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The /etc/netgroup file I'm using works perfectly on another NFS server (Ubuntu 18.04) in production, so this wasn't an immediate suspicion. However, if I use this /etc/exports: /srv/nfs @cryo_em(rw,sync,fsid=0,crossmnt,no_subtree_check) /srv/nfs/cryosparc @cryo_em(rw,sync,fsid=2,crossmnt,no_subtree_check) Client mounts fail: root@javelina:~# mount -vvvt nfs4 cerebro:/cryosparc /cryosparc mount.nfs4: timeout set for Tue Jun 15 11:53:22 2021 mount.nfs4: trying text-based options 'vers=4.2,addr=128.xx.xx.xxx,clientaddr=129.xxx.xxx.xx' mount.nfs4: mount(2): Permission denied mount.nfs4: access denied by server while mounting cerebro:/cryosparc and if I switch to specifying the host explicitly: /srv/nfs javelina.my.domain(rw,sync,fsid=0,crossmnt,no_subtree_check) /srv/nfs/cryosparc javelina.mydomain(rw,sync,fsid=2,crossmnt,no_subtree_check) the mount just works. The tcpdump error message isn't terribly helpful here: 11:14:02.856094 IP cerebro.my.domain.nfs > javelina.my.domain.741: Flags [.], ack 281, win 507, options [nop,nop,TS val 791638255 ecr 2576087678], length 0 11:14:02.856178 IP cerebro.my.domain.nfs > javelina.my.domain.741: Flags [P.], seq 1:25, ack 281, win 507, options [nop,nop,TS val 791638255 ecr 2576087678], length 24: NFS reply xid 2752089303 reply ERR 20: Auth Bogus Credentials (seal broken) but after figuring out the cause of the problem, I did find a corroborating RHEL error report (which you'll need a RHEL account to access): https://access.redhat.com/solutions/3563601 I couldn't figure out how to determine the exact version of the NFS server that ships with Ubuntu 20.04. Maybe someone could explain how to do this. Running /usr/sbin/rpc.nfsd --version doesn't do it.