RAW-Sockets should ignore MTU

Question

I have a small problem with my RAW-Socket. I have to capture packets in a network to modify them. For this I have to add special data, like timestamps. Everything works fine, but I have a problem witht the MTU (Linux).

If some of the application of the upper layers generates packets close to the MTU, I can not add my data. So I decide to downsize the MTU of the systems. So i got smaller packets and it is possible to ad my data. But when I try to sendout my packets with my RAW-Socket, it is not working.

I found (by google, in older Stackoverflow threads and in the linux manpages) that the option IP_MTU_DISCOVER could help. But when I use it with setsockopt, nothing changes.

if ((s = socket(AF_PACKET, SOCK_RAW, ETH_P_ALL)) < 0) {
     printf("Error: could not open socket\n");
     return -1;
}

char socket_mtu = IP_PMTUDISC_DONT;
setsockopt(s,IPPROTO_IP,IP_MTU_DISCOVER,socket_mtu, 1);

Maybe it is important. The downsizing of the MTU was done, because when I add my own data, the resulting packet would not be greater then 1500 + ETH-Header. My problem is, to make Raw-Sockets ignore the local MTU of my Linux-System.

Is IP_MTU_DISCOVER the right way to ignore the value set in MTU?
IS IP_MTU_DISCOVER realy usable with my RAW-SOCKET?
Are there any other ways to decrement the segement size of the protocols on layer 3 and higher?

I read, that sendto() returns EMSGSIZE (=90) when the size is too big. So I have done the following test.

int k = sendto(s, frame.buffer, frame_len, 0,
             (struct sockaddr*)&saddrll, sizeof(saddrll));
    printf("K:%i\n", k);

After calling sendto() "k" contains "-1". But if it depends on the packet size, shouldn't it be "90"??

When I set the size of the packet I like to send smaller then the MTU, it works fine.... A little bit strange.

Any help would be nice :-)

Kind regards, Andreas

score 0 · Answer 1 · edited May 23 '17 at 11:44

Ok. I found a solution to change the MTU from my C program.

set MTU in C programmatically

buffer.ifr_mtu = 1500;
  if(!ioctl(s, SIOCSIFMTU, &buffer)) {
      printf("MTU changed\n");
  }

For the first moment it works. But it's only a workarround, it's not realy clean. The problem coul be, that if an application is sending a packet in the moment i resize the MTU, the packet could be too big. Any better ideas?

Kind regards, Andreas

RAW-Sockets should ignore MTU

1 Answers1