connect.c is the simple relaying command to make network connection via SOCKS and https proxy. It is mainly intended to be used as proxy command of OpenSSH. You can make SSH session beyond the firewall with this command,
Features of connect.c are:
Download source code from:
http://www.imasy.or.jp/~gotoh/ssh/connect.c
For windows user, pre-compiled binary is also available:
http://www.imasy.or.jp/~gotoh/ssh/connect.exe (compiled with MSVC)
ssh-askpass command
OpenSSH development team decides to stop supporting SOCKS and any other tunneling mechanism. It was aimed to separate complexity to support various mechanism of proxying from core code. And they recommends more flexible mechanism: 'ProxyCommand' option instead.
Proxy command mechanism is delegation of network stream communication. If 'ProxyCommand' options is specified, SSH invoke specified external command and talk with standard I/O of thid command. Invoked command undertakes network communication with relaying to/from standard input/output including iniitial communication or negotiation for proxying. Thus, ssh can split out proxying code into external command.
'connect.c' was made for this purpose.
Download source code from here.
If you are MS Windows user, you can get pre-compiled binary from
here.
In most environment, you can compile 'connect.c' simply. On UNIX environment, you can use cc or gcc. On Windows environment, you can use Microsoft Visual C, Borland C or Cygwin gcc.
| Compiler | command line to compile |
|---|---|
| UNIX cc | cc connect.c -o connect |
| UNIX gcc | gcc connect.c -o connect |
| Solaris | gcc connect.c -o connect -lnsl -lsocket -lresolv |
| Microsoft Visual C/C++ | cl connect.c wsock32.lib advapi32.lib |
| Borland C | bcc32 connect.c wsock32.lib advapi32.lib |
| Cygwin gcc | gcc connect.c -o connect |
To install 'connect' command, simply copy compiled binary to directory in your PATH (ex. /usr/local/bin). Like this:
$ cp connect /usr/local/bin
Modify your ~/.ssh/config file to use 'connect' command as
'proxy command'. For the case of SOCKS server is running on
firewall host 'socks.local.net' with port 1080, you can add
'ProxyCommand' option in ~/.ssh/config, like this:
Host remote.outside.net ProxyCommand connect -S socks.local.net %h %p
'%h' and '%p' will be replaced on invoking proxy command with
target hostname and port specified to SSH command.
If you hate writing many entries of remote hosts, following example may help you.
## Outside of the firewall, use connect command with SOCKS conenction. Host * ProxyCommand connect -S socks.local.net %h %p ## Inside of the firewall, use connect command with direct connection. Host *.local.net ProxyCommand connect %h %p
If you want to use http proxy, use '-H' option instead of '-S' option in examle above, like this:
## Outside of the firewall, with HTTP proxy Host * ProxyCommand connect -H proxy.local.net:8080 %h %p ## Inside of the firewall, direct Host *.local.net ProxyCommand connect %h %p
After editing your ~/.ssh/config file, you are ready to use ssh.
You can execute ssh without any special options as if remote host is
IP reachable host. Following is an example to execute 'hostname'
command on host 'remote.outside.net'.
$ ssh remote.outside.net hostname remote.outside.net $
If you have trouble, execute 'connect' command from command line
with '-d' option to see what is happened. Some debug message may
appear and reports progress. This information may tell you what is
wrong. In this example, error has occurred on authentication stage of
SOCKS5 protocol.
$ connect -d -S socks.local.net unknown.remote.outside.net 110 DEBUG: relay_method = SOCKS (2) DEBUG: relay_host=socks.local.net DEBUG: relay_port=1080 DEBUG: relay_user=gotoh DEBUG: socks_version=5 DEBUG: socks_resolve=REMOTE (2) DEBUG: local_type=stdio DEBUG: dest_host=unknown.remote.outside.net DEBUG: dest_port=110 DEBUG: Program is $Revision$ DEBUG: connecting to xxx.xxx.xxx.xxx:1080 DEBUG: begin_socks_relay() DEBUG: atomic_out() [4 bytes] DEBUG: >>> 05 02 00 02 DEBUG: atomic_in() [2 bytes] DEBUG: <<< 05 02 DEBUG: auth method: USERPASS DEBUG: atomic_out() [some bytes] DEBUG: >>> xx xx xx xx ... DEBUG: atomic_in() [2 bytes] DEBUG: <<< 01 01 ERROR: Authentication faield. FATAL: failed to begin relaying via SOCKS.
Command line usage is here:
usage: connect [-dnhs45] [-R resolve] [-p local-port] [-w sec] [-H [user@]proxy-server[:port]] [-S [user@]socks-server[:port]] host port
'host' and 'port' is target hostname and port-number to connect.
'-H' option specify hostname and port number of http proxy server to
relay. If port is omitted, 80 is used. You can specify this value by
environment variable HTTP_PROXY and give '-h' option to use it.
'-S' option specify hostname and port number of SOCKS server to
relay. Like '-H' option, port number can be omit and default is 1080.
You can also specify this value pair by environment variable
SOCKS5_SERVER and give '-s' option to use it.
'-4' and '-5' is for specifying SOCKS protocol version. It is valid only using with '-s' or '-S'. Default is '-5' (protocol version 5)
'-R' is for specifying method to resolve hostname. 3 keywords
('local', 'remote', 'both') or dot-notation IP address is
allowed. Keyword 'both' means; "Try local first, then
remote". If dot-notation IP address is specified, use this host as
nameserver (UNIX only). Default is 'remote' for SOCKS5 or 'local'
for others. On SOCKS4 protocol, remote resolving method ('remote'
and 'both') use protocol version 4a.
