Here’s an exceedingly useful feature of SSH which I only discovered recently.
Imagine that you have a single ‘gateway’ machine on your network which you can connect to from outside using SSH; I do this all the time. You can then use that machine to connect to other machines inside your network in a variety of ways: using the port-forwarding abilities of SSH (the -L and -R options), for example, or simply by running another SSH command from the gateway machine once you’ve connected to it.
But there’s a much tidier way to do it, using the ProxyCommand option.
To connect to internalmachine.mynet.com, just add something like the following to your ~/.ssh/config:
Host internalmachine.mynet.com
ProxyCommand ssh gateway.mynet.com exec nc %h %p
then you can ssh directly to internalmachine.mynet.com from outside.
SSH will connect to the gateway machine and run ‘nc’ to forward the SSH
session to the internal machine.And, of course, you can use it for things layered over SSH, like checkouts from Git or Subversion repositories. Very tidy! I also sometimes add -C to the ssh command so that any access done this way is automatically compressed, even in situations where it was hard to specify that explicitly.
If you’re unlucky enough to find yourself stuck behind a web proxy with no other outgoing access, one very nice-looking use of ProxyCommand is the Corkscrew utility by Pat Padgett.
Hope this is helpful to someone!
Update: there are a few useful extra tips in the comments
from http://www.statusq.org/archives/2008/07/03/1916/
---------------------------------------------
SSH Proxy Command -- connect.c
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:
- Supports SOCKS (version 4/4a/5) and https CONNECT method.
- Supports NO-AUTH and USERPASS authentication of SOCKS
- You can input password from tty, ssh-askpass or environment variable.
- Run on UNIX or Windows platform.
- You can compile with various C compiler (cc, gcc, Visual C, Borland C. etc.)
- Simple and general program independent from OpenSSH.
- You can also relay local socket stream instead of standard I/O.
For windows user, pre-compiled binary is also available: http://www.taiyo.co.jp/~gotoh/ssh/connect.exe (compiled with MSVC)
- News
- What is 'proxy command'
- How to Use
- More Detail
- Specifying user name via environment variables
- Specifying password via environment variables
- Limitations
- Tips
- Break The More Restricted Wall
- F.Y.I.
News
- 2004-07-21
- Rev.1.84. Fixed some typo.
- 2004-05-18
- Rev.1.83. Fixed problem not work on Solaris.
- 2004-04-27
- Rev.1.82. Bug fix of memory clear on http proxying.
- 2004-04-22
- Rev. 1.81. Fixed memory violation and memory leak bug. New environment variable SOCKS5_PASSWD for sharing value with NEC SOCKS implementation. And document (this page) is updated.
- 2004-03-30
- Rev. 1.76. Fixed to accept multiple 'Proxy-Authorization' response.
- 2003-01-07
- Rev. 1.68. Fixed a trouble around timeout support.
- 2002-11-21
- Rev. 1.64 supports reading parameters from file /etc/connectrc or ~/.connectrc instead of specifying via environment variables. For examle, you can use this feature to switch setting by replacing file when network environment is changed. And added SOCKS_DIRECT, SOCKS5_DIRECT, SOCKS4_DIRECT, HTTP_DIRECT, SOCKS5_AUTH, environment parameters. (Thanks Masatoshi TSUCHIYA)
- 2002-11-20
- Rev. 1.63 supports some old proxies which make response 401 with WWW-Authenticate: header. And fixed to use username specified in proxy host by -H option correctly. (contributed from Des Herriott, thanks)
- 2002-10-14
- Rev. 1.61 with New option -w for specifying connection timeout. Currently, it works on UNIX only. (contributed from Darren Tucker, thanks)
- 2002-09-29
- Add sample script for switching proxy server advised from Darren Tucker, thanks.
- 2002-08-27
- connect.c is updataed to rev. 1.60.
- 2002-04-08
- Updated "Using OpenSSH through a SOCKS compatible PROXY on your LAN" written by J. Grant. (version 0.8)
- 2002-02-20
- Add link of new document "Using OpenSSH through a SOCKS compatible PROXY on your LAN" written by J. Grant.
- 2002-01-31
- Rev. 1.53 -- On Win32 and with MSVC, handle password input from console correctly.
- 2002-01-30
- Rev. 1.50 -- [Security Fix] Do not print secure info in debug mode.
- 2002-01-09
- Web page was made. connect.c is rev. 1.48.
What is 'proxy 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.
The connect.c program was made for this purpose.
How to Use
Get Source
Download source code from here.If you are MS Windows user, you can get pre-compiled binary from here.
Compile and Install
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 |
| Mac OS/Darwin | gcc connect.c -o connect -lresolv |
$ cp connect /usr/local/bin
Modify your ~/.ssh/config
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 %pIf 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
Use SSH
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 $
Have trouble?
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: 1.20 $ 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.
More Detail
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 porthost 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). Specifying user name via environment variables
There are 5 environemnt variables to specify user name without command line option. This mechanism is usefull for the user who using another user name different from system account.- SOCKS5_USER
- Used for SOCKS v5 access.
- SOCKS4_USER
- Used for SOCKS v4 access.
- SOCKS_USER
- Used for SOCKS v5 or v4 access and varaibles above are not defined.
- HTTP_PROXY_USER
- Used for HTTP proxy access.
