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Chapter 10. dCache as xroot-Server

This chapter explains how to configure dCache in order to access it via the xroot protocol, allowing xroot-Clients like ROOT’s TXNetfile and xrdcp to do file operations against a dCache instance in a transparent manner. dCache implements version 2.1.6 of xroot protocol.



Setting up

To allow file transfers in and out of dCache using xroot, a new xrootd door must be started. This door acts then as the entry point to all xroot requests. Compared to the native xrootd server-implementation (produced by SLAC), the xrootd door corresponds to the redirector node.

To enable the xrootd door, you have to change the layout file corresponding to your dCache-instance. Enable the xrootd-service within the domain that you want to run it by adding the following line

[<domainName>/xrootd]

Example:

You can just add the following lines to the layout file:

[xrootd-${host.name}Domain]
[xrootd-${host.name}Domain/xrootd]

After a restart of the domain running the DOOR-XROOTD, done e.g. by executing

dcache restart xrootd-babelfishDomain
|Stopping xrootd-babelfishDomain (pid=30246) 0 1 2 3 4 5 6 7 done
|Starting xrootd-babelfishDomain done

the xrootd door should be running. A few minutes later it should appear at the web monitoring interface under “Cell Services” (see the section called “The Web Interface for Monitoring dCache”).

Parameters

The default port the xrootd door is listening on is 1094. This can be changed two ways:

  1. Per door: Edit your instance’s layout file, for example /etc/dcache/layouts/example.conf and add the desired port for the xrootd door in a separate line (a restart of the domain(s) running the xrootd door is required):
[xrootd-${host.name}Domain]
[xrootd-${host.name}Domain/xrootd]
xrootd.net.port = 1095
  1. Globally: Edit /etc/dcache/dcache.conf and add the variable xrootd.net.port with the desired value (a restart of the domain(s) running the xroot door is required):
xrootd.net.port=1095

For controlling the TCP-portrange within which xrootd-movers will start listening in the Domain, you can add the properties dcache.net.lan.port.min and dcache.net.lan.port.max to /etc/dcache/dcache.conf and adapt them according to your preferences. The default values can be viewed in /usr/share/dcache/defaults/dcache.properties.

dcache.net.lan.port.min=30100
dcache.net.lan.port.max=30200

QUICK TESTS

The subsequent paragraphs describe a quick guide on how to test xroot using the xrdcp and ROOT clients.

Copying files with xrdcp

A simple way to get files in and out of dCache via xroot is the command xrdcp. It is included in every xrootd and ROOT distribution.

To transfer a single file in and out of dCache, just issue

xrdcp /bin/sh root://<xrootd-door.example.org>/pnfs/<example.org>/data/xrd_test
xrdcp root://<xrootd-door.example.org>/pnfs/<example.org>/data/xrd_test /dev/null

Accessing files from within ROOT

This simple ROOT example shows how to write a randomly filled histogram to a file in dCache:

root [0] TH1F h("testhisto", "test", 100, -4, 4);
root [1] h->FillRandom("gaus", 10000);
root [2] TFile *f = new TXNetFile("root://<door_hostname>//pnfs/<example.org>/data/test.root","new");
061024 12:03:52 001 Xrd: Create: (C) 2004 SLAC INFN XrdClient 0.3
root [3] h->Write();
root [4] f->Write();
root [5] f->Close();
root [6] 061101 15:57:42 14991 Xrd: XrdClientSock::RecvRaw: Error reading from socket: Success
061101 15:57:42 14991 Xrd: XrdClientMessage::ReadRaw: Error reading header (8 bytes)

Closing remote xroot files that live in dCache produces this warning, but has absolutely no effect on subsequent ROOT commands. It happens because dCache closes all TCP connections after finishing a file transfer, while the SLAC xroot client expects to keep them open for later reuse.