The '-p' option will forward a local TCP port instead of using the standard input and output.
The '-w' option specifys timeout seconds for making connection with TARGET host.
The '-a' option specifiys user intended authentication methods
separated by comma. Currently 'userpass' and 'none' are
supported. Default is 'userpass'. You can also specifying this
parameter by the environment variable SOCKS5_AUTH.
The '-d' option is used for debug. If you fail to connect, use this and check request to and response from server.
You can omit 'port' argument when program name is special format containing port number itself. For example,
$ ln -s connect connect-25 $ ./connect-25 smtphost.outside.net 220 smtphost.outside.net ESMTP Sendmail QUIT 221 2.0.0 smtphost.remote.net closing connection $
This example means that the command name "connect-25" contains port number
25 so you can omit 2nd argument (and used if specified explicitly).
Only NO-AUTH and USER/PASSWORD authentications are supported. GSSAPI authentication (RFC 1961) and other draft authentications (CHAP, EAP, MAF, etc.) is not supported.
BASIC authentication is supported but DIGEST authentication is not.
There is no mechanism to switch proxy server regarding to PC environment. This limitation might be bad news for mobile user. Since I do not want to make this program complex, I do not want to support although this feature is already requested. Please advice me if there is good idea of detecting environment to swich and simple way to specify conditioned directive of servers.
One tricky workaround exists. It is replacing ~/.ssh/config file by script on ppp up/down.
There's another example of wrapper script (contributed by Darren Tucker). This script costs executing ifconfig and grep to detect current environment, but it works. (NOTE: you should modify addresses if you use it.)
#!/bin/sh ## ~/bin/myconnect --- Proxy server switching wrapper if ifconfig eth0 |grep "inet addr:192\.168\.1" >/dev/null; then opts="-S 192.168.1.1:1080" elif ifconfig eth0 |grep "inet addr:10\." >/dev/null; then opts="-H 10.1.1.1:80" else opts="-s" fi exec /usr/local/bin/connect $opts $@
In usual, 'connect.c' relays network connection to/from standard input/output. By specifying '-p' option, however, 'connect.c' relays local network stream instead of standard input/output. With this option, 'connect' command waits connection from other program, then start relaying between both network stream.
This feature may be useful for the program which is hard to SOCKSify.
ssh-askpass command
'connect.c' ask you password when authentication is required. If
you are using on tty/pty terminal, connect can input from terminal
with prompt. But you can also use 'ssh-askpass' program to input
password. If you are graphical environment like X Window or MS
Windows, and program does not have tty/pty, and environment variable
SSH_ASKPASS is specified, then 'connect.c' invoke command
specified by environment variable 'SSH_ASKPASS' to input password.
ssh-askpass program might be installed if you are using OpenSSH on
UNIX environment. On Windows environment, pre-compiled binary is
available from
here.
This feature is limited on window system environment.
And also useful on Emacs on MS Windows (NT Emacs or Meadow). It is hard to send passphrase to 'connect' command (and also ssh) because external command is invoked on hidden terminal and do I/O with this terminal. Using ssh-askpass avoids this problem.
Although 'connect.c' is made for OpenSSH, it is generic and independent from OpenSSH. So we can use this for other purpose. For example, you can use this command in Emacs to open network connection with remote host over the firewall via SOCKS or HTTP proxy without SOCKSifying Emacs itself.
There is sample code: http://www.imasy.or.jp/~gotoh/lisp/relay.el
With this code, you can use relay-open-network-stream function
instead of open-network-stream to make network connection. See top
comments of source for more detail.
If you are SOCKS4 user on UNIX environment, you might want specify nameserver to resolve remote hostname. You can do it specifying '-R' option followed by IP address of resolver.
Conbination of ssh and 'connect' command have more interesting usage. Following command makes indirect connection to host2:port from your current host via host1.
ssh host1 connect host2 port
This method is useful for the situations like:
For example, I want to use local NetNews service in my office from home. I cannot make NNTP session directly because NNTP host is barriered by firewall. Fortunately, I have ssh account on internal host and allowed using SOCKS5 on firewall from outside. So I use following command to connect to NNTP service.
$ ssh host1 connect news 119 200 news.my-office.com InterNetNews NNRP server INN 2.3.2 ready (posting ok). quit 205 . $
By combinating hopping connection and relay.el, I can read NetNews using Wanderlust on Emacs at home.
|
External (internet) | Internal (office)
|
+------+ +----------+ +-------+ +-----------+
| HOME | | firewall | | host1 | | NNTP host |
+------+ +----------+ +-------+ +-----------+
emacs <-------------- ssh ---------------> sshd <-- connect --> nntpd
<-- connect --> socksd <-- SOCKS -->
SOCKS version 4 is first popular implementation which is documented here. Since this protocol provide IP address based requesting, client program should resolve name of outer host by itself. Version 4a (documented here) is enhanced to allow request by hostname instead of IP address.
SOCKS version 5 is re-designed protocol stands on experience of version 4 and 4a. There is no compativility with previous versions. Instead, there's some improvement: IPv6 support, request by hostname, UDP proxying, etc.
Many http proxy servers implementation supports https CONNECT method
(SLL). You might add configuration to allow using https. For the
example of DeleGate (
DeleGate is a multi-purpose application level gateway, or a proxy
server) , you should add 'https' to 'REMITTABLE' parameter to
allow HTTP-Proxy like this:
delegated -Pxxxx ...... REMITTABLE='+,https' ...
For the case of Squid, you should allow target ports via https by ACL, and so on.