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Permissions and Authentication

Chapter 17. Permissions and Authentication

The contents of the /etc/lpd.perms file are used to control access to the lpd server facilities. The model used for permission granting is similar to packet filters. An incoming request is tested against a list of rules, and the first match found determines the action to be taken. The action is either ACCEPT or the request is granted, or REJECT and the request is denied. You can also establish a default action.

The following is a sample lpd.perms file.

    # allow root on server to control jobs
    # allow same user on originating host to remove a job
    # allow root on server to remove a job
    # all other operations allowed

Each line of the permissions file is a rule. A rule will ACCEPT or REJECT a request if all of the patterns specified in the rule match. If there is a match failure, the next rule in sequence will be applied. If all of the rules are exhausted, then the last specified default authorization will be used.

The sense of a pattern match can be inverted using the NOT keyword. For example, the rules with ACCEPT NOT REMOTEUSER=john,bill succeeds only if the REMOTEUSER value is defined and is not john or bill.

Each entry in a rule is a keyword which has is assigned a value or list of values followed by an optional set of patterns that are matched against these values. The following table is a summary of the available keywords.

Table 17-1. Permission Keywords and Purpose

Keyword Match
DEFAULT default result
SERVICE Checking lpC, lpR, lprM, lpQ, and Printing
USER P (logname) field name in print job control file.
REMOTEUSER user name in request from remote host.
HOST DNS and IP address information for the H (host) field name in print job control file
REMOTEHOST DNS and IP address information for the connection from the remote host making the request
IP Alias for HOST
REMOTEPORT Originating TCP/IP port for the connection from the remote host making the request
PORT Alias for PORT
UNIXSOCKET Connection is on a UNIX socket, i.e. from localhost
SERVER request originates on lpd server
FORWARD destination of job is not host
REMOTEGROUP REMOTEUSER is in the specified group or netgroup in the lpd server group database.
GROUP USER is in the specified group or netgroup in the lpd server group database.
LPC LPC command in the LPC request.
CONTROLLINE match a line in control file
AUTH authentication type
AUTHUSER authenticated user
AUTHFROM authenticated forwarder
AUTHJOB authenticated job in queue

17.1. Permission Checking Algorithm

Options used:

  • default_permission=Default Permission (accept)

The lpd server uses the following algorithm to do permission checks.

  1. The configuration information initially establishes a default permission using the default_permission configuration value. This is used if an explicit permission is not determined by the other steps in this algorithm.

  2. Each line of the permissions file is a lists of tests (patterns) and a permission value that is used if all of the tests (patterns) on the line are successful. A DEFAULT line sets the default result if all lines fail.

  3. Each line is executed in sequence until a match is found. The first matching line terminates the permission checking and the corresponding permission value is used.

  4. Each keyword has a value (or set of values) that are matched against a set of patterns. If the keyword does not have a value (or the null value) then the match will fail. Initially, all the keywords have a null value.

  5. When a connection is received by the lpd server, REMOTEHOST and REMOTEPORT are set to the the IP addresses and hostnames, and the TCP/IP port of the host originating the IP address respectively. REMOTEIP and IFHP are aliases for REMOTEPORT and PORT is an alias for REMOTEPORT and are provided for backwards compatibility with older versions of LPRng. If the connection was on a UNIX socket, then the UNIXSOCKET flag is set. For example, a request originating from, port 1011 would set REMOTEIP to and PORT to 1011.

  6. The REMOTEHOST value is set to the result of doing a reverse DNS lookup on the REMOTEIP address. This value is the list of names and ip addresses in standard IP notation (nnn.nnn.nnn.nnn) that are returned by the lookup. If the DNS lookup fails then the REMOTEHOST value is set to the REMOTEIP value. For example, lookup of would result in the names h2.private and patrick.private, and the only IP address assigned to it was The REMOTEHOST value would then be the list h2.private,patrick.private,

  7. The SERVICE value is set to X and then the permissions database is scanned for a matching entry. The result is the permission value of the first matching line or the default permission. If the result is REJECT then the connection is closed.

