On Friday 11 June 2004 13:31, Henrik Nordstrom wrote:
> On Fri, 11 Jun 2004, Denis Vlasenko wrote:
> > The problem is, how to _send replies_ with correct src ip?
>
> By using IP_PKTINFO in a sendmsg() call. See ip(7) for details.
>
> You can also use this in recvmsg if you prefer only one extra address
> structure type in sending/receiving messages and less complex code..
>
> Regards
> Henrik
manpage says:
SOCKET OPTIONS
IP supports some protocol specific socket options
that can be set with setsockopt(2) and read with
getsockopt(2). The socket option level for IP is
SOL_IP.
...
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 a
struct in_addr ipi_addr; /* Header Destination ad
};
ipi_ifindex is the unique index of the interface
the packet was received on. ipi_spec_dst is the
destination address of the routing table entry and
ipi_addr is the destination address in the packet
header. If IP_PKTINFO is passed to sendmsg(2) then
the outgoing packet will be sent over the interface
specified in ipi_ifindex with the destination
address set to ipi_spec_dst
i.e. it does not tell anything about using IP_PKTINFO
for sending.
I am trying to figure it out, currently I'm getting
-EINVAL in sendmsg in this code:
# define SRCADDR_SOCKOPT IP_PKTINFO
# define SRCADDR_DATASIZE (CMSG_SPACE(sizeof(struct in_pktinfo)))
# define srcaddr(x) (&(((struct in_pktinfo *)(CMSG_DATA(x)))->ipi_spec_dst))
union control_data {
struct cmsghdr cmsg;
u_char data[SRCADDR_DATASIZE];
};
int socket_send4_src2(int s,const char *buf,int len,const char ip[4],uint16 port,const char ip_src[4])
{
struct sockaddr_in sa;
struct iovec iov[1];
union control_data cmsg;
struct msghdr msg;
struct cmsghdr* cmsgptr;
int r;
byte_zero(&sa,sizeof sa);
sa.sin_family = AF_INET;
uint16_pack_big((char *) &sa.sin_port,port);
byte_copy((char *) &sa.sin_addr,4,ip);
iov[0].iov_base = buf;
iov[0].iov_len = len;
memset(&msg, 0, sizeof msg);
msg.msg_name = &sa;
msg.msg_namelen = sizeof sa;
msg.msg_iov = iov;
msg.msg_iovlen = 1;
msg.msg_control = &cmsg;
msg.msg_controllen = sizeof cmsg;
cmsgptr = CMSG_FIRSTHDR(&msg);
byte_zero(&cmsg,sizeof cmsg);
cmsgptr->cmsg_len = SRCADDR_DATASIZE; /* data byte count, including hdr */
cmsgptr->cmsg_level = IPPROTO_IP; /* originating protocol */
cmsgptr->cmsg_type = SRCADDR_SOCKOPT; /* protocol-specific type */
byte_copy((char *) srcaddr(cmsgptr),4,ip); /* followed by u_char cmsg_data[]; */
{ // FIXME: needs to be done only once...
int sockopt;
sockopt = 1;
if (setsockopt(s, IPPROTO_IP, SRCADDR_SOCKOPT, &sockopt, sizeof sockopt) == -1) {
perror("setsockopt problem");
return -1;
}
}
r = sendmsg(s, &msg, 0);
if(r)
perror("sendmsg problem");
return r;
}
--
vda
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