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COURIERTCPD(1) Double Precision, Inc. COURIERTCPD(1)

NAME

couriertcpd - the Courier mail server TCP server daemon

SYNOPSIS

couriertcpd [-pid= pidfile] [option...] {list} {program} {arg...}
couriertcpd {-pid= pidfile} {-stop}
couriertcpd {-pid= pidfile} {-restart}

DESCRIPTION

 
couriertcpd accepts incoming network connections, and runs program after establishing each network connection. The program´s standard input and output are set to the network connection.
 
list is a comma-separated list of TCP port numbers where incoming connections are created. program is the program to run. If program requires any arguments, they are specified on the command line, after program itself.
Before running program, couriertcpd initializes several environment variables that describe the network connection. The environment inherited by program will be the environment inherited by couriertcpd, plus any additional environment variables initialized by couriertcpd. It is also possible to reject certain network connections. Several options are available to specify which network connections will be rejected.

OPTIONS

-access= filename
Specifies an optional access file. The access file lists the IP addresses from which connections should be accepted or rejected. The access file is also used to initialize environment variables based on the IP address of the connection. filename is a GDBM or DB database file that´s usually created by a script from one or more text files. See "ACCESS FILE" below for more information.
-accesslocal
Lookup the local interface IP and port in the access file, in addition to looking up the remote IP. This gives a mechanism for setting environment variables depending on which IP address and/or port the client connected to. In the access file, "1.2.3.4.25" matches connections to IP address 1.2.3.4 port 25; "1.2.3.4" matches connections to IP address 1.2.3.4 on any port; and "*.25" matches connections to port 25 on any IP address.
-address= n.n.n.n
Accept network connections only to IP address n.n.n.n. If not specified, couriertcpd accepts connections to any IP address that the system accepts connections on. If the system has multiple network interfaces with separate IP addresses, this option makes couriertcpd accept connections only to one specific IP address. Most systems have multiple network interfaces: the loopback interface, plus the local network interface, so that -address=127.0.0.1 accepts connections only from the local system. When multiple port numbers are specified, it is also possible to selectively bind different network addresses to each port number when list specifies more than one port number. See " Multiple port list[1]" below for more information.
-block= zone[,var[/n.n.n.n][,msg]]
Initialize the environment variable var if both of the following conditions are true: var is not already initialized; the connecting IP address can be found in a DNS-based list. See DNS ACCESS LISTS, below. Multiple -block options can be used.
-denymsg= text
Specifies an optional message to be returned to the client if the -access option rejects them. The default is to drop the TCP connection without sending back any messages.
-drop= var
If the environment variable var is set to a nonempty value, terminate immediately. Do not run the program to handle the connection. See DNS ACCESS LISTS, below, for more information. var defaults to “BLOCK”, if not specified.
-group= group
Set couriertcpd´s its group ID. group may be specified numerically, or by its name. Only the superuser may use -group.
-listen= n
Length of the queue which holds pending connections. n is a number. If not specified, the system default is used.
-maxperc= n
Maximum number of connections accepted from the same C network block. Using this option is recommended, because connection slots are limited. Without this option, the same C network block can potentially use up all available connection slots.
-maxperip= n
Maximum number of connections accepted from the same IP address. Use both the -maxperc and -maxperip options to fine tune connection limits. For example, when couriertcpd is listening on the SMTP port it makes sense to set an upper limit on the number of connections from the same C block. Domains that send a large amount of mail often have multiple servers sending outbound mail from the same C block, so it makes sense to set limits on individual C blocks. On the other hand, if couriertcpd is listening on the POP3 port it makes more sense to set limits on individual IP addresses. If a C block of addresses is assigned to a dialup modem pool, it is certainly possible to have many IP addresses within the same C block have connections to the POP3 server at the same time.
-maxprocs= n
Maximum number of connection slots, or the maximum number of processes started. This effectively specifies the maximum number of connections accepted at the same time. After the maximum number of connections has been opened, couriertcpd waits for an existing connection to close, before accepting any more connections.
-warn= n
Log a LOG_WARNING message to syslog when the number of active processes exceeds n. The default is 90% of maxprocs. couriertcpd logs a LOG_ALERT syslog message when the number of active processes reaches the maximum.
-nodnslookup
Do not look up the hostname associated with connecting IP address and the local addres, do not initialize the TCPREMOTEHOST or TCPLOCALHOST environment variables (see below).
-noidentlookup
Do not perform an ident lookup, and do not initialize the TCPREMOTEINFO environment variable.
-pid= filename
If given, couriertcpd puts itself into the background and saves its process ID in this file, usually somewhere in /var/run.
 