  8. Next, a single line is read from the connection. This line contains the request type, the print queue name, and depending on the request type an optional user name and options. The SERVICE value is set to R, Q, M, and C, for a lpR, lpQ, lprM, and lpc request respectively and PRINTER to the print queue name.

  9. If the request is for an lpc operation, the LPC value is set to the name of the operation. For example, and lpc lpd operation

  10. If the request contains a user name then REMOTEUSER is assigned the user name.

  11. If the request originates from the lpd server as determined by the connection arriving from the localhost address or an address assigned to one of the network interfaces for this host then the SERVER value is set to true (or matches).

  12. If the request is for an authenticated transfer, (see Authentication and Encryption ), then the authentication procedures are carried out. After they have been performed, the AUTH value is set to true, AUTHTYPE is set to the name of the authentication method, AUTHUSER to the authenticated identifier of the originator of the request, and AUTHFROM to the authenticated identifier of the originator of the connection.

  13. Other matching keywords such as REMOTEGROUP use values set at this time. These are discussed in the next section.

  14. The permission database is rescanned, this time to see if there is permission to operate on the specified spool queue. The permission database is first checked to see if the requesting user has control (SERVICE=C) permission. If they do, then they can perform any operation on the spool queue. The scan is then repeated for the actual request.

  15. If there is no permission to perform the operation then an error code and messages is returned on the requesting connection.

  16. If the operation is for a spool queue or server, no other permissions checking is done. This includes the lpq command, and most of the lpc commands control queue operations.

  17. If the operation is for for individual jobs in a spool queue, then the queue is scanned and job information is extracted for each job in the queue. The USER value is set to the job control file P line. The value of the H line in the control file is used to perform a DNS lookup, and the HOST value is set to the results of this lookup. IP is an alias for HOST, and is retained for backwards compatibility.

  18. The SAMEUSER value is set to true (or match) if the REMOTEUSER value is identical to the USER value. Similarly, SAMEHOST is set to true if the REMOTEHOST value matches the HOST value. See the following sections for other keywords such as GROUP.

  19. The permission checking is done for each individual job in a spool queue, and if it succeeds the action is carried out on the job.

These checks are applied on the arrival of a job from an external connection. Unfortunately, there are a set of print spooler implementations that do not handle job rejection due to lack of permissions. These printers will continually and repeatedly attempt to send a job for which there is no printing permission until the job is removed by administrative action. To accommodate these printers, we must accept jobs for printing and then dispose of them. This is done by using the SERVICE=P (printing) checks. These checks are performed after the job has been accepted.

  1. When a print spool is active and is printing or forwarding jobs, before it processes a job it will read the job control file and set the USER and HOST values as discussed in the previous sections. It will also set the AUTH, AUTHUSER, and AUTHJOB values as well, if the job was spooled by using an authenticated method.

  2. The permissions database will be scanned and the resulting permission determined. Note that the values of the REMOTE keys are undefined, and tests using them will have unpredictable effects.

  3. If the job does not have permission to be printed, it will normally be removed from the spool queue.

While this model is very simple it can handle a wide range of situations. However, it is really based on the simple trust that users will not impersonate other users or hosts. If this is not the case, then more elaborate procedures based on encryption and authentication are called for.

There is a problem with permissions checking for lpq (SERVICE=Q) requests. Since the user name is not passed as part of the request, it is impossible to use the REMOTEUSER clause to restrict lpq operations.

The SERVICE=R and SERVICE=P facilities are provided to handle problems with print spoolers that do not recognize a lack of permission error code, and will indefinitely retry sending a job to the lpd server. If this is the case, then the SERVICE=R clause can be used to accept jobs, and then the SERVICE=P clause will cause the lpd server to remove of the job when it is scheduled for printing.