getsockopt() with IP_PKTINFO not working?

getsockopt() with IP_PKTINFO not working?

[Cord Seele]

I am trying to get the destination address of an incoming udp packet
with getsockopt().
According to the man pages flag IP_PKTINFO should do that. But it
doesn't work:

        struct in_pktinfo pktinfo;
        socklen_t optlen;
        struct in_addr local_addr;

        optlen=(socklen_t)sizeof(pktinfo);   
        syslog (LOG_ERR, "ERR %d",           
          getsockopt(fd, SOL_IP, IP_PKTINFO, &pktinfo, &optlen));
        syslog (LOG_ERR, "LENGTH %d %d", (int)optlen, sizeof(pktinfo));
        local_addr=pktinfo.ipi_addr;                                   
        syslog (LOG_ERR,"ADDR %s",inet_ntoa(local_addr));

results in /var/log/messages:

Dec 19 19:27:49 coda tftpd[20081]: ERR 0
Dec 19 19:27:49 coda tftpd[20081]: LENGTH 4 12
Dec 19 19:27:49 coda tftpd[20081]: ADDR 232.252.255.191

While getsockopt() returns no error, the resulting length is too short
and the addr is
definitely invalid. I would expect either getsockopt() to return -1, it
this is not
implemented or return at least 12 valid bytes.
(I am running a 2.2.16 kernel with glibc 2.1.3.)

I even tried the 'hard way' using recvmsg() but the resulting
msg_controllen == 0. 


Background:
If a machine has more than one address in a single network, i.e.

	eth0   192.168.0.10
	eth0:1 192.168.0.20

a call to bind() normally assigns the primary ip address (.10) to the
socket.
If the server was addressed on his second address (.20) the request is
not answered
and fails. I have this problem with tftpd.
Or is there a better way to get the destination address of an incoming
udp packet?

Thanks for any help.

	Cord Seele

PS: Please CC me since I am not an the list.
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Re: getsockopt() with IP_PKTINFO not working?

[Andi Kleen]

On Wed, Dec 20, 2000 at 05:44:34PM +0100, Cord Seele wrote:
> I am trying to get the destination address of an incoming udp packet
> with getsockopt().
> According to the man pages flag IP_PKTINFO should do that. But it
> doesn't work:
> 
>         struct in_pktinfo pktinfo;
>         socklen_t optlen;
>         struct in_addr local_addr;
> 
>         optlen=(socklen_t)sizeof(pktinfo);   
>         syslog (LOG_ERR, "ERR %d",           
>           getsockopt(fd, SOL_IP, IP_PKTINFO, &pktinfo, &optlen));

You're misreading the manpage. IP_PKTINFO just enables recvmsg() to pass
ancillary messages that contain pktinfo structures. getsockopt IP_PKTINFO
returns the state of the the IP_PKTINFO flag on the socket.

So use setsockopt IP_PKTINFO to set the flag to true and then receive
them using recvmsg().

-Andi
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