Implements PostUp and PostDown commands using /bin/sh

This introduces PostUp and PostDown in dsnet. PostUp and PostDown allow
the user to run arbitrary commands after the device is up or down. These
are typically used to change the firewall rules via iptables. A working
example would be

...
    "PostUp" : "iptables -A FORWARD -i dsnet -j ACCEPT; iptables -A FORWARD -o dsnet -j ACCEPT; iptables -t nat -A POSTROUTING -o ens2 -j MASQUERADE ",
    "PostDown" : "iptables -D FORWARD -i dsnet -j ACCEPT; iptables -D FORWARD -o dsnet -j ACCEPT; iptables -t nat -D POSTROUTING -o ens2 -j MASQUERADE ",
...

All commands are executed by `/bin/sh` and no filtering or sandboxing is
applied. Users of this should know what they are doing.

Fixes https://github.com/naggie/dsnet/issues/16

CONFIG.md

@@ -66,6 +66,14 @@ with [hugo](https://gohugo.io/)
 network overview page. The shortcode file is included in this repository under
 `etc/`.
 
+        "PostUp": ""
+        "PostDown": ""
+
+Allows a user to specify commands to run after the device is up or down. This is
+typcially a collection of `iptables` invocations. The commands are executed by
+`/bin/sh`. *NOTE* These commands run as root, so make sure you check that they
+are secure.
+
         "PrivateKey": "uC+xz3v1mfjWBHepwiCgAmPebZcY+EdhaHAvqX2r7U8=",
 
 The server private key, automatically generated and very sensitive!

README.md

@@ -73,6 +73,8 @@ Main (automatically generated) configuration example:
         "Networks": [],
         "ReportFile": "/var/lib/dsnetreport.json",
         "PrivateKey": "uC+xz3v1mfjWBHepwiCgAmPebZcY+EdhaHAvqX2r7U8=",
+        "PostUp": "",
+        "PostDown" "",
         "Peers": [
             {
                 "Hostname": "test",

add.go

@@ -151,7 +151,6 @@ func PrintPeerCfg(peer PeerConfig, conf *DsnetConfig) {
 		wgifSeed += int(b)
 	}
 
-
 	t := template.Must(template.New("peerConf").Parse(peerConf))
 	err := t.Execute(os.Stdout, map[string]interface{}{
 		"Peer":        peer,

configtypes.go

@@ -53,6 +53,8 @@ type DsnetConfig struct {
 	// TODO Default subnets to route via VPN
 	ReportFile string  `validate:"required"`
 	PrivateKey JSONKey `validate:"required,len=44"`
+	PostUp   string
+	PostDown string
 	Peers    []PeerConfig `validate:"dive"`
 }
 

down.go

@@ -7,6 +7,11 @@ import (
 func Down() {
 	conf := MustLoadDsnetConfig()
 	DelLink(conf)
+	RunPostDown(conf)
+}
+
+func RunPostDown(conf *DsnetConfig) {
+	ShellOut(conf.PostDown, "PostDown")
 }
 
 func DelLink(conf *DsnetConfig) {

up.go

@@ -10,6 +10,11 @@ func Up() {
 	conf := MustLoadDsnetConfig()
 	CreateLink(conf)
 	ConfigureDevice(conf)
+	RunPostUp(conf)
+}
+
+func RunPostUp(conf *DsnetConfig) {
+	ShellOut(conf.PostUp, "PostUp")
 }
 
 func CreateLink(conf *DsnetConfig) {

util.go

@@ -4,6 +4,7 @@ import (
 	"bufio"
 	"fmt"
 	"os"
+	"os/exec"
 	"strings"
 )
 
@@ -35,6 +36,17 @@ func ExitFail(format string, a ...interface{}) {
 	os.Exit(1)
 }
 
+func ShellOut(command string, name string) {
+	if command != "" {
+        fmt.Printf("Running %s commands:\n %s", name, command)
+		shell := exec.Command("/bin/sh", "-c", command)
+		err := shell.Run()
+		if err != nil {
+			ExitFail("%s '%s' failed", name, command, err)
+		}
+	}
+}
+
 func ConfirmOrAbort(format string, a ...interface{}) {
 	fmt.Fprintf(os.Stderr, format+" [y/n] ", a...)