diff --git a/client.c b/client.c
index 5eee530..5601ebd 100644
--- a/client.c
+++ b/client.c
@@ -121,8 +121,8 @@ int main(int argc, const char *argv[]){
     struct sockaddr_in addr;
     const char * filename;
     const char * server_name;
-    in_port_t server_port = SERVER_PORT;
-    int sock;
+    in_port_t server_port;
+    int sock, err;
     if (argc != 4){
         fprintf(stderr,"invaild arguments number.");
         return 1;
@@ -140,17 +140,24 @@ int main(int argc, const char *argv[]){
         perror("sock create fail");
         return 1;
     }
-    {
-        int option = 1;
-        if(setsockopt(sock,SOL_SOCKET,SO_REUSEADDR,&option,sizeof(option)) < 0){
-            perror("setsockopt");
+    err = getsockaddrbyname(AF_INET,SOCK_STREAM,0,server_name,&addr);
+    if (err != 0){
+        int check;
+        fprintf(stderr,"netdb fail: %s\n",gai_error(err));
+        //assume that sernmae is *.*.*.* and try to parse addr
+        check = inet_pton(AF_INET,server_name,&addr.sin_addr);
+        assert(check != -1);
+        if (check == 0){
+            fprintf(stderr,"parsing fail : invaild format\n");
+            close(sock);
+            return 1;
         }
     }
-    addr.sin_addr.s_addr = inet_addr(server_name);
     addr.sin_family = AF_INET;
     addr.sin_port = htons(server_port);
     if(connect(sock,(struct sockaddr *)&addr,sizeof(addr)) < 0){
         perror("connect failed");
+        close(sock);
         return 1;
     }
     if(sendReadOp(sock,filename) == 0){
diff --git a/server.c b/server.c
index a3297ee..474de73 100644
--- a/server.c
+++ b/server.c
@@ -163,6 +163,12 @@ int main(int argc, const char *argv[]){
         perror("sock create fail");
         return 1;
     }
+    else {
+        int option = 1;
+        if(setsockopt(sock,SOL_SOCKET,SO_REUSEADDR,&option,sizeof(option)) < 0){
+            perror("setsockopt");
+        }
+    }
     bufsize = getBufferSizeFrom(sock);
     buf = malloc(bufsize * sizeof(*buf));
     if (buf == NULL){
diff --git a/socket_wrapper.c b/socket_wrapper.c
index dab2d4f..4e8ef57 100644
--- a/socket_wrapper.c
+++ b/socket_wrapper.c
@@ -5,7 +5,8 @@
 #include <stdlib.h>
 #include <unistd.h>
 #include <poll.h>
-
+#include <netdb.h>
+#include <string.h>
 #include "socket_wrapper.h"
 
 int getBufferSizeFrom(int sock){
@@ -60,4 +61,33 @@ ssize_t recv_until_byte(int fd,void * buf, size_t n,int timeout){
     }
     assert(cur == n);
     return cur;
+}
+
+int getsockaddrbyname(int domain,int type, int protocol, const char * hostname_str, struct sockaddr * retaddr){
+    struct addrinfo hints;
+    struct addrinfo * result, *entry;
+    int ret, i;
+    char hostname[_SC_HOST_NAME_MAX];
+    strncpy(hostname,hostname_str,sizeof(hostname));
+    memset(&hints,0,sizeof(hints));
+    hints.ai_family = domain;
+    hints.ai_socktype = type;
+    hints.ai_flags = 0;
+    hints.ai_protocol = protocol;
+
+    //try 5 times
+    for (size_t i = 0; i < 5; i++)
+    {
+        ret = getaddrinfo(hostname,NULL,&hints,&result);
+        if(ret == EAI_AGAIN) continue;
+        else if (ret != 0) return ret;
+        break;
+    }
+    if (ret != 0) return ret; //maybe name server doesn't work properly.
+    
+    for (entry = result; entry != NULL; entry = entry->ai_next){
+        memcpy(retaddr,entry->ai_addr,sizeof(*retaddr));   
+    }
+    freeaddrinfo(result);
+    return ret;
 }
\ No newline at end of file
diff --git a/socket_wrapper.h b/socket_wrapper.h
index 7b41c2b..aae57ba 100644
--- a/socket_wrapper.h
+++ b/socket_wrapper.h
@@ -5,6 +5,7 @@
 #include <sys/socket.h>
 #include <netinet/in.h>
 #include <arpa/inet.h>
+#include <netdb.h>
 
 #ifdef USE_SENDFILE
 #include <sys/sendfile.h>
@@ -54,10 +55,18 @@ int getBufferSizeFrom(int sock);
  * thread safe
 */
 ssize_t timeout_recv(int fd,void * buf,size_t n,int timeout);
-/*
+/**
  * recieve data to buf until all bytes.
  * thread safe
 */
 ssize_t recv_until_byte(int fd,void * buf, size_t n,int timeout);
 
+/**
+ * find sockaddr by hostname
+ * it perform like `gethostbyname`
+ * hostname could not be greater than _SC_HOST_NAME_MAX.
+ * return 0 on success, otherwise return ecode.
+ * ecode can be converted to string by `gai_strerror`. 
+*/
+int getsockaddrbyname(int domain,int type, int protocol, const char * hostname_str, struct sockaddr * retaddr);
 #endif
\ No newline at end of file