Re: Sending a packet to localhost?

[ronnie sahlberg <ronniesahlberg () gmail com>]

On Tue, Feb 24, 2009 at 8:17 AM, Oliver Zheng <
mailinglists+tcpdump () oliverzheng com<mailinglists%2Btcpdump () oliverzheng com>
> wrote:

> Thanks for the response Aaron.
>
> On Mon, Feb 23, 2009 at 11:34 AM, Aaron Turner <synfinatic () gmail com>
> wrote:
> > In my experience, sending packets on eth0 causes the packet to bypass
> > the TCP/IP stack and be sent out sight unseen.  Hence, you won't be
> > able to inject packets into a TCP stream with the target of the local
> > host.
>
> Well that kind of sucks. =( It seems weird that an outgoing packet
> like this could be filtered by the kernel. What criteria does a packet
> like this fit for it to be filtered out? (e.g. it doesn't match a TCP
> connection, but obviously it does in this case.) The only thing I can
> think of is that the kernel uses a different incoming adapter (really
> low level?) than libpcap, which sounds wrong since libpcap should be
> operating at the lowest possible layer, right?



Do you want to send the packet to lo specifically or do you want to
inject/send it to a socket endpoint attached locally?
I.e. to a local ip address, be it attached to eth0 or elsewhere?


You can do that using a raw socket.
Well, I do that with no problem.
This code is from ctdb ( http://samba.ctdb.org   common/system_linux.c )
and does just that.

(this is part of the ctdb "tickle-ack" and "tcp socketkiller" which ctdb
uses to make sure that failover and
session recoveries are as fast as possible. It does this by
injecting/spoofing carefully crafted TCP packets
and sending them to the locally established socket.)



        /* open a raw socket to send this segment from */
        s = socket(AF_INET, SOCK_RAW, htons(IPPROTO_RAW));
        if (s == -1) {
            DEBUG(DEBUG_CRIT,(__location__ " failed to open raw socket
(%s)\n",
                 strerror(errno)));
            return -1;
        }

        ret = setsockopt(s, SOL_IP, IP_HDRINCL, &one, sizeof(one));
        if (ret != 0) {
            DEBUG(DEBUG_CRIT,(__location__ " failed to setup IP headers
(%s)\n",
                 strerror(errno)));
            close(s);
            return -1;
        }

        set_nonblocking(s);
        set_close_on_exec(s);

        ret = sendto(s, &ip4pkt, sizeof(ip4pkt), 0, &dest->ip,
sizeof(dest->ip));
        close(s);
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