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Environment Variables

Authenticating via Apache will set a number of environment variables, depending on the configuration and the authentication method. I'm skipping digest authentication because that is not commonly used.

Common variables

A number of useful variables are set by Apache itself, they include:

DOCUMENT_ROOT The directory the current file/script is executing
HTTP_ACCEPT Contents of Accept: header
HTTP_HOST Contents of Host: header
HTTP_USER_AGENT Contents of User-Agent: header
QUERY_STRING The query string, if any
REMOTE_ADDR The IP address of the client
REMOTE_PORT The port of the client machine
REQUEST_METHOD The HTTP request method
REQUEST_SCHEME The scheme of the reqeust (e.g. http)
REQUEST_URI The requested URI
SERVER_ADDR The IP address of the server
SERVER_NAME The hostname of the server or virtual host
SERVER_PORT The port on the server
SERVER_PROTOCOL Name and version of the request protocol
SERVER_SIGNATURE Server version and virtual host name added to server-generated pages
SERVER_SOFTWARE Server identification string
UNIQUE_ID ID set to be unique across requests. mod_unique_id

Basic Authentication

Basic authentication is managed by Apache.

  • AUTH_TYPE=Basic
  • REMOTE_USER=username


The configuration may look something like:

 AuthType Basic
 AuthName "Restricted Files"
 AuthBasicProvider file
 AuthUserFile /etc/httpd/conf/passwords
 Require valid-user

The user database would be created with this:

htpasswd -c /etc/httpd/conf/passwords testuser

Kerberos Authentication

Kerberos authentication is managed by mod_auth_gssapi or mod_auth_kerb.

  • AUTH_TYPE=Negotiate
  • REMOTE_USER=admin@EXAMPLE.COM

If delegation is enabled in the client and the server configuration includes KrbSaveCredentials on, then KRB5CCNAME will be set pointing to the user's keytab.

The configuration may look something like

 AuthType GSSAPI
 AuthName "Kerberos Login"
 GssapiCredStore keytab:/etc/http.keytab

for mod_auth_gssapi or for mod_auth_kerb:

 AuthType Kerberos
 AuthName "Kerberos Login"
 KrbMethodNegotiate on
 KrbMethodK5Passwd off
 KrbServiceName HTTP
 KrbAuthRealms EXAMPLE.COM
 Krb5KeyTab /etc/httpd/conf/ipa.keytab
 KrbSaveCredentials on

X.509 Authentication

X.509 authentication is managed by either mod_nss or mod_ssl (or mod_gnutls about which I know very little).

No specific AUTH_TYPE is set, see https://issues.apache.org/bugzilla/show_bug.cgi?id=45058

The value of REMOTE_USER is dependent upon the configuration. If SSLUserName or NSSUserName is set then that component of the client certificate DN is set. The exception is when FakeBasicAuth is set, in which case the full DN is set.

By default only the standard CGI environment variables are included, plus HTTPS.

A number of SSL-specific variables are set if ExportCertData is enabled in SSLOptions or NSSOptions.

There may be some slight differences in the variables available in mod_ssl and mod_nss. For example, SSL_TLS_SNI is not available in mod_nss.

<Directory "/var/www/secure">
    NSSOptions +StdEnvVars
    NSSVerifyClient Require
</Directory>

The mod_ssl configuration is similar. Replace NSS with SSL.

The variables are named the same between mod_ssl and mod_nss though the contents may differ slightly. The set of variables available in httpd 2.4 are.

HTTPS HTTPS is being used.
SSL_PROTOCOL The SSL protocol version (SSLv2, SSLv3, TLSv1, TLSv1.1, TLSv1.2)
SSL_SESSION_ID The hex-encoded SSL session id
SSL_CIPHER The cipher specification name
SSL_CIPHER_EXPORT true if cipher is an export cipher
SSL_CIPHER_USEKEYSIZE Number of cipher bits (actually used)
SSL_CIPHER_ALGKEYSIZE Number of cipher bits (possible)
SSL_COMPRESS_METHOD SSL compression method negotiated
SSL_VERSION_INTERFACE The mod_ssl program version
SSL_VERSION_LIBRARY The OpenSSL program version
SSL_CLIENT_M_VERSION The version of the client certificate
SSL_CLIENT_M_SERIAL The serial of the client certificate
SSL_CLIENT_S_DN Subject DN in client's certificate
SSL_CLIENT_S_DN_x509 Component of client's Subject DN
SSL_CLIENT_I_DN Issuer DN of client's certificate
SSL_CLIENT_I_DN_x509 Component of client's Issuer DN
SSL_CLIENT_V_START Validity of client's certificate (start time)
SSL_CLIENT_V_END Validity of client's certificate (end time)
SSL_CLIENT_V_REMAIN Number of days until client's certificate expires
SSL_CLIENT_A_SIG Algorithm used for the signature of client's certificate
SSL_CLIENT_A_KEY Algorithm used for the public key of client's certificate
SSL_CLIENT_CERT PEM-encoded client certificate
SSL_CLIENT_CERT_CHAIN_n PEM-encoded certificates in client certificate chain
SSL_CLIENT_VERIFY NONE, SUCCESS, GENEROUS or FAILED:reason
SSL_SERVER_M_VERSION The version of the server certificate
SSL_SERVER_M_SERIAL The serial of the server certificate
SSL_SERVER_S_DN Subject DN in server's certificate
SSL_SERVER_S_DN_x509 Component of server's Subject DN
SSL_SERVER_I_DN Issuer DN of server's certificate
SSL_SERVER_I_DN_x509 Component of server's Issuer DN
SSL_SERVER_V_START Validity of server's certificate (start time)
SSL_SERVER_V_END Validity of server's certificate (end time)
SSL_SERVER_A_SIG Algorithm used for the signature of server's certificate
SSL_SERVER_A_KEY Algorithm used for the public key of server's certificate
SSL_SERVER_CERT PEM-encoded server certificate
SSL_TLS_SNI Contents of the SNI TLS extension (if supplied with ClientHello)

