codingfreak - Developer IT

Debugging rules in Iptables

- by codingfreak

Hi, How can I know how many packets were dropped by my iptables rules ?? Is there any debugging mechanism to see which rule is Dropping my packet or Accepting it ??

iptables and snatting to different networks

linuxbox (p.q.r.t) | | INTERNAL ------ ABCD ----- INTERNET (p.q.r.s) (m.n.o.k) ABCD has 3 interfaces connected to linuxbox, INTERNAL N/W, INTERNET. Linuxbox has a private address (p.q.r.t). At present I am snatting the packets from linuxbox to INTERNET at ABCD. I have a small doubt regarding the FTP from linuxbox since I have to support ftp from linuxbox to both INTERNAL N/W as well as in INTERNET. How can I right a rule in iptables present in ABCD where it can decide if the destination ip-address of ftp server is within INTERNAL N/W or in INTERNET and do natting accordingly.

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Manually closing a port from commandline

- by codingfreak

Hi I want to close an open port which is in listening mode between my client and server application. Is there any manual command line option in Linux to close a port ?? NOTE: I came to know that "only the application which owns the connected socket should close it, which will happen when the application terminates." I dont understand why it is only possible by the application which opens it ... But still eager to know if there is any another way to do it ??

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Postrouting rule in NAT table

- by codingfreak

Hi I have a strange question regarding NAT using iptables. When I do SNAT in a postrouting chain in NAT table at the end of the rule should I give -J ACCEPT? I see counters on the postrouting rule getting incremented but no packet leaving the machine. So does it mean the packet is DROPPED automatically?

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Manually closing a port from commandline

- by codingfreak

I want to close an open port which is in listening mode between my client and server application. Is there any manual command line option in Linux to close a port ?? NOTE: I came to know that "only the application which owns the connected socket should close it, which will happen when the application terminates." I dont understand why it is only possible by the application which opens it ... But still eager to know if there is any another way to do it ??

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fork() within a fork()

- by codingfreak

Hi Is there any way to differentiate the child processes created by different fork() functions within a program. global variable i; SIGCHLD handler function() { i--; } handle() { fork() --> FORK2 } main() { while(1) { if(i<5) { i++; if( (fpid=fork())==0) --> FORK1 handle() else (fpid>0) ..... } } } Is there any way I can differentiate between child processes created by FORK1 and FORK2 ?? because I am trying to decrement the value of global variable 'i' in SIGCHLD handler function and it should be decremented only for the processes created by FORK1 .. I tried to use an array and save the process id [this code is placed in fpid0 part] of the child processes created by FORK1 and then decrement the value of 'i' only if the process id of dead child is within the array ... But this didn't work out as sometimes child processes dead so fastly that updating the array is not done perfectly and everything messed up. So is there any better solution for this problem ??

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cscope with Clearcase codebase

- by codingfreak

Hi I have a small doubt in using cscope with clearcase views. Before I used to use ctags and used to run script which creates tag files within the directories of codebase for each clearcase view as a result I can use multiple clearcase views with different codebases at once. But in case of cscope, database is getting created in my home directory as a result I have to build cscope database everytime and not able to use parallely different clearcase views with different codebases. Is there any solution for this problem ??

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Checking Linux Kernel Debugging option

- by codingfreak

How can I know if the standard kernel provided in my linux flavour has got DEBUG KERNEL ENABLED flags selected or not ?? I think DEBUG option should be enabled inorder to use tools like kprobe, systemtap ??

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Manually closing a port from commandline

- by codingfreak

Hi I want to close an open port which is in listening mode between my client and server application. Is there any manual command line option in Linux to close a port ?? NOTE: I came to know that "only the application which owns the connected socket should close it, which will happen when the application terminates." I dont understand why it is only possible by the application which opens it ... But still eager to know if there is any another way to do it ??

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inet_ntoa problem

- by codingfreak

Hi I am declaring following variables unsigned long dstAddr; unsigned long gateWay; unsigned long mask; These variables contains ipaddresses in network byte order. So when I am trying to print them using inet_ntoa function for mask variable sometimes it is printing strange values printf("%s\t%s\t%s\t",inet_ntoa(dstAddr),inet_ntoa(gateWay),inet_ntoa(mask)); 192.168.122.0 0.0.0.0 0.255.255.255 but it should be 192.168.122.0 0.0.0.0 255.255.255.0 So is this because of inet_ntoa ??

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