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My doubt is quite strange ..

On my Linux server a DHCP server application is waiting on the port number 547 for dhcp client messages.

Even though I am seeing messages from dhcp client on wireshark (running on my Linux server) .. dhcp server is not replying back.

Is there anyway to confirm the packet is actually processed by dhcp server or dropped in between by Linux ip stack ?

DHCP server is proprietary code so that I cant modify it to add more logs or check at recv function.

Just consider it as a generic question based on sockets rather than DHCP specific. My intrest is how can we know from an stats or other stuff that packet has been received in appropriate socket buffers.

codingfreak
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2 Answers2

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You can do so by building the DHCP server with debugging symbols - your distribution maybe already has a *-dbg package ready for that. By attaching a debugger like gdb to the process, you may intercept messages and debug it to see where the problem is.

However, I'd first try to work with the usual possibilities, namely log-messages. Have you tried to activate Debug logging in your DHCP daemon yet?

Lars
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  • I presume codingfreak is hacking an embedded appliance has limited means to modify the environment. – Steve-o Sep 10 '11 at 12:58
  • I am trying to use the already available DEBUG messages which are part of the application. As per the application help if u enable some options at startup u can see each and every message processed by the application. But I am not seeing in LOG so it means DHCP server hasnt received any right ? – codingfreak Sep 10 '11 at 13:00
  • doesn't have to, depending on the how talkative your dhcp-server is. If Steve-O is correct and you are using a proprietary server, you may be out of luck. I assume you did try the usual dance of restarting everything? – Lars Sep 10 '11 at 13:05
  • I even tried STRACE tool and found out dhcpserver is waiting on select command .. so it means all those solicit messages from client are not reaching the server :) – codingfreak Sep 10 '11 at 13:33
  • hrm... might be a problem with the kernel interface after all. I would have suggested a configuration problem with a CLIENT_ID or something similar, but if it's the select, it might not get the message after all. Did you also strace any child processes, if it's built that way? Did the daemon ever work at all? – Lars Sep 10 '11 at 13:45
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If you ran strace on the dhcp server and it's not getting anything back from select then it's most likely NOT receiving the frame. A couple of things to check

  • Are you sure your client is sending IPv6 DHCP requests instead of IPv4 (port 67)?
  • Make sure no other process is listening on port 547 on the server.

Recheck your strace command line options and enable debug and verbosity. One more thing to make sure of when you use strace is to follow forks... -Ff

Hope that helps

Steve Lazaridis
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  • In my wireshark I an clearly seeing a DHCPv6 solicit message which is designated for port .. I will try the fork options which u have shared .. thanx :) – codingfreak Sep 11 '11 at 03:30