Networks Are Smart!

It's been a long time since my last post, life sometimes lead people to unexpected situations...

But now I am back for a while...

These days I am working on developing a firewall module based on netfilter/iptables. To test my module I tried to prepare a test environment containing three linux machines (both on the same local network). My test scenario is something like that:

• Machine A: Client machine
• Machine B: The machine where my module works
• Machine C: Server machine

I declared B as default gateway for C on A as:

route add -net C netmask 255.255.255.0 gw B dev eth0

And make necessary configuration on B as:

/proc/sys/net/ipv4/ip_forward
/etc/sysctl.conf -- net.ipv4.ip_forward = 1
/etc/sysconfig/network -- FORWARD_IPV4=true

Everything seems perfect but when I observe the routing of packets from A to C, I observed that after the transmission of the first packet, A detects that both A and C are on the same local network, so it bypasses B (the gateway).

I tried to find a solution by Googling but couldn't find a suitable one, so began to read documentations and found out a solution using ip masquerading. On machine B make the following configuration:

iptables -t nat -A POSTROUTING -d C -o eth0 -j MASQUERADE

And everything works as desired (each packet from A to C routed through B)

The Core Rules

I first read the following text from a blog years ago (for the writer of that blog, I am sorry I cant recall your name), and since then I always placed it where I work:

Be humble. Always first presume that you're wrong. While developers do make mistakes, and as a new hire you should certainly assist others in catching and correcting mistakes, you should try to ensure that you're certain of your observation before proudly declaring your find. It is enormously damaging to your credibility when you cry wolf.

Be discreet with constructive criticism. A developer is much more likely to be accept casual suggestions and quiet leading questions than they are if the same is emailed to the entire group. Widening the audience is more likely to yield defensiveness and retribution. The team is always considering what your motives are, and you will be called on it and exiled if you degrade the work of others for self-promotion.

The best way to earn credibility and respect is through hard work and real results. Cheap, superficial substitutes - like best practice emails sent to all, or passing comments about how great it would be to implement some silver bullet - won't yield the same effect, and are more easily neutralized.

Actions speak louder than words. Simply talking about implementing a team blog, or a wiki, or a new source control mechanism, or a new technology, is cheap. Everyone knows that you're just trying to claim ownership of the idea when someone eventually actually does the hard work of doing it, and they'll detest you for it. If you want to propose something, put some elbow grease behind it. For instance, demonstrate the foundations of a team blog, including preliminary usage guidelines, and a demonstration of all of the supporting technologies. This doesn't guarantee that the initiative will fly, and the effort might for the naught, but the team will identify that it's actual motivation and effort behind it, rather than an attempt at some easy points.


There is no one-size-fits-all advice. Not every application is a high-volume e-commerce site. Just because that's the most common best-practices subject doesn't mean that it's even remotely the best design philosophies for the group you're joining.

Canvas.drawLine vs Canvas.drawPath

In Android, if you are in a situation in which you are drawing lots of connected lines (like routes on MapView) try to use "canvas.drawPath". In one of my applications I am drawing nearly 500 routes containing 10.000 points and using "canvas.drawPath" instead of "canvas.drawLine" decreases the drawing time as up to %30.

Android - Baloon Display On Map

I see several posts in Android forums about people asking how to display baloon structures on mapviews. In one of my draft applications I declare a simple solution for this (for source codes and any other issues related to this post, feel free to write me at serkan.dogantekin@gmail.com):

- Define a layout for baloon structure and its "dispatchDraw" draw the baloon itself like:

package com.dogantekin.baloon;

public class BaloonLayout extends LinearLayout {

    .......

    @Override
    protected void dispatchDraw(Canvas canvas) {       
        Paint panelPaint  = new Paint();
        panelPaint.setARGB(0, 0, 0, 0);
               
        RectF panelRect = new RectF();
        panelRect.set(0,0, getMeasuredWidth(), getMeasuredHeight());
        canvas.drawRoundRect(panelRect, 5, 5, panelPaint);
       
        RectF baloonRect = new RectF();
        baloonRect.set(0,0, getMeasuredWidth(), 2*(getMeasuredHeight()/3));
        panelPaint.setARGB(230, 255, 255, 255);       
        canvas.drawRoundRect(baloonRect, 10, 10, panelPaint);
       
        Path baloonTip = new Path();
        baloonTip.moveTo(5*(getMeasuredWidth()/8), 2*(getMeasuredHeight()/3));
        baloonTip.lineTo(getMeasuredWidth()/2, getMeasuredHeight());
        baloonTip.lineTo(3*(getMeasuredWidth()/4), 2*(getMeasuredHeight()/3));
       
        canvas.drawPath(baloonTip, panelPaint);
               
        super.dispatchDraw(canvas);
    }
}

- Define a layout xml for this layout and add views that will be inside this layout like:

<?xml version="1.0" encoding="utf-8"?>
<com.dogantekin.baloon.BaloonLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/transparent_panel"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:paddingLeft="5px"
    android:paddingTop="5px"
    android:paddingRight="5px"
    android:paddingBottom="5px">
    <RelativeLayout
      xmlns:android="http://schemas.android.com/apk/res/android"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content">
        <TextView
            android:id="@+id/note_label"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_alignParentLeft="true"
            android:text="@string/note"
        />
        <ImageView
            android:id="@+id/close_button"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_alignParentRight="true"
            android:paddingRight="10px"       
            android:src="@drawable/close"
            android:clickable="true"
        />
        <TextView
            android:layout_height="wrap_content"
            android:layout_width="fill_parent"
            android:paddingLeft="5px"
            android:paddingRight="5px"
            android:text=""
            android:id="@+id/note_text"
            android:layout_below="@+id/note_label"
            android:layout_centerHorizontal="true"
            android:minLines="4"
            android:maxLines="4"
            android:maxLength="160"   
            android:textSize="5pt"   
        />
    </RelativeLayout>
</com.dogantekin.baloon.BaloonLayout>

- In your map activity (where you will have your mapview) create an instance of layout we defined:


        LayoutInflater              layoutInflater = (LayoutInflater) this.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        noteBaloon = (BaloonLayout) layoutInflater.inflate(R.layout.baloon, null);
        RelativeLayout.LayoutParams layoutParams   = new RelativeLayout.LayoutParams(200,100);
        layoutParams.addRule(RelativeLayout.CENTER_VERTICAL);
        layoutParams.addRule(RelativeLayout.CENTER_HORIZONTAL);
        noteBaloon.setLayoutParams(layoutParams);   

- Now whenever you need to display the baloon, just call mapview's addView method as:

mapView.removeView(noteBaloon);
noteBaloon.setVisibility(View.VISIBLE);
                     ((TextView)noteBaloon.findViewById(R.id.note_text)).setText(msg.getData().getString(HANDLER_MESSAGE_AUTHOR)+"\n"+msg.getData().getString(HANDLER_MESSAGE_NOTE));
mapController.animateTo(noteOverlay.getTapPoint());
mapView.addView(noteBaloon, new MapView.LayoutParams(200,200,noteOverlay.getTapPoint(),MapView.LayoutParams.BOTTOM_CENTER));
mapView.setEnabled(false);       
 
- When  you need to remove baloon, just make it invisible:

noteBaloon.setVisibility(View.GONE);                    
mapView.setEnabled(true);     



Below you can find how it is looking:

• initial mapview

• just make a tap

• touch the close image on baloon

• insert new items in layout xml for richer baloon displays