Given the interfaces on a firewall:

eth0 Link encap:Ethernet HWaddr 02:40:67:34:F5:47
inet addr:192.168.0.1 Bcast:192.168.0.255 Mask:255.255.255.0

eth1 Link encap:Ethernet HWaddr 86:23:98:45:35:56
inet addr:123.45.240.69 Bcast:255.255.255.255 Mask:255.255.252.0

eth2 Link encap:Ethernet HWaddr 86:23:98:45:35:56
inet addr:123.45.240.70 Bcast:255.255.255.255 Mask:255.255.252.0

eth3 Link encap:Ethernet HWaddr 12:73:00:c4:47:56
inet addr:192.168.1.1 P-t-P:192.168.1.2 Mask:255.255.255.255

and the routing table:
Destination Gateway Genmask Iface
192.168.1.2 0.0.0.0 255.255.255.255 eth3
192.168.0.0 0.0.0.0 255.255.255.0 eth0
123.45.240.0 0.0.0.0 255.255.0.0 eth1
123.45.240.0 0.0.0.0 255.255.254.0 eth2
131.244.0.0 0.0.0.0 255.254.0.0 eth3
0.0.0.0 203.45.240.1 0.0.0.0 eth1



If I want to determine the interface which following packets:

SRC ADDR DST ADDR
192.168.0.3 123.45.241.30
240.18.10.3 131.244.12.23
10.4.5.6 10.4.5.7


what is the interface for these three pairs? and the most important thing is how to recognize it?

Did you try route or netstat -r ?

Oh, I see you did

You could traceroute to the IP address and find the interface of your machine that way ....

BTW, the first one seems to be eth0 ....