To read it back into ROOT from dCache:

root [7] TFile *reopen = TXNetFile ("root://<door_hostname>//pnfs/<example.org>/data/test.root","read");
root [8] reopen->ls();
TXNetFile**             //pnfs/<example.org>/data/test.root
TXNetFile*             //pnfs/<example.org>/data/test.root
KEY: TH1F     testhisto;1     test

XROOT security

Read-Write access (legacy)

Legacy default for dCache xroot is restricted to read-only, because plain xroot was originally completely unauthenticated.

To enable read-write access, add the following line to ${dCacheHome}/etc/dcache.conf

xrootd.authz.anonymous-operations=FULL

and restart any domain(s) running a xroot door.

Please note that due to the unauthenticated nature of this access mode, files can be written and read to/from any subdirectory in the pnfs namespace (including the automatic creation of parent directories). If there is no user information at the time of request, new files/subdirectories generated through xroot will inherit UID/GID from its parent directory. The user used for this can be configured via the xrootd.authz.user property.

Permitting read/write access on selected directories

To overcome the security issue of uncontrolled xroot read and write access mentioned in the previous section, it is possible to restrict read and write access on a per-directory basis (including subdirectories).

To activate this feature, a colon-seperated list containing the full paths of authorized directories must be added to /etc/dcache/dcache.conf. You will need to specify the read and write permissions separately.

xrootd.authz.read-paths=/pnfs/<example.org>/rpath1:/pnfs/<example.org>/rpath2
xrootd.authz.write-paths=/pnfs/<example.org>/wpath1:/pnfs/<example.org>/wpath2

A restart of the xroot door is required to make the changes take effect. As soon as any of the above properties are set, all read or write requests to directories not matching the allowed path lists will be refused. Symlinks are however not restricted to these prefixes.

Strong authentication

The xroot-implementation in dCache includes a pluggable authentication framework. To control which authentication mechanism is used by xroot, add the xrootd.plugins option to your dCache configuration and set it to the desired value.

GSI

To enable GSI authentication in xroot, add the following line to /etc/dcache/dcache.conf (globally) or to the layout for the specific door:

xrootd.plugins=gplazma:gsi

When using GSI authentication, depending on your setup, you may or may not want dCache to fail if the host certificate chain can not be verified against trusted certificate authorities. Whether dCache performs this check can be controlled by setting the option dcache.authn.hostcert.verify for all of dCache. For enabling or disabling specifically for xroot,

xrootd.gsi.hostcert.verify=true

Authorization of the user information obtained by strong authentication is performed by contacting the gPlazma service. Please refer to Chapter 10, Authorization in dCache for instructions about how to configure gPlazma.

SECURITY CONSIDERATION

In general GSI on xroot is not secure. It does not provide confidentiality and integrity guarantees and hence does not protect against man-in-the-middle attacks.

As of 6.2, this can be mitigated by using GSI in conjunction with TLS.

ZTN

Scitokens/JWT bearer tokens (see below) are for authorization; however, the (SLAC) xroot protocol also defines an authentication equivalent, ZTN, where a token is passed as a credential at authentication (just after login).

Originally, this was a countermeasure taken to prevent stray clients from accessing the vanilla server via methods where there was no path (and thus no CGI authz element). However, recent changes to the vanilla client and server will allow a ZTN token to be used as a fallback authorization token as well, without further need to express a base64-encoded token as part of the path query.

dCache now supports this strategy. To illustrate, here are two different door configurations.

This one:

[xrootd-1095-${host.name}Domain]
dcache.java.memory.heap=2048m
dcache.java.memory.direct=2048m
[xrootd-1095-${host.name}Domain/xrootd]
xrootd.cell.name=xrootd-1095-${host.name}
xrootd.net.port=1095
xrootd.authz.write-paths=/
xrootd.authz.read-paths=/
xrootd.plugins=gplazma:none,authz:scitokens
xrootd.security.tls.mode=STRICT
xrootd.security.tls.require-login=true
xrootd.plugin!scitokens.strict=true

indicates that any client will be allowed through with anonymous identity and restrictions at authentication time, but ultimately will need a token on the path in order to be authorized, with the subject and claim being converted into dCache user and restrictions at the time of the request containing the path (usually ‘open’ on the file).

