How To Implementation External Authentication Programs
for mod_authnz_external or mod_auth_external
Version 3.2.0
LANGUAGES
External authenticators can be written in almost any language. The sample
authenticators in the 'test' directory are in Perl. The 'pwauth'
authenticator is in ANSI C. The example code fragments in this document
are in C.
If the authenticator is a script rather than a compiled program, it normally
has to start with a "#!/bin/sh" or "#!/usr/bin/perl" type directive. Scripts
without such directives may get interpreted by the shell, or may just not
work, depending on your installation.
SECURITY
The authenticator program should be written with great care because it runs
as a privileged user and handles privileged data. A poorly written
authenticator could substantially compromise the security of your system.
You get points for paranoia. Some notes:
- Don't make any assumptions about the length of the login names and
passwords given by the user. I *think* Apache will never pass you ones
that are longer than 8192 characters, but don't depend this. Check very
carefully for buffer overflows.
- Think about locking. It is possible to get lots of hits at your website
very fast, so there may be many programs simultaneously reading your
authentication database, plus updates may be going on at the same time.
Probably some form of locking is needed to make all this work right.
- Think about core dumps. On some systems core dump files can be publically
readable. A core dump from your authenticator is likely to contain the
user's plain text password, and may include large chunks of your password
database that may have been in buffers. For C programs on most versions of
Unix, it is possible to disable core dumps by doing something like:
rlim.rlim_cur = rlim.rlim_max = 0;
(void)setrlimit(RLIMIT_CORE, &rlim);
It may not hurt to spend a little time looking at the features of the pwauth
authenticator, which is the most secure external authenticator that I have
written.
PASSWORD AUTHENTICATORS
Authenticators communicate their result by the exit status code they return.
A value of 0 indicates that the password is correct. Other values indicate
that the password is incorrect, or something else is wrong. It can be
useful to return different error codes for different kinds of errors. These
will be logged in the Apache error log file, and can be helpful in diagnosing
problems. This version of mod_authnz_external does not have any provision for
returning textual error messages from the external authenticator. You might
be able to use syslog() for this. This might be improved in future releases.
Returned error codes should not be negative. Negative values are used
internally to mod_authnz_external to indicate problems launching your program.
How the external authentication program gets its arguments depends on
the method used. The method used is determined by the 'SetExternalAuthMethod'
command in your Apache configuration file. You need implement only the
method that you plan to use in your configuration.
PIPE METHOD
In the "pipe" method, the arguments are read from standard input. The
user name will be on the first line, and the password will be on the
second. Here's a typical chunk of C code to read that:
main()
{
char user[100], password[100], *p;
if (fgets(user, sizeof(user), stdin) == NULL) exit(2);
if ((p= strchr(user, '\n')) == NULL) exit(4)
*p= '\0';
if (fgets(password, sizeof(password), stdin) == NULL) exit(3);
if ((p= strchr(password, '\n')) == NULL) exit(5)
*p= '\0';
if (check_password(user, password) == OK)
exit(0); /* Good Password */
else
exit(1); /* Incorrect Password */
}
Here we simply read two lines from stdin, being careful not to allow
buffer overflows and stripping off trailing newlines.
We assume "check_password()" is some function that checks the validity of a
password and returns 'OK' if it is good.
Note that we exit with different non-zero error codes in different error
cases. This will be helpful for debugging, as those values will be logged
when authentication fails, giving you some clue as to what went wrong.
It'd really be better for check_password() to return more detailed error
codes, but I wanted to keep the example simple.
CHECKPASSWORD METHOD
The "checkpassword" method is identical to the "pipe" method, except
that the user name and password are terminated by NUL ('\0') characters
instead of newline characters, and they must be read from file descriptor
3 instead of standard input. Documentation for the checkpassword
interface is at http://cr.yp.to/checkpwd.html.
ENVIRONMENT METHOD
In the "environment" method, the arguments are passed in environment
variables. The user id and the clear-text password are passed in the
USER and PASS environment variables respectively.
Note that the environment method has fundamental security weaknesses,
and should probably not be used. Use the pipe method instead.
A typical chunk of C code to authenticate with the environment method
might be like:
main()
{
char *user, *password;
if ((user= getenv("USER")) == NULL) exit(2);
if ((password= getenv("PASS")) == NULL) exit(3);
if (check_password(user, password) == OK)
exit(0); /* Good Password */
else
exit(1); /* Incorrect Password */
}
GROUP AUTHENTICATORS
Security is generally less of a issue with group authenicators, since they
are not handling any data as sensitive as clear-text passwords. They are
only passed a user name (presumably already authenticated), and a list of
group names. They exit with status code 0 if that user is in one of those
groups, and a non-zero code otherwise.
In versions of mod_auth_external before 2.1.8, external authenticators were
always passed just one group name. If the Apache "require group" directive
listed more than one group, then the external authenticator would be called
once with each group name, which could be inefficient if you have a large
number of groups. Mod_auth_external will still behave this way if you
issue the "GroupExternalManyAtOnce off" directive.
