How to set the don’t fragment (DF) flag on a socket?

by

You do it with the setsockopt() call, by using the IP_DONTFRAG option:

int val = 1;
setsockopt(sd, IPPROTO_IP, IP_DONTFRAG, &val, sizeof(val));

Here’s a page explaining this in further detail.

For Linux, it appears you have to use the IP_MTU_DISCOVER option with the value IP_PMTUDISC_DO (or IP_PMTUDISC_DONT to turn it off):

int val = IP_PMTUDISC_DO;
setsockopt(sd, IPPROTO_IP, IP_MTU_DISCOVER, &val, sizeof(val));

I haven’t tested this, just looked in the header files and a bit of a web search so you’ll need to test it.

As to whether there’s another way the DF flag could be set:

I find nowhere in my program where the “force DF flag” is set, yet tcpdump suggests it is. Is there any other way this could get set?

From this excellent page here:

IP_MTU_DISCOVER: Sets or receives the Path MTU Discovery setting for a socket. When enabled, Linux will perform Path MTU Discovery as defined in RFC 1191 on this socket. The don’t fragment flag is set on all outgoing datagrams. The system-wide default is controlled by the ip_no_pmtu_disc sysctl for SOCK_STREAM sockets, and disabled on all others. For non SOCK_STREAM sockets it is the user’s responsibility to packetize the data in MTU sized chunks and to do the retransmits if necessary. The kernel will reject packets that are bigger than the known path MTU if this flag is set (with EMSGSIZE).

This looks to me like you can set the system-wide default using sysctl:

    sysctl ip_no_pmtu_disc

returns "error: "ip_no_pmtu_disc" is an unknown key" on my system but it may be set on yours. Other than that, I’m not aware of anything else (other than setsockopt() as previously mentioned) that can affect the setting.

More Related Contents:

  1. Structure size so big, need optimization
  2. C++ Socket Error (new initializer expression list treated as compound expression)
  3. Detecting TCP Client Disconnect
  4. How can I get the IP Address of a local computer?
  5. What are the use cases of SO_REUSEADDR?
  6. Send and Receive a file in socket programming in Linux with C/C++ (GCC/G++)
  7. How to set a timeout on blocking sockets in boost asio?
  8. will casting around sockaddr_storage and sockaddr_in break strict aliasing
  9. Handling partial return from recv() TCP in C
  10. How to link winsock.lib?
  11. Boost::asio – how to interrupt a blocked tcp server thread?
  12. How do you throttle the bandwidth of a socket connection in C?
  13. Problems with SO_BINDTODEVICE Linux socket option
  14. C++ OpenCV image sending through socket
  15. Sending UDP Packet in C#
  16. c++ how to use select to see if a socket has closed
  17. How to exit a blocking recv() call? [duplicate]
  18. Where does one get the “sys/socket.h” header/source file?
  19. How to Run Only One Instance of Application
  20. Why same char is not equal [closed]
  21. Returning arrays from a function in c++
  22. How to compile for Windows on Linux with gcc/g++?
  23. Template deduction for function based on its return type?
  24. How can I compose output streams, so output goes multiple places at once?
  25. What’s the difference between long long and long
  26. How many spaces for tab character(\t)?
  27. Singleton pattern in C++
  28. Fast division/mod by 10ˣ
  29. Where are string literals stored, and can I modify them?
  30. C++ trying to swap values in a vector

Leave a Comment