This configuration,

[xrootd-1095-${host.name}Domain]
dcache.java.memory.heap=2048m
dcache.java.memory.direct=2048m
[xrootd-1095-${host.name}Domain/xrootd]
xrootd.cell.name=xrootd-1095-${host.name}
xrootd.net.port=1095
xrootd.authz.write-paths=/
xrootd.authz.read-paths=/
xrootd.plugins=gplazma:ztn,authz:scitokens
xrootd.security.tls.mode=STRICT
xrootd.security.tls.require-login=true
xrootd.plugin!scitokens.strict=false

on the other hand, turns on ZTN in the door. For seamless functioning, this should be coupled with a loosening of the strict requirement on the CGI/path token. With this configuration, the client will need to be provided a ZTN token via an environment variable, e.g.,

XDG_RUNTIME_DIR=/run/user/8773

it will look for a file named ‘bt_<uid>’ in that directory. With that token in hand, authorization will also take place. A second token can still be passed as the path query CGI element (authz=Bearer%20<base64-encoded string>), and will override the original if present, but this is treated as optional, not required.

ZTN requires TLS. One has the option to enforce this up front via the STRICT setting (see further below), but a check will be made at authentication time as well, and an authentication error returned if TLS is not on.

Token-based authorization (SciTokens/JWT)

The xroot dCache implementation includes a generic mechanism to plug in different authorization handlers.

As of 6.2, xroot authorization has been integrated with gPlazma SciToken/JWT bearer token support.

Add

auth    sufficient	scitoken

to the gplazma.conf configuration file in order to enable authorization.

The token for xroot is passed as an ‘authz=Bearer%20’ query element on paths.

For example,

xrdcp -f xroots:///my-xroot-door.example.org:1095///pnfs/fs/usr/scratch/testdata?authz=Bearer%20eyJ0eXAiOiJKV1QiLCJhb... /dev/null

dCache will support different tokens during the same client session, as well as different tokens on source and destination endpoints in a third-party transfer.

To enable scitoken authorization on an xroot door, use “authz:scitokens” to load the plugin. See above (under ZTN) for an example layout configuration.

Note that the above configuration enforces TLS (STRICT); this is highly recommended with SciToken authorization as the token hash is not secure unless encrypted. While it is not strictly required to start TLS at login (since the actual token is not passed until a request involving a path, in this case, ‘open’) –– xrootd.security.tls.require-session=true would have been sufficient –– the extra protection on login of course will not hurt. (The same applies to GSI: TLS is of course redundant for the handshake, but can further guarantee data protection and integrity if on thereafter. For GSI-only doors, then, one can also opt to start the TLS session after login using ‘session’.)

The xroot protocol states that the server can specify supporting different authentication protocols via a list which the client should try in order (again, see below on multi-protocol doors). Authorization, on the other hand, takes place after the authentication phase; the current library code assumes that the authorization module it loads is the only procedure allowed, and there is no provision for passing a failed authorization on to a successive handler on the pipeline.

We thus make provision here for failing over to “standard” behavior via xrootd.plugin!scitokens.strict. If it is true, then the presence of a scitoken is required. If false, and the token is missing, whatever restrictions that are already in force from the login apply.

A Note on Pool configuration with Scitokens

JWT/Scitoken authorization also requires TLS to protect the token.

SECURITY CONSIDERATION

The client will pass the token in the open request’s path query regardless of whether the server supports TLS or has indicated that it should be turned on. While a check for TLS when authenticating/authorizing via token on the door is made, and the attempt rejected if TLS is off, on the pool no such check occurs, since no further authorization takes place.

In order to protect the bearer token when passed on redirect to the pool, then, the client should always require TLS by using ‘xroots’ as the URL schema. By using ‘xroots’, the client guarantees TLS will be activated by dCache at login or the connection will fail.

