screend(8) System Manager's Manual screend(8)
Name
screend - Internet (IP) gateway screening daemon
Syntax
/usr/etc/screend [ -d ] [ -c ] [ -l ] [ -f configfile ] [ -L logfile ] [ -s ] [ -r ]
Description
The daemon is used in conjunction with the gateway screen facility to decide which IP packets should be forwarded when the system is acting
as an IP gateway. Only the super-user may run this program.
Before using you must add the following line to your system configuration file:
pseudo-device gwscreen
After editing the system configuration file you must rebuild the kernel, and reboot the system. For information on rebuilding the kernel
see the Guide to Configuration File Maintenance and the reference page.
When is started, it reads the configuration file specified (configfile) and then processes packets according to the instructions in that
file.
The kernel screening facility must be enabled using before has any effect. (When screening is disabled, packets are forwarded by the ker-
nel according to its usual procedures.)
It is possible to run more than one copy of at a time, although it is not recommended. You might do this, however, when the configuration
file is changed. Because the configuration file is read only at program startup, you must restart when the file is changed. To avoid any
service interruption, you should start a new instance of before killing the old one.
Options
-d Prints large amounts of debugging information. This is not meant for normal use.
-c Checks the syntax of the configuration file but does not actually process any packets.
-l Turns on logging for all packets (normally, packets are logged only if requested in the configuration file).
-f configfile
Specifies the configuration file. If not specified, the default, is used.
-L logfile
Specifies that logging records should be appended to the given file. There is no default logging file. This may be specified simul-
taneously with in which case each logging record is stored by both means.
-s Specifies that logging records should be logged using
-r Specifies that logging records should include the rule number of the configuration file rule responsible for the action logged. This
is useful for debugging configuration file problems. See the section in this reference page on Rule Numbers.
Configuration File
This is an informal guide to the grammar of the configuration file. It is intended for readers who are familiar with the basic concepts of
the IP protocol family, including the distinction between the terms ``network'' and ``subnet.''
Lexical structure:
Comments
Can either be C-style comments, delimited by /* and */ or csh-style comments begun with a number sign (#) and terminated by
the end of a line. Comments do not nest.
Case Significant in reserved words (all are lower-case). This is actually a benefit, because if a host name happens to conflict
with a reserved word, you can use the host name in upper-case.
Host names
Must begin with a letter but may contain digits, minus signs (-), dots ( . ), and underscores (_ ). The same is true of net-
work, subnet, and netmask names. Hosts can also be identified by their IP address, in dotted quad notation (for example,
``128.45.40.15'').
Numbers
May be in decimal or in hex (0x0 notation). Octal notation is not allowed. Decimal notation is the preferred method.
Protocol names
Specified as they are found in These can also be given as numbers.
Port names
For TCP or UDP, specified as they are in These can also be given as numbers (host byte order).
ICMP type codes
Must be chosen from the following list, or given as numbers:
echo echoreply sourcequench redirect unreachable timeexceeded parameterprob-
lem timestamp timestampreply informationrequest informationreply addressmaskrequest addressmaskreply
White space
All white space is the same (including newlines).
General syntax rules:
The configuration file consists of specifications terminated by semicolons.
There are three kinds of specifications:
default-action specification
There should only be one of these (the last one is the one that counts); it specifies what action to take if no action speci-
fication matches a packet.
subnet mask specifications
Specifies the subnet mask used for a given network.
action specifications
Specifies a class of packets and the action to take when such a packet is received.
Specifications can appear in any order, but the evaluation order of action specifications is the order in which they appear in the
file.
In BNF, this is:
<configuration-file> ::= { <specification> | <configuration-file> <specification> }
<specification> ::= { <default-action> | <subnet-spec> | <action-spec> }
The syntax for a default-action specification is:
<default-action> ::= default {accept | reject} [notify] [log] ;
Note that is not legal. If not specified, the default-action is
The syntax for subnet mask specifications is:
<subnet-spec> ::= for <network> netmask is <maskval> ;
The <network> is either a network name or a dotted-quad address, such as ``36.0.0.0''. The number ``36'' is not a reasonable value.
<Maskval> is either a name (treated as a hostname) or a dotted-quad address, such as ``255.255.255.0'' (bits are on for the network and
subnet fields.)
The syntax for action specifications is:
<action-spec> ::= from <object> to <object> {accept | reject} [notify] [log] ;
Such a specification says that packets flowing this way between this pair of objects (defined below) should either be accepted or rejected.
If is specified, when a packet is rejected an ICMP error message is returned to the source. If is specified, this packet and its disposi-
tion are logged.
