diff --git a/asyncutils.nim b/asyncutils.nim
index f00fbe7..1813486 100644
--- a/asyncutils.nim
+++ b/asyncutils.nim
@@ -23,17 +23,29 @@ from posix import
   sendmsg,
   recvmsg
 
-proc asyncExecCmd*(command: string): Future[int] =
-  let event = newAsyncEvent()
-  let future = newFuture[int]("asyncExecCmd")
-  proc execCmdBackground(event: AsyncEvent, command: string): int =
-    result = execCmd(command)
-    event.trigger()
-  let flowVar = spawn execCmdBackground(event, command)
-  proc callback(fd: AsyncFD): bool =
+proc asyncExecCmd*(command: string): Future[string] =
+  let successEvent = newAsyncEvent()
+  let failureEvent = newAsyncEvent()
+  let future = newFuture[result.T]("asyncExecCmd")
+  proc execCmdBackground(successEvent: AsyncEvent, failureEvent: AsyncEvent,
+                         command: string): string =
+    var exitCode: int
+    (result, exitCode) = execCmdEx(command)
+    if exitCode != 0:
+      failureEvent.trigger()
+      successEvent.close() # FIXME: is close the right way to cancel an event?
+    else:
+      successEvent.trigger()
+      failureEvent.close() # FIXME: is close the right way to cancel an event?
+  let flowVar = spawn execCmdBackground(successEvent, failureEvent, command)
+  proc successCallback(fd: AsyncFD): bool =
     future.complete(^flowVar)
-    true                                                                                
-  addEvent(event, callback)
+    true
+  proc failureCallback(fd: AsyncFD): bool =
+    future.fail(newException(OSError, ^flowVar))
+    true
+  addEvent(successEvent, successCallback)
+  addEvent(failureEvent, failureCallback)
   return future
 
 type ControlMessage* = object
diff --git a/tcp_syni.nim b/tcp_syni.nim
index 4b58568..5a9804f 100644
--- a/tcp_syni.nim
+++ b/tcp_syni.nim
@@ -35,8 +35,9 @@ proc addFirewallRule(srcIp: IpAddress, srcPort: Port,
   --ctorigdst {dstIp} \
   --ctorigdstport {dstPort.int} \
   -j DROP"""
-  let exitcode = await asyncExecCmd(firewall_cmd)
-  if exitcode != 0:
+  try:
+    discard await asyncExecCmd(firewall_cmd)
+  except OSError:
     raise newException(PunchHoleError, "cannot add firewall rule")
 
 proc delFirewallRule(srcIp: IpAddress, srcPort: Port,
@@ -53,8 +54,9 @@ proc delFirewallRule(srcIp: IpAddress, srcPort: Port,
   --ctorigdst {dstIp} \
   --ctorigdstport {dstPort.int} \
   -j DROP"""
-  let exitcode = await asyncExecCmd(firewall_cmd)
-  if exitcode != 0:
+  try:
+    discard await asyncExecCmd(firewall_cmd)
+  except OSError:
     raise newException(PunchHoleError, "cannot delete firewall rule")
 
 proc captureSeqNumbers(puncher: TcpSyniPuncher, rawFd: AsyncFD,