This option must also be present when using the -restart and -stop options.
-restart
Send a SIGHUP to an existing couriertcpd process. Specify the same -pid argument as the one that was used to start couriertcpd. The process ID is read from the -pid file, and the couriertcpd receives a SIGHUP signal.
-stderr=socket
Set program´s standard error to the network connection, just like its standard input and output.
-stderr= logfile
Set program´s standard error to the specified file, logfile. The file is created, if necessary, and is opened in append mode.
-stderrlogger= logprogram
Set program´s standard error to a pipe, which is read by logprogram. Only one instance of logger is started, which receives standard error from every instance of program. The specified logger is executed with the output end of the stderr pipe connected as standard input. logprogram is executed with one argument - program´s name.
-stderrloggername=name
Use name as the argument to logprogram, instead of the program´s name.
-stop
Stop (kill) an existing couriertcpd process. Specify the same -pid argument as the one that was used to start couriertcpd. The process ID is read from the -pid file, and the couriertcpd process is killed. All child processes of couriertcpd will receive a SIGTERM signal.
-user= user
Set couriertcpd´s user ID. Also, the group ID is set to the user´s group ID. Using both -group and -user is not necessary. Only the superuser can specify -user.

MULTIPLE PORT LIST

The list argument can be a comma-separated list of multiple port numbers. couriertcpd will create network connections on any listed port. Each port number can be optionally specified as "address.port", for example:
 
couriertcpd -pid=/var/run/smtp.pid 127.0.0.1.25,999  program
This instance accepts network connections to either port 25 or port 999, however connections on port 25 are created only on the IP address 127.0.0.1, the loopback interface.
Whenever an IP address is not specified, network connections are accepted to any IP address (called "wildcarding"). On IPv6-capable systems, couriertcpd will attempt to create two incoming network connection ports, if an IP address is not specified. After creating the first port as an IPv6 wildcard port, couriertcpd will then attept to create an IPv4 wildcard port, with the same port number. Some BSD-derived systems must use separate IPv6 and IPv4 wildcard ports to create incoming network connections. Most other systems only need an IPv6 port to create both IPv6 and IPv4 incoming network connections. couriertcpd quietly ignores a failure to create an IPv4 wildcard port, as long as an IPv6 wildcard was succesfully created.
The -address option can be used to default a specific IP address for every listed port number. For example:
 
couriertcpd -pid=/var/run/smtp.pid 127.0.0.1.25,127.0.0.1.999  program
and
 
couriertcpd -pid=/var/run/smtp.pid -address=127.0.0.1 25,999  program
will create network connections on ports 25 and 999 of the IP address 127.0.0.1.

ACCESS FILE

The access file lists IP addresses that couriertcpd will accept or reject connections from. An access file is optional. Without an access file couriertcpd accepts a connection from any IP address.
Both IPv4 and IPv6 addresses can be specified, if IPv6 support is available. A non-standard syntax is currently used to specify IPv6 addresses. This is subject to change in the near future. IPv6 support is currently considered to be experimental.
The access file is a binary database file that´s usually created by a script, such as makesmtpaccess(8)[2], from one or more plain text files. Blank lines in the text file are ignored. Lines that start with the # character are also ignored.

