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get default gateway
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| /* | |
| Licensed under the Apache License, Version 2.0 (the "License"); | |
| you may not use this file except in compliance with the License. | |
| You may obtain a copy of the License at | |
| http://www.apache.org/licenses/LICENSE-2.0 | |
| Unless required by applicable law or agreed to in writing, software | |
| distributed under the License is distributed on an "AS IS" BASIS, | |
| WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. | |
| See the License for the specific language governing permissions and | |
| limitations under the License. | |
| */ | |
| #include <sys/socket.h> | |
| #include <stdlib.h> | |
| #include <stdio.h> | |
| #include <string.h> | |
| #include <linux/netlink.h> | |
| #include <linux/rtnetlink.h> | |
| #include <arpa/inet.h> | |
| #include <unistd.h> | |
| #include <net/if.h> | |
| #define BUFFER_SIZE 4096 | |
| int getgatewayandiface() | |
| { | |
| int received_bytes = 0, msg_len = 0, route_attribute_len = 0; | |
| int sock = -1, msgseq = 0; | |
| struct nlmsghdr *nlh, *nlmsg; | |
| struct rtmsg *route_entry; | |
| // This struct contain route attributes (route type) | |
| struct rtattr *route_attribute; | |
| char gateway_address[INET_ADDRSTRLEN], interface[IF_NAMESIZE]; | |
| char msgbuf[BUFFER_SIZE], buffer[BUFFER_SIZE]; | |
| char *ptr = buffer; | |
| struct timeval tv; | |
| if ((sock = socket(AF_NETLINK, SOCK_RAW, NETLINK_ROUTE)) < 0) { | |
| perror("socket failed"); | |
| return EXIT_FAILURE; | |
| } | |
| memset(msgbuf, 0, sizeof(msgbuf)); | |
| memset(gateway_address, 0, sizeof(gateway_address)); | |
| memset(interface, 0, sizeof(interface)); | |
| memset(buffer, 0, sizeof(buffer)); | |
| /* point the header and the msg structure pointers into the buffer */ | |
| nlmsg = (struct nlmsghdr *)msgbuf; | |
| /* Fill in the nlmsg header*/ | |
| nlmsg->nlmsg_len = NLMSG_LENGTH(sizeof(struct rtmsg)); | |
| nlmsg->nlmsg_type = RTM_GETROUTE; // Get the routes from kernel routing table . | |
| nlmsg->nlmsg_flags = NLM_F_DUMP | NLM_F_REQUEST; // The message is a request for dump. | |
| nlmsg->nlmsg_seq = msgseq++; // Sequence of the message packet. | |
| nlmsg->nlmsg_pid = getpid(); // PID of process sending the request. | |
| /* 1 Sec Timeout to avoid stall */ | |
| tv.tv_sec = 1; | |
| setsockopt(sock, SOL_SOCKET, SO_RCVTIMEO, (struct timeval *)&tv, sizeof(struct timeval)); | |
| /* send msg */ | |
| if (send(sock, nlmsg, nlmsg->nlmsg_len, 0) < 0) { | |
| perror("send failed"); | |
| return EXIT_FAILURE; | |
| } | |
| /* receive response */ | |
| do | |
| { | |
| received_bytes = recv(sock, ptr, sizeof(buffer) - msg_len, 0); | |
| if (received_bytes < 0) { | |
| perror("Error in recv"); | |
| return EXIT_FAILURE; | |
| } | |
| nlh = (struct nlmsghdr *) ptr; | |
| /* Check if the header is valid */ | |
| if((NLMSG_OK(nlmsg, received_bytes) == 0) || | |
| (nlmsg->nlmsg_type == NLMSG_ERROR)) | |
| { | |
| perror("Error in received packet"); | |
| return EXIT_FAILURE; | |
| } | |
| /* If we received all data break */ | |
| if (nlh->nlmsg_type == NLMSG_DONE) | |
| break; | |
| else { | |
| ptr += received_bytes; | |
| msg_len += received_bytes; | |
| } | |
| /* Break if its not a multi part message */ | |
