Re: sendmsg and IP_PKTINFO

[Neil Brown]

On Tuesday February 18, [e-mail] wrote:
>    From: Neil Brown <[e-mail]>
>    Date: Tue, 18 Feb 2003 22:00:37 +1100
>    
>    It does go on to say that the outgoing packet will be sent over the
>    same interface, however I feel that is an illogical conclusion given
>    the description of the meaning of the field.
>    
>    So yes, the current behaviour seems to match part of the
>    documentation.  However I argue that the documented behaviour is
>    irrational.
> 
> Alexey and myself totally disagree.  We have described for you
> the intended purpose of this feature.  Please do not try to use
> it in some other way, it may prove to be painful :-)

Thankyou for making that clear.

I am currently working towards testing a patch that will fix the
behaviour of glibc.

Currently the sunrpc/svc_udp.c code asks for an IP_PKTINFO from
recvmsg, and passes it verbatim down through sendmsg.
My patch checks that the returned data looks believable and, if it
does, zeros the ipi_ifindex field.

NeilBrown

--- sunrpc/svc_udp.c.orig	2003-02-19 11:25:20.000000000 +1100
+++ sunrpc/svc_udp.c	2003-02-19 14:28:46.000000000 +1100
@@ -256,8 +256,26 @@
       mesgp->msg_controllen = sizeof(xprt->xp_pad)
 			      - sizeof (struct iovec) - sizeof (struct msghdr);
       rlen = recvmsg (xprt->xp_sock, mesgp, 0);
-      if (rlen >= 0)
-	len = mesgp->msg_namelen;
+      if (rlen >= 0) {
+	      struct cmsghdr *cmsg;
+	      len = mesgp->msg_namelen;
+	      cmsg = CMSG_FIRSTHDR(mesgp);
+	      if (cmsg == NULL ||
+		  CMSG_NXTHDR(mesgp, cmsg) != NULL ||
+		  cmsg->cmsg_level != SOL_IP ||
+		  cmsg->cmsg_type != IP_PKTINFO ||
+		  cmsg->cmsg_len != sizeof(struct in_pktinfo)) {
+		      /* Not a simple IP_PKTINFO, ignore it */
+		      mesgp->msg_control = NULL;
+		      mesgp->msg_controllen = 0;
+	      } else {
+		      /* it was a simple IP_PKTIFO as we expected,
+		       * Discard the interface field 
+		       */
+		      struct in_pktinfo *pkti = CMSG_DATA(cmsg);
+		      pkti->ipi_ifindex = 0;
+	      }
+      }
     }
   else
 #endif