As of: C-Kermit 8.0, 12 Dec 2001
This page last updated: 12 Dec 2001

The Internet Kermit Service Deamon (IKSD) is C-Kermit 7.0 or later, started in a special way. At this writing, only the UNIX version of C-Kermit can act as IKSD, but IKSD will also be available soon in the Windows version, and can be added to the VMS, VOS, and other versions if there is sufficient demand. The Internet Kermit Service is specified in RFC2839 and RFC2840.

CONTENTS

  1. Why IKSD?
  2. System Configuration
  3. Compile-Time Configuration Options
  4. Runtime Configuration Options
  4.1. Command-Line Options
  4.2. System Logging
  4.3. The IKSD Log File
  4.4. IKSD Configuration File
  4.5. Initialization File
  5. Access to Services
  5.1. Automatic Settings
  5.2. Authentication
  5.3. The DISABLE Command
  5.4. Shell Access
  5.5. Anonymous Users
  5.6. Management Information
  6. Open Issues
  6.1. Connection Establishment
  6.2. Shell Access
  6.3. External and/or non-Kermit Protocols
  6.4. Additional Administrative Controls
  6.5. Known Bugs
  7. Monitoring
  7.1. Database Record Format
  7.2. The Display Module
  7.3. Database Management
  8. Testing

1. Why IKSD?

• Like FTP, Kermit provides a service that can be accessed from many different platforms with a consistent set of commands, but unlike FTP, these commands include programming constructions such as variables, arrays, looping and selection mechanisms, and local and remote procedure calls, thus allowing file transfer and management operations of arbitrary complexity to be easily automated.

• Like FTP, Kermit provides both text- and binary-mode data transfer, as well as file management capabilities. But Kermit also offers numerous features lacking from FTP, such as:
  • character-set translation;
  • flexible file selection mechanisms;
  • preservation of timestamps and permissions;
  • automatic per-file text/binary mode switching (important in cross-platform transfers);
  • cross-platform directory-tree transfer; and:
  • a selection of file collision options including "make backup copy of existing file and accept incoming file", "reject incoming file", "accept incoming file only if newer than existing file", etc.

• Unlike FTP, Kermit supports atomic file movement, in which the source file can be deleted (or renamed, or moved) when and only when it has been transferred successfully, as part of the transfer operation itself.

• Unlike most FTP implementations, Kermit supports recovery of interrupted transfers from the point of failure. You can even use it to recover interrupted FTP transfers when FTP itself does not offer this option.

• Unlike standard FTP, Kermit can transfer data through firewalls or proxies or NATs on a single well-known port (unless the TCP port for the connection has been explicitly filtered).

• Unlike FTP, Kermit can transfer data across a combination of transports (e.g. dial-up to a terminal server and thence to an Internet host).

• Authentication and data transfer can take place over secure connections (mutually authenticated and encrypted) using established Telnet authentication and encryption options.

• Unlike traditional Kermit use over Telnet, anonymous access is possible, and the considerable overhead of the intervening Telnet server and pseudoterminal service is eliminated.

• Unlike the FTP daemon (ftpd), the IKSD can be monitored in real time by the system administrator (see Section 7).

A prototype IKSD is available for public access at:

  kermit.columbia.edu port 1649

This service is based in part on a new Telnet Kermit Option described in

  RFC2839
  RFC2840

2. System Configuration

Directory organization, file system, naming conventions, and other administrative details vary from system to system and site to site, so specific instructions can not be given here. In general, however, the steps are:

1. The C-Kermit executable should be copied to the directory that is normally used for launching Internet services, and renamed to "iksd". Normally the owner and group should be root and the permission 0750.

2. The following entry should be added to the Internet services file, normally /etc/services:

     kermit   1649/tcp
   

3. An entry must be made in the inetd configuration file, normally /etc/inetd.conf. See your system-specific documentation for the name, location, and format of this file, e.g. "man inetd" and "man inetd.conf". Typical example:

     kermit  stream  tcp  nowait  root  iksd -A other-iksd-options
   

   Or, with TCP wrappers:

     kermit  stream  tcp  nowait  root  tcpd iksd -A other-iksd-options
   

   Include full pathnames for tcpd and iksd if required. See Section 4 for iksd-options. NOTE: Different platforms might require different syntax, e.g.:

     kermit stream tcp nowait root /usr/sbin/iksd iksd -A other-iksd-options
   

4. When using TCP wrappers, make the appropriate entries in its configuration file(s), such as /etc/hosts.allow and /etc/hosts.deny. See the TCP wrappers documentation for further info.

5. Make any required hard or soft links to the iksd executable.

6. If desired, create an IKSD configuration file (/etc/iksd.conf by default).

This setup allows multiple simultaneous IKSD sessions. A fresh IKSD process is started for each session, and disappears when IKSD exits, automatically closing the session and the connection.

To kill a particular instance of IKSD, find its pid with "ps ax | grep iksd" (or the equivalent command to list all processes) and then "kill -9 pid" (as root).

To update the IKSD program, simply replace the executable in the "launch" directory ((a) above).

To discontinue IKSD service quickly, delete the iksd executable, or (to avoid unwanted console logging) replace it with (say) /bin/false.

3. Compile-Time Configuration Options

The following compile-time options are specific to or of special relevance to the IKSD:

TNCODE - Required for IKS.

Include Telnet protocol code, even if networking code is not included. Defined by default if TCPSOCKET defined, otherwise must be defined explicitly.

IKS_OPTION - Required for IKS.

Include code for the new Internet Kermit Service Option. Currently defined automatically if TNCODE is defined.

CK_LOGIN - Required for IKS.

Include user authentication code. Defined automatically for UNIX unless NOLOGIN defined.

