netdev - Re: [PATCH net] ip: report the original address of ICMP messages

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Date:	Tue, 23 Jun 2015 23:06:28 +0300 (EEST)
From:	Julian Anastasov <ja@....bg>
To:	Willem de Bruijn <willemb@...gle.com>
cc:	David Miller <davem@...emloft.net>,
	Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH net] ip: report the original address of ICMP messages


	Hello,

On Tue, 23 Jun 2015, Willem de Bruijn wrote:

> >         It is using IP_RECVERR and may be relying only on
> > original address returned in msg_name from MSG_ERRQUEUE.
> 
> It's not very important, in that even if ping does not use this
> msg_name, another tool very well might. But from ping.c in that
> srpm:
> 
>         res = recvmsg(icmp_sock, &msg, MSG_ERRQUEUE|MSG_DONTWAIT);
> ...
>         e = NULL;
>         for (cmsg = CMSG_FIRSTHDR(&msg); cmsg; cmsg = CMSG_NXTHDR(&msg, cmsg)) {
>                 if (cmsg->cmsg_level == SOL_IP) {
>                         if (cmsg->cmsg_type == IP_RECVERR)
>                                 e = (struct sock_extended_err *)CMSG_DATA(cmsg);
>                 }
>         }
> ...
>         } else if (e->ee_origin == SO_EE_ORIGIN_ICMP) {
>                 struct sockaddr_in *sin = (struct sockaddr_in*)(e+1);
> ...

	It is used, it compares the embedded original address
from error message here (msg.msg_name = (void*)&target):

	target.sin_addr.s_addr != whereto.sin_addr.s_addr

and it does not match because kernel failed to fill the
msg_name buffer. In fact, kernel returns msg_namelen=0 but
it is not checked here, so may be ping.c compares random memory.

>                         printf("From %s icmp_seq=%u ",
> pr_addr(sin->sin_addr.s_addr), ntohs(icmph.un.echo.sequence));

	What you are referring here is the offender address (sin),
the sender of the ICMP error, it is just printed...

Regards

--
Julian Anastasov <ja@....bg>
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