I have used both modes on my Canon 550D and found the results to be quite different, particularly indoors under fluorescent light. Auto white balance seems to produce yellowish images at times, while using the custom white balance seems to produce more neutral colours.

How do these two modes work, and why doesn't the auto white balance do an equivalent job?

I use an image of white objects taken with auto white balance settings in the same lighting conditions to set the custom white balance

P.S. I will try to add images to show the difference


Question: How to modify source port in mount application linux
Answer: I already solve this, the answer was to create a new struct sockaddr, open a socket for that struct bind it to the port you want and then call connect to the struct that the mount client is using and with you socket. That way you force the kernel not to choose the src port for you.
Note that you will have to do the same when you are going to ask the server (Where the filesystem you are trying to mount is) for the file handles. 
Also in the end of the mount client calls the mount function from the kernel and it puts a random port again for the src port, so this approach is good if you want to modify the kernel as well.

Maybe I did not explain myself well, when I made the question but what I was trying to accomplish was to mount a filesystem in Computer A from Computer B.
Computer A is behind a router 
Computer B is on the router's outside network or like the isp network

For Example 

Computer A will have lets said 192.168.1.99
Router (Gateway) 192.168.1.1
Router Wan Ip 10.0.0.26
Computer B 10.0.0.55

so in order for computer B to talk to computer A there have to be a port opened in Router to allow the traffic to go through. And when you issue a mount the src port is generated randomly in the range 512-1023 and happens to be the same port that computer B sends info to.
I solve this by open it up that range on the router, It was not an option for me to modify the kernel. I hope this helps @mikea