IP_BINDANY: return packets not accepted by kernel

[Rajkumar S]

Hello all,

I am testing IP_BINDANY functionality with a small C program. I can
bind to a foreign (non existing) address, and syn packets are being
sent with the bound source IP address and port. Return (ACK) packets
are reaching the host (I can see the SYN-ACK packet in tcpdump), but
the third packet in TCP handshake is not getting generated. It looks
like the second SYN-ACK is not getting accepted by the kernel.

My test setup consists of 2 machines, one freebsd 8.2 where I am
running my test program has an ip address of 192.168.3.83, and an echo
server running in the second machine with ip address 192.168.3.18. The
souce ip address of connection originating from my test program is
192.168.9.5 and echo server is having a route for 192.168.9.0/24 to
192.168.3.83.

Since there is a route for 192.168.9.5 from  192.168.3.18 there is no
need for any additional pf rules for redirecting and packets can be
seen in tcpdump.

Netstat gives the following output. The state remains SYN_SENT and the
kernel is not able to "see" the SYN-ACK.

tcp4       0      0 192.168.9.5.7          192.168.3.18.7         SYN_SENT

tcpdump gives the following output (first 2 lines)

17:39:54.745332 00:40:f4:bd:3d:1c > 00:27:0e:26:d9:66, ethertype IPv4
(0x0800), length 74: 192.168.9.5.7 > 192.168.3.18.7: Flags [S], seq
587594188, win 65535, options [mss 1460,nop,wscale 3,sackOK,TS val
92954112 ecr 0], length 0

17:39:54.745446 00:27:0e:26:d9:66 > 00:40:f4:bd:3d:1c, ethertype IPv4
(0x0800), length 74: 192.168.3.18.7 > 192.168.9.5.7: Flags [S.], seq
399460822, ack 587594189, win 5792, options [mss 1460,sackOK,TS val
4341497 ecr 92954112,nop,wscale 7], length 0

arp -na (abbreviated)
? (192.168.3.83) at 00:40:f4:bd:3d:1c on re0 permanent [ethernet]
? (192.168.3.18) at 00:27:0e:26:d9:66 on re0 expires in 1133 seconds [ethernet]

Am I missing some thing here? I am running all programs as root, so I
do have the privileges. I guess there are no sysctrl to be set. Any
help to get this working is much appreciated.

with regards,

raj