CODEWITHSUNDEEP
csocketsnetwork-programming
RHLK
RHLK

Reputation: 347

Port that we bind in server is not same as with netstat

In the server program I'm binding IP 127.0.0.1 and port 1234.

However when I use netstat -lntpu to find what process is listening to what port..I find my server program listed but strikingly the port number is not what I've assigned !

Here's the line what I'm referring to

Proto | Recv-Q | Send-Q | Local Address    |     Foreign Address  | State  | PID/Program_name 

tcp   | 0      |  0     | 127.0.0.1:53764  |       0.0.0.0:*      | LISTEN |   10545/server

So if I binded the socket with 1234 why is the netstat listing 53764 ?

I'm able to transfer data successfully from client to server and back. But this is looking strange here !!

Here's the binding code

struct sockaddr_in ser_addr

ser_addr.sin_family=AF_INET;
ser_addr.sin_port=1234;
ser_addr.sin_addr.s_addr=inet_addr("127.0.0.1");

sd=socket(AF_INET,SOCK_STREAM,0);
ret=bind(sd,(struct sockaddr*)&ser_addr,sizeof(ser_addr));
listen(sd,3);

Upvotes: 0

Views: 350

Answers (1)

paxdiablo
paxdiablo

Reputation: 881443

It has to to do with the byte ordering of your port setting.

The value 1234 is made up of the two octets {4, 210} while {210, 4} gives you your 53764 value (a).

When specifying the port address, you should be aware that it is a 16-bit port number, network byte ordered (from W. Richard Stevens' UNIX Network Programming (b)). Network byte ordering is a big-endian scheme whereas your implementation appears to have a little-endian scheme, hence you need to switch the octets around:

ser_addr.sin_port = htons (1234);

In fact, you should do it that way even if the byte ordering is the same since it becomes, in that case, a no-operation. The htons is how you (quoting from the same book mentioned above) "convert host-to-network, short integer".

(a) This is so because the order of the octets within a data type dictate how the value is calculated:

  4 * 256 + 210 =  1234  // or use (x <<   8 | y) rather than 
210 * 256 +   4 = 53764  //        (x *  256 + y) if you wish

(b) A book you really should have if you're going to be doing any serious networking development.

Upvotes: 4

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