- CONNECT_USER
- Used for all type of access if all above are not defined.
| SOCKS v5 | SOCKS v4 | HTTP proxy | |
|---|---|---|---|
| 1 | SOCKS5_USER | SOCKS4_USER | HTTP_PROXY_USER |
| 2 | SOCKS_USER | ||
| 3 | CONNECT_USER | ||
| 4 | (query user name to system) | ||
Specifying password via environment variables
There are 5 environemnt variables to specify password. If you use this feature, please note that it is not secure way.- SOCKS5_PASSWD
- Used for SOCKS v5 access. This variables is compatible with NEC SOCKS implementation.
- SOCKS5_PASSWORD
- Used for SOCKS v5 access if SOCKS5_PASSWD is not defined.
- SOCKS_PASSWORD
- Used for SOCKS v5 (or v4) access all above is not defined.
- HTTP_PROXY_PASSWORD
- Used for HTTP proxy access.
- CONNECT_PASSWORD
- Used for all type of access if all above are not defined.
| SOCKS v5 | HTTP proxy | |
|---|---|---|
| 1 | SOCKS5_PASSWD | HTTP_PROXY_PASSWORD |
| 2 | SOCKS_PASSWORD | |
| 3 | CONNECT_PASSWORD | |
| 4 | (ask to user interactively) | |
Limitations
SOCKS5 authentication
Only NO-AUTH and USER/PASSWORD authentications are supported. GSSAPI authentication (RFC 1961) and other draft authentications (CHAP, EAP, MAF, etc.) is not supported.HTTP authentication
BASIC authentication is supported but DIGEST authentication is not.Switching proxy server
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 $@
Tips
Proxying socket connection
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.
Use with 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 usessh-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.
Use for Network Stream of Emacs
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.taiyo.co.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. Remote resolver
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.Hopping Connection via SSH
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 portThis method is useful for the situations like:
- You are outside of organizasion now, but you want to access an internal host barriered by firewall.
- You want to use some service which is allowed only from some limited hosts.
$ ssh host1 connect news 119 200 news.my-office.com InterNetNews NNRP server INN 2.3.2 ready (posting ok). quit 205 . $By combinating hopping connection and relay.el, I can read NetNews using Wanderlust on Emacs at home.
|
External (internet) | Internal (office)
|
+------+ +----------+ +-------+ +-----------+
| HOME | | firewall | | host1 | | NNTP host |
+------+ +----------+ +-------+ +-----------+
emacs <-------------- ssh ---------------> sshd <-- connect --> nntpd
<-- connect --> socksd <-- SOCKS -->
As an advanced example, you can use SSH hopping as fetchmail's plug-in program to access via secure tunnel. This method requires that connect program is insatalled on remote host. There's example of .fetchmailrc bellow. When fetchmail access to mail-server, you will login to remote host using SSH then execute connect program on remote host to relay conversation with pop server. Thus fetchmail can retrieve mails in secure. poll mail-server protocol pop3 plugin "ssh %h connect localhost %p" username "username" password "password"
Break The More Restricted Wall
If firewall does not provide SOCKS nor HTTPS other than port 443, you cannot break the wall in usual way. But if you have you own host which is accessible from internet, you can make ssh connection to your own host by configuring sshd as waiting at port 443 instead of standard 22. By this, you can login to your own host via port 443. Once you have logged-in to extenal home machine, you can execute connect as second hop to make connection from your own host to final target host, like this:$ cat ~/.ssh/config Host home ProxyCommand connect -H firewall:8080 %h 443
Host server ProxyCommand ssh home connect %h %p ... internal$ ssh home You are logged in to home! home# exit internal$ ssh server You are logged in to server! server# exit internal$This way is similar to "Hopping connection via SSH" except configuring outer sshd as waiting at port 443 (https). This means that you have a capability to break the strongly restricted wall if you have own host out side of the wall.
|
Internal (office) | External (internet)
|
+--------+ +----------+ +------+ +--------+
| office | | firewall | | home | | server |
+--------+ +----------+ +------+ +--------+
<------------------ ssh --------------------->sshd:443
<-- connect --> http-proxy <-- https:443 --> any
connect <-- tcp --> port
NOTE: If you wanna use this, you should give up hosting https service at port 443 on you external host 'home'. F.Y.I.
Difference between SOCKS versions.
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.
Configuration to use HTTPS
Many http proxy servers implementation supports httpsCONNECT 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.
SOCKS5 Servers
- NEC SOCKS Reference Implementation
- Reference implementation of SOKCS server and library.
- Dante
- Dante is free implementation of SOKCS server and library. Many enhancements and modulalized.
- DeleGate
- DeleGate is multi function proxy service provider. DeleGate 5.x.x or earlier can be SOCKS4 server, and 6.x.x can be SOCKS5 and SOCKS4 server. and 7.7.0 or later can be SOCKS5 and SOCKS4a server.
Specifications
- socks4.protocol.txt
- SOCKS: A protocol for TCP proxy across firewalls
- socks4a.protocol.txt
- SOCKS 4A: A Simple Extension to SOCKS 4 Protocol
- RFC 1928
- SOCKS Protocol Version 5
- RFC 1929
- Username/Password Authentication for SOCKS V5
- RFC 2616
- Hypertext Transfer Protocol -- HTTP/1.1
- RFC 2617
- HTTP Authentication: Basic and Digest Access Authentication
Related Links
Similars
- Proxy Tunnel -- Proxying command using https CONNECT.
- stunnel -- Proxy through an https tunnel (Perl script)
