Getting gateway to use for a given ip in ANSI C

Question

I have looked around like crazy but don't get a real answer. I got one example, but that depended on the individuals own library so not much good.

At first I wanted to get the default gateway of an interface, but since different IP's could be routed differently I quickly understood that what I want it get the gateway to use for a given destination IP by using an AF_ROUTE socket and the rtm_type RTM_GET. Does anyone have an example where I actually end up with a string containing the gateways IP (or mac address)? The gateway entry seem to be in hex but also encoded in /proc/net/route, where I guess the AF_ROUTE socket get's it info from (but via the kernel I guess).

Thanx in advance

and p.s. I just started using stack overflow and I must say, all of you guys are great! Fast replies and good ones! You are my new best friends ;)

Answer 1

This is OS specific, there's no unified(or ANSI C) API for this.

Assuming Linux, the best way is to just parse /proc/net/route , look for the entry where Destination is 00000000 , the default gateway is in the Gateway column , where you can read the hex representation of the gateway IP address (in big endian , I believe)

If you want to do this via more specific API calls, you'll have to go through quite some hoops, here's an example program:

#include <netinet/in.h> #include <net/if.h> #include <stdio.h> #include <string.h> #include <stdlib.h> #include <unistd.h> #include <sys/socket.h> #include <sys/ioctl.h> #include <linux/netlink.h> #include <linux/rtnetlink.h> #include <sys/types.h> #include <sys/socket.h> #include <arpa/inet.h>   #define BUFSIZE 8192 char gateway[255];  struct route_info {     struct in_addr dstAddr;     struct in_addr srcAddr;     struct in_addr gateWay;     char ifName[IF_NAMESIZE]; };  int readNlSock(int sockFd, char *bufPtr, int seqNum, int pId) {     struct nlmsghdr *nlHdr;     int readLen = 0, msgLen = 0;   do {     /* Recieve response from the kernel */         if ((readLen = recv(sockFd, bufPtr, BUFSIZE - msgLen, 0)) < 0) {             perror("SOCK READ: ");             return -1;         }          nlHdr = (struct nlmsghdr *) bufPtr;      /* Check if the header is valid */         if ((NLMSG_OK(nlHdr, readLen) == 0)             || (nlHdr->nlmsg_type == NLMSG_ERROR)) {             perror("Error in recieved packet");             return -1;         }      /* Check if the its the last message */         if (nlHdr->nlmsg_type == NLMSG_DONE) {             break;         } else {     /* Else move the pointer to buffer appropriately */             bufPtr += readLen;             msgLen += readLen;         }      /* Check if its a multi part message */         if ((nlHdr->nlmsg_flags & NLM_F_MULTI) == 0) {            /* return if its not */             break;         }     } while ((nlHdr->nlmsg_seq != seqNum) || (nlHdr->nlmsg_pid != pId));     return msgLen; } /* For printing the routes. */ void printRoute(struct route_info *rtInfo) {     char tempBuf[512];  /* Print Destination address */     if (rtInfo->dstAddr.s_addr != 0)         strcpy(tempBuf,  inet_ntoa(rtInfo->dstAddr));     else         sprintf(tempBuf, "*.*.*.*\t");     fprintf(stdout, "%s\t", tempBuf);  /* Print Gateway address */     if (rtInfo->gateWay.s_addr != 0)         strcpy(tempBuf, (char *) inet_ntoa(rtInfo->gateWay));     else         sprintf(tempBuf, "*.*.*.*\t");     fprintf(stdout, "%s\t", tempBuf);      /* Print Interface Name*/     fprintf(stdout, "%s\t", rtInfo->ifName);      /* Print Source address */     if (rtInfo->srcAddr.s_addr != 0)         strcpy(tempBuf, inet_ntoa(rtInfo->srcAddr));     else         sprintf(tempBuf, "*.*.*.*\t");     fprintf(stdout, "%s\n", tempBuf); }  void printGateway() {     printf("%s\n", gateway); } /* For parsing the route info returned */ void parseRoutes(struct nlmsghdr *nlHdr, struct route_info *rtInfo) {     struct rtmsg *rtMsg;     struct rtattr *rtAttr;     int rtLen;      rtMsg = (struct rtmsg *) NLMSG_DATA(nlHdr);  /* If the route is not for AF_INET or does not belong to main routing table then return. */     if ((rtMsg->rtm_family != AF_INET) || (rtMsg->rtm_table != RT_TABLE_MAIN))         return;  /* get the rtattr field */     rtAttr = (struct rtattr *) RTM_RTA(rtMsg);     rtLen = RTM_PAYLOAD(nlHdr);     for (; RTA_OK(rtAttr, rtLen); rtAttr = RTA_NEXT(rtAttr, rtLen)) {         switch (rtAttr->rta_type) {         case RTA_OIF:             if_indextoname(*(int *) RTA_DATA(rtAttr), rtInfo->ifName);             break;         case RTA_GATEWAY:             rtInfo->gateWay.s_addr= *(u_int *) RTA_DATA(rtAttr);             break;         case RTA_PREFSRC:             rtInfo->srcAddr.s_addr= *(u_int *) RTA_DATA(rtAttr);             break;         case RTA_DST:             rtInfo->dstAddr .s_addr= *(u_int *) RTA_DATA(rtAttr);             break;         }     }     //printf("%s\n", inet_ntoa(rtInfo->dstAddr));      if (rtInfo->dstAddr.s_addr == 0)         sprintf(gateway, (char *) inet_ntoa(rtInfo->gateWay));     //printRoute(rtInfo);      return; }   int main() {     struct nlmsghdr *nlMsg;     struct rtmsg *rtMsg;     struct route_info *rtInfo;     char msgBuf[BUFSIZE];      int sock, len, msgSeq = 0;  /* Create Socket */     if ((sock = socket(PF_NETLINK, SOCK_DGRAM, NETLINK_ROUTE)) < 0)         perror("Socket Creation: ");      memset(msgBuf, 0, BUFSIZE);  /* point the header and the msg structure pointers into the buffer */     nlMsg = (struct nlmsghdr *) msgBuf;     rtMsg = (struct rtmsg *) NLMSG_DATA(nlMsg);  /* Fill in the nlmsg header*/     nlMsg->nlmsg_len = NLMSG_LENGTH(sizeof(struct rtmsg));  // Length of message.     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.  /* Send the request */     if (send(sock, nlMsg, nlMsg->nlmsg_len, 0) < 0) {         printf("Write To Socket Failed...\n");         return -1;     }  /* Read the response */     if ((len = readNlSock(sock, msgBuf, msgSeq, getpid())) < 0) {         printf("Read From Socket Failed...\n");     return -1;     } /* Parse and print the response */     rtInfo = (struct route_info *) malloc(sizeof(struct route_info)); //fprintf(stdout, "Destination\tGateway\tInterface\tSource\n");     for (; NLMSG_OK(nlMsg, len); nlMsg = NLMSG_NEXT(nlMsg, len)) {         memset(rtInfo, 0, sizeof(struct route_info));         parseRoutes(nlMsg, rtInfo);     }     free(rtInfo);     close(sock);      printGateway();     return 0; }