CK_PAM

The IKS is to be compiled to use PAM (Pluggable Authentication Modules) to authenticate the user for interactive logins instead of Unix password files. The default PAM service type for IKSD is "kermit". Might also require -lpam.

PAM_SERVICE_TYPE

If CK_PAM is defined and you wish to use a service type other than "kermit" you can define PAM_SERVICE_TYPE to equal the string you wish to use.

CK_SHADOW

If your host utilizes the shadow password system for storing user passwords the IKS must be compiled with support for shadow passwords. This might also require some additional libraries added the LIBS clause, such as -lgen, for loading the getspnam() routine.

CKSYSLOG

Include code for making syslog entries. This symbol is defined automatically if (a) CK_LOGIN is defined; (b) UNIX is defined; and (c) NOSYSLOG is not defined.

CKWTMP

Include code for making wtmp log entries. This symbol is defined automatically if (a) CK_LOGIN is defined; (b) UNIX is defined; and (c) NOWTMP is not defined.

WTMPFILE

String containing pathname of the default wtmp logfile. If not specified, a default is supplied, currently /var/log/wtmp on Linux and /usr/adm/wtmp elsewhere. The wtmp file can also be specified at runtime; see next section.

CK_CURSES

Need not be defined in an IKSD-only build, since curses is used only in local mode, and the IKSD is always in remote mode, plus the fact that curses initialization can cause spurious error messages in IKSD since it might not have a terminal type (TERM variable) in its environment.

NOLOCAL

This may be defined to exclude all code relating to the making of both serial and network connections from the C-Kermit/IKSD executable. This code is presently not needed, and is never used, in the IKSD. Compiling it out reduces the size of the executable and eliminates any possibility that a loophole could be exploited to use the IKSD as a relay.

NOPUSH

This symbol may be defined to exclude all code that is used to access local shell or system functions, such as the RUN (!) command, the server end of the REMOTE HOST command, file-transfer pipes and filters, etc. Note that such code is disabled at runtime in the IKS anyway; compiling it out completely reduces the size of the executable and the risk of loopholes.

NOICP

Disables the interactive command parser as well as long-form command-line options (Section 4.1). In this configuration, the IKS runs only in Kermit Server mode, and the size of the executable is reduced dramatically. WARNING: NOICP also inhibits extended command-line options (the ones that start with "--"), since these share the same parsing routines as the interactive parser.

NOSPL

Disables the script programming language (INPUT, OUTPUT, FOR, WHILE, IF, SWITCH, macros, variables, etc) without disabling the interactive command parser.

NOGETUSERSHELL

Add this if the link steps fails because it can't resolve getusershell() and/or endusershell(). Normally, IKSD will refuse a login if the user's login shell is not one of those returned by getusershell() (normally, getusershell() simply lists the shells listed in /etc/shells, but we can't depend on this). Thus if your site "invalidates" user IDs by setting their login shell to something that is not really a shell (like /etc/nologin), but does not have a getusershell() function, IKSD will still let such users log in unless (a) you put them in the forbidden users file (see Section 4.1), or (b) add some code to ckufio.c to check for whatever invalid or valid shells might apply at your site.

NOIKSD

This defines the symbols NOLOGIN, NOSYSLOG, NOWTMP, NOPRINTFSUBST, which in turn prevent the definition of symbols (CK_LOGIN, CKSYSLOG, etc) that cause IKSD-specific code to be included.

CK_SYSINI

CK_DSYSINI

System-wide initialization file; see Section 4.4.

IKSD

Is defined automatically if NOIKSD is not defined.

IKSDCONF

The name of the IKSD configuration file. In UNIX this is "/etc/iksd.conf" by default; elsewhere "iksd.cfg".

In an IKSD-only build, you should omit the termcap/termlib and (n)curses libraries, and set up the link step to make sure the IKSD is linked with the same user authorization libraries used by your local login program. Here is a sample for SunOS 4.1.3 at a particular site:

sunos41giks:
	@echo Making C-Kermit $(CKVER) for SunOS 4.1 with gcc for IKS...
	$(MAKE) wermit "CC= gcc" "CC2= gcc" \
	"CFLAGS= -O -DSUNOS41 -DNDGPWNAM \
	-DTNCODE -DNOLOCAL -DNOPUSH $(KFLAGS)" \
	"LIBS= -L/usr/local/lib -lpwent -lcrypt"

in which:

• -DNOLOCAL excludes all code that is used to make connections.
• -DNOPUSH excludes all code that accesses local system shells and commands.
• -DTNCODE includes Telnet protocol code even though -DNOLOCAL is included.

At sites that use shadow passwords, you'll need to add -DCK_SHADOW to CFLAGS and perhaps -lgen to LIBS.

Also note that the crypt() function, which is used for verifying passwords, is normally found in libc, but on some platforms it has been broken out into a separate library, such as -lcrypt, and in that case must be included in the LIBS clause.

For PAM installations, here's an example /etc/pam.d/kermit file that uses EPS password files for user authentication and the standard unix modules for account and session management:

  auth      required       /lib/security/pam_eps_auth.so
  password  required       /lib/security/pam_eps_passwd.so
  account   required       /lib/security/pam_unix_acct.so
  session   required       /lib/security/pam_unix_session.so

Kerberos IV, Kerberos V, SRP, and TLS can also be selected at build time if you have the appropriate libraries and header files installed on your system. See the Kermit Security Reference for further details.

For a build that excludes all IKSD functions, it is normally sufficient to add NOIKSD to the CFLAGS, e.g.:

  make aix41 KFLAGS=-DNOIKSD

4. Runtime Configuration Options

4.1. Command-Line Options

In addition to -A and the command-line options described in Using C-Kermit (2nd Edition), plus any other new ones described in the C-Kermit 7.0 Update Notes and C-Kermit 8.0 Update Notes, a selection of new long-form options apply specifically to the IKSD. Long-form options work as follows:

1. All long-form options must begin with "--" (two hyphens) or "-+". If the option begins with "--" it is executed before or after the C-Kermit initializaton file, according to its internal keyword flags. If it begins with "-+", it is executed before the init file, regardless of the keyword flags. There is no way to force an argument to be executed after the init file if it has the CM_PRE bit set (see table below).

