Iam using RHL 9, kernel 2.4.20-8.
I have two machines. Machine 1 (IP 172.27.22.201) has two IP Aliases.
10.0.1.1 and 10.0.1.2.
Machine 2 (IP 172.27.22.232) has a IP alias 10.0.2.1.
(IP Alias is created using cmd #ifconfig eth0:1 10.0.0.1)
I want to send a packet from 10.0.1.1 (m/c 1) --> 10.0.2.1 (m/c
2) --> 10.0.1.2 (m/c 3).
In Machine 1, Iam capturing a packet from 10.0.1.1 to 10.0.1.2 at
netfilter hook NF_IP_LOCAL_OUT. First to send the packet to 10.0.2.1
(m/c 2), I need to send the packet to Machine 2.
For sending the packet to machine 2 Iam doing the following changes
in sk_buff structure of the packet.
sb->dev = dev_get_by_name("eth0");
Previously I captured the object of struct dst_entry * from a
skbuffer of a packet going to Machine 2(for m/c 2 IP. Not to its
alias) . Now, for the currently outgoing packet to m/c 2 IP Alias, Iam
filling the sk_buff field struct dst_entry * with the previously
captured dst_entry * object.
sb->dst = my_dst_entry; // previously captured dst_entry
Next Iam doing the following changes in the packet Header.
Iam adding another IP Header (i.e. IP-over-IP) with src IP 10.0.1.1
and dst IP 10.0.2.1. Iam changing the protocol field in the outer IP
Header to IP-over-IP value. I changed the total len field and
calculated the new checksum.
Next Iam filling the MAC Header fields. src MAC as the MAC address of
m/c 1 and dst MAC as the MAC address of m/c 2 and type of packet to
IP.
Now iam returning NF_ACCEPT.
Iam receiving the packet at machine 2. Now I captured the packet at
netfilter-hook NF_IP_PRE_ROUTING. Now I want to send the packet to
10.0.1.2 (m/c 1). For this Iam changing the sk_buff fields, outer IP
Header fileds and MAC Header fields as before.
Next Iam calling the dev_queue_xmit() to send the packet to machine 1.
Iam receiving the packet at m/c 1 netfilter hook NF_IP_PRE_ROUTING.
But the packet is *not* reaching the Application.
Why it is not reaching the application ? Whether I need to fill any
other fields in the sk_buff structure.
Iam using i) ping command ii) udp sockets to send packets at the
application layer.
Thanks n Regards
Sridhar.
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