Precedence of security mechanisms

The previously explained methods to restrict access via xroot can also be used together. The precedence applied in that case is as follows:

With strong authentication like GSI, (as well as the Kerberos protocols) trust in remote authorities is required; however, this only affects user authentication, while authorization decisions can be adjusted by local site administrators by adapting the gPlazma configuration. Hence, the permission check executed by an authorization plugin (if one is installed), or acquired at login via GSI, is given the lowest priority; access control is ultimately determined by the file catalogue (global namespace).

This may also be true for some token authorization schemas; however, as implemented, the JWT/ scitoken authorization (see below) can in fact override the local controls via the scope and group claims the token carries.

To allow local site’s administrators to override remote security settings, write access can be further restricted to few directories (based on the local namespace, the pnfs). Setting xroot access to read-only has the highest priority, overriding all other settings.

TLS

As of 6.2, dCache supports TLS according to the protocol requirements specified by the xroot Protocol 5.+.

The xroot protocol allows a negotiation between the client and server as to when to initiate the TLS handshake. The server-side options are explained in the xrootd.properties file. Currently supported is the ability to require TLS on all connections to the door and pool, or to make TLS optional, depending on the client. For the former, one can also specify whether to begin TLS before login or after. The “after” option is useful in the case of TLS being used with a strong authentication protocol such as GSI, in which case it would make sense not to protect the login as GSI already requires a Diffie-Hellman handshake to protect the passing of credential information; it can also be used if token authorization is coupled with anonymous authentication (gplazma:none). For ZTN, TLS should begin at login.

For third-party, the dCache embedded client (on the destination server) will initiate TLS if (a) TLS is available on the destination pool (not turned off), and (b) the source server supports or requires it. In the case that the source does not support TLS, but the triggering client has expressed ‘tls.tpc=1’ (requiring TLS on TPC), the connection will fail.

As of 6.2, dCache has not yet implemented the GP file or data channel options; stay tuned for further developments in those areas.

A note on TLS configuration for the pools

Given that pools may need to service clients that do not support TLS (they may, for instance, be using a non-xroot protocol), it is probably not practical to make the pools require TLS by setting pool.mover.xrootd.security.tls.mode=STRICT.

Multiple authentication protocol chaining and defaults

As of 8.1, dCache now supports the chaining of authentication plugins/protocols on the door. This means that a single door can tell the client to try each or any of the protocols indicated.

The defaults have been modified so that dCache can be used out of the box in most cases without further concern to configure doors and pools. To summarize, for the door:

xrootd.plugins=gplazma:gsi,gplazma:ztn,gplazma:none,authz:scitokens
xrootd.security.tls.mode=OPTIONAL
xrootd.plugin!scitokens.strict=false

These defaults guarantee that the client

  1. will try GSI without TLS first if it has a certificate;
  2. if it tries ZTN it must turn on/request TLS or that protocol will be rejected (xroots:// must be used)
  3. if those fail, it can be logged in anonymously and potentially receive further authorization downstream from another token;
  4. if ZTN succeeds and there is no authorization token on the path URL, the ZTN token will be used as fallback (the scitoken requirement is not strict);
  5. third-party-copy will succeed with dCache doors as sources because the third-party client can connect using the rendezvous token without further authentication (gplazma:none).
  6. NOTE: it is possible to use GSI rather than ZTN and als provide a scitoken/JWT token on the path URL; TLS must be activated in this case. The usage scenario for this would, however, be rare.

Of course, these remain configurable in case of special requirements.

The pool defaults

pool.mover.xrootd.tpc-authn-plugins=gsi,unix,ztn

have been updated to load the ZTN module, but currently this results in a NOP because there is no agreed-upon strategy for getting the token to the third-party client; this may change in the near future.

Tried-hosts

Xrootd uses the path URL CGI “tried” and “triedrc” as hints to the redirector/manager not to reselect a data source because of some error condition or preference. dCache provides limited support for this attribute. In particular, it will honor it in the case that the indicated cause suggests some error previously encountered that suggests an IO malfunction on the node.