2. No spaces are allowed in the keyword.

3. If an operand contains spaces, it (or the entire option) must be quoted according to the rules of the shell from which Kermit is invoked.

4. Case doesn't matter in the option keyword (unlike with regular single-letter options).

5. If an operand is required, it must be included. If no operand is required, no operand must be included.

6. The operand follows the keyword, separated by a colon (:) or equal sign (=). There are no spaces between the separator and the keyword or operand.

7. Long-form option names may be abbreviated according to the same rules as interactive keywords.

8. On/Off operands can be On, Off, Yes, No, True, False, 0, 1, or OK (case of letters does not matter).

9. Long-form options are available only when the interactive command parser is built-in (i.e. not NOICP).

The new long-form options are:

  Option        Values    Default      CM_PRE   Effect                  

  --anonymous   on/off    on             yes    anonymous login allowed
  --bannerfile  filename  (none)         no     greeting message
  -+cdfile      filelist  (see below)    no     CD message filename
  -+cdmessage   on/off    on             no     CD messages on/off
  -+cdmsg       on/off    on             no     synonym for cdmessage
  --help        (none)    (n/a/)         no     list available --options
  --helpfile    filename  (none)         no     custom text for HELP command
  --initfile    filename  /.kermrc       yes    Init file for anonymous users
  --nointerrupt (none)    (see below)    yes    Disables SIGINT and SIGTSTP
  --permissions octalnum  0040           yes    anonymous upload permissions
  --perms       octalnum  0040           yes    synonym for permissions
  --root        directory /pub/ftp       yes    root for anonymous users
  --syslog      number    3              yes    level of syslogging    
  --timeout     number    300            yes    time limit for login
  --userfile    filename  /etc/ftpusers  yes    forbidden local users list
  --wtmplog     on/off    on             yes    wtmp logging on/off
  --wtmpfile    filename  (see below)    yes    pathname of wtmp logfile
  --xferlog     on/off    off            yes    use an ftpd-logfile
  --xferfile    filename  (see below)    yes    pathname of ftpd-like logfile
                                                                        

Note: in all options that take a filename, except --cdfile, relative filenames or filenames that start with ~ are expanded to full pathnames.

In more detail:

--nointerrupts

Disables keyboard interrupts that are normally enabled. In IKSD, when it is in interactive prompting state, commands can normally be interrupted with Ctrl-C. Suspending (normally via Ctrl-Z) is always disabled in IKSD.

--help

Lists the extended command-line options that are available in your version of C-Kermit. If any options seem to be missing, that is because your copy of C-Kermit was built with compile-time options to deselect them.

--helpfile:filename

Specifies the name of a file to be displayed if the user types HELP (not followed by a specific command or topic), in place of the built-in top-level help text. The file need not fit on one screen; more-prompting is used if the file is more than one screen long if COMMAND MORE-PROMPTING is ON, as it is by default.

--anonymous:{on,off}

Whether anonymous logins are allowed. By default they are allowed, so this option need be included only to disallow them (or for clarity, to emphasize that they are allowed). Anonymous login occurs when the username "anonymous" or "ftp" is given, with any password (as with ftpd).

--timeout:number

This sets a limit (in seconds) on the amount of time the client has to log in once the connection is made. If successful login does not occur within the given number of seconds, the connection is closed. The default timeout is 300 seconds (5 minutes). A value of 0 or less indicates there is to be no limit.

--bannerfile:filename

The name of a file containing a message to be printed after the user logs in, in place of the normal message (Copyright notice, "Type HELP or ? for help", "Default transfer mode is...", etc).

--permissions:octalnum

The permissions given to any file that is uploaded by an anonymous user. 0040 is the default and recommended value. The default permits only group-read access, and therefore prevents anonymous users from uploading files (e.g. pirated sofware) to be downloaded by other anonymous users. Synonym: --perms.

--initfile:filename

The initialization file to be executed for anonymous logins. By default it is /.kermrc, where "/" is the root of the chroot'd file system. Any filename that you specify here must be specified in the notation of the the chroot'd file system. This option is independent of the '-y' option (alternative init file), which applies only to real users.

--cdmessage:{on,off,0,1,2}

For use in the Server-Side Server configuration; whenever the client tells the server to change directory, the server sends the contents of a "read me" file to the client's screen. This feature is On by default, and operates only in client/server mode when ON or 1. If set to 2 or higher, it also operates when the CD command is given at the IKSD> prompt. Synonym: --cdmsg.

--cdfile:filename (or list)

When cdmessage is on, this is the name of the "read me" file to be sent. Normally you would specify a relative (not absolute) name, since the file is opened using the literal name you specified, after changing to the new directory. Example:

  --cdfile:READ.ME

You can also give a list of up to 8 filenames by (a) enclosing each filename in braces, and (b) enclosing the entire list in braces. Example: --cdfile:{{./.readme}{READ.ME}{aaareadme.txt}{README}{read-this-first}} When a list is given, it is searched from left to right and the first file found is displayed. The default list for UNIX is:

  {{./.readme}{README.TXT}{READ.ME}}

--database:{on,off}

Whether an active-sessions database should be kept. On by default. If "on", but --dbfile is not specified, /var/log/iksd.db is used.

--dbfile:filename

Use this option to specify an iksd database file name. If you include this option, it implies .

