The term btree stands for "balanced tree." It is a grow-only form of database. Lookups and insertions are fast, but deletions do not shrink the database. A good description of this form of database can be found in The Art of Computer Programming, Vol. 3: Sorting and Searching, D.E. Knuth, 1968, pp. 471-480. The btree class is available only if sendmail was compiled with NEWDB defined and the new Berkeley db library linked in.

See Table 33.5 in Section 33.3.4 for a list of the K command switches that can be used with this class and the meaning of each.

The bestmx map class looks up a hostname as the key and returns the current best MX record as the value. Internally, a call is made to getmxrr() to get a list of MX records for the host. That list is sorted in order of the best to the worst, and bestmx returns the first. Because bestmx is a class, not a map, you need to declare it with a K configuration command before you can use it:

Kbestmx bestmx

One use for this class might be to see whether a particular host has a usable MX at all:

Kbestmx bestmx
...
R$*< @ $+ > $*                $: $1<@$2>$3 <$(bestmx $2 $: NO $)>
R$*< @ $+ > $* < NO >         $#smtp  $@ $2 $: $1 < @ $2 > $3
R$*< @ $+ > $* < $* >         $: $1<@ $[ $2 $] > $3

In the first rule we look up the host part of an address (which has already been focused by rule set 3) with the bestmx database map. The result of the lookup is surrounded with angle brackets and appended to the original address. The second rule looks for the NO caused by an unsuccessful lookup (the $:). The original address is then sent with the smtp delivery agent. If the hostname inside the appended angle braces is not NO, the host part of the original address is canonicalized with the $[ and $] operators.

This bestmx class is a special internal one that can take advantage of only two of the K command switches: the -a (as you saw) and the -q (to suppress dequoting the key). This class can be watched with the -d8 debugging switch (see Section 37.5.30, -d8.1).

The dbm class, which is really the ndbm form of database, is the traditional form of UNIX database. Data are stored in one file, keys in another. The data must fit in blocks of fixed sizes, so there is usually a limit on the maximum size (1 kilobyte or so) on any given stored datum. The dbm class is available only if sendmail was compiled with NDBM declared (see Section 18.8.24, NDBM).

This is the class of database traditionally used with alias files. Because of the limit on the size of a datum, you should consider using one of the db(3) hash or btree classes instead.

See Table 33.5 in Section 33.3.4 for a list of the K command switches that can be used with the class and the meaning of each switch.

V8 sendmail can remove quotation marks from around tokens by using the special dequote class. Because dequote is a class, not a map, you need to declare it with a K configuration command before you can use it:

Kunquote dequote

This declares a map named unquote of the class dequote. Once a map name has been declared, the dequote class can be used in the RHS of rules to remove quotation marks. It is used with $( and $), just like database lookups:

$(unquote tokens $)

Here, arbitrary tokens are looked up in the database named unquote. That database is special because it is of the class dequote. Instead of really being looked up in a database, tokens will just have any surrounding quotation marks removed:

"A.B.C"           becomes   A.B.C
"A"."B"."C"       becomes   A.B.C
"A B"             becomes   "A B"
"A,B"             becomes   "A,B"
"A>B"             becomes   "A>B"

The first example shows that surrounding quotation marks are removed. The second shows that multiple quoted tokens are all de-quoted. The last three show that sendmail refuses to dequote any tokens that will form an illegal or ambiguous address when dequoted.

As an aid to understanding this dequoting process, run the following two-line configuration file in rule-testing mode:

V7
Kdequote dequote

You can then use the -bt /map command to try various dequoting possibilities:

> /map dequote "A.B.C"
map_lookup: dequote ("A.B.C") returns A.B.C (0)
> /map dequote "A"."B"."C"
map_lookup: dequote ("A"."B"."C") returns A.B.C (0)
> /map dequote "A B"
map_lookup: dequote ("A B") no match (0)

Note that beginning with V8.7, specifying the -s switch causes the space character to be replaced with another character before dequoting (see Section 33.3.4.10).

