This issue is related to nfs-utils 1.2.2 against amd64 (x86_64) architecture.* This test works against the x86 (32 bit) architecture.
When using nfs-kernel-server on amd64, files cannot be written even though the group permission is set correctly.* This looks like a group permission issue.
Test setup are two servers running Lenny with the nfs-common and nfs-kernel-server version 1.2.2 from backport.
Server 1 (nfs_client)
---------------------
# dpkg -l nfs-common
Desired=Unknown/Install/Remove/Purge/Hold
+++-=============================================-=============================================-================================================== ================================================== ======
ii* nfs-common*********************************** 1:1.2.2-1~bpo50+1**************************** NFS support files common to client and server
ii* nfs-common*********************************** 1:1.2.2-1~bpo50+1**************************** NFS support files common to client and server
ii* nfs-kernel-server**************************** 1:1.2.2-1~bpo50+1**************************** support for NFS kernel server
/etc/exportfs from nfs_server:
/data** *(rw,sync,no_root_squash,no_subtree_check)
The test from nfs_client:
root@nfs_client# mount -t nfs -o vers=3 nfs_server:/data /mnt
root@nfs_client# mount | grep mnt
nfs_server:/data on /mnt type nfs (rw,vers=3,addr=<hidden>)
root@nfs_client# mkdir /mnt/adm
root@nfs_client# chgrp adm /mnt/adm
root@nfs_client# chmod g+w /mnt/adm
root@nfs_client# ls -ld /mnt/adm
drwxrwxr-x 2 root adm 4096 2010-06-08 11:23 /mnt/adm
root@nfs_client# groups user
user
root@nfs_client# adduser user adm
Adding user `user' to group `adm' ...
Adding user user to group adm
Done.
root@nfs_client# groups user
user adm
root@nfs_client# su - user
user@nfs_client$ ls -ld /mnt/adm
drwxrwxr-x 2 root adm 4096 2010-06-08 11:23 /mnt/adm
user@nfs_client$ cd /mnt/adm
user@nfs_client:/mnt/adm$ ls -ald .
drwxrwxr-x 2 root adm 4096 2010-06-08 11:23 .
user@nfs_client:/mnt/adm$ touch a
This was also tested against the Squeeze distribution with the same result.* Downgrading nfs-kernel-server and nfs-common on nfs_server to 1.1.2 works.