--syslog:number or {on,off}

Whether and what level of syslog entries should be made. A level of 0 (or "off" or "false" or "no") means no syslogging. A level of "on" (or "yes" or "true") invokes the default syslogging level. To choose a non-default level of syslogging, specify a number (see Section 4.2).

--wtmplog:{on,off}

Whether wtmp log entries should be made. On by default. An entry is made when the user logs in and when the session is closed. The "tty name" field is given as "iks_xxxx", where xxxx is the process ID (pid) of the IKSD. Thus IKSD session history can be monitored via "last | grep iks_". Anonymous logins are shown with a user ID of "ftp".

--wtmpfile:filename

The default wtmp log file is /var/log/wtmp in Linux, and /usr/adm/wtmp elsewhere. In case your system's wtmp log file does not match the default, use this option to specify the appropriate filename.

--userfile:filename

This file contains a list of local usernames that are to be denied access to Internet Kermit Service. The default is /etc/ftpusers. This can be the same file that is used by wuftpd, and the syntax is the same: one username per line; lines starting with "#" are ignored. Use this option to specify the name of a different forbidden-user file, or use --userfile:/dev/null to disable this feature in case there is a /etc/ftpusers file but you don't want to use it.

--xferlog:{on,off}

Whether a wu-ftpd-like log should be kept. Off by default. If "on", but --xferfile is not specified, /var/log/iksd.log is used. This log is explained in Section 4.3.

--xferfile:filename

Use this option to specify an iksd log file name. If you include this option, it implies --xferlog:on.

--database:{on,off}

Whether an active-sessions database should be kept. On by default. If "on", but --dbfile is not specified, /var/log/iksd.db is used.

--dbfile:filename

Use this option to specify an iksd database file name. If you include this option, it implies --database:on.

Some of the traditional options have special significance in the IKS, e.g.:

-x (server mode)

If this option is included on the IKSD command line, the Client Side Server configuration is disabled, and the user will not get a Username: or Password: prompt, and will not be able to access the IKSD command prompt. A FINISH command sent to the IKSD will log it out and close the connection, rather than returning it to its prompt.

-y filename

Execute "filename" rather than the normal initialization file for real users; this option does not apply to anonymous users.

-Y (no init file)

Do not execute an initialization file, even if a real user is logging in.

Examples:

kermit -A

Starts the Internet Kermit Server with all defaults in effect.

iksd

Equivalent to "kermit -A". If the binary is invoked as "iksd" (e.g. through a symlink) rather than "kermit", the -A option is not necessary.

iksd --syslog:5

As above, but with the highest level of syslogging (Section 4.2).

iksd --anonymous:off --cdfile:READ.ME

Starts the IKSD with anonymous access forbidden and changes the name of CD message file from the default list to READ.ME.

iksd -x --anonym:no --cdf:READ.ME

As above, but forces the IKSD to operate only in Kermit server mode, thus preventing user access to the IKSD command prompt. Also illustrates abbreviation of long-form option keywords and use of "no" as a synonym for "off".

Bad syntax in an extended command-line option causes immediate termination with the message "Extended option error", and an exit status code of 1.

Brief help about command line options may be obtained at the C-Kermit prompt with "help options", and for extended options with "help extended-options".

You can prototype the IKSD command line by including any desired extended options, but omitting the -A option, and then when you get the C-Kermit> prompt, use "show extended-options" to check the parsing of the options you have included. Example:

  $ kermit --anonymous:ok --root:/tmp --userfile:/etc/badusers
  (/users/olga/) C-Kermit>show extended-options
   --bannerfile=(null)
   --cdfile:./.readme
   --cdmessage:0
   --syslog:3
   --wtmplog:1
   --wtmpfile=(null)
   --anonymous:1
   --permission:0040
   --initfile:(null)
   --userfile:/etc/badusers
   --root:/tmp
   --xferlog=0
   --xferfile=(null)
   --timeout:300
  (/users/olga) C-Kermit>

Note that this only shows the values that were given as arguments to the extended options, or their default values, if they have any, but it does not show values that will be computed dynamically at runtime in the absence of a command-line value. For example, --wtmplog:1 and --wtmpfile:(null) mean that an appropriate default will be supplied for the wtmp logfile name at runtime.

Also note that SHOW EXTENDED-OPTIONS does not show the IKSD-specific options when this command given at the IKSD prompt by the user. Thus this command is useful only as a prototyping tool. (And as noted elsewhere, the IKSD command-line argument vector is also inaccessible to the IKSD user via other methods such as the \&@[] array.)

4.2. System Logging

All IKSD entries (except debugging, see below) appear in the daemon log, as defined in the syslog.conf file with a tag of "iksd" and the process ID (pid) of the IKSD process, and therefore can be extracted by grepping for "iksd".

The system logging levels are:

  0 = no logging
  1 = Login/out, failed login attempts, failed Kerberos (etc) authentication
  2 = Dialing out (does not apply to IKSD)
  3 = Making any kinds of connections (does not apply to IKSD)
  4 = Creating / receiving / deleting / renaming / copying files
  5 = Sending / typing / reading / transmitting files
  6 = All top-level commands and all server commands sent to iksd
  7 = Commands executed from macros and command files
  8 = Debug

Each level includes all the levels beneath it (except 0 is not included if the logging level is greater than 0).

The default logging level is 5, and is used if the --syslog: option is not included on the command line, or if is given with an affirmative keyword (yes, true, on, ok) rather than a number. If you specify a number higher than the the maximum, it is the same as specifying the maximum.

Syslog entries are at LOG_INFO priority, except for refused logins, which are at LOG_NOTICE priority, and failed Telnet-level authentication (such as Kerberos), which are at LOG_ERR priority.

Note that if C-Kermit is built with -DSYSLOGLEVEL=n (where n is a number) on the cc command line, this turns on syslogging and hardwires it to the given level for all users. See ftp://kermit.columbia.edu/kermit/f/ckuins.txt for further info.