V7
Kdequote dequote -s+

In that case the last example above would become the following:

> /map dequote "A B"
map_lookup: dequote ("A B") returns A+B (0)

Also note that beginning with V8.8, specifying the -a switch causes a suffix of your choice to be appended to a successful match:

V7
Kdequote dequote -a.yes

In that case the "A.B.C" example would become the following:

> /map dequote "A.B.C"
map_lookup: dequote ("A.B.C") returns A.B.C.yes (0)

In addition to removing quotes, the dequote class also tokenizes everything that is returned. It does this because quotes are ordinarily used to mask the separation characters that delimit tokens. For example, consider the $& operator. It prevents a macro in a rule from being expanded when the configuration file is read and always returns a single token, no matter how many tokens it really contains. Consider this configuration file:

V7
DXhost.domain
Kdequote dequote
R$*    $: $&X , $(dequote "" $&X $)

Here, the macro X is assigned host.domain as its value. The only rule in the file (when sendmail is run in rule-testing mode) prints the expression $&X to show that it is a single token, then prints the result of dequoting that same expression. Note that an empty token needs to be dequoted. Putting quotes around $&X itself won't work. The output produced by rule-testing mode looks like this:

> 0 foo
rewrite: ruleset  0   input: foo
rewrite: ruleset  0 returns: host.domain , host . domain
>                                         
                             $X        $X dequoted

No debugging switch is available to watch the actions of the dequote class.

The hash class uses a hashing algorithm for storing data. This approach to a database is described in A New Hash Package for UNIX, by Margo Seltzer (USENIX Proceedings, Winter 1991). The hash class is available only if sendmail was compiled with NEWDB defined and the new Berkeley db library linked in.

The hash class is the default that is used with most of the FEATURES offered by the m4 technique (see Table 33.6 in Section 33.6, "Database Maps and m4"). For example, consider the following:

Kuudomain hash -o /etc/uudomain

Here, a map named uudomain is declared to be of class hash. The -o says that the file /etc/uudomain is optional.

See Table 33.5 in Section 33.3.4 for a list of the K command switches that can be used with this class and the meaning of each.

The hesiod class of map uses the Hesiod system, a network information system developed as Project Athena. Support of hesiod maps is available only if you declare HESIOD when compiling sendmail. (See Section 18.8.10, HESIOD for a fuller description of the Hesiod system.)

A hesiod map is declared like this:

Kname hesiod HesiodNameType

The HesiodNameType must be one that is known at your site, such as passwd or service. An unknown HesiodNameType will yield this error when sendmail begins to run:

cannot initialize Hesiod map (hesiod error number)

One example of a lookup might look like this:

Kuid2name hesiod uid
R$+      $: $(uid2name $1 $)

Here, we declare the map uid2name using the Hesiod-type uid, which converts uid numbers into login names. If the conversion was successful, we use the login name returned; otherwise, we use the original workspace.

See Table 33.5 in Section 33.3.4 for a list of the K command switches that can be used with this class and the meaning of each.

The host class is a special internal database used by sendmail to help it resolve hostnames. It is fully described under the $[ and $] operators in Section 33.4.3.

The -d9 debugging switch (see Section 37.5.37, -d9.1) can be used to watch the actions caused by this host class.

The implicit class refers specifically to aliases(5) files only. It causes sendmail to first try to open a db(3) hash-style alias file, and if that fails or if NEWDB support was not compiled in, it tries to open a ndbm(3)-style database. If that fails, sendmail reads the aliases(5) source file into its internal symbol table.

Although you can declare and use this class in a configuration file, there is no reason to do so. It is of use only to the internals of sendmail. If implicit fails to open an aliases file (probably because of a faulty AliasFile (A) option; see Section 34.8.1), sendmail will issue the following error if it is running in verbose mode:

WARNING: cannot open alias database bad file name

If the source aliases file exists but no database form exists, sendmail will read that source file into its internal symbol table using the stab class (see Section 33.8.16).

LDAP stands for Lightweight Directory Access Protocol and provides access to the X.500 directory. The ldapx class is used to look up items in that directory service. It is declared like this:

Kname ldapx switches

Lookups via LDAP are entirely defined by the switches specified. To illustrate, consider the following X.500 entry:

cn=Full Name, o=Organization, c=US
sn=Name
uid=yourname
cn=Full Name
commonname=Full Name
mail=yourname@mailhub.your.domain
objectclass=person
objectclass=deptperson

To look up a login name in this database and have the official email address for that user returned, you might use a declaration like this:

Kldap ldapx -k"uid=%s" -v"mail" -hldap_host -b"o=Organization, c=US"

Note that the -k switch is in the form of a ldap_search(3) filter, where the key will replace the %s and then the whole expression will be searched for as the key. The -b is required to specify the base from which to search. Note that a base must be selected such that it ensures that sendmail will always get a unique result.

