[SOLVED] tap devices and dhcp

[otto001]

Hi,

I am having a proxmox 7.2-7 node running with several VMs (NO containers!)
For each of these VMs a tap device gets created (for one of those for example - output of ip a):

24: tap102i0: <BROADCAST,MULTICAST,PROMISC,DYNAMIC,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast master vmbr0 state UNKNOWN group default qlen 1000
    link/ether 4a:e3:a9:ec:4d:21 brd ff:ff:ff:ff:ff:ff
    inet 169.254.54.48/16 brd 169.254.255.255 scope global tap102i0
       valid_lft forever preferred_lft forever

So far, so good. But those interfaces are bombing my dhcp-server with requests, eg:

Jul 19 15:25:50 dnsmasq-dhcp[725526]: 870583166 DHCPDISCOVER(ens18) 4a:e3:a9:ec:4d:21
Jul 19 15:25:50 dnsmasq-dhcp[725526]: 870583166 tags: lan1, ens18
Jul 19 15:25:50 dnsmasq-dhcp[725526]: 870583166 DHCPOFFER(ens18) 192.168.1.47 4a:e3:a9:ec:4d:21
Jul 19 15:25:50 dnsmasq-dhcp[725526]: 870583166 requested options: 1:netmask, 3:router, 26:mtu, 252, 42:ntp-server,
Jul 19 15:25:50 dnsmasq-dhcp[725526]: 870583166 requested options: 15:domain-name, 6:dns-server, 12:hostname
Jul 19 15:25:50 dnsmasq-dhcp[725526]: 870583166 next server: 192.168.1.6
Jul 19 15:25:50 dnsmasq-dhcp[725526]: 870583166 broadcast response
Jul 19 15:25:50 dnsmasq-dhcp[725526]: 870583166 sent size:  1 option: 53 message-type  2
Jul 19 15:25:50 dnsmasq-dhcp[725526]: 870583166 sent size:  4 option: 54 server-identifier  192.168.1.6
Jul 19 15:25:50 dnsmasq-dhcp[725526]: 870583166 sent size:  4 option: 51 lease-time  12h
Jul 19 15:25:50 dnsmasq-dhcp[725526]: 870583166 sent size:  4 option: 58 T1  6h
Jul 19 15:25:50 dnsmasq-dhcp[725526]: 870583166 sent size:  4 option: 59 T2  10h30m
Jul 19 15:25:50 dnsmasq-dhcp[725526]: 870583166 sent size:  4 option:  1 netmask  255.255.255.0
Jul 19 15:25:50 dnsmasq-dhcp[725526]: 870583166 sent size:  4 option: 28 broadcast  192.168.1.255
Jul 19 15:25:50 dnsmasq-dhcp[725526]: 870583166 sent size:  4 option:  6 dns-server  192.168.1.6
Jul 19 15:25:50 dnsmasq-dhcp[725526]: 870583166 sent size:  9 option: 15 domain-name  fritz.box
Jul 19 15:25:50 dnsmasq-dhcp[725526]: 870583166 sent size:  4 option: 42 ntp-server  192.168.1.1
Jul 19 15:25:50 dnsmasq-dhcp[725526]: 870583166 sent size:  4 option:  3 router  192.168.1.1

Could anyone please tell me how to turn this of?
In the conf of this machine I have

net0: e1000=6E:E4:3D:FB:DA:C9,bridge=vmbr0

(which is of course NOT the tap device, but the device as seen by the guest)....
Best regards and thanks in advance,
Otto

 

[otto001]

No one? :-(

 

[shrdlicka]

HI,

the interfaces itself shouldn't send DHCP request by themselfs, the OS in the VM is sending those request. You could do something like this:

tcpdump -i [interface] udp port 67 and port 68 -vvv

Then either set the tap device or the outgoing connection and check which MAC address is sending those requests to narrow down who is sending those requests.

 

[otto001]

Hi,
thanks for your answer! But I know, which interface is sending those requests (as seen in my first post).
doing tcpdump just confirmed it.

Then either set the tap device or the outgoing connection and check which MAC address is sending those requests to narrow down who is sending those requests.

Could you please explain what you mean? I know the MAC from dhcp-log...

 

[shrdlicka]

ah ok ... , when you know the interface my next question would be what OS are you running inside the VM?

 

[otto001]

as written... debian 11 without networkmanager and fixed IPs in interfaces....

 

[otto001]

omg. one of the VMs was opnsense - and I had dhcp relaying on.... the solution was as simple as turning it off (the second VM where this appeared was the VM with the dhcp-server. disappeared as well )
thanks for your help anyway!