WARNING

Debug level produces VOLUMINOUS amounts of information -- it is equivalent to (in fact, it is) C-Kermit's debug log. Furthermore, there is a good possibility it will contain sensitive information such as clear-text passwords. Debug records are written to the syslog DEBUG facility, as defined (or not defined) in syslog.conf. If you use this level of logging, make sure that the logfile defined for "daemon.debug" is secured from the public, and that there is plenty of space for it.

ANOTHER WARNING

In some UNIX systems, syslogging does not work once a chroot() has occurred, which is done by iksd just as it is by ftpd for anonymous users, to restrict them to the /pub/ftp (or other desired) directory tree. At present, SunOS and Linux are known to have this deficiency and there probably are many others. THERE IS NO SYSLOGGING FOR ANONYMOUS USERS on these systems. The only way to log the activities of anonymous users on such systems is to specify a transfer iksd log on the command line (Sections 4.1 and 4.3).

4.3. The Transfer Log File

The transfer log has the same format as the wu-ftpd log, and so all the same scripts can be used to process it, collect statistics, etc. In fact, you can even have ftpd and iksd share the same log, in which case records will be intermixed.

The Transfer log can also be used in regular user-mode C-Kermit sessions.

The first field is fixed-length and contains spaces; subsequent fields are variable length, contain no spaces, and are separated by one or more spaces. The fields are:

Timestamp

This is an asctime-style timestamp, example: "Wed Sep 16 20:19:05 1998" It is always exactly 24 characters long, and the subfields are always in fixed positions.

Elapsed time

The whole number of seconds required to transfer the file, as a string of decimal digits, e.g. "24".

Connection

In IKSD, the IP hostname or address of the client. For user-mode C-Kermit transfers, The name of the network host to which C-Kermit is connected, or the name of the serial device through which it has dialed (or has a direct connection), or "/dev/tty" for transfers in remote mode.

Bytes transferred

The number of bytes transferred, decimal digits, e.g. "1537904".

Filename

The full pathname of the file that was transferred, e.g. "/pub/ftp/kermit/a/README.TXT". If the filename contains any spaces or control characters, each such character is replaced by an underscore ('_') character.

Mode

The letter 'b' if the file was transferred in binary mode, or 'a' if it was transferred in text (ASCII) mode.

Options

For compatibility with the wuftpd log. This field always contains an underscore ('_') character.

Direction

The letter 'o' if the file was transferred Out, and 'i' if the file was transferred In.

User class

The letter 'r' for real users, or 'a' for anonymous users.

User identification

The user ID of a real user, or the password given by an anonymous user.

Server identification

The string "iks" (Internet Kermit Server), or if C-Kermit is running in user mode, "kermit". This distinguishes a Kermit transfer log record from a WU-FTPD record, which contains "ftp" in this field.

Authentication class

The digit '1' if we know the user's ID on the client system, otherwise '0'. Currently, always '0'.

Authenticated user

If the authentication class is '1', this is the user's ID on the client system. Otherwise it is an asterisk ('*'). Currently it is always an asterisk.

4.4. The IKSD Configuration File

When C-Kermit is started as IKSD, it always begins by reading the IKSD configuration file, if any. By default this is /etc/iksd.conf (you may change the definition at compile time by adding -DIKSDCONF=\"filename\" to CFLAGS). This file contains ordinary C-Kermit commands. Normally these would be SET or DISABLE commands. In case any commands in this file write to standard output, remember that IKSD's standard output is the connection to the client. For more ideas about what can go in the iksd.conf file, read the next section -- the difference is that when you put commands in the iksd.conf file, they apply not only to anonymous users, but to real users too. So, for example, if you want to disable uploads for all users, you could put DISABLE SEND in the iksd.conf file, whereas if you wanted to disable them only for anonymous users, you would put this command in the anonymous-user initialization file.

The IKSD configuration file is executed before the user logs in (or is otherwise authenticated), before the C-Kermit initialization file, and before any extended-format command line options that start with "--", but after any that are marked CM_PRE in the table in Section 4.1, or that start with "-+". Since execution occurs before login, the \v(user) variable is meaningless here.

4.5. The Initialization File

For anonymous users, the default initialization file, if any, is .kermrc in the chroot'd file system. This default may be overridden with the --initfile:xxx command-line option. The system administrator may include commands in this file to disable selected services for anonymous users, e.g.:

  disable delete  ; Don't let anonymous users delete files
  disable send    ; Don't let anonymous users send files

Of course, any Kermit commands at all may be included: settings, macro definitions, etc. (Also see Section 5.5.)

When the sysadmin specifies the initialization file, this allows a high degree of fine-grained control over who is allowed access to what commands and resources, using standard C-Kermit commands, functions, and variables. The following are particularly useful:

\v(date), \v(ndate)

The current date, in case you want to restrict access by date. (Also read about the new date-related functions in Section 1.6 of the C-Kermit 7.0 Supplement.)

\v(day), \v(nday)

The day of the week, in case you want to restrict access to certain days of the week.

\v(home)

The user's home directory.

\v(host)

The hostname of the IKS.

\v(ipaddress)

The IP address of the IKS. This and/or \v(host) may be used when you are running an IKS on multiple hosts and want to have different setups on each, but still have a common initialization file.

\v(line)

The IP host name or address of the client's host.

\v(time), \v(ntime)

The current time of day, in case you wish to restrict access to certain times of day.

\v(userid)

The ID with which the user logged in to the IKS. For anonymous logins, this is "ftp".

