-1

In order to substitute my ISP router, I decied to build a custom router using ArchLinux and connecting it directly to the ONT. My ISP applies QoS by seperating services into different VLANs. The internet service is provided by VLAN 12.

Because of this, I have created a VLAN interface using the following configurations

File -> /etc/systemd/network/20-enp0s20f0.network

[Match]
Name=enp0s20f0

[Link]
RequiredForOnline=yes

[Network]
DHCP=no
VLAN=enp0s20f0.12
LinkLocalAddressing=no

File -> /etc/systemd/network/20-enp0s20f0.12.netdev

[NetDev]
Name=enp0s20f0.12
Kind=vlan

[VLAN]
Id=12

File -> /etc/systemd/network/20-enp0s20f0.12.network

[Match]
Name=enp0s20f0.12

[Link]
RequiredForOnline=yes

[Network]
DHCP=ipv4
LinkLocalAddressing=no

[DHCPv4]
SendHostname=no
UseNTP=no
UseDNS=no

After restarting systemd-networkd, the vlan interface is created, but a second interface is also created.

enp0s20f0.12: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet xxx.yyy.zzz.iii  netmask 255.255.255.0  broadcast xxx.yyy.zzz.iii
        ether 31:91:8f:92:59:20  txqueuelen 1000  (Ethernet)
        RX packets 78540  bytes 525492249 (501.1 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 98901  bytes 15388297 (14.6 MiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

enp0s20f0.12-if: flags=195<UP,BROADCAST,RUNNING,NOARP>  mtu 1500
        inet6 fe80::ac8a:12ff:fe46:ad6d  prefixlen 64  scopeid 0x20<link>
        ether ae:8a:12:46:ad:6d  txqueuelen 32  (Ethernet)
        RX packets 78511  bytes 526426271 (502.0 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 78511  bytes 526426271 (502.0 MiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

Clearly the second interface receives data but my system is not using it. Can anyone explain to me what this extra interface is? Thank you!

1

The second interface pulls up the required link-local address for your ipv6 interface. For further info see: https://en.wikipedia.org/wiki/Link-local_address#IPv6_address

1
  • But why does it need a second interface for that? I don't remember seeing this on OpenWRT for example. If I want to configure the dhcpv6 client to configure an IPv6 from a network prefix obtained from a router advertisement using SLAAC, should I configure the client to use this extra interface?
    – Claymore
    Sep 16 '19 at 13:40

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.