Conceptually, for each packet the action specifications are searched in the order they appear in the configuration file, until one matches.
The specified action is then performed. If no specification matches, the default action is performed.
To simplify the configuration file, the following syntax may be used to indicate that the same action should be performed on packets flow-
ing in either direction between the specified pair of objects:
<action-spec> ::= between <object> and <object> {accept | reject} [notify] [log] ;
Note that this has the same effect as specifying the two unidirectional rules, with the forward direction listed first.
An object is a specification of the source or destination of a packet. The syntax for object specifications is somewhat complex, since
certain fields are optional:
<object> ::= { <address-spec> | <port-spec> | <address-spec> <port-spec> }
If the <address-spec> is not given, any host will match. If the <port-spec> is not given, any protocol and port will match.
<address-spec> ::= { <net-spec> | <subnet-spec> | <host-spec> | any }
<net-spec> ::= { net <name-or-addr> | net-not <name-or-addr> }
<subnet-spec> ::= { subnet <name-or-addr> | subnet-not <name-or-addr> }
<host-spec> ::= { host <name-or-addr> | host-not <name-or-addr> }
The convention means that the object specification matches if the specified field does not have the specified value. In the following
example, packets not from nic.ddn.mil are dropped.
from host-not nic.ddn.mil to host any reject;
The ``subnet'' and ``subnet-not'' forms match against the entire address under the subnet mask (for example, if the netmask for net 36 is
``255.255.0.0'', then ``subnet 36.8.0.0'' matches a packet address of 36.8.0.1).
<name-or-addr> ::= { <name> | <dotted-quad> | any }
<port-spec> ::= { proto <proto-name-or-number>
| icmp type <type-name-or-number> | icmp type-not <type-name-or-number>
| tcp port <port-name-or-number> | tcp port-not <port-name-or-number>
| udp port <port-name-or-number> | udp port-not <port-name-or-number> }
<proto-name-or-number> ::= { <name> | <number> }
<type-name-or-number> ::= { <name> | <number> | any | infotype }
<port-name-or-number> ::= { <name> | <number> | any | reserved | xserver }
``Reserved'' ports are those reserved by 4.2BSD Unix for privileged processes. ``Xserver'' ports are those used by X11 window system
servers. ``Infotype'' ICMP packets are those that are purely informational: echo, timestamp, information, and addressmask requests, and
the corresponding replies.
Restrictions
IP gateways are allowed to fragment IP datagrams if they are too large to be forwarded in one piece. Only the first fragment of a datagram
carries enough information to make certain kinds of accept/reject decisions. The daemon can only handle fragments if it sees the first
fragment of a datagram before it sees any subsequent fragments. Also, only a limited rate of fragmented packet arrival can be accommodated
by the program (fragmentation is, in general, a bad idea). Finally, if more than one instance of is running, most likely this will result
in significant loss of fragments.
The current implementation does not forward packets that contain IP header options. This is because several of these options can be used
to subvert checks based on the IP header destination address.
If a host name given in an object specification has more than one IP address associated with it, does not understand that all these
addresses should be checked. Only the first (primary) address of the host is used. This may lead to erroneous operation in some cases
(possibly including a security hole), so a warning is printed if the configuration file contains such names. (Note that you probably will
not see this warning if is only started in
Examples
This following is an example of the syntax; it is not intended to be used in an actual installation:
# Example configuration file
default reject;
for 36.0.0.0 netmask is 255.255.0.0;
from subnet 36.8.0.0 to net milnet reject notify;
from host nic.ddn.mil to host any accept;
from host any to net arpanet tcp port telnet accept;
from host any to host any icmp type redirect reject log;
from host any to subnet 36.10.0.0 tcp port-not reserved reject;
Rule Numbers
If the option is given, log records contain a notation of the rule number responsible for the action being logged. A rule is a ``from ...
to ...'' specification in the configuration file; rules are numbered in order starting with zero. Note that ``between ... and ...'' speci-
fications expand to two ``from ... to ... '' rules, each numbered individually. The default action, whether explicitly stated or not, is
not numbered; it is referred to distinctively in the log.
Diagnostics
During argument processing and configuration file parsing, various diagnostics may be issued. During normal operation, only serious inter-
nal inconsistencies result in diagnostics. (See the Restrictions section about warning messages in some borderline cases.) Except in
debug mode ( most diagnostics are logged using
Once an hour, a statistics report is made using that shows the number of packets processed since the program was started, the hit rate of
an internal cache buffer, and the number of packets dropped because they arrived too rapidly.
Files
default configuration file
See Also
screen(2), screenmode(8), screenstat(8)
screend(8)