IKSD - The Internet Kermit Service Daemon


The Kermit Project
Columbia University

As of: C-Kermit 8.0, 12 Dec 2001
This page last updated: 23 March 2003

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.


  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?

Why run an Internet Kermit Service Daemon when you are already running an FTP server?

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


2. System Configuration

WARNING: Do not attempt to install a pre-7.0 version of C-Kermit as an IKSD. Earlier versions do not perform the necessary authentication and security functions, and therefore will leave your system wide open to anonymous root access. (Don't worry, the installation procedure described here, if you follow it, and the interface to IKSD, prevent you from starting an earlier version of C-Kermit as an IKSD, since it lacks the needed command-line options and will halt with a usage message if you try to use them.)

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

You can use any C-Kermit configuration as an IKSD, ranging from a fully configured version, to a special IKSD-Only version, to any of the various "stripped-down" versions, as described in the C-Kermit Configuration Guide (ckccfg.txt) and/or Installation Instructions (ckuins.txt).

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

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.

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.

System-wide initialization file; see Section 4.4.

Is defined automatically if NOIKSD is not defined.

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:

	@echo Making C-Kermit $(CKVER) for SunOS 4.1 with gcc for IKS...
	$(MAKE) wermit "CC= gcc" "CC2= gcc" \
	"LIBS= -L/usr/local/lib -lpwent -lcrypt"

in which:

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

The IKSD can be configured at runtime with command-line options or by an initialization file or other auxilliary file(s), in any combination. The SHOW IKSD command lists the IKSD-relevant parameters and their values.

4.1. Command-Line Options

To start C-Kermit as an Internet Kermit Service Daemon, include the '-A' command-line option. This option not only configures C-Kermit as an IKSD, but it also ensures that you are running a version of C-Kermit that can do this, i.e. C-Kermit 7.0 or later built with all the necessary compile-time options (Section 3). If either of these conditions does not apply, the '-A' option will be invalid and the Kermit program will halt immediately.

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:

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.

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.

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.

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).

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.

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).

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.

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.

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:


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:


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

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).

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".

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.

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.

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.

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

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

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.


kermit -A
Starts the Internet Kermit Server with all defaults in effect.

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
  (/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

System logging in UNIX is via the standard syslog() facility ("man syslog" and/or "man syslogd" for further info).

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.

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.

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 is disabled by default; it must be enabled on the command line (Section 4.1).

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:

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".

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".

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.

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

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

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

The IKSD configuration file is a place to put commands that should always be executed for every client, real or anonymous.

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

When a real user logs in to the IKSD, the C-Kermit initialization file is executed in the normal manner, depending on the compile-time configuration of the IKSD. Normally, the user's own .kermrc file is executed, and this, in turn, executes the user's customization file, .mykermrc. If the IKSD was built with the CK_SYSINI or CK_DSYSINI compile-time options (described in ckccfg.txt), a single, shared system-wide initialization file is executed instead; this may, in turn, execute a customization file out of the user's home directory. You may override C-Kermit's automatic selection of initialization with the regular C-Kermit -y or -Y options on the IKSD command line, described in the C-Kermit manual.

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.

The user's home directory.

The hostname of the IKS.

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.

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.

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

4.6. Security

4.6.1. Introduction

The Internet Kermit Service when built with support for Kerberos 4, Kerberos 5, Secure Remote Password, or OpenSSL's SSL/TLS can be configured to accept, request, or require secure (authenticated, encrypted, integrity protected) connections utilizing Telnet Security options.

Kermit 95 2.1 is shipped with support for all of the security options. The availability of various options is determined by the options selected at install time.

Instructions for building C-Kermit 8.0 with arbitrary combinations of security options are available in the C-Kermit Security documentation.


Most installations of IKSD utilize the TELNET START_TLS option to provide encrypted and integrity protected connections in conjunction with X.509 certificate based server-side authentication. START_TLS is often combined with Kerberos, Secure Remote Password, or X.509 client certificates to provide secure client side authentication. Username and Password prompts are issued over the TLS connection when secure authentication of the client is unavailable.

4.6.2. Configuring IKSD for use with TELNET START-TLS

  1. Retrieve or generate Server Side Certificate and Private Key

  2. In your IKSD configuration file, \v(common)IKSD.KSC (on Windows) or /etc/iksd.conf (on Unix), specify these options:

  3. To request or require the use of TLS, add:


4.6.3. Configuring IKSD TELNET START-TLS utilizing X.509 Certificates

  1. Kermit supports the installation of both RSA and DSS certificates. If you have an RSA-based server certificate then you must specify the file containing the certificate as well as the file containing the private key. The private key file must not be encrypted; otherwise, it will not be able to be loaded by IKSD.


  2. If your certificate is not directly signed by the Root CA, you will need to include the intermediary CA certificates in a Certificate Chain file:


  3. If you have an DSS-based server certificate then you must specify the file containing the certificate as well as the file containing the private key. The private key file must not be encrypted; otherwise, it will not be able to be loaded by IKSD.


  4. If your certificate is not directly signed by the Root CA, you will need to include the intermediary CA certificates in a Certificate Chain file:


  5. If you wish to restrict the ciphers used to secure the connection you may do so with the SET AUTH TLS CIPHERS command. If you do not have any certificates at all to use, specify the use of Anonymous-Diffie-Hellman ciphers:


4.6.4. Configuring IKSD to accept Client Side Certificates

  1. If you wish to accept client certificates you must specify:


  2. If you wish to require the use of client certificates specify:


    In addition, you must configure where the validation information for the client side certificates is located. First, the location of the CA certificates for the validation chain must be specified. This is done with:


    The location of Certificate Revocation Files must also be specified:

Read about these commands in
http://www.columbia.edu/kermit/security.html for information on the details of how Certificates and CRLs are stored in files and directories for use with OpenSSL.

