iptables reject and redirect to url

自己寫的，較舊版本的iptable沒有redirect url功能。

//Reference:
//http://gpl.back2roots.org/source/puma5/netgear/CG3300D_V1.00.15_src/apps/fw_modules/ipt_REJECT.c
//http://bbs.chinaunix.net/thread-1960367-1-1.html
//http://blog.chinaunix.net/uid-27087754-id-3279302.html
//http://blog.csdn.net/yadon_z/article/details/7983402
//http://code.google.com/p/wl500g/source/browse/branches/rt-n/kernel-2.6/606-netfilter-webstr.patch?r=2456
#define MAX_HTTP_REDIRECT_CONTENT 512
static void send_http_redirect(struct sk_buff *oldskb, int hook, const char *data){   char http_content[MAX_HTTP_REDIRECT_CONTENT], *pHttp_content;   sprintf(http_content, "HTTP/1.1 307 FoundrnServer: ZyXEL: Content Filter/1.03nLocation:%srn", data);
   strcat(http_content, "Date: Thu, 17 Feb 2000 12:46:46 GMTrnContent-Type: text/htmlrnAccept-Ranges: bytesrnContent-Length: 256rn");
   strcat(http_content, "Cache-control: no-cachernPragma: no-cachernExpires: 0rnConnection: closernrn");   struct sk_buff *nskb;
	const struct iphdr *oiph;
	struct iphdr *niph;
	struct tcphdr *oth;
	struct tcphdr _otcph, *tcph;
	unsigned int addr_type;
   unsigned int otcplen;   /* IP header checks: fragment. */
	if (ip_hdr(oldskb)->frag_off & htons(IP_OFFSET))
		return;   oth = skb_header_pointer(oldskb, ip_hdrlen(oldskb),
				 sizeof(_otcph), &_otcph);
   otcplen = oldskb->len - ip_hdr(oldskb)->ihl*4;
	if (oth == NULL)
		return;   /* No RST for RST. */
	if (oth->rst)
		return;   /* Check checksum */
	if (nf_ip_checksum(oldskb, hook, ip_hdrlen(oldskb), IPPROTO_TCP))
		return;
	oiph = ip_hdr(oldskb);     unsigned char *http_data = (void *)oth + oth->doff*4;
   unsigned int datalen = (oldskb)->len - (oiph->ihl*4) - (oth->doff*4);   /*debug message*/
   /*
   char show_data[20];
   memset(show_data, 0, sizeof(show_data));
   memcpy(show_data, http_data, sizeof(show_data) - 1);
   printk(KERN_EMERG "__[Kernel]datalen=%d http data=%sn", datalen,show_data);   if (datalen < 10){
      return;
   }
   */   if(memcmp(http_data, "GET", sizeof("GET") - 1) != 0 &&
      memcmp(http_data, "POST", sizeof("POST") - 1) != 0){
      return;
   }   nskb = alloc_skb(sizeof(struct iphdr) + sizeof(struct tcphdr) +
			 LL_MAX_HEADER + strlen(http_content), GFP_ATOMIC);
	if (!nskb)
		return;   skb_reserve(nskb, LL_MAX_HEADER);   skb_reset_network_header(nskb);
	niph = (struct iphdr *)skb_put(nskb, sizeof(struct iphdr));
	niph->version	= 4;
	niph->ihl	= sizeof(struct iphdr) / 4;
	niph->tos	= 0;
	niph->id	= 0;
	niph->frag_off	= htons(IP_DF);
	niph->protocol	= IPPROTO_TCP;
	niph->check	= 0;
	niph->saddr	= oiph->daddr;
	niph->daddr	= oiph->saddr;   tcph = (struct tcphdr *)skb_put(nskb, sizeof(struct tcphdr));
	memset(tcph, 0, sizeof(*tcph));
	tcph->source	= oth->dest;
	tcph->dest	= oth->source;
	tcph->doff	= sizeof(struct tcphdr) / 4;     pHttp_content = (char *)skb_put(nskb, strlen(http_content));
   strcpy(pHttp_content, http_content);   tcph->seq = oth->ack_seq;
	tcph->ack_seq = htonl(ntohl(oth->seq) + oth->syn + oth->fin
				      + otcplen - (oth->doff<<2));   tcph->fin = 1;
   tcph->psh = 1;
   tcph->ack = 1;   tcph->rst	= 0;   nskb->csum = csum_partial((char *)tcph + tcph->doff*4,
			strlen(http_content), 0);   tcph->check	= tcp_v4_check(sizeof(struct tcphdr) + strlen(http_content),
				       niph->saddr, niph->daddr,
				       csum_partial(tcph,
						    sizeof(struct tcphdr), nskb->csum));   addr_type = RTN_UNSPEC;
	if (hook != NF_INET_FORWARD
#ifdef CONFIG_BRIDGE_NETFILTER
	    || (nskb->nf_bridge && nskb->nf_bridge->mask & BRNF_BRIDGED)
#endif
	   )
		addr_type = RTN_LOCAL;   /* ip_route_me_harder expects skb->dst to be set */
	dst_hold(oldskb->dst);
	nskb->dst = oldskb->dst;   if (ip_route_me_harder(nskb, addr_type))
		goto free_nskb;   niph->ttl	= dst_metric(nskb->dst, RTAX_HOPLIMIT);
	nskb->ip_summed = CHECKSUM_NONE;   /* "Never happens" */
	if (nskb->len > dst_mtu(nskb->dst))
		goto free_nskb;   nf_ct_attach(nskb, oldskb);   ip_local_out(nskb);   return;   free_nskb:
	kfree_skb(nskb);   }  

