We have two SL5.1 x86_64 systems running kernel 2.6.18-128.1.1.el5. 
I'll call them "A" and "B".  Each exports two file systems, and each runs 
amd to mount whatever filesystems are requested from elsewhere. 
Filesystems requested from SL3.0.9 systems mount without problem, and 
filesystems requested from the SL5.1 systems also mounted without problem 
until recently. But recently attempts to access from "A" a filesystem 
exported by "B" or access from "B" a filesystem exported by "A" started 
being met with a message "Input/output error".  Similar requests on an 
SL3.0.9 system to view a SL5.1 exported one give "Permission denied". I'd 
appreciate advice.  I'll give some more information in the following, and 
will be glad to add more depending on what others think might be useful.
     The /etc/hosts.allow files allow portmap, mountd, rquotad, and statd 
to a set of computers including "A" and "B".
     Unless I've made a mistake, the firewall is open between "A" and "B".
     In the following is what went into /var/log/messages on "A" and "B" at 
the time of an attempt to look from "A" at a filesystem exported by "B", 
with a perhaps ineffectual paranoid attempt to maintain a low profile by 
replacing computer names and IP's with "A" and "B".
     On "A" at the time of the "Input/output error", a set of lines went to 
/var/log/messages all beginning with "Apr 9 12:04:34 "A" amd[12252]: " and 
otherwise containing

get_nfs_version: returning NFS(3,tcp) on host "B"
get_nfs_version: returning NFS(3,udp) on host "B"
Using NFS version 3, protocol tcp on host "B"
initializing "B"'s pinger to 30 sec
creating mountpoint directory '/.automount/"B"/root'
file server "B", type nfs, state starts up
Flushed /net/"B"; dependent on "B"
recompute_portmap: NFS version 3 on "B"
Using MOUNT version: 3
amfs_host_mount: NFS version 3
fetch_fhandle: NFS version 3
mountd rpc failed: RPC: Can't decode result
fetch_fhandle: NFS version 3
mountd rpc failed: RPC: Can't decode result
/net/"B": mount (amfs_cont): Input/output error


    On "B" at that time lines in messages.log began with "Apr 9 12:04:34 
"B" mountd[9831]: " and otherwise contained:

authenticated mount request from "A":1023 for /data (/data)
authenticated mount request from "A":1023 for /scratch (/scratch)


Steven Yellin