The following rule can be used with the above declaration to look up the preferred mail address for a user:

R$* <@ $+ > $*       $: $(ldap $1 $: $1<@$2>$3 $)

Here we presume that this rule was preceded by a call to rule set 3 to focus on the host part. If the lookup succeeds, the new (unfocused) address is returned from the mail= line in the database. Otherwise, the original address is returned.

A few errors can occur during sendmail's startup that indicate a faulty K command:

LDAP map: -h flag is required
LDAP map: -b flag is required
No return attribute in map name

The first two show that those switches are mandatory. The third prints only to show that the -v switch is mandatory if the -o switch is absent.

In addition, each successful lookup can cause a line like the following to be logged via syslog(3):

qid: ldap key => value

See Table 33.5 in Section 33.3.4 for a list of the K command switches that can be used with this class and the meaning of each. Also, Table 33.6 lists several nonstandard switches that are used by this ldapx class. Finally, note that the ldapx class can be used only if LDAPMAP was defined when sendmail was compiled (see Section 18.8.15, LDAPMAP).

NetInfo is NeXT's implementation of a network-based information service. The netinfo class expects a map declaration to be of the following form:

Kname netinfo map

Other switches may be used with this class, and they have their normal meanings (see Table 33.5 in Section 33.3.4).

Support of netinfo maps is available only if you declare NETINFO when compiling sendmail (see Section 18.8.27, NETINFO).

Sun Microsystems offers a network information service called NIS. It provides the ability to look up various kinds of information in network databases. The nis class allows you to access that network information by way of rules in rule sets. You declare an nis class map like this:

Kname nis map

Here, name is the identifier that you will later use in rule sets. The map is any nis map. Lookups will occur in the default nis domain. If you wish to specify some other domain, you may append an @ character and the domain name to the map:

Kname nis map@domain

To illustrate, consider the need to look up the name of the central mail server for your department. If such a map were called mailservers, you could use the following configuration file line to look up your domain in that map:

Kmailservers nis -o mailservers
...
R$* <@ $+ > $*          $: $1<@$2>$3 <$(mailservers $2 $)>
R$* <@ $+ > $* <$+>     $#smtp $@ $4 $: $1 < @ $2 > $3
...

Here, we look up the host part of an address ($2) in the mailservers nis map. The -o makes the existence of the map optional. If the host part is found, it is rewritten to be the name of the mail server for that host. In the last rule we forward the original address to that server.

Without the -o, the nonexistence of a map will cause this error to be logged:

Cannot bind to map name in domain domain: reason here

If nis is not running at all or if sendmail cannot bind to the domain specified or the default domain, the following error is logged:

NIS map name specified, but NIS not running

The nis class is available only if sendmail is compiled with NIS defined (see Section 18.8.29, NIS).

See Table 33.5 in Section 33.3.4 for a list of the K command switches that can be used with this class and the meaning of each.

Sun Microsystems's NIS+ is a complete redo of its earlier nis system. The nisplus class allows you to look up information using NIS+. The form of that class declaration looks like this:

Kname nisplus map.domain

Here, the map is a NIS+ map name, such as mail_aliases. [7] If the domain or .domain is missing, the nisplus default domain is used. If the entire map.domain is missing, the default becomes mail_aliases.org_dir. The domain org_dir contains all the systemwide administration tables.

[7] Note that under NIS+ map names cannot contain a dot, whereas under NIS they could - for example, mail_aliases for NIS+ but mail.aliases for NIS.

Any lookup failures that can be retried will automatically be retried up to five times, with a sleep(3) of 2 seconds between each try. If the map.domain doesn't exist in the local nisplus system, this error is printed when sendmail starts:

Cannot find table map.domain: reason for failure here

This error is suppressed if the original K command declaration included the -o switch.

Two other errors can happen during startup:

map.domain: reason for failure here is not a table
nisplus_map_open(map): can not find key column -k column name here

You can use the -k switch to specify a key column to look up. Under nisplus, columns are named, so the -k must be followed by a valid name, or the last error above will be printed. You can also use the -v switch to specify the value column, also a name. If the -v is omitted, the last column becomes the default. See Table 33.5 in Section 33.3.4 for a list of the other K command switches that can be used with this class and the meaning of each.

The null class is an internal database that always returns a failed lookup. It is useful for replacing other classes to force failures without errors. Normally, the null class is used internally only.

Consider a tiny configuration file that does not need the use of the aliases facilities. One way to declare aliases would be like this:

O AliasFile=null:

This tells sendmail to use the null class for looking up aliases. Therefore no aliases will ever be found.

None of the K command switches may be used with the null class. If you try to use any, they will be silently ignored. No debugging switch is available to watch this null class.