So, for example, if the sysadmin wishes to prevent user "olga" from using the IKS on Mondays, the initialization file could contain a command like:

  if equal \v(user) olga -
    if equal \v(nday) 1 -
       exit 1 Sorry Olga - please come back another day

Or suppose it is desirable to block access from all xyzcorp.com hosts between 9:00am and noon:

  if match \v(line) *.xyzcorp.com -
    if lgt \v(time) 09:00:00 -
      if llt \v(time) 12:00:00 -
         exit 1 Sorry - Please come back after noon

Or suppose a certain user is to be allowed to GET files from the server, but not SEND, PRINT, or MAIL them:

  xif equal \v(user) ivan {
      disable send
      disable print
      disable mail
      disable enable
  }

5. Access to Services

5.1. Automatic Settings

1. Login (authentication) is required.
2. Shell access is disabled.
3. Server-side Telnet negotiation is enabled.
4. SET RELIABLE ON (see the C-Kermit 7.0 Supplement).
5. FAST file-transfer settings, including "cautious" unprefixing.
6. No flow control, no parity.

Items d-f can be overridden with command-line options and/or in the initialization file.

5.2. Authentication

When the IKSD has been started without the '-x' command-line option, it issues a Username: prompt. The user may type a username at the prompt, in which case a Password: prompt is issued, and the user must enter a password. Three login attempts are allowed, with a pause enforced between each one. If all three fail, the connection is closed.

The user may also authenticate from the client by sending a [ REMOTE ] LOGIN command (again, only 3 tries are allowed), or by Telnet Authentication negotiations. Prior to authentication, the IKSD responds to only the following client commands:

  [ REMOTE ] LOGIN
  [ REMOTE ] LOGOUT
  REMOTE HELP (or RHELP)
  REMOTE EXIT (or REXIT)
  BYE

Once authenticated, the user may not re-authenticate or change identities.

The connection persists until it is broken in any of the following ways:

1. Client sends BYE or REMOTE EXIT (REXIT) or [ REMOTE ] LOGOUT to IKSD.
2. Client sends FINISH to IKSD that has been started with "-x".
3. User gives HANGUP or CLOSE command to the client.
4. User gives EXIT, QUIT, or LOGOUT command at IKSD prompt.

The connection is also closed if the user exits from the client, but only if it was an end-to-end Telnet connection. There can be no guarantee that exiting from a serial communication program will close and hang up the serial connection.

5.3. The DISABLE Command

The DISABLE ENABLE command is irreversible; once this command is given, the ENABLE command can not be re-enabled, and therefore no other disabled commands can be enabled either. ENABLE is DISABLEd automatically for anonymous users, so any DISABLE commands in the anonymous-user initialization file (Section 4.4) are also irreversible.

5.4. Shell Access

5.5. Anonymous Users

Anonymous users are handled in about the same way as they are by ftpd. Their effective user ID is set to "ftp" and their file-system access is restricted to the tree rooted at the home directory of the "ftp" user, normally /pub/ftp, via chroot. If /pub/ftp does not exist, login fails. In that case (or for any other desired reason), the anonymous root directory can be changed with the --root: option, e.g. --root:/tmp. Note that, unlike ftpd, iksd does not necessarily require any particular binaries (such as ls) to be in the chroot'd tree. DLLs might be another story, but so far these have not been needed, even with dynamically linked Kermit binaries (at least in Linux and Solaris). Should DLLs be a problem, use a statically linked Kermit binary (if possible), or install the needed DLLs in the anonymous root.

File access is according to user "ftp" and the directory and file permissions of the /pub/ftp tree. If anonymous uploads are to be allowed, then usually only specific directories (often with a name like "incoming") are given write permission, and any files that are created in such a directory have owner "ftp", with the group set to the group of the directory. As noted in Section 4.1, the permission for all files created by anonymous users is 0040 (group-read only), or whatever else the sysadmin has specified in the --permissions: command-line option.

Thus, if you already have a public ftp server, most likely everything is set up appropriately already">up appropriately already. If anonymous ftp is allowed (by the presence of a valid user "ftp" in your system's password file), it will be allowed also for IKSD unless explicitly disallowed on the IKSD command-line (or if /pub/ftp doesn't exist and you did not supply a --root: option on the command line).

In addition to the FTP-like restrictions, certain Kermit services are always denied to anonymous users. These include:

• PRINT (at IKSD prompt) and REMOTE PRINT (from client)
• MAIL (or SEND /MAIL) at IKSD prompt and from client.
• Creation of any logs (transaction, debug, packet, etc).
• No file may be deleted, including implicitly, e.g. by the COPY command.
• FILE COLLISION is set to RENAME and may not be changed.

The latter three provisions mean that anonymous users can not delete, overwrite, rename, or alter any existing files in any way, whether by file transfer or with the DELETE or RENAME command.

Note that IKSD, like FTPD, does not allow directory creation by anonymous users, even when file/directory permissions would otherwise allow it. To change this, add:

  enable mkdir    ; Enable directory creation

to /pub/ftp/.kermrc (or whatever other initialization file you have designated for anonymous users with --initfile). Similarly for directory removal:

  enable rmdir    ; Enable directory removal

Of course directories can be removed only if (a) they are empty, and (b) their permissions allow it.

5.6. Management Information

6. Open Issues

6.1. Connection Establishment

6.2. Shell Access

6.3. External and/or non-Kermit Protocols

• When the user is 'anonymous' the file system is chroot'd, which means that any external executables must exist in the chroot'd file system. This leaves open the possibility that executables might be replaced by a different file of the same name that has been uploaded either on top of the existing file or just into the same directory. If "." is in the PATH for the process anything the user uploads with the name 'rz' or 'sz' might be executed in its place. Therefore, the IKSD does not use any external commands of any kind. Not 'ls', not 'cp', not 'df', not anything. To allow the execution of external programs is a major security hole in this setting.