The property

xrootd.enable.tried-hosts

is true by default. When it is off, the ‘tried’ element on the path is simply ignored. dCache also ignored the tried hosts when ‘triedrc’ is not provided, or when it is not ‘enoent’ or ‘ioerr’. In the latter two cases, the xroot door will forward the list of previously tried hosts to the Pool Manager to ask that they be excluded from selection.

See xrootd.properties for further information.

Other configuration options

The xrootd-door has several other configuration properties. You can configure various timeout parameters, the thread pool sizes on pools, queue buffer sizes on pools, the xroot root path, the xroot user and the xroot IO queue. Full descriptions on the effect of those can be found in /usr/share/dcache/defaults/xrootd.properties.

XROOTD Third-party Transfer

Starting with dCache 4.2, native third-party transfers between dCache and another xroot server (including another dCache door) are possible.

To enforce third-party copy, one must execute the transfer using

xrdcp --tpc only <source> <destination>

One can also try third party and fail over to one-hop two-party (through the client) by using

xrdcp --tpc first <source> <destination>

TPC from dCache to another xroot server

Very few changes in the dCache door were needed to accomplish this. If dCache is merely to serve as file source, then all that is needed is to update to version 4.2+ on the nodes running the xrootd doors.

TPC from another xroot server to dCache, or between dCache instances

As per the protocol, the destination pulls/reads the file from the source and writes it locally to a selected pool. This is achieved by an embedded third-party client which runs on the pool. Hence, using dCache as destination means the pools must also be running dCache 4.2+.

Pools without the additional functionality provided by 4.2+ will not be able to act as destination in a third-party transfer and a “tpc not supported” error will be reported if --tpc only is specified.

Unauthenticated TPC using the xrootd-generated rendezvous key.

The xrootd protocol allows the third-party-copy client to read the source file under the following conditions:

  1. The initiating client has successfully opened the source file;
  2. The initiating client has been authorized to write the requested path on the destination;
  3. The third-party client has successfully connected to the source.

Note that (3) does not strictly require authentication, much less further authorization. This is achieved by the generation of an internal opaque key called a ‘rendezvous token’ which is handed off by the client to both source and destination, with the destination forwarding this key to the third-party-client. When the third-party-client connects, the source server checks the key against the one it was given by the initiating client, and if they match, the transfer proceeds.

To allow for unauthenticated TPC, the gplazma:none plugin must be active on the door.

Unauthenticated TPC (or “rendezvous TPC”) is done using the --tpc only <source> <destination> directive on the client command.

Authenticated TPC using GSI

The only way currently available to enforce authentication by the TPC client is to use GSI (neither dCache nor plain xrootd currently support passing the ZTN token to the TPC client, though this is subject to change in the future).

To enable GSI on the TPC client, on all pools that will receive files through xroot TPC, the GSI service provider plugin must be loaded by including this in the configuration or layout:

pool.mover.xrootd.tpc-authn-plugins=gsi

(Note that this is already loaded by default.)

There are two modes for using GSI to authenticate the TPC client.

GSI using a generated proxy

This method may be useful in the case of communication with pre-4.9 xrootd or pre-5.2 dCache instances, or when using a pre-4.9 xroot client.

First, enable this alternative by setting xrootd.gsi.tpc.delegation-only to false.

There are two ways of providing authentication capability to the pools in this case:

  • Generate a proxy from a credential that will be recognized by the source (e.g., a robocert) and arrange to have it placed (and periodically refreshed) on each pool that may be the recipient of files transfered via xrootd TPC. The proxy path must be indicated to dCache by setting this property:
xrootd.gsi.tpc.proxy.path={path-to-proxy}
  • If this property is left undefined, dCache will attempt to auto-generate a proxy from the hostcert.pem / hostkey.pem of the node on which the pool is running. While this solution means no cron job is necessary to keep the proxy up to date, it is also rather clunky in that it requires the hostcert DNs of all the pools to be mapped on the source server end.

Once again, this mode requires the --tpc only <source> <destination> directive.

