However, the server I’m testing with runs Arch Linux; virt-manager, libvirt and its dependencies were installed through AUR; In the list of dependencies the OpenBSD implementation of netcat wasn’t listed. I’m aware Debian has ported it. The special feature in this netcat re-write is support for UNIX socket. As libvirt uses UNIX socket to communicate, the gnu-netcat cannot be used; it doesn’t understand the -U flag. The error happens when you try to connect to a remote server through a SSH tunnel:
virsh -d 5 -c qemu+ssh://somehost/system list

If the netcat binary on the remote host doesn’t understand -U, it will fail.

A quick dirty fix, more like a workaround, is to use a wrapper. socat‘s speciality is socket. We can use it like this:

• rename original netcat: mv /usr/bin/netcat /usr/bin/netcat.orig
• create the wrapper /usr/bin/netcat with as content:

	#!/bin/sh
	if [ "$1" == "-U" ]; then socat - unix-client:$2
	else netcat.orig $@
	fi

• chmod 755 /usr/bin/netcat

And you’re ready to rock

On the long run libvirt shouldn’t rely on the OpenBSD implementation of netcat. As there’s only a few Linux distributions out there that adopted this rewrite. Perhaps give the user the freedom to choose which socket-tool to use in the configuration file?

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