata kebijakan firewall secara default memblokir semua lalu lintas yang mencoba "menyeberang"/FORWARD antar interface. Karena VM berada di virbr0 dan ingin keluar melalui WiFi/Ethernet, paket datanya langsung dibuang (DROP) oleh rule tersebut.

this is full script to create virtbr0, I am using bash.

script.sh

# pastikan modul tun sudah aktif
sudo modprobe tun
# akan otomatis membuat bridge 'virtbr0'
sudo virsh net-start default

#sudo virsh net-autostart default
ip a show virbr0

Di script ini pertama-tama akan mengaktifkan modul tun di system linux aku lalu membuat bridge virtbr0 atau virtual bridge.

Note

Sebelum menjalankan script dan VM, pastikan tambah rules di iptables untuk meloloskan packet data dari lewat di virtbr0 atau virtual bridge, dengan command ini.

sudo iptables --list

this is its output and will be distict in your system.

# sudo iptables --list 
Chain INPUT (policy ACCEPT)
target     prot opt source               destination         

Chain FORWARD (policy DROP)
target     prot opt source               destination         
ACCEPT     all  --  anywhere             anywhere            
ACCEPT     all  --  anywhere             anywhere            
DOCKER-USER  all  --  anywhere             anywhere            
DOCKER-FORWARD  all  --  anywhere             anywhere            

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination         

Chain DOCKER (4 references)
target     prot opt source               destination         
DROP       all  --  anywhere             anywhere            
DROP       all  --  anywhere             anywhere            
DROP       all  --  anywhere             anywhere            
DROP       all  --  anywhere             anywhere            

Chain DOCKER-BRIDGE (1 references)
target     prot opt source               destination         
DOCKER     all  --  anywhere             anywhere            
DOCKER     all  --  anywhere             anywhere            
DOCKER     all  --  anywhere             anywhere            
DOCKER     all  --  anywhere             anywhere            

Chain DOCKER-CT (1 references)
target     prot opt source               destination         
ACCEPT     all  --  anywhere             anywhere             ctstate RELATED,ESTABLISHED
ACCEPT     all  --  anywhere             anywhere             ctstate RELATED,ESTABLISHED
ACCEPT     all  --  anywhere             anywhere             ctstate RELATED,ESTABLISHED
ACCEPT     all  --  anywhere             anywhere             ctstate RELATED,ESTABLISHED

Chain DOCKER-FORWARD (1 references)
target     prot opt source               destination         
DOCKER-CT  all  --  anywhere             anywhere            
DOCKER-INTERNAL  all  --  anywhere             anywhere            
DOCKER-BRIDGE  all  --  anywhere             anywhere            
ACCEPT     all  --  anywhere             anywhere            
ACCEPT     all  --  anywhere             anywhere            
ACCEPT     all  --  anywhere             anywhere            
ACCEPT     all  --  anywhere             anywhere            

Chain DOCKER-INTERNAL (1 references)
target     prot opt source               destination         

Chain DOCKER-USER (1 references)
target     prot opt source               destination         

Policy DROP: Berarti "Larang semua data lewat kecuali ada aturan khusus". virbr0: Karena belum ada aturan khusus yang mengizinkan virbr0, maka installer Ubuntu kamu "terisolasi" di dalam laptop saja.

# Izinkan paket data lewat di bridge virtual
sudo iptables -I FORWARD -o virbr0 -j ACCEPT
sudo iptables -I FORWARD -i virbr0 -j ACCEPT

# cativate IP Forwarding 
sudo sysctl -w net.ipv4.ip_forward=1

Untuk cara alternatifnya yaitu di permanenkan.

yaitu dengan cara

Permanenkan IP Forwarding

Buat file baru.

sudo nano /etc/sysctl.d/30-ipforward.conf

isi dengan net.ipv4.ip_forward=1 untuk ngeforward IP dari virtbr0.