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Subjectgetsockopt() with IP_PKTINFO not working?
Date
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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