Debian – routing a second public IP to multiple VM’s

The added net-update command in virsh should allow an dhcp-host update without restarting the virtual network (I have not tested it yet).

man virsh:

net-update network command section xml [--parent-index index] [[--live] [--config] | [--current]]

Update the given section of an existing network definition, with the changes optionally taking effect immediately, without needing to destroy and re-start the network.

command is one of "add-first", "add-last", "add" (a synonym for add-last), "delete", or "modify".

section is one of "bridge", "domain", "ip", "ip-dhcp-host", "ip-dhcp-range", "forward", "forward-interface", "forward-pf", "portgroup", "dns-host", "dns-txt", or "dns-srv", each section being named by a concatenation of the xml element hierarchy leading to the element being changed. For example, "ip-dhcp-host" will change a <host> element that is contained inside a <dhcp> element inside an <ip> element of the network.

xml is either the text of a complete xml element of the type being changed (e.g. "<host mac="00:11:22:33:44:55' ip='1.2.3.4'/>", or the name of a file that contains a complete xml element. Disambiguation is done by looking at the first character of the provided text - if the first character is "<", it is xml text, if the first character is not "<", it is the name of a file that contains the xml text to be used.

The --parent-index option is used to specify which of several parent elements the requested element is in (0-based). For example, a dhcp <host> element could be in any one of multiple <ip> elements in the network; if a parent-index isn't provided, the "most appropriate" <ip> element will be selected (usually the only one that already has a <dhcp> element), but if --parent-index is given, that particular instance of <ip> will get the modification.

If --live is specified, affect a running network. If --config is specified, affect the next startup of a persistent network. If --current is specified, affect the current network state. Both --live and --config flags may be given, but --current is exclusive. Not specifying any flag is the same as specifying --current.

You're thinking about this the wrong way. Instead of trying to derive the KVM hostname from the guest (impossible by design), try to figure out how the KVM host might inform the guest of its name.

Data can be easily shared with a guest from a KVM host by using the Filesystem Passthrough feature. There's even a GUI option for it in Virtual Machine Manager (See Add Hardware -> Filesystem).

Just create a directory somewhere on the KVM host with whatever stuff in it you want to share with the guest. This can be a text file containing the hostname, some setup or maintenance script, or whatever you want really. Mark it read-only for simplicity and security. Then configure Filesystem Passthrough using this path and mount it somewhere convenient in each guest.

Obviously this approach requires slight modifications to each KVM host and guest, but with a few hundred hosts out there, I assume you have some sort of configuration management in place (like Puppet, Ansible, etc.), thus greatly simplifying the matter.

Sorry this answer is super late, but given that the question has 5,000+ views I figured I should post it anyway.