Rejecting and accepting connections by IP address

The following line instructs couriertcpd to reject all connections from an IP address range:
 
netblock<tab>deny
netblock is an IP address, such as 192.68.0.2. <tab> is the ASCII tab character. There MUST be exactly one tab character after the IP address and the word "deny".
You can also block connections from an entire network C block:
 
192.68.0<tab>deny
This blocks connections from IP addresses 192.68.0.0 through 192.68.0.255. Blocking connections from an entire B or A network block works the same way.
Use the word "allow" instead of "deny" to explicitly allow connections from that IP address or netblock. For example:
 
192.68.0<tab>deny
192.68.0.10<tab>allow
This blocks all connections from 192.68.0.0 to 192.68.0.255 except for 192.68.0.10. These two lines can occur in any order. couriertcpd always uses the line with the most specific IP address.
If the IP address of the connection is not found in the access file the connection is accepted by default. The following line causes unlisted connections to be rejected:
 
*<tab>deny

IPv6 addresses

 
Note
 
 
IPv6 support in the access file is experimental, and is subject to change in a future release. The following syntax is subject to change at any time.
 
The access file can also specify IPv6 addresses, if IPv6 support is available. The existing IPv4 address format is used for IPv6-mapped IPv4 addresses, and no changes are required. For all other IPv6 addresses use the following format:
 
:hhhh:hhhh:hhhh:hhhh:hhhh:hhhh:hhhh:hhhh<tab> action
The IPv6 address must begin with :. The initial : character is not really a part of the IPv6 address, it is only used to designate this record as an IPv6 address, allowing an access file to contain a mixture of IPv4 and IPv6 addresses. The IPv6 address follows the initial : character, and it must be spelled out using zero-padded lowercase hexadecimal digits. For example:
 
:0000:0000:0000:0000:0000:f643:00a2:9354<tab>deny
Netblocks must be specified using even-word boundaries only:
 
:3ffe<tab>deny
This will deny entire 3ffe::/16 (6bone network, which is phased out).
 
:2002:c0a8<tab>deny
This will deny 2002:c0a8::/32 (6to4 addresses derived from private address space).

Setting environment variables

 
allow can be optionally followed by a list of environment variable assignments, separated by commas. The environment variables are set before executing program or checking access lists (see below). For example:
 
192.68.0<tab>allow,RELAYCLIENT
192.68.0.10<tab>allow,RELAYCLIENT,SIZELIMIT=1000000
This sets RELAYCLIENT environment variable for connections from the 192.68.0 block. In addition to that, the SIZELIMIT environment variable is set to 1000000 if the connection comes from the IP address 192.68.0.10.
Note that RELAYCLIENT must be explicitly specified for the IP address 192.68.0.10. The first line is NOT used for connections from this IP address. couriertcpd only reads one entry from the access file, the entry for the most specific IP address.

DNS ACCESS LISTS

An alternative to listing banned IP addresses is to use an external DNS-based IP access list.
There is no provision to support IPv6-based lists, because none yet exist. IPv6-based access list support will be added in the future.
 
couriertcpd´s default configuration does not automatically reject connections from any IP address listed on a DNS-based list. If the connecting IP address is listed couriertcpd simply sets an environment variable. It´s up to the program, run by couriertcpd, to read the environment variable and choose what to do if the environment variable is set.
Please note that if the environment variable is already set, couriertcpd will NOT search the access list. This can be used to override the access list where program only recognizes the access list if the environment variable is not empty. By setting the environment variable to an empty string in the access file (see above), you can override access lists for selected IP addresses.
The -block option queries a DNS list for each connecting IP address. The only required argument to -block is the DNS zone that is used to publish thelist. The name of the zone can optionally be followed by a comma and the name of the environment variable to set if the DNS list includes the IP address. couriertcpd sets the environment variable BLOCK if you do not specify the name yourself.
The name of the environment variable can be optionally followed by a slash and an IP address. Normally couriertcpd sets the environment variable if the access list includes any A record entry for the specified IP address. Some access lists may offer additional information by returning one of several possible A records. If the name of the environment variable is followed by a slash and an IP address, the environment variable will be initialized only if the access list includes an A record containing the indicated IP address.
The contents of the environment variable will be the contents of any TXT record for the listed IP address. var[/n.n.n.n] can be optionally followed by a comma and a text message, which will be used instead of the TXT record. The text message may include a single @ character somewhere in it, which will be replaced by the listed IP address.
When the -drop option is given in addition to -block, couriertcpd drops the connection, rather than running the program. First, all -block options are processed and the environment variables are set, based on the results of any matching DNS lookups. The -drop gets processed after all DNS lookups. -drop takes a list of comma-separated environment variables (if not specified, BLOCK is the default list). If any environment variable named by the -drop option is set to a non-empty string, couriertcpd drops the connection instead of executing the program.

