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Beemo

Member

Registered: 2024-12-20

Posts: 36

[Solved] sd-networkd: One SIT tunnel creates 2 interfaces?

I created a SIT tunnel in systemd-networkd, but 2 interfaces turn up. One is the one I created, the other has the default name "sit0" and is always off, and is created by the default link file not my config. Why?
(If I remove my SIT tunnel both of them go away.)

$ networkctl
IDX LINK        TYPE      OPERATIONAL SETUP     
  1 lo          loopback  carrier     unmanaged
  2 ens3        ether     routable    configured
  3 wireguard-0 wireguard routable    configured
  4 sit0        sit       off         unmanaged
  5 sit-tunnel  sit       routable    configured

5 links listed.

$ networkctl status sit0
● 4: sit0
                   Link File: /usr/lib/systemd/network/99-default.link
                Network File: n/a
                       State: off (unmanaged)
                Online state: unknown
                        Type: sit
                        Kind: sit
                      Driver: sit
            Hardware Address: 00:00:00:00
                         MTU: 1480 (min: 1280, max: 65555)
                       QDisc: noop
IPv6 Address Generation Mode: eui64
    Number of Queues (Tx/Rx): 1/1

$ networkctl status sit-tunnel
● 5: sit-tunnel
                 NetDev File: /etc/systemd/network/20-sit-tunnel.netdev
                   Link File: /usr/lib/systemd/network/99-default.link
                Network File: /etc/systemd/network/20-sit-tunnel.network
                       State: routable (configured)
                Online state: online                                         
                        Type: sit
                        Kind: sit
                      Driver: sit
            Hardware Address: ac:f5:e2:2b
                         MTU: 1480 (min: 1280, max: 65555)
                       QDisc: noqueue
IPv6 Address Generation Mode: none
                       Local: REDACTED
                      Remote: REDACTED
    Number of Queues (Tx/Rx): 1/1
                     Address: REDACTED
                     Gateway: REDACTED
           Activation Policy: up
         Required For Online: yes

Jan 01 15:32:57 vps systemd-networkd[373]: sit-tunnel: netdev ready
Jan 01 15:32:57 vps systemd-networkd[373]: sit-tunnel: netdev ready
Jan 01 15:32:57 vps systemd-networkd[373]: sit-tunnel: Configuring with /etc/systemd/network/20-sit-tunnel.network.
Jan 01 15:32:57 vps systemd-networkd[373]: sit-tunnel: Link UP
Jan 01 15:32:57 vps systemd-networkd[373]: sit-tunnel: Gained carrier

Last edited by Beemo (2025-01-07 13:45:53)

-thc

Member

Registered: 2017-03-15

Posts: 775

Re: [Solved] sd-networkd: One SIT tunnel creates 2 interfaces?

from unix.stackexchange.com:

How sit0 comes into play:

You might wonder what happens when Linux receives a protocol 41 packet that doesn't match any of the sit devices (e.g. it came from some random address). In this case, it gets delivered to sit0.

sit0 is a special "fallback" device set up by the sit kernel module to handle just these packets. It has local and remote addresses set to 0.0.0.0 (i.e. "any") and it's not attached to any particular physical device, so it will accept any protocol 41 packet that's not already handled by some other sit device.

Is this useful? Maybe in specific circumstances, but I imagine in the vast majority of cases, you would want to drop such packets. I'm not sure why it's not left up to the administrator whether such a catch-all device should be created. Perhaps there is some historical reason.