I am having a problem to understand VLAN tagging. I have already done changes in the /etc/network/interface file using this link(http://myhomelab.blogspot.de/2014/01...-in-linux.html). I am using socket programming on Raspbian (Raspberry pi) using C language.

I tried 2 methods:
1. The socket in C is

s = socket(AF_INET, SOCK_RAW, IPPROTO_RAW);

When i send a ethernet packet from the raspberry pi using the above socket i only see the outgoing "arp" messages on the bus.

2. s = socket(AF_PACKET, SOCK_RAW, htons(ETH_P_ALL))

But this creates an error "sendto failed: Invalid argument" or "sendto failed: Network down" during sending the packet, the code to send packet is:

struct sockaddr_in sin;
sin.sin_family = AF_PACKET;
sin.sin_port = htons(30490);
sin.sin_addr.s_addr = inet_addr(destination ip address);

if(sendto(socket, data, iph->tot_len, 0, (struct sockaddr *)&sin, sizeof(sin)) < 0)
{ perror("Sendto failed"); }
else { printf("Packet send"); }

Could anyone help me with the information on how to attach a VLAN tag with every packet that is being send ? Or what i am doing wrong in above code ?

Is there a code which is already implemented for sending packets with VLAN?

(In my setup i am not using any router or switch)

Method 1 should work but you have to build packet from Ethernet header.

I didnt understand "build packet from Ethernet header" means that i should manually attach the MAC address ?

Yes, you have to generate Ethernet header, such as source and destination MAC. For your case, VLAN tag.
Another one, you should use AF_PACKET instead of AF_INET in method #1.

Ok. Now the problem is the destination and source MAC address will not always remain the same. But the destination ip address will always be the same. I am implementing a platform independent application. Now how should i proceed ?

If you don't want to deal with MAC address, you have to set socket option, IP_HDRINCL and SO_BINDTODEVICE.

@nini09 could you please provide an example for this ?

// Set flag so socket expects us to provide IPv4 header.
if (setsockopt (sd, IPPROTO_IP, IP_HDRINCL, &on, sizeof (on)) < 0) {
perror ("setsockopt() failed to set IP_HDRINCL ");
exit (EXIT_FAILURE);
}

// Bind socket to interface index.
if (setsockopt (sd, SOL_SOCKET, SO_BINDTODEVICE, &ifr, sizeof (ifr)) < 0) {
perror ("setsockopt() failed to bind to interface ");
exit (EXIT_FAILURE);
}

The packets are not being send using this method. I only shows the IPv4 packets in Wireshark. not the vlan and udp packet attached to it.

No Ethernet header?