Linux Get gateway Address

Use of the route command: operation or display of the IP routing table
Route:description
Route manipulates the kernel ' s IP routing tables. Its primary use are to set up static routes to
Specific hosts or networks via an interface after it have been configured with the Ifconfig (8) Pro‐gram.

When the Add or Del Options is used, route modifies the routing tables. Without these options,
Route displays the current contents of the routing tables.
Route-n: (for printing routing tables)
Show numerical addresses instead of trying to determine symbolic host names. This is useful
If you is trying to determine, the route to your nameserver have vanished.

First, look at the routing table under Linux:

(1) route-n with command

[Email protected]:~# Route-n Kernel IP routing table Destination gateway subnet Mask flag Hop reference use interface192.168.123.0   0.0.0.0         255.255.255.0U1      0        0eth10.0.0.0         192.168.123.254 0.0.0.0Ug0      0        0Eth1

(2) Cat/pro/net/route

[Email protected]:/proc/net#CatRoute Iface Destination Gateway flags refcnt use Metric Mask MTU Window Irtt Eth1 007ba8c000000000    0001     　　 0     　　 0     　　 100FFFFFF0    0    0eth100000000Fe7ba8c00003     　　 0     　　 0     　　 0     　　 00000000     　　 0    0    0 

Second, the implementation of code:

#include <stdio.h>#include<stdlib.h>#include<fcntl.h>#include<ctype.h>#include<signal.h>#include<unistd.h>#include<string.h>#include<errno.h>#include<arpa/inet.h>intGET_GATEWAY_ADDR (Char*gateway_addr) {    Charbuff[ the]; intNL =0 ; structin_addr GW; intFlgs,ref, use, metric; unsignedLong intd,g,m; unsignedLongaddr; FILE*FP =NULL; FP= fopen ("/proc/net/route","R"); if(fp = =NULL) {        return-1; } NL=0 ; memset (Buff,0,sizeof(Buff));  while(Fgets (Buff,sizeof(buff), fp)! =NULL) {        if(NL) {intIFL =0;  while(buff[ifl]!=' '&& buff[ifl]!='\ t'&& buff[ifl]!=' /') IFL++; BUFF[IFL]=0;/*Interface*/            if(SSCANF (buff+ifl+1,"%LX%LX%X%D%D%D%LX",                   &d, &g, &flgs, &ref, &use, &metric, &m)! =7) {fclose (FP); return-2; } IFL=0;/*Parse Flags*/            //if (flgs&rtf_up)//{            GW.S_ADDR =G; if(d==0) {strcpy (gateway_addr, Inet_ntoa (GW));                    Fclose (FP); return 0; }                            //}} NL++; }            if(FP) {fclose (FP); FP=NULL; }        return-1;}intMain () {Chargateway_addr[ the] = {0};    GET_GATEWAY_ADDR (GATEWAY_ADDR); printf ("gateway_addr:%s\n", GATEWAY_ADDR); return 0;}

Third, the operation result:

GATEWAY_ADDR:192.168. 123.254

Linux Get gateway Address