Also, read the section on how to build C-Kermit for different methods of extracting the userid from the client certificate.

4.6.5. Configuring IKSD TELNET START-TLS Utilizing Kerberos 5 Tickets

When utilizing Kerberos 5 to authenticate the IKSD to the client you must specify the location of the Kerberos Keytab File:


and specify the use of KRB5 ciphers:


NOTE: OpenSSL must have been built with support for MIT Kerberos 5.

Configuring IKSD to utilize TELNET AUTHENTICATION options:

  1. The TELNET AUTH options (KRB4, KRB5, SRP, NTLM) may be used either with or without the START-TLS option. If you wish to request or require the use of the AUTH option:


  2. To specify an authentication type(s) use:


    where type(s) is one or more of KRB4, KRB5, SRP, NTLM.

5. Access to Services

The IKSD behaves at runtime just like the regular C-Kermit program, with any restrictions resulting from compile-time options, and with the differences noted in this section and in Section 4.

5.1. Automatic Settings

When C-Kermit is started as an Internet Kermit Service, the following settings occur automatically:

  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

The IKSD command prompt will not appear, and no commands may be given, before the user is authenticated.

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:


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

In the IKSD, the DISABLE command applies not only to client/server functions, but also to the corresponding commands when given at the prompt. For example, DISABLE DELETE disables not only REMOTE DELETE commands given from the client, but also DELETE commands given at the IKSD's command prompt, as well as implicit forms of file deletion, such as when the target of a COPY command is an existing file.

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

All forms of system and shell access are disabled in the IKS. Thus the user can not execute REMOTE HOST commands from the client, nor access the shell from the IKS command prompt via shell escapes (!), the RUN or PUSH command, or by specifying pipes or filters in file-transfer commands, or by pipe specifications in REMOTE commands, or in any other way. This is true even if the executable was built without the NOPUSH compile-time option.

5.5. Anonymous Users

Anonymous users are allowed by default, but can be denied with --anonymous:no on the command line (Section 4.1).

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. 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:

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

The command-line argument vector, normally accessible in the \&@[] array, the top-level \%0..9 variables, or by other means, is inaccessible to IKSD users. Thus IKSD clients can not discover the IKSD startup path or options, the logfile pathnames or directories, logging level, etc.

6. Open Issues

Several services that are normally provided by C-Kermit are not available when it is an Internet Kermit Service Daemon.

6.1. Connection Establishment

If the user has access to the IKSD command prompt, why not allow her to "set host" or "set line" from there to another place? Obviously this would be a security risk if allowed for anonymous users. For authenticated users, it should be OK, but is not currently possible for Telnet connections since the IKSD is already a Telnet server on the incoming connection, and is not designed to conduct two separate Telnet sessions simultaneously. It might be possible to allow the user to make a dialout connection, but some coding and testing would be needed should this prove desirable.

6.2. Shell Access

Shell access is forbidden to anonymous users for obvious reasons. From a security standpoint, it could be allowed for authenticated users, but there remains a technical obstacle: the absence of a terminal driver for the connection.

6.3. External and/or non-Kermit Protocols

External protocols such as Zmodem can not be expected to perform any of the logging or security functions that are done by C-Kermit itself within its protocol and file modules. Thus external protocols are disabled in the IKSD. Plus the fact that such protocols are likely to be incompatible with Telnet connections in the first place. In more detail:

6.4. Additional Administrative Controls

Certain options available in wu-ftpd are not implemented in iksd:

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

6.5. Known Bugs

7. Monitoring

Unless you disable it, all IKSDs keep current session information in a shared database. The IKSD instances can be within one computer or running on any number of different computers that share the same file system on the same network. The database can be monitored by the sysadmin with a simple "systat"-like display program, which shows who is logged in, from where, and what they're doing. (The idea is easily adapted to other servers, such as FTP.) For maximum portability and reliability, the database is an ordinary file on disk.

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-Use flag is 1 and IP address is ours, but PID is invalid.

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.


  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

A slot is 4K (4096 octets), divided into 4 1K chunks. The first chunk is further subdivided into shorter fields. Numeric fields are coded in hexadecimal, right-adjusted, and left-padded with 0's. Text fields are left-adjusted and right-padded with blanks. Date-time fields are right-adjusted within a field of 18 with the leading blank reserved for Y10K. Date-time format is:

  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)


  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

The straightforward data definitions and formats allow a display module to be written easily in the language of your choice, including scripting languages such as Kermit's own. A sample display module, iksdpy, is provided as a "Kerbang" script, providing a running display of the active IKSD sessions from the database. In its startup mode, it lists each session in the database, refreshing every 4 seconds. Various keystroke commands are available for help, quitting, etc, and to enter the detail screen for a specific session. The detail screen, too, refreshes itself every 4 seconds. Here you have keys to return to the main screen, to pick another session, to cycle through sessions, and so on.

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

The following command-line options let you disable/enable the database and specify its location:

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

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.


In case you want to test IKSD on a port other than 1649, be aware that IKS-aware Kermit clients (such as C-Kermit 7.0 and K95 1.1.18 and later) will not initiate Telnet negotiations with it, since it is not on a Telnet port (i.e. 23 or 1649). To get correct operation you'll need to force the client to negotiate, e.g.:

  telnet hostname 3000
  set host hostname 3000 /telnet

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C-Kermit IKSD Aministration / Columbia University / kermit@columbia.edu / 12 Dec 2001