NFSv4 client identifier

This document explains how the NFSv4 protocol identifies client instances in order to maintain file open and lock state during system restarts. A special identifier and principal are maintained on each client. These can be set by administrators, scripts provided by site administrators, or tools provided by Linux distributors.

There are risks if a client’s NFSv4 identifier and its principal are not chosen carefully.

Introduction

The NFSv4 protocol uses “lease-based file locking”. Leases help NFSv4 servers provide file lock guarantees and manage their resources.

Simply put, an NFSv4 server creates a lease for each NFSv4 client. The server collects each client’s file open and lock state under the lease for that client.

The client is responsible for periodically renewing its leases. While a lease remains valid, the server holding that lease guarantees the file locks the client has created remain in place.

If a client stops renewing its lease (for example, if it crashes), the NFSv4 protocol allows the server to remove the client’s open and lock state after a certain period of time. When a client restarts, it indicates to servers that open and lock state associated with its previous leases is no longer valid and can be destroyed immediately.

In addition, each NFSv4 server manages a persistent list of client leases. When the server restarts and clients attempt to recover their state, the server uses this list to distinguish amongst clients that held state before the server restarted and clients sending fresh OPEN and LOCK requests. This enables file locks to persist safely across server restarts.

NFSv4 client identifiers

Each NFSv4 client presents an identifier to NFSv4 servers so that they can associate the client with its lease. Each client’s identifier consists of two elements:

  • co_ownerid: An arbitrary but fixed string.
  • boot verifier: A 64-bit incarnation verifier that enables a server to distinguish successive boot epochs of the same client.

The NFSv4.0 specification refers to these two items as an “nfs_client_id4”. The NFSv4.1 specification refers to these two items as a “client_owner4”.

NFSv4 servers tie this identifier to the principal and security flavor that the client used when presenting it. Servers use this principal to authorize subsequent lease modification operations sent by the client. Effectively this principal is a third element of the identifier.

As part of the identity presented to servers, a good “co_ownerid” string has several important properties:

  • The “co_ownerid” string identifies the client during reboot recovery, therefore the string is persistent across client reboots.
  • The “co_ownerid” string helps servers distinguish the client from others, therefore the string is globally unique. Note that there is no central authority that assigns “co_ownerid” strings.
  • Because it often appears on the network in the clear, the “co_ownerid” string does not reveal private information about the client itself.
  • The content of the “co_ownerid” string is set and unchanging before the client attempts NFSv4 mounts after a restart.
  • The NFSv4 protocol places a 1024-byte limit on the size of the “co_ownerid” string.

Protecting NFSv4 lease state

NFSv4 servers utilize the “client_owner4” as described above to assign a unique lease to each client. Under this scheme, there are circumstances where clients can interfere with each other. This is referred to as “lease stealing”.

If distinct clients present the same “co_ownerid” string and use the same principal (for example, AUTH_SYS and UID 0), a server is unable to tell that the clients are not the same. Each distinct client presents a different boot verifier, so it appears to the server as if there is one client that is rebooting frequently. Neither client can maintain open or lock state in this scenario.

If distinct clients present the same “co_ownerid” string and use distinct principals, the server is likely to allow the first client to operate normally but reject subsequent clients with the same “co_ownerid” string.

If a client’s “co_ownerid” string or principal are not stable, state recovery after a server or client reboot is not guaranteed. If a client unexpectedly restarts but presents a different “co_ownerid” string or principal to the server, the server orphans the client’s previous open and lock state. This blocks access to locked files until the server removes the orphaned state.

If the server restarts and a client presents a changed “co_ownerid” string or principal to the server, the server will not allow the client to reclaim its open and lock state, and may give those locks to other clients in the meantime. This is referred to as “lock stealing”.

Lease stealing and lock stealing increase the potential for denial of service and in rare cases even data corruption.

Selecting an appropriate client identifier

By default, the Linux NFSv4 client implementation constructs its “co_ownerid” string starting with the words “Linux NFS” followed by the client’s UTS node name (the same node name, incidentally, that is used as the “machine name” in an AUTH_SYS credential). In small deployments, this construction is usually adequate. Often, however, the node name by itself is not adequately unique, and can change unexpectedly. Problematic situations include:

  • NFS-root (diskless) clients, where the local DCHP server (or equivalent) does not provide a unique host name.
  • “Containers” within a single Linux host. If each container has a separate network namespace, but does not use the UTS namespace to provide a unique host name, then there can be multiple NFS client instances with the same host name.
  • Clients across multiple administrative domains that access a common NFS server. If hostnames are not assigned centrally then uniqueness cannot be guaranteed unless a domain name is included in the hostname.

Linux provides two mechanisms to add uniqueness to its “co_ownerid” string:

nfs.nfs4_unique_id
This module parameter can set an arbitrary uniquifier string via the kernel command line, or when the “nfs” module is loaded.
/sys/fs/nfs/client/net/identifier
This virtual file, available since Linux 5.3, is local to the network namespace in which it is accessed and so can provide distinction between network namespaces (containers) when the hostname remains uniform.

Note that this file is empty on name-space creation. If the container system has access to some sort of per-container identity then that uniquifier can be used. For example, a uniquifier might be formed at boot using the container’s internal identifier:

sha256sum /etc/machine-id | awk ‘{print $1}’ \
> /sys/fs/nfs/client/net/identifier

Security considerations

The use of cryptographic security for lease management operations is strongly encouraged.

If NFS with Kerberos is not configured, a Linux NFSv4 client uses AUTH_SYS and UID 0 as the principal part of its client identity. This configuration is not only insecure, it increases the risk of lease and lock stealing. However, it might be the only choice for client configurations that have no local persistent storage. “co_ownerid” string uniqueness and persistence is critical in this case.

When a Kerberos keytab is present on a Linux NFS client, the client attempts to use one of the principals in that keytab when identifying itself to servers. The “sec=” mount option does not control this behavior. Alternately, a single-user client with a Kerberos principal can use that principal in place of the client’s host principal.

Using Kerberos for this purpose enables the client and server to use the same lease for operations covered by all “sec=” settings. Additionally, the Linux NFS client uses the RPCSEC_GSS security flavor with Kerberos and the integrity QOS to prevent in-transit modification of lease modification requests.

Additional notes

The Linux NFSv4 client establishes a single lease on each NFSv4 server it accesses. NFSv4 mounts from a Linux NFSv4 client of a particular server then share that lease.

Once a client establishes open and lock state, the NFSv4 protocol enables lease state to transition to other servers, following data that has been migrated. This hides data migration completely from running applications. The Linux NFSv4 client facilitates state migration by presenting the same “client_owner4” to all servers it encounters.

See Also

  • nfs(5)
  • kerberos(7)
  • RFC 7530 for the NFSv4.0 specification
  • RFC 8881 for the NFSv4.1 specification.