Re: recvfrom and sockaddr_in.sin_port

Re: recvfrom and sockaddr_in.sin_port

[Chris Wedgwood]

On Sun, Jul 08, 2001 at 12:14:47PM -0500, Adam wrote:

    	I have attached a simple program I used for generating those
    	results.

'fromlen' needs to be set to sizeof 'from' before the recvfrom syscall



  --cw

recvfrom and sockaddr_in.sin_port

[Adam]

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hello,
	I'm using recvfrom, and upon return it should fill in
	from argument with sockaddr_in data structure. from
	ip(7) man page I get that it has format of :

        struct sockaddr_in {
             sa_family_t    sin_family; /* address family: AF_INET */
             u_int16_t      sin_port;   /* port in network byte order*/
             struct in_addr  sin_addr;  /* internet address */
        };

	for PF_INET type of socket. Now if I run the program the, data in
	the from field comes out as:

2  0  0  0  192  168  1  4  61  63  140  200  85  214  2

 |         |               |

AF_INET      :2
IP           :192.168.1.4

	and as show in above example and other repeated test, the port
	part is always set to 0. Shouldn't that be set to port number?

	on similar token the "padding" part after the IP is always set
	to the same pattern. Shouldn't it rather be zeroed, or be
	some random data?

	I have attached a simple program I used for generating those
	results.

-- 
Adam
http://www.eax.com      The Supreme Headquarters of the 32 bit registers


[-- Warning: decoded text below may be mangled, UTF-8 assumed --]
[-- Attachment #2: Type: TEXT/x-csrc; name="test.c", Size: 1713 bytes --]

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <stdlib.h>
#include <ctype.h>

int main (int argc, char **argv) {

  size_t buflen = 1024;
  char buf[buflen];
  struct sockaddr_in from;
  socklen_t fromlen = sizeof(struct sockaddr_in);
  int flags;

  int rawsock;
  int bytesread;

  int i;
  unsigned char * ptr;


  rawsock = socket(PF_INET,SOCK_RAW,IPPROTO_TCP);

  if (rawsock == -1)
    perror("socket():");

  flags =  MSG_WAITALL;
  memset(&from,0,fromlen);

  bytesread = recvfrom(rawsock,&buf,buflen,flags,
		       (struct sockaddr*)(&from),&fromlen);

  ptr = (unsigned char*)&from;
  for(i=0;i<fromlen;i++) {
      printf("%i  ",*ptr);
      ptr++;
  }
  printf("\n");

  if (bytesread == -1) {
    printf("err # : %i \n",errno);
    perror("recvfrom():");
    return -1;
 }

 
    if (argc ==2) {
      struct hostent *host;

      printf("msg len: %i \n",bytesread);
      printf("frm len: %i \n",fromlen);
      printf("IPv4 : type:%i port:%X addr:%X \n", 
	     from.sin_family,
	     ntohs(from.sin_port),
	     from.sin_addr.s_addr);

      host=gethostbyaddr(&(from.sin_addr.s_addr),
			 fromlen,from.sin_family);

      if (host == NULL ) {
	printf("err # : %i \n",h_errno);
	herror("gethostbyaddr():");
      }

      printf("host: (");

      ptr = (unsigned char*)(&(from.sin_addr.s_addr));
      for(i=0;i<4;i++) {
	printf("%i",*(ptr++));
	if (i<3)
	  printf(".");
      }
      
      if (host == NULL)
	printf(") \n");
      else
	printf(") %s \n",host->h_name);


    }


  return 0;

}

Re: recvfrom and sockaddr_in.sin_port

[Chris Wedgwood]

On Sun, Jul 08, 2001 at 01:08:37PM -0500, Adam wrote:

    isn't it set? to quote from the example I have attached:
    
      socklen_t fromlen = sizeof(struct sockaddr_in);

sorry, I misread the source (the memset line)

you are using raw sockets, what does port mean for raw sockets?



  --cw

Re: recvfrom and sockaddr_in.sin_port

[Adam]

>     	I have attached a simple program I used for generating those
>     	results.
>
> 'fromlen' needs to be set to sizeof 'from' before the recvfrom syscall

isn't it set? to quote from the example I have attached:

  socklen_t fromlen = sizeof(struct sockaddr_in);

Re: recvfrom and sockaddr_in.sin_port

[Chris Wedgwood]

On Sun, Jul 08, 2001 at 01:42:28PM -0500, Adam wrote:

    so it seem to imply that only tcp packets only are to be passed.
    still group "SOCK_RAW" is subset of the PF_INET group (the way
    I see it), so from ip(7) man page I should use sockaddr_in
    structure, which should be defined in this particular case,
    as it ought be for IPPROTO_UDP.

