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Chapter 11. dCache as xRootd-Server

This chapter explains how to configure dCache in order to access it via the xrootd protocol, allowing xrootd-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 xrootd protocol.

Setting up

To allow file transfers in and out of dCache using xrootd, a new xrootd door must be started. This door acts then as the entry point to all xrootd 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



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


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”).


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):
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):

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.



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

Copying files with xrdcp

A simple way to get files in and out of dCache via xrootd 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 xrootd 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 xrootd 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

XROOTD security

Read-Write access

Per default dCache xrootd is restricted to read-only, because plain xrootd is completely unauthenticated. A typical error message on the clientside if the server is read-only looks like:

xrdcp -d 1 /bin/sh root://ford.desy.de//pnfs/desy.de/data/xrd_test2
|Setting debug level 1
|061024 18:43:05 001 Xrd: main: (C) 2004 SLAC INFN xrdcp 0.2 beta
|061024 18:43:05 001 Xrd: Create: (C) 2004 SLAC INFN XrdClient kXR_ver002+kXR_asyncap
|061024 18:43:05 001 Xrd: ShowUrls: The converted URLs count is 1
|061024 18:43:05 001 Xrd: ShowUrls: URL n.1: root://ford.desy.de:1094//pnfs/desy.de/data/asdfas.
|061024 18:43:05 001 Xrd: Open: Access to server granted.
|061024 18:43:05 001 Xrd: Open: Opening the remote file /pnfs/desy.de/data/asdfas
|061024 18:43:05 001 Xrd: XrdClient::TryOpen: doitparallel=1
|061024 18:43:05 001 Xrd: Open: File open in progress.
|061024 18:43:06 5819 Xrd: SendGenCommand: Server declared: Permission denied. Access is read only.(error code: 3003)
|061024 18:43:06 001 Xrd: Close: File not opened.
|Error accessing path/file for root://ford//pnfs/desy.de/data/asdfas

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


and restart any domain(s) running a xrootd 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 xrootd 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 xrootd 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.


A restart of the xrootd 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.

Token-based authorization

The xrootd dCache implementation includes a generic mechanism to plug in different authorization handlers. The only plugin available so far implements token-based authorization as suggested in http://people.web.psi.ch/feichtinger/doc/authz.pdf.

The first thing to do is to setup the keystore. The keystore file basically specifies all RSA-keypairs used within the authorization process and has exactly the same syntax as in the native xrootd tokenauthorization implementation. In this file, each line beginning with the keyword KEY corresponds to a certain Virtual Organisation (VO) and specifies the remote public (owned by the file catalogue) and the local private key belonging to that VO. A line containing the statement "KEY VO:*" defines a default keypair that is used as a fallback solution if no VO is specified in token-enhanced xrootd requests. Lines not starting with the KEY keyword are ignored. A template can be found in /usr/share/dcache/examples/xrootd/keystore.

The keys itself have to be converted into a certain format in order to be loaded into the authorization plugin. dCache expects both keys to be binary DER-encoded (Distinguished Encoding Rules for ASN.1). Furthermore the private key must be PKCS #8-compliant and the public key must follow the X.509-standard.

The following example demonstrates how to create and convert a keypair using OpenSSL:

Generate new RSA private key

openssl genrsa -rand 12938467 -out key.pem 1024

Create certificate request

openssl req -new -inform PEM -key key.pem -outform PEM -out certreq.pem

Create certificate by self-signing certificate request

openssl x509 -days 3650 -signkey key.pem -in certreq.pem -req -out cert.pem

Extract public key from certificate

openssl x509 -pubkey -in cert.pem -out pkey.pem
openssl pkcs8 -in key.pem -topk8 -nocrypt -outform DER -out <new_private_key>
openssl enc -base64 -d -in pkey.pem -out <new_public_key>

Only the last two lines are performing the actual conversion, therefore you can skip the previous lines in case you already have a keypair. Make sure that your keystore file correctly points to the converted keys.

To enable the plugin, it is necessary to add the following two lines to the file /etc/dcache/dcache.conf, so that it looks like


After doing a restart of dCache, any requests without an appropriate token should result in an error saying “authorization check failed: No authorization token found in open request, access denied.(error code: 3010)”.

If both tokenbased authorization and read-only access are activated, the read-only restriction will dominate (local settings have precedence over remote file catalogue permissions).

Strong authentication

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


For instance, to enable GSI authentication in xrootd, add the following line to /etc/dcache/dcache.conf:


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:


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.


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

Precedence of security mechanisms

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


The xrootd-door can be configured to use either token authorization or strong authentication with gPlazma authorization. A combination of both is currently not possible.

The permission check executed by the authorization plugin (if one is installed) is given the lowest priority, because it can controlled by a remote party. E.g. in the case of token based authorization, access control is determined by the file catalogue (global namespace).

The same argument holds for many strong authentication mechanisms - for example, both the GSI protocol as well as the Kerberos protocols require trust in remote authorities. However, this only affects user authentication, while authorization decisions can be adjusted by local site administrators by adapting the gPlazma configuration.

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 `xrootd access to read-only has the highest priority, overriding all other settings.


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


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 xrootd 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 xrootd root path, the xrootd user and the xrootd 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 xrootd server (including another dCache door) are possible. These can be done either in unauthenticated mode, or with GSI (X509) authentication, using the client provided by SLAC (xrdcp or xrdcopy).

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 xrootd 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 xrootd 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.

Changes to dCache configuration for authenticated (GSI) transfers

For dCache as source, gPlazma configuration is identical to that needed for normal two-party reads and writes, with the caveat that the necessary destination DNs must be mapped on the dCache end. This will depend upon the nature of the proxy credential being used by the source.

To use dCache as TPC destination, some additional steps need to be taken.

First, for all pools that will receive files through xrootd TPC, the GSI service provider plugin must be loaded by including this in the configuration or layout:


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 keeps the proxy in memory and discards it when the session is disconnected.

To indicate that you wish delegation, the xrootd client requires:

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


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.

In the case of communication with pre-4.9 XrootD or pre-5.2 dCache instances, or when using a pre-4.9 XrootD client, one can still make use of third-party copy with a few extra configuration steps.

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, 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:
  • If this property is left undefined, dCache will 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.

  • 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:

      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.


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:


In the case that the latter is set to true, and one anticipates there will be xrootd 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: