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/* Helper for netlink */
/*
* (C) 2014 by Holger Hans Peter Freyther
*
* All Rights Reserved
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU Affero General Public License as published by
* the Free Software Foundation; either version 3 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU Affero General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>.
*
*/
#include <arpa/inet.h>
#include <netinet/ip.h>
#include <sys/socket.h>
#include <linux/netlink.h>
#include <linux/rtnetlink.h>
#include <string.h>
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#define NLMSG_TAIL(nmsg) \
((struct rtattr *) (((void *) (nmsg)) + NLMSG_ALIGN((nmsg)->nlmsg_len)))
/**
* In case one binds to 0.0.0.0/INADDR_ANY and wants to know which source
* address will be used when sending a message this function can be used.
* It will ask the routing code of the kernel for the PREFSRC
*/
int source_for_dest(const struct in_addr *dest, struct in_addr *loc_source)
{
int fd, rc;
struct rtmsg *r;
struct rtattr *rta;
struct {
struct nlmsghdr n;
struct rtmsg r;
char buf[1024];
} req;
memset(&req, 0, sizeof(req));
fd = socket(AF_NETLINK, SOCK_RAW|SOCK_CLOEXEC, NETLINK_ROUTE);
if (fd < 0) {
perror("nl socket");
return -1;
}
/* Send a rtmsg and ask for a response */
req.n.nlmsg_len = NLMSG_LENGTH(sizeof(struct rtmsg));
req.n.nlmsg_flags = NLM_F_REQUEST | NLM_F_ACK;
req.n.nlmsg_type = RTM_GETROUTE;
req.n.nlmsg_seq = 1;
/* Prepare the routing request */
req.r.rtm_family = AF_INET;
/* set the dest */
rta = NLMSG_TAIL(&req.n);
rta->rta_type = RTA_DST;
rta->rta_len = RTA_LENGTH(sizeof(*dest));
memcpy(RTA_DATA(rta), dest, sizeof(*dest));
/* update sizes for dest */
req.r.rtm_dst_len = sizeof(*dest) * 8;
req.n.nlmsg_len = NLMSG_ALIGN(req.n.nlmsg_len) + RTA_ALIGN(rta->rta_len);
rc = send(fd, &req, req.n.nlmsg_len, 0);
if (rc != req.n.nlmsg_len) {
perror("short write");
close(fd);
return -2;
}
/* now receive a response and parse it */
rc = recv(fd, &req, sizeof(req), 0);
if (rc <= 0) {
perror("short read");
close(fd);
return -3;
}
if (!NLMSG_OK(&req.n, rc) || req.n.nlmsg_type != RTM_NEWROUTE) {
close(fd);
return -4;
}
r = NLMSG_DATA(&req.n);
rc -= NLMSG_LENGTH(sizeof(*r));
rta = RTM_RTA(r);
while (RTA_OK(rta, rc)) {
if (rta->rta_type != RTA_PREFSRC) {
rta = RTA_NEXT(rta, rc);
continue;
}
/* we are done */
memcpy(loc_source, RTA_DATA(rta), RTA_PAYLOAD(rta));
close(fd);
return 0;
}
close(fd);
return -5;
}
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