• Most implementations of 'sz' and 'rz' are designed for remote use over a terminal connection. They expect to be run underneath a pseudoterminal (PTY). When IKSD is running there is no PTY. In fact, when IKSD is running there is only a raw socket; the IKSD handles all necessary Telnet protocol translations including Telnet Network Virtual Terminal (NVT) rules regarding quoting of Carriage Return and 0xFF depending upon the state of the BINARY Telnet option in each direction. 'sz' and 'rz' are completely incapable of performing these translations in either direction and any attempt to transfer a file with them will result in a transfer failure and the corruption of the data stream.

• IKSD supports encrypted sessions, in which case it handles the encryption and decryption itself; an external protocol module would not do this, and so could not be used on encrypted channels in any case.

6.4. Additional Administrative Controls

• Ability to select IKSD logging for real vs anonymous users.
• Ability to select IKSD logging for inbound vs outbound files.
  • Currently all transfers are logged or not logged.

• Ability to allow/restrict chmod/delete/overwrite for anonymous users.
  • Currently there is no command for changing file permissions.
  • Guests may not delete files, period.
  • FILE COLLISION is set to RENAME for guests and may not be changed since all the other options allow existing files to be altered.
  • There is no way to grant these capabilities to guests.

• Ability to allow directory creation by anonymous users.
  • Anonymous users may not create or remove directories.

• Ability to allow/specify CD messages on a per-directory basis.
• RFC931 authentication of remote (client) user (but wu-ftpd doesn't either).

These or other controls can be added if there is sufficient reason or demand.

6.5. Known Bugs

• When commands are logged in syslog, the EXIT command appears twice.

• wtmp entries show pids in hex due to length restrictions, so you'll need to convert them to decimal before using them with ps, kill, etc.

7. Monitoring

The IKSD database file should reside in its own directory (to avoid conflicts with other servers that might use the same filename conventions), and this directory and its files should be visible to the sysadmin without privileges (e.g. by group access) but hidden from the public for privacy reasons if other such logs (e.g. the ftpd log, syslog, etc) also are.

The database is a random-access file indexed by "slot number", which is self-assigned during a quick search at startup. Once a slot is claimed, there is no more searching; each IKSD updates its own slot in place and does not touch the others. Slots never move.

When an IKSD instance starts, it must obtain a "slot allocation lock" before it can claim its slot. Otherwise there could be a race condition when another IKSD instance starts at the same time, in which the two could wind up with the same slot.

Since exclusive access is a nonportable concept, we obtain the lock in a crude but portable manner, without recourse to kernel locks, interprocess communication, semaphores, memory-mapped files, or other platform-specific mechanisms:

1. Create a temp file in the database directory called ip.pid,, where ip is the local IP address as 8 hex digits and pid is my own process ID (PID) in hex (no leading zeros). This is guaranteed to be a unique name (and if by chance a file of this name already exists, it can't possibly be valid, so it it's ok to overwrite it).

2. Write my own ID string into the file. The ID string is ip:pid, where ip is (again) my IP address as 8 hex digits, and pid is my PID in decimal (not hex). (The reason for decimal here is readability, so sysadmins can easily enter it into other tools or commands; it has to be hex in the temp filename for compactness in case of 14-character filename limitations.)

3. Try to open iksd.lck, which is the real lockfile. If this succeeds, read its contents (an ip:pid string). If the ip not the same as mine, consider the file locked. If it is the same, extract the PID and check its validity. If it is not valid, delete the lockfile. (Currently this step is skipped by K95 since it can't check PIDs.)

4. At this point the iksd.lck file might or might not exist. Try to rename the temp file to iksd.lck. This will fail if the lockfile exists, in which case we sleep, loop, and try again, up to a certain number of times -- say 16 tries spaced 1 second apart. If we fail to get a lock, we let the user on anyway, in the spirit of service over management. If the rename succeeds, on the other hand, we have a lock.

When and if we have obtained a lock, we open the database file itself and search the whole database (sequentially) for:

1. Free slots (In-Use flag is 0), or:

2. Stale slots(*), or:

3. Any slot with our own IP and PID (a special kind of stale slot).

In C-Kermit only (not K-95) we also perform some housekeeping duties while searching (the Windows APIs include no provisions for this):

1. Whenever a stale slot is found, we free it (set its In-Use flag to 0).

2. We remove all free slots after the last in-use slot by truncating the database file (this keeps the database file from constantly growing as new highwater marks are reached).

Then we claim the first one of these that was found. If no slot was claimed, we add a new slot at the end and claim it. If the database file did not exist, we create it and claim the first slot. (In Windows, the IKSD listener simply deletes the existing database, if any, on initial startup.)

To claim a slot:

1. We set the In-Use flag to 1, fill in the Server PID and IP address fields with our own PID and IP address, fill in the session-start and update time fields, and clear the other fields.

2. Then we release the lock.

From this point, we can write freely into our slot regardless of locks.

Notes:

1. Since all IKSDs follow the same procedure, only one can have a lock a at a time.

2. The lockfile is readable text; it can be typed (cat'd). The contents are the same format on every platform, regardless of byte order or word size.

3. This scheme allows (but does not require) multiple computers that share a common file system to have a single IKSD database, which in turn allows the site manager to monitor all IKSDs on all computers at once. This works if each computer has a unique IP address (which it must if they are on the same network sharing a common file system; nevertheless, care must be taken regarding IP address pools, etc). It also depends on the file- sharing mechanism (such as NFS) to propogate updates promptly and in sequence.

4. While one IKSD is allocating its slot, nothing prevents other IKSD instances that already have their own slots from updating them, since that does not interfere with slot allocation.

5. The tempfile name format is UNIXish and will need modification for file systems with restrictive names, such as FAT, 14-char UNIXes, etc. In such cases we could (a) omit the IP address if we're not concerned about multiple computers sharing a single database, or (b) encode the IP address in Base 64 to make it shorter. But probably none of this will ever come up.

