When using syspatch on OpenBSD, the upgrade sometimes fails with

Relinking to create unique kernel... failed!
!!! "/usr/libexec/reorder_kernel" must be run manually to install the new kernel

This generally happens after a system upgrade, or an otherwise manual change of kernel. This fix is to update the kernel hash, before re-running reorder_kernel.

# sha256 /bsd > /var/db/kernel.SHA256
# /usr/libexec/reorder_kernel 

PXE-booting OpenBSD is easy and well documented. Unfortunately, it is not the case when more than just one OpenBSD install has to be made available through PXE. Using PXELINUX for multiplexing purposes has potential but the documentation is scarce, unsuccessful, or require patching the PXE loader.

One trick which works is to boot from a floppy disk image, using memdisk.

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