The program class allows you to perform lookups via arbitrary external programs. The form for the declaration of this class looks like this:

Kname program /path arg1 arg2 ...

The /path must be the full pathname to the program. Relative paths will not work, and attempts to use them will log the following error and cause the lookup to fail:

NOQUEUE: SYSERR(bcx): relative name: cannot exec: No such file or directory

The program is run under the uid and gid of the person who ran sendmail. But if that person is root, the program is instead run as the user and group specified by the DefaultUser(u) option (see Section 34.8.15, DefaultUser (g)(u)).

The arguments to the program always have the key to be looked up added as a final argument:

Kname program /path arg1 arg2 ...
                                    
                                    key added here

This is the only way that the key can be passed to the program. The key will specifically not be piped to the program's standard input.

The value (result of the lookup) is read from the program's standard output. Only the first MAXLINE-1 characters are read (where MAXLINE is defined in conf.h, currently as 2048). The read result is processed as an address and placed into the workspace (unless the -m switch is used with the K command).

To illustrate, consider the need to look up a user's preferred address in an external relational database:

Kilook program /usr/lib/ingres_lookup -d users.database

This program has been custom written to accept the key as its final argument. To prevent spurious errors, it exits with a zero value whether the key is found or not. Any system errors cause it to exit with a value selected from those defined in <sysexits.h> (those recognized by sendmail). Error messages are printed to the standard error output, and the found value (if there was one) is printed to the standard output.

In general, it is better to use one of the database formats known to sendmail than to attempt to look up keys via external programs. The process of fork(2)ing and exec(2)ing the program can become expensive if it is done often, slowing down the handling of mail.

See Table 33.5 in Section 33.3.4 for a list of the K command switches that can be used with this class and the meaning of each.

The sequence class is a more general form of the implicit class described for use with alias files. The sequence class allows you to declare a single name that will be used to search a series of databases. It is declared like this:

Kname sequence map1 map2 ...

Here, a key will be looked up first in the map named map1, and if not found there, it will be looked up in the map named map2. The class of each of the listed maps should logically relate but need not be the same. Consider, for example, a rule's LHS that will match if the workspace contains either a user's login name or the name of a host, with the hostname taking precedence:

Khosts host  -a+ /etc/hosts
Kpasswd user -a- /etc/passwd
Kboth sequence hosts passwd

R$-     $: $(both $1 $)

Here, we say that the map named both is of type sequence. Any single token in the LHS will be looked up first in the map named hosts, and if it is found there the hostname will be returned with a + appended. If it is not found in the hosts map, it will be next looked up in the passwd map. If it is found there, the original workspace will be returned with a - appended. If the workspace is not found in either map, the lookup fails and the workspace remains unchanged.

If any map in the series of maps declared with the K command does not exist:

Kboth sequence hosts passwd badname

the following error is printed, and that map is ignored:

Sequence map both: unknown member map badname

If the number of maps that are sequenced exceeds the maximum allowed (MAXMAPSTACK in conf.h, currently 12), the following error is printed, and the overflow of maps is ignored:

Sequence map name: too many member maps (max max)

None of the K command switch may be used with the sequence class. If you try to use any, they will be wrongly interpreted as map names.

The stab class is used internally by sendmail to load the raw aliases(5) file into its internal symbol table. [8] This is a fallback position that is taken if no database form of aliasing is found.

[8] As such it is somewhat misnamed. One might reasonably expect a class named stab to provide access to the symbol table, but alas, it is not so.

The stab class should never be used in configuration files.

The switch class is used internally by sendmail to create sequence classes of maps based on external service-switch files. Recall that the lines inside a service-switch file look like this:

service  how how

as, for example:

aliases   files nis

This line tells sendmail to search for its aliases first in files then in NIS.

To illustrate the switch class, consider the need to look up aliases inside rule sets in the same way that sendmail looks up its own aliases. To do this, you would declare a switch map, as, for example:

Kali switch aliases

This causes sendmail to search for the service named aliases in the service-switch file. In this example it finds such a line, so for each how that follows the aliases in that line, sendmail creates a new map with the name ali followed by a dot and the how: [9]

[9] Your switch map declaration references the new maps named ali.files and ali.nis. These must be declared before the switch map is declared. Note that switch map declarations always reference other map names!

aliases   files    becomes    ali.files
aliases   nis      becomes    ali.nis

These named maps are then sequenced for you. Recall that sequence maps are declared like this:

Kname sequence map1 map2,...

