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Failed PXE Installations

Although the article is lengthy, it is highly recommended to read it completely as it provides detailed explanations of all common PXE issues and their corresponding solutions.

Issues with PXE booting or DHCP are very unlikely to be related to Tenantos, but typically network-related issues.

Installation failed because of network connectivity issues

Typical error messages

This only affects dedicated servers. PXE installations for virtual servers are usually not affected.

  • no response after x secs - giving up
  • No configuration methods succeeded ( [Screenshot]
  • Failed to retrieve the preconfiguration file [Screenshot]
  • TFTP open timeout
  • Media test failure, check cable
  • Connection timed out (


In most cases, the server can obtain an IP from the DHCP server during PXE boot, but re-requesting the IP address fails either during iPXE chainloading or within the OS installer.

The problem mostly occurs on Debian installations, because the Debian-Installer expects a fast IP address assignment. Anaconda-based installations (e.g. CentOS), on the other hand, usually work, although even that must not be the case if PXE chainloading already fails.

Possible causes

The issue is nearly always tied to either a failure in network connectivity or a misconfiguration of the switch. We try to increase the default timeouts if technically possible, but if the network is not configured optimally, problems may still occur.

The most common cause for issues like these is that the interface is in a blocking state due to STP.

  • If, during the PXE boot process, the server successfully obtains an IP address via DHCP, but then encounters a failure when chainloading a newer iPXE version (i.e., the PXE process restarts and fails to renew the IP request via DHCP), this is usually indicative of a problem with the STP configuration.
  • If the server receives an IP address via DHCP during PXE boot, but the process times out during the HTTP data download, this scenario also often points to potential issues with the STP configuration.

The following article delves into all common causes and provides further information about the STP below.

No DHCP lease received

If no DHCP lease has been received, a network-related issue is likely. Please verify the following:

  • If multiple VLANs are used, ensure that a DHCP relay has been configured or an additional remote agent has been installed in the VLAN.
  • Verify that your network satisfies the DHCP requirements and no firewall is blocking DHCP packets. For example, a firewall configuration may exist to block rogue DHCP servers.
  • Verify that the DHCP server is running by executing ps aux | grep dhcpd. This command must be executed on the agent assigned as a DHCP server for the server in Tenantos. Keep in mind that depending on the configuration, the DHCP server may only be running during a server reinstall.

After these points have been checked, further troubleshooting proceeds much like any other network problem, and a network administrator should be able to identify the cause. For completeness, some technical points and potential troubleshooting paths are explained:

The DHCP logs can be located on the respective agent or Tenantos server in the file /var/log/syslog. The first step is to initiate a new installation and monitor the logs by executing tail -f /var/log/syslog to check if DHCP requests are being received. A successful DHCP lease looks like this:

Jun 19 12:25:16 platform dhcpd[743083]: DHCPDISCOVER from 32:b3:59:22:c2:92 via ens19
Jun 19 12:25:16 platform dhcpd[743083]: DHCPOFFER on to 32:b3:59:22:c2:92 via ens19
Jun 19 12:25:19 platform dhcpd[743083]: DHCPREQUEST for ( from 32:b3:59:22:c2:92 via ens19
Jun 19 12:25:19 platform dhcpd[743083]: DHCPACK on to 32:b3:59:22:c2:92 via ens19

Log entries like "network tenantos: no free leases" are normal and can be ignored.

  • If the "DHCPDISCOVER" line is not visible, the server is not receiving a DHCP request. If this line is missing, the standard procedure is to use a packet capture tool like tcpdump to investigate where the DHCP packets are ending up in the network. As example: tcpdump -i eth0 port 67 or port 68 -envv

  • If a DHCP request comes in but "DHCPNAK" appears in the logs, it means that another DHCP server exists in the network, which has given a lease for the MAC of the server that's being installed.

  • If not all four log lines (DHCPDISCOVER, DHCPOFFER, DHCPREQUEST, and DHCPACK) are present, it means that the DHCP handshake hasn't been completed. If only the DHCPDISCOVER and DHCPOFFER lines are visible, it means that the agent successfully offered a DHCP lease, but the acknowledgment packets didn't reach back to the requesting server. This is often due to issues with the DHCP relay or firewall configuration.

