Linux C: Get the default interface IP address

Question

My question is regarding the following code (in this link ):

#include <stdio.h> #include <sys/types.h> #include <ifaddrs.h> #include <netinet/in.h> #include <string.h> #include <arpa/inet.h> int main (int argc, const char * argv[]) { struct ifaddrs * ifAddrStruct = NULL; struct ifaddrs * ifa = NULL; void * tmpAddrPtr = NULL; getifaddrs(&ifAddrStruct); for (ifa = ifAddrStruct; ifa != NULL; ifa = ifa->ifa_next) { if (ifa ->ifa_addr->sa_family==AF_INET) { // Check it is // a valid IPv4 address tmpAddrPtr = &((struct sockaddr_in *)ifa->ifa_addr)->sin_addr; char addressBuffer[INET_ADDRSTRLEN]; inet_ntop(AF_INET, tmpAddrPtr, addressBuffer, INET_ADDRSTRLEN); printf("%s IP Address %s\n", ifa->ifa_name, addressBuffer); } else if (ifa->ifa_addr->sa_family==AF_INET6) { // Check it is // a valid IPv6 address tmpAddrPtr = &((struct sockaddr_in6 *)ifa->ifa_addr)->sin6_addr; char addressBuffer[INET6_ADDRSTRLEN]; inet_ntop(AF_INET6, tmpAddrPtr, addressBuffer, INET6_ADDRSTRLEN); printf("%s IP Address %s\n", ifa->ifa_name, addressBuffer); } } if (ifAddrStruct != NULL) freeifaddrs(ifAddrStruct); return 0; } 

This code output is as follows:

 lo IP Address 127.0.0.1 wlan0 IP Address 172.28.1.89 (I want to only this) lo IP Address ::1 wlan0 IP Address fe80::6e71:d9ff:fe1d:b0 

How to get the IP address of the default interface? (You can give an answer with another piece of code.)

Answer 1

This code will do the following:

 #include <stdio.h> #include <unistd.h> #include <string.h> /* For strncpy */ #include <sys/types.h> #include <sys/socket.h> #include <sys/ioctl.h> #include <netinet/in.h> #include <net/if.h> #include <arpa/inet.h> int main() { int fd; struct ifreq ifr; fd = socket(AF_INET, SOCK_DGRAM, 0); /* I want to get an IPv4 IP address */ ifr.ifr_addr.sa_family = AF_INET; /* I want an IP address attached to "eth0" */ strncpy(ifr.ifr_name, "eth0", IFNAMSIZ-1); ioctl(fd, SIOCGIFADDR, &ifr); close(fd); /* Display result */ printf("%s\n", inet_ntoa(((struct sockaddr_in *)&ifr.ifr_addr)->sin_addr)); return 0; } 

Result:

 192.168.5.27 RUN SUCCESSFUL (total time: 52ms) 

---

Alternatively, you can also use an IP address mask. For example, this will be printed only if the mask is different from 255.0.0.0 (loop mask)

 #include <stdio.h> #include <sys/types.h> #include <ifaddrs.h> #include <netinet/in.h> #include <string.h> #include <arpa/inet.h> int main(int argc, const char * argv[]) { struct ifaddrs * ifAddrStruct = NULL, * ifa = NULL; void * tmpAddrPtr = NULL; getifaddrs(&ifAddrStruct); for (ifa = ifAddrStruct; ifa != NULL; ifa = ifa->ifa_next) { if (ifa ->ifa_addr->sa_family == AF_INET) { // Check it is IPv4 char mask[INET_ADDRSTRLEN]; void* mask_ptr = &((struct sockaddr_in*) ifa->ifa_netmask)->sin_addr; inet_ntop(AF_INET, mask_ptr, mask, INET_ADDRSTRLEN); if (strcmp(mask, "255.0.0.0") != 0) { printf("mask:%s\n", mask); // Is a valid IPv4 Address tmpAddrPtr = &((struct sockaddr_in *) ifa->ifa_addr)->sin_addr; char addressBuffer[INET_ADDRSTRLEN]; inet_ntop(AF_INET, tmpAddrPtr, addressBuffer, INET_ADDRSTRLEN); printf("%s IP Address %s\n", ifa->ifa_name, addressBuffer); } else if (ifa->ifa_addr->sa_family == AF_INET6) { // Check it is // a valid IPv6 Address. // Do something } } } if (ifAddrStruct != NULL) freeifaddrs(ifAddrStruct); return 0; } 

Result:

 mask:255.255.255.0 eth0 IP Address 192.168.5.27 RUN SUCCESSFUL (total time: 53ms) 

Answer 2

A method that covers more situations will include something like the following:

• analyze the file / proc / net / route to see which interface is "standard",
• use your code snippet from the first message (using getifaddrs ) to find out the IP address for this interface.

Parsing a route file might look like this (C ++ code for short):

 std::string defaultInterface; std::ifstream routeFile(NET_ROUTE_FILEPATH, std::ios_base::in); if (!routeFile.good()) { return; } std::string line; std::vector<std::string> tokens; while(std::getline(routeFile, line)) { std::istringstream stream(line); std::copy(std::istream_iterator<std::string>(stream), std::istream_iterator<std::string>(), std::back_inserter<std::vector<std::string> >(tokens)); // the default interface is the one having the second // field, Destination, set to "00000000" if ((tokens.size() >= 2) && (tokens[1] == std::string("00000000"))) { defaultInterface = tokens[0]; break; } tokens.clear(); } routeFile.close(); 

Then in your code in a for loop, iterating over the ifAddrStruct structures, you can add a test for ifa->ifa_name as the defaultInterface defined above.