add setup open wrt dns use case

cli/openwrt_dns.go

@@ -0,0 +1,63 @@
+package main
+
+import (
+	"bytes"
+	"fmt"
+	"log"
+	"os"
+
+	"golang.org/x/crypto/ssh"
+)
+
+const (
+	sshKeyPathEnvVarName       = "SSH_KEY"
+	setupOpenWRTDNSFlagName    = "setup-openwrt-dns"
+	setupOpenWRTNewDNSFlagName = "new-dns"
+
+	openWRTUser = "root"
+	openWRTHost = "192.168.1.1:22" // Using IP because assuming DNS is down
+)
+
+var (
+	sshKeyPath, _ = os.LookupEnv(sshKeyPathEnvVarName)
+)
+
+// SetOpenWRTDNS ssh-es into `host` and sets `dns` as it's primary dns for dnsmasq
+func SetOpenWRTDNS(privateKey []byte, dns string) (string, error) {
+	signer, err := ssh.ParsePrivateKey(privateKey)
+	if err != nil {
+		log.Fatalf("unable to parse private key: %v", err)
+	}
+
+	config := &ssh.ClientConfig{
+		User: openWRTUser,
+		Auth: []ssh.AuthMethod{
+			ssh.PublicKeys(signer),
+		},
+		HostKeyCallback: ssh.InsecureIgnoreHostKey(),
+	}
+	client, err := ssh.Dial("tcp", openWRTHost, config)
+	if err != nil {
+		log.Fatal("Failed to dial: ", err)
+	}
+	defer client.Close()
+
+	session, err := client.NewSession()
+	if err != nil {
+		log.Fatal("Failed to create session: ", err)
+	}
+	defer session.Close()
+
+	cmd := openwrtDNSUpdateCmd(dns)
+	var b bytes.Buffer
+	session.Stdout = &b
+	if err := session.Run(cmd); err != nil {
+		log.Fatal("Failed to run: " + err.Error())
+	}
+	fmt.Println(b.String())
+	return "", nil
+}
+
+func openwrtDNSUpdateCmd(newDNS string) string {
+	return fmt.Sprintf("sed -i \"s/\\slist server.*/ list server '%s'/\" /etc/config/dhcp && /etc/init.d/dnsmasq reload", newDNS)
+}