GSI using credential (proxy) delegation

With the 5.2.0 release, full GSI (X509) credential delegation is available in dCache. This means that the dCache door, when it acts as destination, will ask the client to sign a delegation request.

If both endpoints support delegation (dCache 5.2+, XrootD 4.9+), nothing further need be done by way of configuration. dCache caches the proxy in memory and discards it when the session is disconnected.

To indicate that you wish delegation, the xroot client requires this directive:

xrdcp --tpc delegate only <source> <destination>

or

xrdcp --tpc delegate first <source> <destination>

Like the xrootd server and client, dCache can determine whether the endpoint with which it is communicating supports delegation, and fail over to the pre-delegation protocol if not.

NOTE: For reading the file in dCache (dCache as TPC source), the third-party server needs only a valid certificate issued by a recognized CA; anonymous read access is granted to files (even privately owned) on the basis of the rendezvous token submitted with the request.

Proxy delegation and host aliasing

A feature of the xrootd client is that it will refuse to delegate a proxy to a server endpoint if the hostname of the host credential is unverified.

This can occur if hostname aliasing is used but the host certificate was not issued with the proper SAN extensions. This is because the xrootd client by default does not trust the DNS service to resolve the alias.

In the case where dCache is the destination of a third-party transfer and the client does not delegate a proxy to the door, one may thus see an error on the pool due to the missing proxy. It is possible to configure dCache to attempt to generate a proxy from the pool host certificate in this case, but one may similarly see an error response from the source if the host DN is not mapped there.

Short of having the host certificate reissued with a SAN extension for the alias, DNS lookup can be forced in the client by setting this environment variable:

XrdSecGSITRUSTDNS
      0    do not use DNS to aide in certificate hostname validation.
      1    use DNS, if needed, to validate certificate hostnames.
      Default is 0.

WARNING: this is considered to be a security hole. The recommended solution is to issue the certificate with SAN extensions.

Please consult the xrootd.org document for further information; this policy may be subject to change in the future.

https://xrootd.slac.stanford.edu/doc/dev50/sec_config.htm

Client timeout control

The Third-party embedded client has a timer which will interrupt and return an error if the response from the server does not arrive after a given amount of time.

The default values for this can be controlled by the properties

pool.mover.xrootd.tpc-server-response-timeout

and

pool.mover.xrootd.tpc-server-response-timeout.unit.

These are set to 2 seconds to match the aggressive behavior of the SLAC implementation. However, dCache allows you to control this dynamically as well, using the admin command:

\s <xrootd-door> xrootd set server response timeout

This could conceivably be necessary under heavier load.

Signed hash verification support

The embedded third-party client will honor signed hash verification if the source server indicates it must be observed.

Starting with dCache 5.0, the dCache door/server also provides the option to enable signed hash verification.

However, there is a caveat here. Since dCache redirects reads from the door to a selected pool, and since the subsequent connection to the pool is unauthenticated (this has always been the case; the connection fails if the opaque id token dCache gives back to the client is missing), the only way to get signed hash verification on the destination-to-pool connection is to set the kXR_secOFrce flag. This means that the pool will then require unix authentication from the destination and that it will expect unencrypted hashes.

While the usefulness of unencrypted signed hash verification is disputable, the specification nevertheless provides for it, and this was the only way, short of encumbering our pool interactions with yet another GSI handshake, to allow for sigver on the dCache end at all, since the main subsequent requests (open, read, etc.) are made to the pool, not the door.

dCache 5.0 will provide the following properties to control security level and force unencrypted signing:

dcache.xrootd.security.level={0-4}
dcache.xrootd.security.force-signing={true,false}

In the case that the latter is set to true, and one anticipates there will be xroot TPC transfers between two dCache instances or two dCache doors, one also would need to include the unix service provider plugin in all the relevant pool configurations:

pool.mover.xrootd.tpc-authn-plugins=gsi,unix

As of 6.2, TLS is the preferred way of establishing a secured connection. Signed has verification has not been officially deprecated, however, and the choice to use it is still available.