vm: add tools and docs for convenient ssh integration

README.md

@@ -99,6 +99,11 @@ By default, Virter uses the libvirt network named `default`.
 
 Check out [`doc/networks.md`](./doc/networks.md) for more details on VM networking.
 
+### Connecting with `ssh`
+
+SSH can be configured for convenient access to virtual machines created by Virter.
+See [`doc/ssh.md`](./doc/ssh.md) for details.
+
 ### DHCP Leases
 
 Libvirt produces some weird behavior when MAC or IP addresses are reused while

cmd/vm.go

@@ -19,6 +19,8 @@ func vmCommand() *cobra.Command {
 
 	vmCmd.AddCommand(vmCommitCommand())
 	vmCmd.AddCommand(vmExecCommand())
+	vmCmd.AddCommand(vmExistsCommand())
+	vmCmd.AddCommand(vmHostKeyCommand())
 	vmCmd.AddCommand(vmRmCommand())
 	vmCmd.AddCommand(vmRunCommand())
 	vmCmd.AddCommand(vmSSHCommand())

cmd/vm_exists.go

@@ -0,0 +1,30 @@
+package cmd
+
+import (
+	"os"
+
+	log "github.com/sirupsen/logrus"
+	"github.com/spf13/cobra"
+)
+
+func vmExistsCommand() *cobra.Command {
+	existsCmd := &cobra.Command{
+		Use:   "exists vm_name",
+		Short: "Check whether a VM exists",
+		Long:  `Check whether a VM exists and was created by Virter.`,
+		Args:  cobra.ExactArgs(1),
+		Run: func(cmd *cobra.Command, args []string) {
+			v, err := InitVirter()
+			if err != nil {
+				log.Fatal(err)
+			}
+			defer v.ForceDisconnect()
+
+			if err := v.VMExists(args[0]); err != nil {
+				os.Exit(1)
+			}
+		},
+		ValidArgsFunction: suggestVmNames,
+	}
+	return existsCmd
+}

cmd/vm_hostkey.go

@@ -0,0 +1,33 @@
+package cmd
+
+import (
+	"fmt"
+
+	log "github.com/sirupsen/logrus"
+	"github.com/spf13/cobra"
+)
+
+func vmHostKeyCommand() *cobra.Command {
+	hostKeyCmd := &cobra.Command{
+		Use:   "host-key vm_name",
+		Short: "Get the host key for a VM",
+		Long:  `Get the host key for a VM in the format of an OpenSSH known_hosts file.`,
+		Args:  cobra.ExactArgs(1),
+		Run: func(cmd *cobra.Command, args []string) {
+			v, err := InitVirter()
+			if err != nil {
+				log.Fatal(err)
+			}
+			defer v.ForceDisconnect()
+
+			knownHostsText, err := v.VMGetKnownHosts(args[0])
+			if err != nil {
+				log.Fatal(err)
+			}
+
+			fmt.Print(knownHostsText)
+		},
+		ValidArgsFunction: suggestVmNames,
+	}
+	return hostKeyCmd
+}

doc/ssh.md

@@ -0,0 +1,58 @@
+# Virter SSH Integration
+
+The simplest way to connect to a virtual machine that was started with Virter
+is to use `virter vm ssh ...`. However, you may also choose to use `ssh` and
+related tools to connect. This can be made very convenient by adding the
+following to your `~/.ssh/config`:
+
+```
+Match exec "virter vm exists %h"
+    User root
+    IdentityAgent none
+    IdentityFile ~/.config/virter/id_rsa
+    KnownHostsCommand /usr/bin/env virter vm host-key %n
+```
+
+Now you can easily connect:
+
+```
+$ ssh foo
+[root@foo ~]#
+```
+
+## Name resolution
+
+Depending on your configuration, `ssh` may or may not be able to resolve the VM
+name to a hostname. If not, you will see an error similar to:
+
+```
+$ ssh foo
+ssh: Could not resolve hostname foo: Name or service not known
+```
+
+Fix this by installing and configuring the [libvirt NSS
+modules](https://libvirt.org/nss.html). In particular, you will need to install
+a package such as `libvirt-nss` or `libnss-libvirt`. Then add `libvirt_guest`
+to the `hosts:` configuration in the file `/etc/nsswitch.conf`.
+
+## SSH integration with qualified names
+
+If you have configured a network domain in your libvirt network, you can also
+connect to the VM using the fully qualified domain name (FQDN). For instance,
+with the domain name `test`, you can use this configuration in your
+`~/.ssh/config`:
+
+```
+Host *.test
+    User root
+    IdentityAgent none
+    IdentityFile ~/.config/virter/id_rsa
+    KnownHostsCommand /bin/bash -c 'virter vm host-key "$(basename "%n" .test)"'
+```
+
+Now you can easily connect:
+
+```
+$ ssh foo.test
+[root@foo ~]# 
+```

internal/virter/vm.go

@@ -1,6 +1,7 @@
 package virter
 
 import (
+	"bytes"
 	"context"
 	"fmt"
 	"net"
@@ -20,6 +21,14 @@ import (
 	"github.com/LINBIT/virter/pkg/sshkeys"
 )
 
+func (v *Virter) VMExists(vmName string) error {
+	if _, err := v.getMetaForVM(vmName); err != nil {
+		return fmt.Errorf("failed to get VM metadata: %w", err)
+	}
+
+	return nil
+}
+
 func (v *Virter) anyImageExists(vmConfig VMConfig) (bool, error) {
 	vmName := vmConfig.Name
 	imgs := []string{
@@ -502,6 +511,19 @@ func (v *Virter) getKnownHostsFor(vmNames ...string) (sshkeys.KnownHosts, error)
 	return knownHosts, nil
 }
 
+func (v *Virter) VMGetKnownHosts(vmName string) (string, error) {
+	knownHosts, err := v.getKnownHostsFor(vmName)
+	if err != nil {
+		return "", fmt.Errorf("failed to fetch host keys: %w", err)
+	}
+
+	buf := new(bytes.Buffer)
+	if err := knownHosts.AsKnownHostsFile(buf); err != nil {
+		return "", fmt.Errorf("failed to write known hosts file: %w", err)
+	}
+	return buf.String(), nil
+}
+
 func (v *Virter) getSSHUserName(vmName string) string {
 	meta, err := v.getMetaForVM(vmName)