Ubuntu Manpage: socks.conf - SOCKS clients configuration file

Provided by: socks4-clients_4.3.beta2-19build1_amd64

NAME

       socks.conf - SOCKS clients configuration file

SYNOPSIS

       /etc/socks.conf

DESCRIPTION

All SOCKS client programs use this file to determine whether to use direct or proxy connection to a given
destination host, and to exert access control based on the destination host, the requested service (port
number on the destination host), and the effective user-id of the requesting local user. If this file is
absent, SOCKS clients will only try direct connections, making them behave like their regular
counterparts.

Each line in the file may be up to 1024 characters long. Lines starting with a # are comments. Non-
comment lines must be of one of the three forms:

deny [*=userlist] dst_addr dst_mask [op dst_port] [: shell_cmd]
direct [*=userlist] dst_addr dst_mask [op dst_port] [: shell_cmd]
sockd [@=serverlist] [*=userlist] dst_addr dst_mask [op dst_port] [: shell_cmd]

A deny line tells the SOCKS clients when to reject a request. A direct lines tells when to use a direct
connection. A sockd line indicates when to use a proxy connection and, optionally, which SOCKS proxy
server or servers it should try.

Spaces and tabs separate the fields. Fields enclosed in square brackets are optional.

The userlist field, when present, consists of one or more user-ids or filenames, with comma as separator.
No spaces or tabs are allowed in the list. The user-ids should be ids of users on the local host, not
those on the destination host or the SOCKS server host. The filenames must be full pathnames with the
leading /. Inside the specified files, user-ids may be listed one or several per line, with any
combination of blanks, tabs, and commas as separators. The appearance of # marks the remainder of the
line as comment. Each line in the files may be up to 1023 characters long. If the *=userlist field is
omitted, the line applies to all user-ids.

The dst_addr field specifies either the IP address of a host, a network, or a subnet in the usual dotted
form, e.g., 129.201.4.0, or a doamin name, e.g., internic.net. dst_mask specifies mask for the IP address
used in dst_addr. Bits in dst_mask that are set to 0 indicate the bit positions to be ignored during
comparison of IP addresses. So, specifying 255.255.255.255 in dst_mask demands an exact match with
dst_addr, whereas 0.0.0.0 in dst_mask causes a matching with any given destination address regardless of
what is specified for dst_addr. If a domain name is used for dst_addr, the contents of dst_mask are
ignored, though it must still be supplied (simply use 0.0.0.0). If the domain name starts with a period,
it specifies a zone and matches all domain names within that zone, otherwise it matches only the domain
name itself. For example, xyz.com matches only xyz.comP, while .xyz.com macthes not only xyz.com, but
also abc.xyz.com and this.and.that.xyz.com, among others. The special symbol ALL (which must be entirely
in uppercase) matches everything. Domain names are otherwise case-insentive.

When using a domain name in dst_addr, you have be very careful in maintaining your DNS setup. See the
last few paragraphs in sockd.conf(5).

The op field must be eq, neq, lt, gt, le, or ge, for the condition of equal, not equal, less than,
greater than, less than or equal, and greater than or equal, respectively. The dst_port field can be
either a port number, e.g., 23, or the equivalent service name as specified in file /etc/services, e.g.,
telnet for port number 23. If this pair is omitted, the line applies to all services.

The serverlist, which may only be used in a sockd line, consists of one or more SOCKS proxy servers,
which the client program should try to use (in the indicated order) for establishing a proxy connection.
Only commas can be used as separator, no spaces or tabs are allowed in the list. Domain names of the
servers may be used in the list, though it is probably more prudent to specify IP addresses. If this
field is omitted, the client program will use the default SOCKS proxy server, which is determined by the
environment variable SOCKS_SERVER if it exists, or the name compiled into the SOCKS client program
otherwise.

Consider

sockd @=1.2.3.4 *=boss,root 11.12.13.14 255.255.255.255 eq telnet

To match the condition indicated in this line, a request must come from a local user whose effective id
is either boss or root, the destination IP address must be 11.12.13.14 exactly, and the service requested
must be telnet. In that case, connection to host 11.12.13.14 should be done via a SOCKS proxy server on
host 1.2.3.4.

Every time a SOCKS client has to make a network connection, it checks the pending request against the
file /etc/socks.conf, one line at a time. Once it finds a line with conditions that are matched by the
request, the action specified on that line is taken. The remaining lines of file /etc/socks.conf are
skipped. So the order of the lines in the file is extremely important; switch two lines and you may have
entirely different results. If no matching line is found throughout the file, the request is denied.

The shell_cmd field specifies a command string that is executed when the conditions on that line are
satisfied. The following substitutions occur before the string is presented to the Borne shell for
execution:

%A -- replaced by the client host's domainname if known, by its IP address otherwise
%a -- replaced by the client host's IP address
%c -- replaced by "connect" or "bind"
%p -- replaced by the process id of the client program
%S -- replaced by the service name (e.g., ftp) if known, by the destination port number otherwise
%s -- replaced by the destination port number
%U -- replaced by the user-id at login
%u -- replaced by the effective user-id
%Z -- replaced by the destination host's domainname if known, by its IP address otherwise
%z -- replaced by the destination host's IP address
%% -- replaced by a single %

Several shell commands can be strung together in the usual way with `|', `;', etc.

Although there is an implied 'deny all' at the end of the control file, you may supply one explicitly so
as to take some specific action when requests are so rejected, e.g.,

deny 0.0.0.0 0.0.0.0 : /usr/ucb/mail -s 'SOCKS: rejected %S from %u to %Z' root

Unlike the previous version, connection to address 127.0.0.1 or 0.0.0.0 is always done directly to
localhost, so there is no need to specify either of them in /etc/socks.conf.

You have the option of using the frozen file /etc/socks.fc instead of /etc/socks.conf. The frozen file is
produced by make_socksfc and is essentially the memory image of the parsed configuration file. using it
can reduced the start-up delay of SOCKS client applications since no parsing is needed. Because SOCKS
client applications always look for /etc/socks.fc first, be sure that you always run make_socksfc every
time after you modify /etc/socks.conf.

ENVIRONMENT

       SOCKS_SERVER, if defined, specifies the name or IP address  of  the  SOCKS  proxy  server  host  to  use,
       overriding the default server compiled into the programs.

NAME

SYNOPSIS

DESCRIPTION

ENVIRONMENT

SEE ALSO