[systemd-devel] networkd: Link local static IP address behind NAT

[Andrei Borzenkov]

On 18.01.2023 17:12, Thomas Burghout wrote:
> On 18.01.20233 04:06, Andrei Borzenkov wrote:
>> On 17.01.2023 18:28, Thomas Burghout wrote:
>>>       inet 169.254.146.171/16 brd 169.254.255.255 scope link eth0
>>
>> Is it output from the correct system? Because address is different. I do
>> not see how "ping -I 169.254.1.2" can work with this.
> 
> That is unfortunate, I copied the wrong notes indeed. Apologies. The
> following output should completely describe the configuration of the
> system:
> 
> 
> $ cat /usr/lib/systemd/network/10-lan.network
> [Match]
> Name=eth0
> 
> [Network]
> Address=169.254.1.2/16
> DNS=169.254.1.1
> Gateway=169.254.1.1
> $ ip route
> default via 169.254.1.1 dev eth0
> 169.254.0.0/16 dev eth0 scope link  src 169.254.1.2
> $ ip address
> 1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue qlen 1000
>      link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
>      inet 127.0.0.1/8 scope host lo
>         valid_lft forever preferred_lft forever
>      inet6 ::1/128 scope host
>         valid_lft forever preferred_lft forever
> 2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq qlen 1000
>      link/ether e2:26:9e:11:ae:da brd ff:ff:ff:ff:ff:ff
>      inet 169.254.1.2/16 brd 169.254.255.255 scope link eth0
>         valid_lft forever preferred_lft forever
>      inet6 fe80::e026:9eff:fe11:aeda/64 scope link
>         valid_lft forever preferred_lft forever
> 3: usb0: <BROADCAST,MULTICAST> mtu 1500 qdisc noop qlen 1000
>      link/ether b6:c8:ab:ac:44:7f brd ff:ff:ff:ff:ff:ff
> 4: sit0 at NONE: <NOARP> mtu 1480 qdisc noop qlen 1000
>      link/sit 0.0.0.0 brd 0.0.0.0
> $ ip route get 8.8.8.8
> 8.8.8.8 via 169.254.1.1 dev eth0
> $ ping 8.8.8.8
> PING 8.8.8.8 (8.8.8.8): 56 data bytes
> ^C
> --- 8.8.8.8 ping statistics ---
> 9 packets transmitted, 0 packets received, 100% packet loss
> $ ping -I 169.254.1.2 8.8.8.8
> PING 8.8.8.8 (8.8.8.8) from 169.254.1.2: 56 data bytes
> 64 bytes from 8.8.8.8: seq=0 ttl=116 time=12.576 ms
> 64 bytes from 8.8.8.8: seq=1 ttl=116 time=8.341 ms
> 64 bytes from 8.8.8.8: seq=2 ttl=116 time=9.124 ms
> ^C
> --- 8.8.8.8 ping statistics ---
> 3 packets transmitted, 3 packets received, 0% packet loss
> round-trip min/avg/max = 8.341/10.013/12.576 ms
> 
> 
> The commands I included for "interactive" were also wrong. The
> following commands produce a working configuration:
> 
> 
> ip address flush dev eth0
> ip route flush dev eth0
> ip address add 169.254.1.2/16 brd + dev eth0

This adds address with global scope.

> ip route add 169.254.1.1 dev eth0
> ip route add default via 169.254.1.1 dev eth0
> 
> 
> Most notably, ip route now includes the following line as well:
> 169.254.1.1 dev eth0 scope link
> 

This is red herring. What happens - kernel needs to set source address 
when you did not specify any. Because route to 8.8.8.8 has global scope 
and the only available address has link scope, this address is ignored 
and so no packet can be sent.

When you explicitly set source address with -I option kernel simply is 
using it.

If you do

ip address add 169.254.1.2/16 brd + dev eth0 scope link

you will observe exactly the same behavior.

> Adding an extra [Route] section with this address does not fix the
> networkd configuration file.

Of course not. But using

[Address]
Address=169.254.1.2/16
Scope=global

does, although I am not sure about possible implications.