Newer versions of mod_auth_external and mod_authnz_external will pass all
group names, separated by spaces. There will only be multiple calls if more
than one "require group" directive applies to the same program (e.g., if
different parent directories contain such directives in their .htaccess
files - for efficiency, this should be avoided). The list of group names
is passed in exactly as they appear on the "require group" directive - if
your program can't handle multiple spaces between group names, don't put
them there.
Arguments are passed in a manner similar to password authenticators. The
method used is determined by the 'SetExternalGroupMethod' command in your
Apache configuration file.
ENVIRONMENT METHOD
In the "environment" method, the arguments are passed in environment
variables. The user id and the group names are passed in the USER and
GROUP environment variables respectively. A typical chunk of C code to
fetch the arguments and check each group might be like:
main()
{
char *user, *groups, *group;
if ((user= getenv("USER")) == NULL) exit(2);
if ((groups= getenv("GROUP")) == NULL) exit(3);
group= strtok(groups, " ");
while (group != NULL)
{
if (check_group(user, group) == OK)
exit(0); /* User is in group */
group= strtok(NULL, " ");
}
exit(1); /* User is not in any group */
}
Here "check_group()" is some function that looks in your database to see if
user is in group and returns 'OK' if he is.
PIPE METHOD
In the "pipe" method, the arguments are read from standard input. The
user name will be on the first line, and the group name will be on the
second. Here's a typical chunk of C code to read that:
main()
{
char user[100], groups[100], *group, *p;
if (fgets(user, sizeof(user), stdin) == NULL) exit(2);
if ((p= strchr(user, '\n')) == NULL) exit(4)
*p= '\0';
if (fgets(groups, sizeof(groups), stdin) == NULL) exit(3);
if ((p= strchr(groups, '\n')) == NULL) exit(5)
*p= '\0';
group= strtok(groups, " ");
while (group != NULL)
{
if (check_group(user, group) == OK)
exit(0); /* User is in group */
group= strtok(NULL, " ");
}
exit(1); /* User is not in any group */
}
Here we simply read two lines from stdin, being careful not to allow
buffer overflows and stripping off trailing newlines. We loop through
all groups, checking each.
CHECKPASSWORD METHOD
Mod_auth_external will happily try to do group authentication via the
checkpassword method, piping NUL terminated user and group names to
the child process's file descriptor 3, but this isn't actually allowed
for in the checkpassword protocol specification, so I don't recommend it.
OTHER ENVIRONMENT VARIABLES
In all cases (pipe or environment method, password or group authentication),
the following additional environment variables will be supplied to the
authenticator:
AUTHTYPE either "PASS" or "GROUP" depending on whether we are doing
password or group authentication. This is handy if you are
using one program to do both.
CONTEXT a string whose value is set by an "AuthExternalContext"
directive in the .htaccess file or "<Directory>" block for
the directory. This can be used to select different
authentication behaviors in different directories. It is
undefined if there is no "AuthExternalContext" directive.
IP the client's ip-address.
HOST the client's host name, if Apache has "HostnameLookups On".
PATH the httpd's path environment variable.
COOKIE all cookie values passed in by the client.
HTTP_HOST the server's host name, as given in the HTTP request. May
be useful if you have multiple virtual hosts sharing an
authenticator.
URI the document requested. This is the URL including any extra
path information, but not including the hostname or any CGI
arguments.
These may be useful for logging, or you may want to accept logins from
certain users only if they are connecting from certain locations or requesting
certain documents.
Note that if you have configured Apache with "HostnameLookups Off" then HOST
will usually not be set. If you really want hostnames, either turn on
HostnameLookups or do your own gethostbyaddr() calls from the authenticator
when HOST is not defined. Note that if the user is coming from an
unresolvable IP, then hostname lookups can be very slow.
Note that using IP addresses to track a user through your site is not
reliable. Users of services like AOL and WebTV use proxy servers, so that
their IP addresses appear to change constantly since each request may come
through a different proxy. A single user's requests for successive pages,
or for different images on the same page may all come from different IP
addresses.
The PATH environment variable passed to the authenticator is just whatever
PATH was in effect when Apache was launched, and may differ if the server
was launched automatically during a reboot or manually by an admin.
Probably your program should set its own PATH if it needs one.
The COOKIE environment variable contains all cookies set in the current
request. This has the same format as the HTTP_COOKIES ("key=val;key=val")
passed to a CGI program. This should be used with caution. Cookies come
from the user's computer and might have been created, editted or deleted
by the user rather than your website. This severely limits their use for
authentication. It is not possible to set cookies from an authentication
module.
The URI variable is there because various people want it. Mostly it
is useful not for authentication ("who is this person?") but for access
control ("is this person permitted to do this?"), and good design usually
dictates separating those functions. Strictly speaking, an authenticator
is not the right place to be doing access control. However,
mod_authnz_external is 50% a kludge-builder's tool, so we won't fuss if you
want to break the rules.