have you dumped the packet contents? presumably its a raw packet,
protocol 6, and the header and such like is returned

i see no reason, for raw packets to fill out sin_port whatoever, it
just doesn't make sense that raw packets have a port



  --cw

Re: recvfrom and sockaddr_in.sin_port

[Adam]

>     isn't it set? to quote from the example I have attached:
>       socklen_t fromlen = sizeof(struct sockaddr_in);
> sorry, I misread the source (the memset line)
> you are using raw sockets, what does port mean for raw sockets?

well, from raw(7) man page:

       raw_socket = socket(PF_INET, SOCK_RAW, int protocol);
       [...]
       All packets [...] matching the protocol number  speci
       fied  for the raw socket are passed to this socket.

so it seem to imply that only tcp packets only are to be passed.
still group "SOCK_RAW" is subset of the PF_INET group (the way
I see it), so from ip(7) man page I should use sockaddr_in
structure, which should be defined in this particular case,
as it ought be for IPPROTO_UDP.

Re: recvfrom and sockaddr_in.sin_port

[Andi Kleen]

Adam <adam@eax.com> writes:
> 
> 	and as show in above example and other repeated test, the port
> 	part is always set to 0. Shouldn't that be set to port number?

SOCK_RAW has no ports, so no.

> 
> 	on similar token the "padding" part after the IP is always set
> 	to the same pattern. Shouldn't it rather be zeroed, or be
> 	some random data?

It should stay at the old value or alternatively be zeroed. That's a 
kernel bug.

Here is a patch that implements the first alternative. David, please consider
applying.

diff -burp linux/include/linux/in.h linux-work/include/linux/in.h
--- linux/include/linux/in.h	Thu Jul  5 13:18:27 2001
+++ linux-work/include/linux/in.h	Sun Jul  8 19:05:30 2001
@@ -109,6 +109,10 @@ struct in_pktinfo
 };
 
 /* Structure describing an Internet (IP) socket address. */
+/* Note here and in others places is the assumption made that there
+   are no holes in in this structure. This is ok for all architectures
+   which layout structures members at worst to their natural
+   alignment. -AK */
 #define __SOCK_SIZE__	16		/* sizeof(struct sockaddr)	*/
 struct sockaddr_in {
   sa_family_t		sin_family;	/* Address family		*/
@@ -119,7 +123,7 @@ struct sockaddr_in {
   unsigned char		__pad[__SOCK_SIZE__ - sizeof(short int) -
 			sizeof(unsigned short int) - sizeof(struct in_addr)];
 };
-#define sin_zero	__pad		/* for BSD UNIX comp. -FvK	*/
+#define sin_zero	__pad
 
 
 /*
diff -burp linux/net/ipv4/af_inet.c linux-work/net/ipv4/af_inet.c
--- linux/net/ipv4/af_inet.c	Wed Jul  4 17:21:32 2001
+++ linux-work/net/ipv4/af_inet.c	Sun Jul  8 19:05:31 2001
@@ -729,7 +729,7 @@ static int inet_getname(struct socket *s
 		sin->sin_port = sk->sport;
 		sin->sin_addr.s_addr = addr;
 	}
-	*uaddr_len = sizeof(*sin);
+	*uaddr_len = sizeof(offsetof(struct sockaddr_in, sin_zero));
 	return(0);
 }
 
diff -burp linux/net/ipv4/raw.c linux-work/net/ipv4/raw.c
--- linux/net/ipv4/raw.c	Wed Jul  4 17:21:32 2001
+++ linux-work/net/ipv4/raw.c	Sun Jul  8 19:06:10 2001
@@ -493,7 +493,7 @@ int raw_recvmsg(struct sock *sk, struct 
 		goto out;
 
 	if (addr_len)
-		*addr_len = sizeof(*sin);
+		*addr_len = offsetof(struct sockaddr_in, sin_zero);
 
 	if (flags & MSG_ERRQUEUE) {
 		err = ip_recv_error(sk, msg, len);
diff -burp linux/net/ipv4/udp.c linux-work/net/ipv4/udp.c
--- linux/net/ipv4/udp.c	Thu Apr 12 21:11:39 2001
+++ linux-work/net/ipv4/udp.c	Sun Jul  8 19:05:31 2001
@@ -635,7 +635,7 @@ int udp_recvmsg(struct sock *sk, struct 
 	 *	Check any passed addresses
 	 */
 	if (addr_len)
-		*addr_len=sizeof(*sin);
+		*addr_len=offsetof(struct sockaddr_in, sin_zero);
 
 	if (flags & MSG_ERRQUEUE)
 		return ip_recv_error(sk, msg, len);


-Andi