In modern networking setups, systems often have multiple interfaces where IPv6 Stateless Address Autoconfiguration (SLAAC) might not be desirable for all of them. For instance, in Debian-based appliances, you may want eth0 to obtain an IPv6 address via SLAAC while restricting other interfaces (eth1, wlan0, etc.) to link-local or manually configured addresses.

Debian uses sysctl and NetworkManager or systemd-networkd for network configurations. To disable SLAAC on specific interfaces, you can tweak kernel parameters or use network configuration files.

Method 1: Using sysctl (Temporary Solution)

To disable SLAAC for a specific interface (e.g., eth1), run:

echo 0 > /proc/sys/net/ipv6/conf/eth1/accept_ra

This prevents Router Advertisements (RAs) from triggering SLAAC on eth1.

Method 2: Persistent Configuration via systemd-networkd

Edit or create /etc/systemd/network/10-eth1.network:

[Network]  
Description=Disable SLAAC on eth1  
[IPv6AcceptRA]  
UseAutonomousPrefix=false

After saving, restart the network service:

systemctl restart systemd-networkd

Method 3: Using NetworkManager (if installed)

For NetworkManager-managed interfaces, modify /etc/NetworkManager/conf.d/no-slaac.conf:

[keyfile]  
unmanaged-devices=interface-name:eth1  

[connection-ethernet-eth1]  
ipv6.addr-gen-mode=stable-privacy

Then reload NetworkManager:

nmcli connection reload

Check if SLAAC is disabled on the target interface while still active on eth0:

ip -6 addr show eth1  
ip -6 addr show eth0

Only eth0 should display a globally scoped SLAAC-derived address.

If the changes don’t take effect:

• Ensure no conflicting DHCPv6 client (dhclient) is running.
• Check kernel logs for RA-related errors: dmesg | grep -i ipv6.

---

In modern Debian systems with multiple network interfaces, there are legitimate cases where administrators need fine-grained control over IPv6 Stateless Address Autoconfiguration (SLAAC). The typical scenario involves:

• Primary interface (eth0) using SLAAC for automatic configuration
• Secondary interfaces (eth1, eth2, etc.) requiring only link-local or static IPv6 addresses
• Preventing unwanted global IPv6 addresses on non-primary interfaces

Method 1: sysctl Interface-Specific Controls

The most precise approach uses per-interface sysctl parameters. Create or modify /etc/sysctl.d/60-ipv6-slaac.conf:

# Enable SLAAC on eth0
net.ipv6.conf.eth0.accept_ra = 1
net.ipv6.conf.eth0.autoconf = 1

# Disable SLAAC on all other interfaces
net.ipv6.conf.default.accept_ra = 0
net.ipv6.conf.default.autoconf = 0
net.ipv6.conf.all.accept_ra = 0
net.ipv6.conf.all.autoconf = 0

# Explicitly disable for specific interfaces (e.g., eth1)
net.ipv6.conf.eth1.accept_ra = 0
net.ipv6.conf.eth1.autoconf = 0

Apply changes immediately with: sysctl --system

Method 2: NetworkManager Configuration (If Used)

For systems using NetworkManager, create interface-specific profiles:

[connection]
id=eth1-no-slaac
type=ethernet
interface-name=eth1

[ipv6]
method=disabled
# OR for link-local only:
method=link-local

After configuration, verify with these commands:

ip -6 addr show dev eth0  # Should show SLAAC addresses
ip -6 addr show dev eth1  # Should only show link-local
cat /proc/sys/net/ipv6/conf/eth1/autoconf  # Should show 0

For older Debian versions using /etc/network/interfaces:

iface eth1 inet6 manual
    up ip link set dev eth1 up
    up sysctl -w net.ipv6.conf.eth1.autoconf=0
    up sysctl -w net.ipv6.conf.eth1.accept_ra=0

When dealing with unpredictable interface naming (e.g., predictible network interface names):

# Disable for all except primary in a dynamic way
for IFACE in $(ls /sys/class/net/ | grep -v eth0); do
    sysctl -w net.ipv6.conf.$IFACE.autoconf=0
    sysctl -w net.ipv6.conf.$IFACE.accept_ra=0
done