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ssh-proxy

ssh ProxyCommand script to smartly proxy only when necessary.

Simple shell wrapper to proxy or tunnel or hop through intermediate ssh servers, SOCKS proxies, or HTTP proxies (e.g. corkscrew), when ssh would be unable to connect directly.

Most often useful on portable clients (e.g. laptops or tablets), especially when only SOMETIMES behind a draconian firewall or outside a network with limited ssh connections inbound. In all cases, if you have a VPN option, that will probably be better, but even in those cases, this script should not interfere and allows you to be "lazy" in your VPN usage.

Intended for use inside your ~/.ssh/config file, like these examples:

Outbound http proxy (e.g., behind a restrictive outbound firewall):

Host *
  ProxyCommand $HOME/.ssh/proxy -p httpproxy.foo.com:8080 %h %p

If not specified on the command line, the proxy will default to the first non-empty evironment variables, in order:

  • http_proxy
  • HTTP_PROXY
  • https_proxy
  • HTTPS_PROXY

Inbound hop, when limited inbound ssh connections allowed:

Host *
  ProxyCommand $HOME/.ssh/proxy -h ssh-hop.foo.com:8080 %h %p

In either case, the script will first attempt a direct connection, avoiding any overhead.

Another feature enables you to setup "pseudo" hostnames in your ~/.ssh/config file and use a sed regexp to modify the destination hostname before attempting a connection. For example:

Host prefix-*
  ProxyCommand $HOME/.ssh/proxy -S 's/^prefix-//' -h subnet-bastion %h %p

In those cases, the destiation hostname would have the prefix- removed before attempting to connect to the destiation server. This can be helpful if an entire subnet of hosts is behind a special bastion server, and you need or want rather just have hostname resolution happen on the bastion. Side-effect: your ~/.ssh/known_hosts file will be populated with keys that still include the prefix-.

In general, this script assumes ssh, netcat (nc) and corkscrew (an http-proxy available from http://www.agroman.net/corkscrew). As an alternative to corkscrew, some people have repoted success using desproxy (http://sourceforge.net/projects/desproxy/).

Netcat is used both to detect the availablility of the destination or proxy hosts as well as for direct connections. Both are assumed to be resident in your PATH. If not, you may specify a different http-proxy or alternate location for netcat on the command line (in your ssh config) OR, of course, "use the source, Luke"...

Comments, suggestions, questions welcome.