improve on MTU-check

packages/mtu-check/default.nix

@@ -1,17 +1,29 @@
-{ pkgs, stdenv, ... }:
-let
-  mtu-check-skript = pkgs.writeShellScriptBin "mtu-check" ''
-    # start with a known good MTU
-    MTU=1380
-    IP=1.1.1.1
+# 2. Check 5 times before giving up
+{ writeShellScriptBin, iputils, ... }:
+writeShellScriptBin "mtu-check" ''
+  # start with a known good MTU
+  MTU=1380
+  
+  IP=1.1.1.1
+  
+  # if argument is given, use it as IP
+  if [ -n "$1" ]; then
+      IP=$1
+  fi
 
-    # check maximum transmission unit (MTU) of a network
-    # loop until we get an error
-    while true; do
-        # try to ping with the current MTU
-        echo "Trying MTU of $MTU"
-        ping -c 1 -M do -s $MTU $IP >/dev/null 2>&1
-        if [ $? -ne 0 ]; then
+  # counter for failed pings in a row
+  counter=0
+
+  # check maximum transmission unit (MTU) of a network
+  # loop until we get an error
+  while true; do
+      # try to ping with the current MTU
+      echo "Trying MTU of $MTU"
+      ${iputils}/bin/ping -c 1 -M do -s $MTU $IP >/dev/null 2>&1
+      if [ $? -ne 0 ]; then
+          counter=$(($counter + 1))
+          # if we failed 5 times in a row, we assume the MTU is too big
+          if [ $counter -eq 5 ]; then
             # ping failed, so the MTU is too big
             echo "Ping with MTU size $MTU failed..."
             echo "Adding 28 Bytes for ICMP and IP header to the previous value..."
@@ -20,25 +32,14 @@ let
             MTU=$(($MTU + 28 - 1))
             echo "MTU of network is $MTU Bytes"
             exit 0
-        else
-            # ping succeeded, so the MTU is too small
-            # increase it by 1 and try again
-            MTU=$(($MTU + 1))
-        fi
-    done
-  '';
-in
-stdenv.mkDerivation {
-
-  pname = "mtu-check";
-  version = "0.1.0";
-
-  # Needed if no src is used. Alternatively place script in
-  # separate file and include it as src
-  dontUnpack = true;
-
-  installPhase = ''
-    cp -r ${mtu-check-skript} $out
-  '';
-}
+          fi
+      else
+          # ping succeeded, so the MTU is too small
+          # increase it by 1 and try again
+          MTU=$(($MTU + 1))
 
+          # reset counter if we had a successful ping
+          counter=0
+      fi
+  done
+''