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From: Denis Vlasenko <vda@port.imtp.ilyichevsk.odessa.ua>
To: Herbert Xu <herbert@gondor.apana.org.au>
Subject: Re: UDP sockets bound to ANY send answers with wrong src ip address
Date: Fri, 11 Jun 2004 15:27:45 +0300
Cc: yoshfuji@linux-ipv6.org, netdev@oss.sgi.com, linux-net@vger.kernel.org,
       davem@redhat.com, pekkas@netcore.fi, jmorris@redhat.com,
       linux-kernel@vger.kernel.org
References: <E1BYjMY-0002CE-00@gondolin.me.apana.org.au>
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On Friday 11 June 2004 13:34, Herbert Xu wrote:
> Denis Vlasenko <vda@port.imtp.ilyichevsk.odessa.ua> wrote:
> > I looked into sendmsg(). Looks like ther is no way to
> > indicate source ip.
> >
> > Shall I use some other technique?
>
> IP_PKTINFO works just as well there.  Look at the ip_cmsg_send call
> in udp_sendmsg.

int udp_sendmsg(...)
{
...
        ipc.addr = inet->saddr;

        ipc.oif = sk->sk_bound_dev_if;
        if (msg->msg_controllen) {
                err = ip_cmsg_send(msg, &ipc);
                if (err)
                        return err;
                if (ipc.opt)
                        free = 1;
                connected = 0;
        }
        if (!ipc.opt)
                ipc.opt = inet->opt;

        saddr = ipc.addr;
...

int ip_cmsg_send(struct msghdr *msg, struct ipcm_cookie *ipc)
{
...
                case IP_PKTINFO:
                {
                        struct in_pktinfo *info;
                        if (cmsg->cmsg_len != CMSG_LEN(sizeof(struct in_pktinfo)))
                                return -EINVAL;
                        info = (struct in_pktinfo *)CMSG_DATA(cmsg);
                        ipc->oif = info->ipi_ifindex;
                        ipc->addr = info->ipi_spec_dst.s_addr;

manpage:
IP_PKTINFO
Pass an IP_PKTINFO ancillary message that contains a pktinfo structure that supplies some information  about  the
incoming packet. This only works for datagram oriented sockets.

struct in_pktinfo
{
    unsigned int   ipi_ifindex;  /* Interface index */
    struct in_addr ipi_spec_dst; /* Routing destination address */
    struct in_addr ipi_addr;     /* Header Destination address */
};

Hmmm... do I have to set a *routing dest address* field
to set src ip address of my UDP packet?
-- 
vda
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