Troubleshooting using tcpdump

As mentioned earlier, tcpdump is a useful tool for troubleshooting DHCP issues. This process involves performing a PXE boot while simultaneously running tcpdump (refer to the command mentioned above) on the assigned Tenantos agent.

  • If you see DHCP traffic, ensure that the packets are from the server undergoing PXE boot. This can be determined from the "Client Ethernet Address" field. Other DHCP traffic may be present, but is not relevant for troubleshooting.
  • If only DHCPDISCOVER packets are seen for the MAC address of the server without a DHCPOFFER response, verify that the "Gateway IP" value matches with the gateway of the assigned primary IP. If the "Gateway IP" does not match, add a subnet that includes the IP from the "Gateway IP" field. The need for this step depends on the specific network configuration.
  • If the DHCP server sends a DHCPOFFER packet following the DHCPDISCOVER, but no DHCPREQUEST is received, this could indicate that a firewall or network configuration is either discarding or mishandling the packets.
  • If you do not see any DHCP traffic, it means no DHCP traffic is reaching the agent. This is a firewall or network configuration problem.

Media test failure, check cable

If you encounter the "Media test failure, check cable" error message, it usually means that the NIC currently being used for PXE booting is not connected to the network.

To fix the issue, access the server's BIOS and enable PXE boot for the appropriate network card.

PXE boot hangs at "Start PXE over IPv4 on MAC:" / "TFTP open timeout"

When encountering the message "Start PXE over IPv4 on MAC:" and the PXE boot process does not proceed, the initial step should be to check if the server has obtained a DHCP lease. If so, the problem often lies with either the STP configuration or an unreachable TFTP server.

For the "TFTP open timeout" message, the server already received a DHCP lease and there is no need to verify that the server has received a DHCP lease. The issue typically relates to either STP configuration problems or an inaccessible TFTP server.

The TFTP address is the IP address of the assigned PXE agent. The cause of the issue might be:

  • TFTP service being inaccessible due to firewall rules or ACL configurations on a switch/router. If you have virtualized the agent, the hypervisor could block the TFTP port as well.
  • An incorrect IP address has been specified on the remote agent page in Tenantos.

To PXE boot servers, the server must be able to establish a connection with the specified agent IP. This connectivity can be tested as follows:

:~$ tftp AGENT_IP
tftp> get snponly.efi
Transfer timed out.

This test must be executed from a server other than the agent, as otherwise the test would bypass the network. Ideally, the affected server is booted into a rescue system via an ISO and the test is executed from there.

Should the output be "Transfer timed out", typically a firewall is blocking the TFTP traffic.

STP Configuration (Cisco: PortFast)

A common cause is the STP configuration on the side of the switch. If you don't use RSTP (or adequate protocols like MSTP), the network traffic to the server's interface will be blocked for up to 50 seconds when the interface comes up. This is too long for some installers and a timeout occurs.

Each switch manufacturer describes in its documentation how to enable RSTP. If STP is currently used and everything works, we still recommend switching to RSTP (or a similar protocol) as this will result in faster PXE boots and most switch vendors advise the same.

Juniper Networks has an excellent article explaining the benefits of RSTP, you can find the article here.

Please note that RSTP should only be enabled for edge ports. Edge ports are ports that connect an end device. For example, if the cable from interface ge-0/0/1 is plugged into a server, it is an edge port.

Click to view additional information.
Also click if you still have issues, even after activating RSTP.

Sometimes it is assumed that a correct STP configuration is in place. However, when troubleshooting the issue, it often becomes apparent that this is not the case. In addition to checking the logs on the switch to verify that the server's interface is not in a blocking state, you can manually check how long it takes for an IP address to become accessible once it has been brought up:

  1. Boot the server into a rescue system such as grml. This can be achieved in several ways: By trying to start grml via PXE, manually mounting the ISO through your PC, or using Tenantos's ISO functionality to mount the grml ISO via the NoVNC console.
  2. After the server has booted, enter the ifconfig command to determine the name of the interface. Then, take down the interface using ifconfig <name> down and wait for up to 1 minute.
  3. From your PC, start pinging the server IP by executing ping <server-ip>
  4. Manually bring up the IP address on the server. This can be done by executing ifconfig <name> <ip>/<cidr> up; route add default gw <gateway-ip> (e.g. ifconfig ens18 up; route add default gw
  5. Begin counting the time it takes for the IP to respond to pings.