6. The design accommodates 64-bit IPv6 addresses, but for now the software uses only 32 bits.

7.1. Database Record Format

  yyyymmdd hh:mm:ss

where yyyymmdd are the numeric year, month, and day, and hh:mm:ss are the hour (24-hour clock), minute, and second. Months and days are 1-based, leading 0's are supplied where needed.

The layout of each slot is as follows (fields and byte positions are numbered from 0):

   0. FLAGS (Slot/Session Flags)
      Start:     0
      Length:    4
      Type:   Bit Mask
      Format: Hex digits.   
      Bit Values:
       1: 1 = Slot in use, 0 = Slot is free.
       2: 1 = Real user, 0 = Anonymous user.
       4: 1 = Logged in, 0 = Not logged in.

   1. AUTHTYPE (Authorization Type)
      Start:     4
      Length:    4
      Type:   Number
      Format: Hex digits.
      Values:
       0: None          8: (reserved)
       1: Kerberos IV   9: (reserved)
       2: Kerberos V   10: LOKI
       3: SPX          11: SSA
       4: MINK         12: KEA_SJ
       5: SRP          13: KEA_INTEG
       6: RSA          14: DSS
       7: SSL          15: NTLM

   2. AUTHMODE (Authorization Type)
      Start:     8
      Length:    4
      Type:   Number
      Format: Hex digits.
      Values:
       0: Rejected
       1: Unknown
       2: Other
       3: User
       4: Valid

   3. STATE (IKSD State)
      Start:    12
      Length:    4
      Type:   Bit Mask
      Format: Hex digits.
      Bit Values:
       1: Initializing
       2: Sending a file
       4: Receiving a file
       8: Executing a REMOTE command
      32: At command prompt

   4. PID (IKSD's Process ID)
      Start:    16
      Length:   16
      Type:   Number
      Format: Hex digits.

   5. SERVER IP (IKSD's IP Address)
      Start:    32
      Length:   16
      Type:   IP address as a series of numeric octets in network byte order
      Format: Hex digits

   6. CLIENT IP (Client's IP Address)
      Start:    48
      Length:   16
      Type:   IP address as a series of numeric octets in network byte order
      Format: Hex digits

   7. SESSION START (Date and Time session started)
      Start:    64
      Length:   18
      Type:   Date-time
      Format: Date-time string

   8. LAST UPDATE (Date and Time this record was last updated)
      Start:    82
      Length:   18
      Type:   Date-time
      Format: Date-time string

   9. ULENGTH (Username length)
      Start:   100
      Length:    4
      Type:   Number
      Format: Hex digits, right adjusted

  10. DLENGTH (Length of current directory)
      Start:   104
      Length:    4
      Type:   Number
      Format: Hex digits, right adjusted

  11. ILENGTH (Length of state-specific information)
      Start:   108
      Length:    4
      Type:   Number
      Format: Hex digits, right adjusted

  12. RESERVED
      Start:   112
      Length:  912
      Type:   None
      Format: Filled with blanks

  13. USERNAME (Username; if anonymous "anonymous:password")
      Start:  1024
      Length: 1024
      Type:   None
      Format: Text, ULENGTH sigificant chars, right-filled with blanks

  14. DIRECTORY (Current directory)
      Start:  2048
      Length: 1024
      Type:   None
      Format: Text, DLENGTH sigificant chars, right-filled with blanks

  15. INFO (State-specific information)
      Start:  3072
      Length: 1024
      Type:   None
      Format: Text, ILENGTH sigificant chars, right-filled with blanks

The state-specific information tells you whether IKSD is at its command prompt or in server command wait, if it is sending or receiving a file (in which case the filename is shown), it is executing a REMOTE command (in which case the command is shown), or how it was terminated (BYE, REMOTE EXIT, disconnect, etc)

Notes:

1. There are no control characters (CR, LF, NUL, etc) or 8-bit characters; only ASCII graphic characters are used in the database. Fields are left- or right-padded with Space or '0' as indicated.

2. Since the USERNAME, DIRECTORY, and INFO fields are right-padded with spaces, it would be laborious to isolate their actual values by trimming spaces from the right (since we must allow for internal spaces); hence the ULENGTH, DLENGTH, and ILENGTH fields.

3. The USERNAME, DIRECTORY, and INFO fields are 1K each since 1K is the maximum pathname length found on common UNIX platforms (even though many platforms have smaller maximums). Thus, for example, the current directory string might be exactly 1K long. This is why its length is kept outside the block. Also the division of the record into four 1K blocks tends to make for more efficient i/o.

4. The IKSD database does not provide a permanent record or give complete information. That's what the syslog entries and the IKSD log are for.

7.2 The Display Module

When a slot is freed, its information is left intact so you can still see who was using it, when, and from where, and the last thing they did before logging out. The old info persists until the slot is removed or reclaimed, which will happen when the next new IKSD session starts.

The iksdpy script is short, simple, and easily extensible. For example, while watching a detail screen and you see somebody doing something bad, you might like to have a "K" key to kill the session (the script already knows the pid, so it need simply form a "!kill -9" command, including the pid).

7.3. Database Management

--database:{on,off}

Whether an active-sessions database should be kept. On by default. If "on", but --dbfile is not specified, /var/log/iksd.db is used.

--dbfile:filename

Use this option to specify an iksd database file name. If you include this option, it implies --database:on.

In UNIX, the system startup procedure can delete the database file before starting inetd, since nothing in it is valid; however, this is not necessary since normal cleanup procedures will do the job too. In Windows, the IKSD listener takes care of it.

To capture a snapshot of IKSD usage, simply copy the database file.

8. TESTING

  telnet hostname 3000
  set host hostname 3000 /telnet

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