Passing string as -D compiler flag to .c

有時候有些字串需要compiler time時決定。通常前期先用menuconfig之類的工具設定好，再進行compile動作，此時新字串就會自動帶進.c中。

要做到這樣的效過，在gcc中可以用-D方式指定。

gcc編譯中加入 -DNEW_STRING=\"xxxxxxx\"。

在c原始碼內，利用一些字串連接的技巧串起來。或者可以自己加入#define中。

char a* = "I got "NEW_STRING"!!!!";

這樣目的就達成了。

參考資料
[1] Passing string as -D compiler option

Linux get ipv6 default route

在ipv6情況下可以從/proc/net/ipv6_route印出當前default route資訊。試了一下發現內容順序以及值會跟隨目前routing作改變。在參考了route_linux.c以及how to get ipv6 default gateway from proc in linux之後，改寫出以下範例。

int getIpv6DefaultGatway(char *ifname, char *dgw, int dgw_size){
   struct in6_addr src_addr;
   int src_prefix;
   struct in6_addr dst_addr;
   int dst_prefix;
   struct in6_addr gw_addr;
   struct in6_addr any_addr;
   unsigned int metrix;
   unsigned int ref_cnt;
   unsigned int usr_cnt;
   unsigned int flag;
   char devname[32];
   FILE *fp;
   int ret;
   char dst[64];
   char src[64];
   char gw[64];

   if(ifname == NULL){
      return 0;
   }

   strcpy(dgw, "");

   fp = fopen("/proc/net/ipv6_route", "r");
   if(fp!=NULL){            
      memset(&any_addr, 0, sizeof(any_addr));                        
      while((ret=fscanf(fp, "%s %x %s %x %s %x %x %x %x %s",
         dst, &dst_prefix, src, &src_prefix, gw,
         &metrix, &ref_cnt, &usr_cnt, &flag, devname)) != EOF){ 
                       
                         
         /*return value must be parameter's number*/                            
         if(ret != 10){                                
            continue;
         }
         
         /*default route?*/
         if(!strncmp(gw, "fe80", 4) 
            && !strcmp(dst, "00000000000000000000000000000000") && dst_prefix == 0
            && !strcmp(ifname, devname)){
            

            sprintf(gw, "%c%c%c%c:%c%c%c%c:%c%c%c%c:%c%c%c%c:%c%c%c%c:%c%c%c%c:%c%c%c%c:%c%c%c%c",
                gw[0], gw[1], gw[2], gw[3], gw[4], gw[5], gw[6], gw[7], gw[8], gw[9],
                gw[10], gw[11], gw[12], gw[13], gw[14], gw[15], gw[16], gw[17], gw[18], gw[19],
                gw[20], gw[21], gw[22], gw[23], gw[24], gw[25], gw[26], gw[27], gw[28], gw[29],
                gw[30], gw[31]);           
            
            inet_pton(PF_INET6, gw, &gw_addr);
            inet_ntop(PF_INET6, &gw_addr, gw, sizeof(gw));

            snprintf(dgw, dgw_size, "%s", gw);

            //DO NOT BREAK to get last gateway
         }
       }                                                
   }
   fclose(fp);
   
   if(!strcmp(dgw, "")){
      return 0;
   }

   return 1;
}

exec, spawn, system不同點

根據stackoverflow中Difference b/w exec( ) and system( ) in unix這篇文章中討論到的

exec: 覆蓋原本的程式，執行完即結束。

spawn: 啟動一個新的子程式(fork效用相當)，並且和原本的程式同時進行。

system: 啟動一個新的子程式(fork效用相當)，並且原本程式會等待該子程式執行完後才結束。

Find process and send signal to it in Kernel

工作上覺得還蠻實用的程式片段。

struct task_struct *p;  
struct siginfo info;
      
pid_t pid = -1;  
     
for_each_process(p) { 
   if (!strcmp(p->comm, "process name")){  
      pid = p->pid;      
      break;  
   }  
}


if(pid != -1){
   
   if(p != NULL) {
      info.si_signo = SIGUSR1;
      info.si_code = SI_QUEUE;
      info.si_int = 0x3;
      send_sig_info(SIGUSR1, &info, p);

      printk(KERN_EMERG "Send SIGUSR1 with value %d to %s(%d)\n", info.si_int, "process name", pid);
   }        