If it takes longer than a few seconds for the IP to become pingable, it is likely that the issue lies with the STP configuration. Although other causes will be discussed further in this article, it is impossible to list every single cause. However, a network administrator should be able to identify and solve the issue.

protocols {
    rstp {
        interface ge-0/0/1.0 {
interface Ethernet2
spanning-tree portfast

Slow port speed auto-negotiation

This is rather rare, but the network timeout may also be related to a slow port speed auto-negotiation. If the STP mode has already been changed and the problem persists, switching to a fixed port speed might solve the problem.

LACP Configuration

If you have configured LACP for the affected server on the switch, it is important to also configure LACP fallback. This is because during PXE boot, no BPDU packets can be transmitted. BPDUs are used to exchange information between switches in a network using STP. Without BPDUs, the switch cannot determine the topology of the network and may not allow traffic to flow through the ports.

To establish network connectivity, LACP fallback must be configured. The specific steps for configuring LACP fallback do vary depending on the manufacturer and model of the switch. Please consult the documentation provided by the manufacturer for detailed instructions.

Windows installation hangs at "Prepare Networking"

Screenshot (the error message can also be different)

This problem is mostly related to missing drivers for the network card and is often caused by an incorrect cache directory configuration. If the cache directory is correctly defined, Tenantos will automatically add common drivers to the Windows installer (Tenantos automatically suggests a correct cache directory when adding Windows profiles).

If the problem occurs at virtual servers, you can also try changing the NIC model. Drivers for Virtio are added by default (provided the cache directory is correct). If it works with another NIC, you can be sure that the necessary drivers are missing and add them as described on the page linked above.

Error: "No space left on device" (affects only VMs)


This error message is typically encountered by virtual machines that have not been allocated enough RAM. In contrast to an ISO installation, a PXE installation loads all installation files into the RAM, and there's no feasible workaround for this.

To solve this issue, more RAM needs to be assigned to the virtual machine. Smaller virtual machines with, for example, only 1 GB of RAM, are unable to be provisioned via PXE due to insufficient memory.

For example, Ubuntu 22.04, with its new "autoinstall" system, currently demands the highest amount of memory (~3.5 GB RAM) during the PXE installation process, as it loads the entire ISO into RAM. Conversely, other operating systems, like CentOS 8, require less memory - CentOS 8, for instance, needs 2103 MiB of RAM. Please keep in mind that these RAM requirements are approximations and can vary with updates to the installation files.

The recommendation is to allocate at least 4 GB of RAM to virtual machines to be ready for future operating systems as well. However, this recommendation is without guarantee, as it is not possible to predict or influence the system requirements of the operating systems.

If enough RAM has been assigned

If the issue persists despite having sufficient memory and if Proxmox is being used, it might be due to the "Enable Hotplug" setting being activated in the "Memory Configuration". As explained in the tooltip at the VPS plan configuration, hotplugged memory may result in only a portion of the RAM being available to the installer system.

Please note that disabling this setting in the VPS plan does not automatically update existing VMs. You either need to re-apply the VPS plan to the affected VM, or change the hotplug setting directly in Proxmox.

"But the installation via ISO file works"

As outlined above, the system requirements for ISO and PXE installations differ. We do not have control over these system requirements, and the VMs must be equipped with sufficent memory to successfully carry out PXE installations.

If there's enough demand for template-based installations on virtual servers, we can integrate this feature. Please vote for this in the feature tracker. However, bear in mind that while template-based installations may be quicker and less memory-intensive, PXE installations offer significantly greater flexibility.

Proxmox: Kernel panic when installing RHEL 9 (and derivatives such as AlmaLinux 9)


When attempting to install RHEL 9, or its derivatives such as AlmaLinux 9 or Rock Linux 9, on Proxmox, the installation process may fail with a kernel panic if the CPU type is set to "kvm64" or to a type that does not support the "x86-64-v2" CPU flag. RHEL 9 and its derivatives require the "x86-64-v2" CPU flag for a successful installation.

To resolve this issue, please adjust the CPU type of the affected VM and set the CPU type in the VPS plan configuration to a type that supports "x86-64-v2", such as "host".

Please note that changing this setting in the VPS plan does not automatically update existing VMs. You will either need to re-apply the VPS plan to the affected VM, or change the CPU type directly in Proxmox.

For additional information, please refer to this discussion on the Proxmox forum.