The name given to the sequence is ali. In our example the following sequence is automatically created for you from your original switch declaration:

Kali sequence ali.files ali.nis

In rule sets, when you look up aliases with the ali map:

R...        $( ali $1 $)
                
          the sequence named ali

you will use the sequence named ali that was automatically built for you from a combination of your original switch definition and your service-switch file's aliases line. That is, you declare a switch, but you use a sequence.

The text class allows you to look up keys in flat text files. This technique is vastly less efficient than looking up keys in real databases, but it can serve as a way to test rules before implementing them in database form.

For the text map, columns for the key and value are both measured as an index. That is, the first column is number 0. To illustrate, consider the following miniconfiguration file that can be used to check spelling:

Kspell text /usr/dict/words
Spell
R$-      $: $( spell $1 $: not in dictionary $)

The /usr/dict/words file contains only a single column of words. The above rule shows that the key is (by default) the first column (index 0). And the value is (by default) also the first column (index 0).

For more sophisticated applications you can specify the key's column (with the -k switch) the value's column (with the -v switch) and the column delimiter (with the -z switch). To illustrate, consider the need to look up a uid in the /etc/passwd file and to return the login name of the user to whom it belongs:

Kgetuid text -k2 -v0 -z: /etc/passwd
R$-      $: $( getuid $1 $)

The lines of a password file look like this:

ftp:*:1092:255:File Transfer Protocol Program:/u/ftp:/bin/sh

The third column (where the columns are separated by colons) is the uid field. The first is the login name. Note that the -k and -v switches show these fields as indexes, where the first is 0 and the third is 2.

See Table 33.5 in Section 33.3.4 for a list of the K command switches that can be used with this class and the meaning of each. No debugging switch is available to watch this text class.

The userdb class allows you to look things up in the User Database (see Section 33.5, "The User Database" for a full description of the User Database). The userdb class is declared with the K command like this:

Kname userdb switches keyword      V8.7 and above

Here, the keyword is the name of the field to search and is either maildrop or mailname (see Section 33.5). There is no need to list any files or servers with this command. Those should already have been declared with the UserDatabaseSpec (U) option (see Section 34.8.75).

One possible use for a userdb map might be to check for a local account in the check_rcpt rule set (see \#sRULESETS_check_rcpt). In this example, all valid incoming recipient addresses are listed with the User Database:

Kislocal userdb maildrop
Scheck_rcpt
R$*                     $: $>3 $1               focus on host
R$* <@ $+ > $*          $: $1                   discard host
R$+                     $: $(islocal $1 $: nope $)
Rnope                   $#error $@ 5.1.3 $: "Recipient is not local"

See Table 33.5 in Section 33.3.4 for a list of the K command switches that can be used with this class and the meaning of each. The -d28 debugging switch (see Section 37.5.97, -d28.1) can be used to watch this userdb class in action.

The user class is used to look up passwd(5) information using getpwent(3). A password entry typically looks like this:

ftp:*:1092:255:File Transfer Protocol Program:/u/ftp:/bin/sh

Here, there are seven fields, each separated from the others by colon characters. The key is always compared to the first field. The value returned is (by default) the first field unless you specify another field with a -v switch:

Kname user -vfield

Here, field can be either a number 1 through 7 or one of the names name, passwd, uid, gid, gecos, dir, or shell, which correspond to the numbers. For example, to look up usernames and get the full name (gecos) field returned, you could use something like this:

Kgetgecos user -vgecos
...
R$-        $: $( getgecos $1 $)

Note that this returns the full gecos field in its rawest form. It is not cleaned up to provide a reliable full name, as is the $x macro (see Section 31.10.42, $x).

One possible application for the user class is in conjunction with the check_rcpt rule set (see \#sRULESETS_check_rcpt). In the following we check to see whether a recipient is a local user and reject the mail if that is not so:

Kislocal user
Scheck_rcpt
R$*                     $: $>3 $1               focus on host
R$* <@ $+ > $*          $: $1                   discard host
R$-                     $: $(islocal $1 $: nope $)
Rnope                   $#error $@ 5.1.3 $: "Recipient is not local"

Here, we focus on the host part with rule set 3, then discard all but the user part in the second rule. The third rule performs the lookup. If the user is found, that username is returned unchanged. If, on the other hand, the user is not found, the token nope is returned. The last rule rejects any SMTP RCPT command that contains a nonlocal user part.

See Table 33.5 in Section 33.3.4 for a list of the K command switches that can be used with this class and the meaning of each.