segmentation fault when using gethostbyname - c

Question

I am trying to create a proxy server which accepts requests from client and sends the response back to client after receiving message from the server.
But the program gives segmentation fault.
My code
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
int main(int argc,char *argv[])
{
struct sockaddr_in server,client;
struct hostent *host;
int s1,s2,s3,len;
int port;
char ip[100];
char path[100];
int p;
int n;
char buffer[500];
char site[500];
port = atoi(argv[1]);
bzero((char *)&server,sizeof(server));
server.sin_port = htons(port);
server.sin_addr.s_addr = INADDR_ANY;
server.sin_family = AF_INET;
s1 = socket(AF_INET,SOCK_STREAM,0);
if(bind(s1,(struct sockaddr *)&server,sizeof(server)) == -1) {
perror("can't bind");
exit(1);
}
if(listen(s1,10) == -1) {
perror("coudnt't listen");
exit(1);
}
len = sizeof(client);
s2 = accept(s1,(struct sockaddr *)&client,&len);
if(s2 == -1) {
perror("coudnt aceept");
exit(1);
}
memset(buffer,0,sizeof(buffer));
n = recv(s2,buffer,500,0);
if(n < 0) {
perror("not recieved");
exit(1);
}
sscanf(buffer, "http://%99[^:]:%99d/%99[^\n]", ip, &p, path);
bzero((char *)&client,sizeof(client));
client.sin_port = htons(80);
host = gethostbyname(ip);
bcopy((char *)host->h_addr,(char *)&client.sin_addr.s_addr,host->h_length);
client.sin_family = AF_INET;
s3 = socket(AF_INET,SOCK_STREAM,0);
if(connect(s3,(struct sockaddr *)&client,sizeof(client)) == -1) {
perror("can't connect\n");
exit(1);
}
sprintf(site,"GET http://%s/ HTTP/1.0\n\n",ip);
n = send(s3,site,strlen(site),0);
if(n < 0) {
perror("message not sent");
exit(1);
}
while(1) {
memset(site,0,sizeof(site));
n = recv(s3,site,500,0);
if(n < 0) {
perror("coudnot read");
exit(1);
}
site[n] = '\0';
send(s2,site,strlen(site),0);
}
close(s1);
close(s2);
return 0;
}
The mistake seems to be coming from gethostbyname(ip) because when I use gethostbyname("www.iiita.ac.in"), it works.

Answer 1

gethostbyname returns a null pointer in case of error and, thus, host->h_addr is a null dereference.
You need to check whether host is NULL or not before using it.
See http://man7.org/linux/man-pages/man3/gethostbyname.3.html