MULTIPLE DNS LISTS

Multiple -block options can be used. The connecting IP address gets looked up in multiple access lists. This is implemented as follows.
 
couriertcpd processes all -block options one at a time. If the indicated environment variable is already set, couriertcpd skips the DNS list lookup (this is also true if only one -block option is specified). Therefore, if multiple -block options are used, and an IP address is found in the first access list, the remaining lists that use the same environment variable will not be checked. But other lists that use a different environment variable WILL be checked.
The same zone can be specified more than once, with different environment variables and different IP addresses. For example:
 
couriertcpd -block=block.example.org,BLOCK1/127.0.0.2 \
            -block=block.example.org,BLOCK2/127.0.0.3
If the specified access list contains an A record for the listed address, and the A record contains the IP address 127.0.0.2, couriertcpd initializes the BLOCK1 environment variable. If the A record contains the IP address 127.0.0.3, couriertcpd initializes BLOCK2. If both records are present, both variables are initialized.
 
couriertcpd uses the following logic to determine what kind of DNS query to issue:
If neither the IP address, nor msg is specified, couriertcpd will query for existence of TXT records, for the IP address.
If only msg is specified, couriertcpd looks up the existence of A records, for the IP address.
If /n.n.n.n is used, and msg is not specified for at least one -block option for this same zone, couriertcpd will query for existence of ANY records, which should return both TXT and all the A records for this IP address.
If /n.n.n.n is used, and msg is specified for every -block option for this same zone, couriertcpd will query for existence of A records only.

ENVIRONMENT VARIABLES

 
couriertcpd also initializes the following environment variables prior to running program:
TCPLOCALHOST
The name of the host on the local end of the network connection, looked up in DNS. TCPLOCALHOST will not be set if the IP address of the network connection´s local end cannot be found in DNS, or if -nodnslookup option is specified. TCPLOCALHOST will be set to the string softdnserr if the DNS lookup fails with a temporary error (so you cannot tell if the IP address has a valid host name associated with it), or if the reverse and forward DNS lookups do not match. TCPLOCALHOST will not be set if the reverse DNS lookup fails completely.
TCPLOCALIP
The IP address of the local end of the network connection.
TCPLOCALPORT
Rhe number of the port of the local end of the network connection.
TCPREMOTEHOST
The hostname of the connecting host. Like TCPLOCALHOST, but for the connecting IP address.
TCPREMOTEIP
Connecting IP address.
TCPREMOTEINFO
Identification string received from the IDENT server on the remote IP address. Not set if the IDENT server returned an error, or if the -noidentlookup option was specified.
TCPREMOTEPORT
TCP port of the remote end of the network connection.

SEE ALSO

 
courier(8)[3].

AUTHOR

Sam Varshavchik
Author

NOTES

1.
Multiple port list
[set $man.base.url.for.relative.links]/#list
2.
makesmtpaccess(8)
[set $man.base.url.for.relative.links]/makesmtpaccess.html
3.
courier(8)
[set $man.base.url.for.relative.links]/courier.html
04/04/2011 Courier Mail Server