ICMP's & IP src addrs


Tait Cyrus (hi.unm.edu!cyrus@hc.dspo.gov)
11 Sep 88 20:32:41 GMT


I have been playing with `ping' (ECHO,TIME,MASK, & INFO) lately. I am
seeing different machines respond with different IP src fields (assuming
I was `pinging to broadcast').

My question then is:
        "Should the src field in the returning IP header be the same
         address that was in the dst field in the outgoing (requesting)
         IP packet or should the src field be the IP address of the
         responding machine?"

Ok, that is a mouth full, so let me give you some more information.

Given the following information:

        Class B address: 129.24
        Net Mask: 255.255.248.0 (0xfffff800)

a `ping {-t,-n,-i,} 129.24.15.255' sometimes yields 129.24.15.255 as
the src IP address on the packet that is being returned to me and
sometimes the ping yields 129.24.#.# as the src IP address where #.# is
the correct IP address of the responding machine.

What is interesting is that some machines respond using 129.24.15.255
for ECHO/TIME/INFO requests, but use 129.24.#.# for MASK requests.
Some machines use different src IP addresses for different requests.

I can't find an RFC which talks about what src IP address to use when
echoing packets, so if one exists, I would greatly appreciate it if
someone could point me towards it.

Any other information about the "correct" way to handle this would also
be greatly appreciated.

Thanks in advance.....

    @__________@ W. Tait Cyrus (505) 277-0806
   /| /| University of New Mexico
  / | / | Dept of EECE - Hypercube Project
 @__|_______@ | Albuquerque, New Mexico 87131
 | | | |
 | | hc | | e-mail:
 | @.......|..@ cyrus@hc.dspo.gov
 | / | /
 @/_________@/



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