TCP Port Scanner in Bash

Source: TCP Port Scanner in Bash

I just had this quick idea to write a tcp port scanner in bash. Bash supports the special /dev/tcp/host/port file that you can read/write. Writing to this special file makes bash open a tcp connection to host:port. If writing to the port succeeds, the port is open, else the port is closed.

So at first I wrote this quick script:

for port in {1..65535}; do
  echo >/dev/tcp/google.com/$port &&
    echo "port $port is open" ||
    echo "port $port is closed"
done

This loops over ports 1-65535 and tries to open google.com:$port. However this doesn’t work that well because if the port is closed, it takes bash like 2 minutes to realize that.

To solve this I needed something like alarm(2) to interrupt bash. Bash doesn’t have a built-in alarm function, so I had to write my own using Perl:

alarm() {
  perl -e '
    eval {
      $SIG{ALRM} = sub { die };
      alarm shift;
      system(@ARGV);
    };
    if ($@) { exit 1 }
  ' "$@";
}

This alarm function takes two args: seconds for the alarm call, and the code to execute. If the code doesn’t execute in the given time, the function fails.

Once I had this, I could take my earlier code and just call it through alarm:

for port in {1..65535}; do
  alarm 1 "echo >/dev/tcp/google.com/$port" &&
    echo "port $port is open" ||
    echo "port $port is closed"
done

This is working! Now if bash freezes because of a closed port, the alarm 1 will kill the probe in 1 second, and the script will move to the next port.

I went ahead and turned this into a proper scan function:

scan() {
  if [[ -z $1 || -z $2 ]]; then
    echo "Usage: $0 <host> <port, ports, or port-range>"
    return
  fi

  local host=$1
  local ports=()
  case $2 in
    *-*)
      IFS=- read start end <<< "$2"
      for ((port=start; port <= end; port++)); do
        ports+=($port)
      done
      ;;
    *,*)
      IFS=, read -ra ports <<< "$2"
      ;;
    *)
      ports+=($2)
      ;;
  esac


  for port in "${ports[@]}"; do
    alarm 1 "echo >/dev/tcp/$host/$port" &&
      echo "port $port is open" ||
      echo "port $port is closed"
  done
}

You can run the scan function from your shell. It takes two arguments: the host to scan, and a list of ports to scan (such as 22,80,443), or a range of ports to scan (such as 1-1024), or an individual port to scan (such as 80).

Here is what happens when I run scan google.com 78-82:

$ scan google.com 78-82 
port 78 is closed
port 79 is closed
port 80 is open
port 81 is closed
port 82 is closed

Similarly you can write an udp port scanner. Just replace /dev/tcp/ with /dev/udp/.

Update

It turns out GNU’s coreutils include timeout utility that runs a command with a time limit. Using timeout we can rewrite the tcp proxy without using Perl for SIGALRM:

$ timeout 1 bash -c "echo >/dev/tcp/$host/$port" &&
    echo "port $port is open" ||
    echo "port $port is closed"

===============================================

This is how to write alarm in bash:

function alarm() {
    timeout=$1; shift;
    bash -c "$@" &
    pid=$!
    {
      sleep $timeout
      kill $pid 2> /dev/null
    } &
    wait $pid 2> /dev/null
    return $?
  }

alarm 1 "echo >/dev/tcp/google.com/230" && echo "Y" || echo "N"

//this prints 'N' in ~1s

alarm 60 "echo >/dev/tcp/google.com/80" && echo "Y" || echo "N"

//this still returns almost immediately and prints "Y"

===============================================

Here’s a pure bash version I came up with a while ago, abusing the timeout in read to wait less than a second while still only using bash builtins 🙂

function portscan() {
  for p in {0..65535}; do
    (
      (
         bash -c "(>/dev/tcp/$1/$p)" 2> /dev/null && echo open: $p
      ) &
      read -t0.1
      kill $! 2>/dev/null
    ) 2>/dev/null
  done
}