LDAP authentication

Apache provides the module mod_authnz_ldap to perform authentication and authorization over LDAP.

A simple configuration looks like:

   AuthType Basic
   AuthName "LDAP Protected"
   AuthBasicProvider ldap
   AuthLDAPURL ldap://127.0.0.1/dc=example,dc=com?uid?one
   Require valid-user

Authorization can be done by specifying the allowed users, groups, attribute with in an entry or even a filter.

Attributes can be specified in the AuthLDAPURL value such that those values are set as environment variables of the form "AUTHENTICATE_", so any arbitrary list of values may be provided.

Proposed Additional Variables

When Apache module is used for authentication, the authentication result is passed to the application typically in the form of environment variable REMOTE_USER. Current web applications however want and need to create the user record in their internal databases so that foreign keys validate, and applications also want to do access control checks (authorizations) -- applications typically don't rely on Apache modules for authorization.

We are in need of a way for Apache modules to pass information about the authenticated user beyond the login name (in REMOTE_USER) to the application. That way the applications do not need to implement all possible authentication mechanisms (Kerberos, SAML, LDAP, ...) and can depend on specialized mod_auth_* modules to do it, while being able to know what user to populate and maintain in their internal user database.

We propose Apache modules that wish to pass information about users to applications adopt the following environment variable names:

Variable name Semantics Possible source Example mod_lookup_identity configuration
REMOTE_USER_GROUPS colon-separated list of group names the user is in POSIX call getgrouplist; sssd dbus call org.freedesktop.sssd.infopipe.GetUserGroups LookupOutputGroups REMOTE_USER_GROUPS :
REMOTE_USER_GROUP_N, REMOTE_USER_GROUP_1, REMOTE_USER_GROUP_2, ... number of user groups and individual group names alternate way to get the list of groups, avoiding the split needed with REMOTE_USER_GROUPS LookupUserGroupsIter REMOTE_USER_GROUP
REMOTE_USER_GECOS Equivalent of the GECOS value from the password file, could be full name. pw_gecos field of result of POSIX call getpwname; IPA attribute gecos, sssd dbus call org.freedesktop.sssd.infopipe.GetUserAttr gecos LookupUserGECOS REMOTE_USER_GECOS or LookupUserAttr gecos REMOTE_USER_GECOS
REMOTE_USER_DOMAIN domain the user was authenticated in (could be the domain in sssd, nss, LDAP, etc.)
REMOTE_USER_EMAIL user's email address IPA attribute mail, sssd-dbus attribute mail LookupUserAttr mail REMOTE_USER_EMAIL
REMOTE_USER_GROUPS_JSON list of groups the user is in, formatted as JSON string
REMOTE_USER_FIRSTNAME user's first name IPA attribute givenname, sssd-dbus attribute givenname LookupUserAttr givenname REMOTE_USER_FIRSTNAME
REMOTE_USER_MIDDLENAME user's middle name
REMOTE_USER_LASTNAME user's last name IPA attribute sn, sssd-dbus attribute sn LookupUserAttr sn REMOTE_USER_LASTNAME
REMOTE_USER_FULLNAME user's full name formatted as one string (similar to and possibly the same as REMOTE_USER_GECOS) IPA attribute cn or displayname, sssd-dbus attribute cn or displayname LookupUserAttr cn REMOTE_USER_FULLNAME or LookupUserAttr displayname REMOTE_USER_FULLNAME
REMOTE_USER_ORGUNIT organizational unit to which the user belongs IPA attribute ou, sssd-dbus attribute ou LookupUserAttr ou REMOTE_USER_ORGUNIT
REMOTE_USER_EXTERNAL_ID SID, GUID, or other unique identifier from the external identity provider; used to reconcile account after login change IPA attribute ipaUniqueId, 389 DS attribute nsUniqueID, AD attribute objectSid LookupUserAttr ipaUniqueId REMOTE_USER_EXTERNAL_ID
EXTERNAL_AUTH_ERROR when external authentication fails (and REMOTE_USER is not set), this variable can contain error describing the reason

The character set for values should be UTF-8.

The list above is not exhaustive, authentication and identity modules can provide additional variables with other values and meanings and applications are welcome to use them.

Module mod_lookup_identity (documentation, git repo) has been created as a proof of concept for this way of information passing. The full functionality depends on the sssd-dbus package (not yet released, in testing).

Module mod_intercept_form_submit (documentation, git repo) has been created as a proof of concept for PAM authentication based on form submission and it supports the REMOTE_USER and EXTERNAL_AUTH_ERROR outputs, plus mod_lookup_identity can work based on the mod_intercept_form_submit authentication result (latest versions of both modules required).