| if ((nlmsg->nlmsg_flags & NLM_F_MULTI) == 0) | |
| break; | |
| } | |
| while ((nlmsg->nlmsg_seq != msgseq) || (nlmsg->nlmsg_pid != getpid())); | |
| /* parse response */ | |
| for ( ; NLMSG_OK(nlh, received_bytes); nlh = NLMSG_NEXT(nlh, received_bytes)) | |
| { | |
| /* Get the route data */ | |
| route_entry = (struct rtmsg *) NLMSG_DATA(nlh); | |
| /* We are just interested in main routing table */ | |
| if (route_entry->rtm_table != RT_TABLE_MAIN) | |
| continue; | |
| route_attribute = (struct rtattr *) RTM_RTA(route_entry); | |
| route_attribute_len = RTM_PAYLOAD(nlh); | |
| /* Loop through all attributes */ | |
| for ( ; RTA_OK(route_attribute, route_attribute_len); | |
| route_attribute = RTA_NEXT(route_attribute, route_attribute_len)) | |
| { | |
| switch(route_attribute->rta_type) { | |
| case RTA_OIF: | |
| if_indextoname(*(int *)RTA_DATA(route_attribute), interface); | |
| break; | |
| case RTA_GATEWAY: | |
| inet_ntop(AF_INET, RTA_DATA(route_attribute), | |
| gateway_address, sizeof(gateway_address)); | |
| break; | |
| default: | |
| break; | |
| } | |
| } | |
| if ((*gateway_address) && (*interface)) { | |
| fprintf(stdout, "Gateway %s for interface %s\n", gateway_address, interface); | |
| break; | |
| } | |
| } | |
| close(sock); | |
| return 0; | |
| } | |
| int main(int argc, char **argv) | |
| { | |
| getgatewayandiface(); | |
| return 0; | |
| } |
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
| /* | |
| Licensed under the Apache License, Version 2.0 (the "License"); | |
| you may not use this file except in compliance with the License. | |
| You may obtain a copy of the License at | |
| http://www.apache.org/licenses/LICENSE-2.0 | |
| Unless required by applicable law or agreed to in writing, software | |
| distributed under the License is distributed on an "AS IS" BASIS, | |
| WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. | |
| See the License for the specific language governing permissions and | |
| limitations under the License. | |
| */ | |
| #include <stdio.h> | |
| #include <string.h> | |
| #include <unistd.h> | |
| #include <errno.h> | |
| #include <fcntl.h> | |
| #include <ctype.h> | |
| #include <netinet/in.h> | |
| #include <arpa/inet.h> | |
| #include <string.h> | |
| #include <net/if.h> | |
| #define BUFFER_SIZE 4096 | |
| int getgatewayandiface(in_addr_t * addr, char *interface) | |
| { | |
| long destination, gateway; | |
| char iface[IF_NAMESIZE]; | |
| char buf[BUFFER_SIZE]; | |
| FILE * file; | |
| memset(iface, 0, sizeof(iface)); | |
| memset(buf, 0, sizeof(buf)); | |
| file = fopen("/proc/net/route", "r"); | |
| if (!file) | |
| return -1; | |
| while (fgets(buf, sizeof(buf), file)) { | |
| if (sscanf(buf, "%s %lx %lx", iface, &destination, &gateway) == 3) { | |
| if (destination == 0) { /* default */ | |
| *addr = gateway; | |
| strcpy(interface, iface); | |
| fclose(file); | |
| return 0; | |
| } | |
| } | |
| } | |
| /* default route not found */ | |
| if (file) | |
| fclose(file); | |
| return -1; | |
| } | |
| int main(int argc, char **argv) | |
| { | |
| in_addr_t addr = 0; | |
| char iface[IF_NAMESIZE]; | |
| memset(iface, 0, sizeof(iface)); | |
| getgatewayandiface(&addr, iface); | |
| printf("%s\n", inet_ntoa(*(struct in_addr *) &addr)); | |
| printf("%s\n", iface); | |
| return 0; | |
| } |
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