clientsystemdidentifierdhcpcoreos

how can I prevent systemd-networkd from sending client identifier?

I have a machine with CoreOS 1800(or 1855) installed onto disk, and with following systemd-networkd config (there is only one network interface in the machine):

$ cat /etc/systemd/network/zz-default.network
[Network]
DHCP=yes

[DHCP]
ClientIdentifier=mac
UseMTU=true
UseDomains=true

Another notable thing is that this machine is also configured with PXE boot but PXE server will reject boot so it will finally boot from disk.

When I restart the machine, there will be two DHCP IPs allocated for it, I confirmed it by checking /var/lib/dhcpd.leases in DHCP server:

lease 100.79.223.152 { 
  starts 5 2018/09/28 02:34:00; ends 6 2018/09/29 02:33:59; tstp 6 2018/09/29 02:33:59; cltt 5 2018/09/28 02:34:00; 
  binding state active; next binding state free; rewind binding state free; 
  hardware ethernet 08:9e:01:d9:28:64; 
  option agent.circuit-id 0:5:8:b9:1:0:29;
}
lease 100.79.223.150 { 
  starts 5 2018/09/28 02:34:29; ends 6 2018/09/29 02:34:28; tstp 6 2018/09/29 02:34:28; cltt 5 2018/09/28 02:34:29; 
  binding state active; next binding state free; rewind binding state free; 
  hardware ethernet 08:9e:01:d9:28:64; uid "001010236001331(d"; 
  option agent.circuit-id 0:5:8:b9:1:0:29;
}

All seems fine, but the PXE lease record 100.79.223.152 cause other problem (when really PXE boot the machine and deploy another OS to it then it will get the 100.79.223.152 instead of 150, then cause other private problem).

If I install other OS which does not use systemd-networkd, then the reboot only cause 1 lease record.

You can see the lease 100.79.223.150 has a field uid "001010236001331(d", which means let DHCP server allocate IP by the uid (client identifier), currently it is actually same content of mac address, just be printed as octet.

This is the root cause of two IPs.

To prevent this two IP problem, I've tried to set deny duplicates in /etc/dhcp/dhcpd.conf in DHCP server, but nothing changes.

I was wandering that if it is possible to tell systemd-networkd not to send uid (client identifier). According to source of systemd, it is intentionally implemented to "always send client identifier",

given such condition, how can I prevent systemd-networkd from sending client identifier?

EDIT 2019/02/17: I found that I misunderstood the meaning of deny duplicates, it does not help this problem.

I remembered I had ever tested another option first but not works.

ignore-client-uids on;

The ignore-client-uids statement

ignore-client-uids flag;

If the ignore-client-uids statement is present and has a value of true or on, the UID for clients will not be recorded. If this statement is not present or has a value of false or off, then client UIDs will be recorded.

https://www.isc.org/wp-content/uploads/2017/08/dhcp43.html

The DHCP server version is isc-dhcpd-4.2.4

EDIT 2019-03-12: I had some mistaken and found something, so answered this question myself. Simple answer is ignore-client-uids true; on server side works well, ClientIdentifier=mac on client side does not work well.

Solution

  • After many times of experiments, I found that only ignore-client-uids true; works constantly, all mystery disappeared., when you set it, you can confirm that no uid "....." appear in /var/lib/dhcp/dhcpd.leases` file, the server completely ignore the client identifier sent from client and just use MAC to determine how to allocate IP.

    If you insist on using ClientIdentifier=mac, you can take a look at what I found:

    /lib/systemd/network/zz-default.network

    [Network]
DHCP=yes

[DHCP]
UseMTU=true
UseDomains=true

    After I change above file to 

    [Network]
DHCP=no

    I got only 1 and same IP as before.

    That is all I found, hope it helpful.