autofs/automount failing to mount nfs exports from localhost in Debian 12

We have a setup where various systems export scratch directories to each other for convenience. An export is set up like:

/local/scratch -rw,root_squash,no_subtree_check,sync

Autofs maps come from LDAP – the autofs configuration is:

timeout = 300
browse_mode = no
mount_nfs_default_protocol = 4

ldap_uri = ldap:///dc%3Dldap%2Cdc%3Dyour%2Cdc%3Dserver%2Cdc%3Ddns%2Cdc%3Dname

search_base = ou=maps,ou=subsubunit,ou=subunit,o=organisation

map_object_class = nisMap
entry_object_class = nisObject
map_attribute = nisMapName
entry_attribute = cn
value_attribute = nisMapEntry

[ amd ]
dismount_interval = 300

If server A (ip address exports the directory, server B (ip address that also gets the autofs maps automounts this as expected when accessing the desired directory. The findmnt command shows:

└─/scratch                           auto.scratch    autofs     rw,relatime,fd=25,pgrp=2511731,timeout=300,minproto=5,maxproto=5,indirect,pipe_ino=1385785421
  └─/scratch/serverA                 serverA:/local/scratch     nfs4       rw,nosuid,nodev,relatime,vers=4.2,rsize=1048576,wsize=1048576,namlen=255,hard,proto=tcp,timeo=600,retrans=2,sec=sys,clientaddr=,local_lock=none,addr=

If server A is a Debian 11 system and we access /scratch/serverA on this system the resulting findmnt output looks like

└─/scratch                           auto.scratch                              autofs     rw,relatime,fd=25,pgrp=2511731,timeout=300,minproto=5,maxproto=5,indirect,pipe_ino=1385785421
  └─/scratch/serverA                 /dev/nvme0n1p1                            ext4       rw,relatime

The automounter has realised that the export comes from the same system, and does a bind mount instead. In this case /local/scratch is where the partition /dev/nvme0n1p1 is mounted (this appears further up the findmnt tree). The automounter appears to follow the chain back and do a bind mount of the base filesystem to the new location. Note that this behaviour can be changed with the nobind option in the autofs maps – see the auto.master man page.

If server A is then upgraded or reinstalled as Debian 12, the behaviour changes. Initially, the automount fails. When we examine the logs we see:

rpc.mountd[2437]: refused mount request from for /local/scratch (/): not exported

We can add the loopback address to the list of allowed addresses in /etc/exports, and the automount then works. findmnt shows that:

└─/scratch                                               auto.scratch            autofs      rw,relatime,fd=18,pgrp=5367,timeout=300,minproto=5,maxproto=5,indirect,pipe_ino=32005
  └─/scratch/serverA                                     serverA:/local/scratch  nfs4        rw,nosuid,nodev,relatime,vers=4.2,rsize=1048576,wsize=1048576,namlen=255,hard,proto=tcp,timeo=600,retrans=2,sec=sys,clientaddr=,local_lock=none,addr=

So the automount can be made to work as an actual network mount going through the loopback address.

This actually appears to be the expected behaviour for nfs4 – see this bug report from 2006(!). Notably:

Jeff Moyer 2006-04-17 21:52:34 UTC
In the event that you have “fstype=nfs4” in your options, the automounter should
not attempt a bind mount when the server is localhost. The reason for this is
that the automounter would need nfsv4 specific information to determine where
the mount actually comes from. As you noted, it’s not straight-forward as it is
in v2 and v3.

So, for your nfsv2 and nfsv3 mounts, the automounter should continue to use bind
mounts. If you specify -fstype=nfs4, then you should get a mount via nfs4.

Clear as mud?

So the question should probably be – why does the automounter do bind mounts in Debian 11, even if it is nominally dealing with nfs4? Falling back to v3 somehow. Debian-specific patch? Whatever it is I can’t find any Debian-specific discussion of the change (and there was only a minor version bump of autofs from 11 to 12).

Published by

Jamie Scott

IT Administrator at the Institute for Gravitational Research, University of Glasgow