Ubuntu Manpage: sockd - Internet firewall secure socket server (proxy server)

Provided by: socks4-server_4.3.beta2-20build1_amd64

NAME

       sockd - Internet firewall secure socket server (proxy server)

SYNOPSIS

       sockd [ -ver | -i | -I ]

DESCRIPTION

sockd is an internet secure socket server, often referred to as a proxy server. It was
designed primarily to provide hosts within a firewall access to resources outside of the
firewall.

Normally, hosts inside a firewall has no IP-accessibility to the network outside of the
firewall. This reduces the risk of being intruded by unauthorized people from the
Internet. Unfortunately, without IP-accessibility users on the inside hosts can no longer
use many of the important tools such as telnet, ftp, xgopher, Mosaic, etc. to access the
tremendous resources available in the Internet.

With sockd installed on a server host, users on the other inside hosts can gain back the
lost functionalities by using clients programs designed to work with sockd proxy server,
e.g, rtelnet in place of telnet, rftp in place of ftp, rfinger in place of finger, etc.
Since these client programs work like their normal counterparts without requiring direct
IP-connectivity to the Internet, convenience to the users is accomplished without
breaching the security. The server host that runs sockd does have to be open to the
Internet, and it therefore requires special attention to make sure that it is secure.

A configuration file /etc/sockd.fc (or /etc/sockd.conf) is used to control access to sockd
and its services. Permission and denial of a service request can be decided based on
various combinations of the requesting host, the destination host, the type of service
(destination port number), as well as the requesting user. (See sockd.conf(5) and
sockd.fc(5).)

If the server host is multi-homed, i.e., having more than one network interface and with
its IP_FORWARDING turned off, and the server support RBIND operation, then it must run a
multi-homed version of sockd, which requires another control file /etc/sockd.fr (or
/etc/sockd.route) to decide which interface to use for connection to any given destination
host. See sockd.route(5) and sockd.fr(5). A multi-homed sockd can be run on a single-homed
host as well if necessary; you just have to set up /etc/sockd.route to direct all traffic
through the host's one and only network interface.

sockd uses syslog with facility daemon and level notice to log its activities and errors.
Typical lines look like

Apr 11 08:51:29 eon sockd[636]: connected -- Connect from don(don)@abc.edu to wxy.com (telnet)
Apr 11 09:24:59 eon sockd[636]: terminated -- Connect from don(don)@abc.edu to wxy.com (telnet)
Apr 11 09:24:59 eon sockd[636]: 1048 bytes from abc.edu, 285143 bytes from wxy.com
Jun 22 18:24:54 eon sockd[884]: refused -- Connect from sam(unknown)@big.com to small.com (ftp)

In these lines, the first user-id is the one reported by the client program, the second
one (within the parentheses) is what is reported by identd on the client host. These log
lines usually appear in file /var/adm/messages though that can be changed by modifying
/etc/syslog.conf. (See syslogd(8) and syslog.conf(5).)

If you allow access to infosystems such as Gopher or WWW, you should be aware that they by
nature would tend to get connections to hosts all over the world and would use not only
Gopher and WWW ports but possibly also ports for finger, telnet, ftp, nntp, etc. as well
as non-privileged ports ( > 1023).

For a stand-alone sockd, /etc/sockd.fc (or /etc/sockd.conf) and /etc/sockd.fr (or
/etc/sockd.route), if required, are only read and parsed once at the beginning of program
execution. If you change the contents of either file and want to make the running sockd
use the new contents, you must send a SIGHUP signal to the running sockd process. Sending
a running stand-alone sockd a SIGUSR1 signal causes it to record on the systems's log file
the effective contents of configuration and route files that it is currently using. You
can find the process id of the stand-alone sockd in /etc/sockd.pid.

Rather than using plain-text configuration file /etc/sockd.conf and route file
/etc/sockd.route, sockd now looks for the corresponding frozen files /etc/sockd.fc and
/etc/sockd.fr first. The plain-text files are used only if the corresponding frozen files
are not found. Use commands make_sockdfc and make_sockdfr to produce the frosen files. Use
commands dump_sockdfc and dump_sockdfr to examine the contents of frozen files. (See
make_sockdfc(8), make_sockdfr(8), dump_sockdfc(8), and dump_sockdfr(8).) Using frozen
configuration and route files can save a lot of overhead at start-up of sockd.

OPTIONS

The options are mutually exclusive and thus may only be used one at a time.

-ver With this option, sockd prints its own version number, the version number of the
SOCKS protocol, whether it is SOCKSified, whether it is a standalone daemon or must
be run under inetd, whether it support RBIND, and whether a route file is required.

-I Use identd (RFC 1413) to verify the requester's user-id. Deny access if connection
to client's identd fails or if the result does not match the user-id reported by
the client program. Client hosts without a properly installed identd daemon will
not be served. User verification is done before and in addition to the normal
access control. This can be overridden in the sockd.conf file on a line by line
basis.

-i Similar to -I but more lenient. Access is denied only if client's identd reports a
user-id that's different from what the client program claims. This can be
overridden in the sockd.conf file on a line by line basis.

Log entries similar to the following are produced upon failure of user-id verification:

Apr 15 14:42:51 eon sockd[729]: cannot connect to identd on big.edu
Apr 15 14:42:51 eon sockd[729]: refused -- Connect from bob(unknown)@big.edu to xyz.com (ftp)
Jul 15 12:23:06 eon sockd[832]: *Alert*: real user is sam, not jim
Jul 15 12:23:06 eon sockd[832]: refused -- Connect from jim(sam)@abc.org to bad.place.com (WWW)

FILES

       /etc/sockd.fc,    /etc/sockd.conf,   /etc/sockd.fr,   /etc/sockd.route,   /etc/inetd.conf,
       /etc/services, /var/adm/messages, /etc/syslog.conf

AUTHOR