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" 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),
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:
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:
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:
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.
