sendmsg(), IPv4, and ancilliary socket data

[Gert Doering <gert%greenie.muc.de@localhost>]

Hi,

I would appreciate a pointer to documentation here - "man ip" isn't
helping, "man sendmsg" isn't helping either...

What I want to do is: 

 - have an UDP socket that is bound to INADDR_ANY
 - receive the destination address for incoming packets via recvmsg()
   (this works, using IP_RECVDSTADDR setsockopt())
 - send the reply packet with sendmsg(), ensuring the source address is
   where the incoming packet was sent to

I have this working for IPv6, and for IPv4 on "about everything but
NetBSD"

On "the other BSDs", using 

        mesg.msg_controllen = CMSG_SPACE(sizeof (struct in_addr));
        cmsg = CMSG_FIRSTHDR (&mesg);
        cmsg->cmsg_len = CMSG_LEN(sizeof(struct in_addr));
        cmsg->cmsg_level = IPPROTO_IP;
        cmsg->cmsg_type = IP_RECVDSTADDR;
        *(struct in_addr *) CMSG_DATA (cmsg) = to->pi.in4;

(where to->pi.in4 is a struct in_addr)

works nicely for sending, but NetBSD 7.0.1 returns EINVAL.

Reading through
  
   http://www.nerv.org/~ryo/files/netbsd/sendfromto/sockfromto.c

made me try IP_PKTINFO instead

        mesg.msg_controllen = CMSG_SPACE(sizeof (struct in_pktinfo));
        cmsg = CMSG_FIRSTHDR (&mesg);
        cmsg->cmsg_len = CMSG_LEN(sizeof (struct in_pktinfo));
        cmsg->cmsg_level = IPPROTO_IP;
        cmsg->cmsg_type = IP_PKTINFO;
        
        struct in_pktinfo *pkti;
        pkti = (struct in_pktinfo *) CMSG_DATA (cmsg);
        pkti->ipi_ifindex = 0;
        pkti->ipi_addr = to->pi.in4;

... but that yields EINVAL just as well.


So, right now I'm wondering if this is a case of "not implemented for
sending" (as per ryo's comment: "socket(), bind() and sendto()"), or 
whether I'm doing something wrong - which might well be.

Thus: a pointer to the part of the documentation that documents what
sort of ancilliary socket options sendmsg() will accept would be 
highly appreciated...

(in parallel, I'm digging through src/sys/ to see if I can find the
answer there, but a concise document would be nicer :) )

thanks,

gert

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Gert Doering - Munich, Germany                             gert%greenie.muc.de@localhost
fax: +49-89-35655025                        gert%net.informatik.tu-muenchen.de@localhost