}

Netlink 使用方法

Kernel Korner - Why and How to Use Netlink Socket這篇寫得非常好，關於netlink的使用方法，範例也非常簡潔易懂。

這裡就貼上關於不使用group方式process送訊息至kernel。

User Space

#include <sys/socket.h>
#include <linux/netlink.h>

#define MAX_PAYLOAD 1024  /* maximum payload size*/
struct sockaddr_nl src_addr, dest_addr;
struct nlmsghdr *nlh = NULL;
struct iovec iov;
int sock_fd;

void main() {
 sock_fd = socket(PF_NETLINK, SOCK_RAW,NETLINK_TEST);

 memset(&src_addr, 0, sizeof(src_addr));
 src__addr.nl_family = AF_NETLINK;
 src_addr.nl_pid = getpid();  /* self pid */
 src_addr.nl_groups = 0;  /* not in mcast groups */
 bind(sock_fd, (struct sockaddr*)&src_addr,
      sizeof(src_addr));

 memset(&dest_addr, 0, sizeof(dest_addr));
 dest_addr.nl_family = AF_NETLINK;
 dest_addr.nl_pid = 0;   /* For Linux Kernel */
 dest_addr.nl_groups = 0; /* unicast */

 nlh=(struct nlmsghdr *)malloc(
                 NLMSG_SPACE(MAX_PAYLOAD));
 /* Fill the netlink message header */
 nlh->nlmsg_len = NLMSG_SPACE(MAX_PAYLOAD);
 nlh->nlmsg_pid = getpid();  /* self pid */
 nlh->nlmsg_flags = 0;
 /* Fill in the netlink message payload */
 strcpy(NLMSG_DATA(nlh), "Hello you!");

 iov.iov_base = (void *)nlh;
 iov.iov_len = nlh->nlmsg_len;
 msg.msg_name = (void *)&dest_addr;
 msg.msg_namelen = sizeof(dest_addr);
 msg.msg_iov = &iov;
 msg.msg_iovlen = 1;

 sendmsg(fd, &msg, 0);

 /* Read message from kernel */
 memset(nlh, 0, NLMSG_SPACE(MAX_PAYLOAD));
 recvmsg(fd, &msg, 0);
 printf(" Received message payload: %s\n",
    NLMSG_DATA(nlh));

 /* Close Netlink Socket */
 close(sock_fd);
}

Kernel Space

struct sock *nl_sk = NULL;

void nl_data_ready (struct sock *sk, int len)
{
  wake_up_interruptible(sk->sleep);
}

void netlink_test() {
 struct sk_buff *skb = NULL;
 struct nlmsghdr *nlh = NULL;
 int err;
 u32 pid;

 nl_sk = netlink_kernel_create(NETLINK_TEST,
                                   nl_data_ready);
 /* wait for message coming down from user-space */
 skb = skb_recv_datagram(nl_sk, 0, 0, &err);

 nlh = (struct nlmsghdr *)skb->data;
 printk("%s: received netlink message payload:%s\n",
        __FUNCTION__, NLMSG_DATA(nlh));

 pid = nlh->nlmsg_pid; /*pid of sending process */
 NETLINK_CB(skb).groups = 0; /* not in mcast group */
 NETLINK_CB(skb).pid = 0;      /* from kernel */
 NETLINK_CB(skb).dst_pid = pid;
 NETLINK_CB(skb).dst_groups = 0;  /* unicast */
 netlink_unicast(nl_sk, skb, pid, MSG_DONTWAIT);
 sock_release(nl_sk->socket);
}

ipv6 pton空字串

當inet_pton進一個空字串再反向inet_ntop回來時，值會變怎樣呢？
範例程式碼如下

#include <sys/socket.h>
#include <netinet/in.h>
#include <stdio.h>
#include <string.h>
int main()
{
    unsigned char a[16];
    char b[64] = "";
    char c[64] = "";
    
    
    inet_pton(PF_INET6, b, &a);
    inet_ntop(PF_INET6, b, c, sizeof(c));
    
    fprintf(stderr, "c is %s\n", c);
    
    return 0;
}

結果：

./ntop_test 
c is ::

2012.7.12更新

在這個範例中inet_pton轉換空字串其實是失敗的，並沒有寫入a中。

其實::的結果是c為空字串轉換回去的結果。