On Wed, 20 Oct 2004 20:17:08 -0500
Eric Bambach <eric@cisu.net> wrote:
> Hello,
>
> I am trying very hard to understand the userspace view of Netlink sockets.
> However, my programming is not suffiecient enough to fully understand the man
> pages or the source(iproute2/zebra) and work off of that. I have tried many
> times but there seems no good documentation or implementation to help me. I
> have found this implementation written by someone on LKML 2 years ago,
> however the code produces no output. If I can get the code to produce some
> and the expected output im sure I can work and understand the code from
> there. However, because I did not write this code nor do I fully understand
> netlink sockets I am at a loss of what to do. Can someone explain to me if
> this code is correct for the 2.6.x series kernel? Furthermore, the author did
> not define replybuf nor byes nor netlink_fd so I have guessed as to their
> types. Ideally in the future Id like to use netlink to gather interface
> information and modify routing tables, however without a solid base to learn
> and adapt, I am at a loss.
The file libnetlink.c in iproute2 sources is what you want to look at:
The problem with your code is it isn't look at the length of each sub-element
of the response message and stepping appropriately.
The similar loop in libnetlink.c (notice more error checking)....
status = recvmsg(rth->fd, &msg, 0);
if (status < 0) {
if (errno == EINTR)
continue;
perror("OVERRUN");
continue;
}
if (status == 0) {
fprintf(stderr, "EOF on netlink\n");
return -1;
}
if (msg.msg_namelen != sizeof(nladdr)) {
fprintf(stderr, "sender address length == %d\n", msg.msg_namelen);
exit(1);
}
h = (struct nlmsghdr*)buf;
while (NLMSG_OK(h, status)) {
int err;
if (nladdr.nl_pid != 0 ||
h->nlmsg_pid != rth->local.nl_pid ||
h->nlmsg_seq != rth->dump) {
if (junk) {
err = junk(&nladdr, h, arg2);
if (err < 0)
return err;
}
goto skip_it;
}
if (h->nlmsg_type == NLMSG_DONE)
return 0;
if (h->nlmsg_type == NLMSG_ERROR) {
struct nlmsgerr *err = (struct nlmsgerr*)NLMSG_DATA(h);
if (h->nlmsg_len < NLMSG_LENGTH(sizeof(struct nlmsgerr))) {
fprintf(stderr, "ERROR truncated\n");
} else {
errno = -err->error;
perror("RTNETLINK answers");
}
return -1;
}
err = filter(&nladdr, h, arg1);
if (err < 0)
return err;
skip_it:
h = NLMSG_NEXT(h, status);
}
if (msg.msg_flags & MSG_TRUNC) {
fprintf(stderr, "Message truncated\n");
continue;
}
if (status) {
fprintf(stderr, "!!!Remnant of size %d\n", status);
exit(1);
}
}
}
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