Be confused with msg_name field in msghdr structure

In user space, I encapsulated a L3 packet using sock_raw (including IP header) and send to kernel space using sock_sendmsg() using msghdr structure

struct msghdr {
    void         *msg_name;       /* optional address */
    struct iovec *msg_iov;        /* scatter/gather array */
    ...
};

I cannot understand clearly the roles of msg_name. I already specified the source IP and dest IP in L3 header. Why do I need msg_name?

Answer

The msg_name and msg_namelen fields of struct msghdr have the same function as the dest_addr and addrlen arguments to sendto: they specify the destination address. They are intended to be used with normal unconnected datagram sockets. For instance, when sending UDP packets with sendmsg on an AF_INET/SOCK_DGRAM socket, you supply only the payload, not the headers, in the iovec, and the destination address goes in msg_name + msg_namelen.

raw(7), the manpage describing SOCK_RAW sockets, indicates that you are allowed to put the header into the iovec when using raw sockets (note in particular the discussion of IP_HDRINCL) but does not make clear what you should set msg_name and msg_namelen to in that case. I would recommend you try setting both of them to 0 and see if that works.