Sending frames of size 1518 using raw sockets

[yalakvar]

Hi,

I wrote an application that receives packets on one interface and sends them to another interface after it added a vlan header. Both the sending and the receiving is done using raw sockets.

Everything seems to work fine until I get TCP packets that are of size 1514 (MTU). Once I add a vlan header to the packet, its size becomes 1518 and when I try to send it I get the returned value -1 and errno=90 (message too long).

I tried to change the MTU of the NIC to a value that is bigger than 1500 but that fails.
If I create a bridge using brctl and vconfig between the NICs I can see that my NIC does sends packets of size 1518.

What do I need to in order to make my NIC to send packets of size 1518?

Thanks in advance,
Mickey

[chaosless]

you opened the socket raw (SOCK_RAW)? you receive on a TCP stream (SOCK_STREAM)? then are sending out again on TCP? or UDP?

then are getting EMSGSIZE from sendmsg() ?

when i'd needed to do something similar - was using udp - and istr that had to rewrite settings to one of the file in /proc/sys/net - will poke around and see if i still have that somewhere...

[chaosless]

should be one of the conf. settings you can find with this:

sysctl -a | grep -i mtu

[chaosless]

i suppose you know 1518 is actually not available in the standard ethernet packet... so presumably you have gigE and jumbo packets available to where you are sending this repeater?

[yalakvar]

Thx for answering.
The socket I opened is SOCK_RAW. I receive on a raw socket and send on another one.
Both NICs are 100 Mb ethernet interfaces. I didn't change any configuration regarding the interfaces. I also wasn't sure if my NIC supports frames of size 1518, but when I setup a bridge and config a vlan (using brctl and vconfig) between them I see that my NIC is able to send such frames of size 1518. For some reason when my application does it, it fails.

I will try what you said regarding one of the conf. settings.

[yalakvar]

Hi,

If it helps, this is the creation code of my socket:

int CreateRawSocket (void)
{
	int iSockHandle = socket(AF_PACKET, SOCK_RAW, htons(ETH_P_ALL));
	if (iSockHandle == -1)
	{
		return -1;
	}

	struct sockaddr_ll sll;
	struct ifreq ifr;

	bzero(&sll, sizeof(sll));
	bzero(&ifr, sizeof(ifr));

	/* First Get the Interface Index  */

	strncpy((char *)ifr.ifr_name, "eth2", IFNAMSIZ);
	if ((ioctl (iSockHandle, SIOCGIFINDEX, &ifr)) == -1)
	{
		return -1;
	}

	// add the promiscuous mode
	struct packet_mreq mr;
	memset (&mr, 0, sizeof(mr));
	mr.mr_ifindex = ifr.ifr_ifindex;
	mr.mr_type = PACKET_MR_PROMISC;

	if (setsockopt (iSockHandle, SOL_PACKET, PACKET_ADD_MEMBERSHIP, (char *)&mr, sizeof(mr)) < 0)
	{
	    perror("Failed to add the promiscuous mode");
	    return (-1);
	}

	/* Bind our raw socket to this interface */

	sll.sll_family = AF_PACKET;
	sll.sll_ifindex = ifr.ifr_ifindex;
	sll.sll_protocol = htons (ETH_P_ALL);

	if ((bind (iSockHandle, (struct sockaddr *)&sll, sizeof(sll)))== -1)
	{
		return -1;
	}

	return iSockHandle;
}