PLIP no longer working after 6.1 upgrade

[nolty its caltech edu]

Hi all --

Until recent days, I had a laptop running RedHat 5.2, which I could
network over PLIP with a desktop running RedHat 6.0.  A few days ago I
upgraded both machines to RedHat 6.1, and I can no longer get the PLIP
interface to work.  The strange thing is, the low-level networking seems
to be working, but I see no evidence that either kernel is responding
at a higher level to the packets it receives.

I connect the cable, and on both machines I run

# insmod parport
# insmod parport_pc io=0x378 irq=7
# insmod plip

Then I run on the appropriate machines
# ifconfig plip0 plip-host pointopoint plip-client
or
# ifconfig plip0 plip-client pointopoint plip-host

Now my routing table on both machines looks good.  On either machine, I
can ping itself:

# ping plip-host
PING plip-host (192.168.68.1) from 192.168.68.1 : 56(84) bytes of data.
64 bytes from 192.168.68.1: icmp_seq=0 ttl=255 time=0.1 ms
64 bytes from 192.168.68.1: icmp_seq=1 ttl=255 time=0.0 ms

(this actually goes over the loopback interface, as I verfied with
tcpdump, but it proves that the kernel recognizes the IP address for
plip-host as its own.)

However, I cannot get the machines to communicate with each other at a
high level.  If I ping plip-host from plip-client, all the packets are
unreturned.  If I try to open a telnet session, there is no response.
However, the packets are getting through the cable.  When I check with
ifconfig plip0, I see that the number of transmitted packets on one
machine is equal to the number of received packets on the other machine,
and no packets are shown as dropped.  Also, with tcpdump -i plip0, I can
see the packets arriving.

I did ipchains -L, and it showed all chains set to receive all packets.
Nothing shows up in the syslog  during plip exchanges.  Can anyone
suggest a solution, or know of another place I can look where the kernel
may be programmed to drop certain packets?  Incidentally, I have an
ethernet on the desktop machine which works perfectly (I'm using it to
send this news!)

Thanks,
Bob
nolty lbay com