Proxies, jump hosts and tunnels

When the server is not directly reachable, linscp can go through something in between: a proxy server, a command that opens the connection for it, or an SSH tunnel through another host. All three are set per site, on the Proxy and Tunnel pages under Connection in Advanced Site Settings.

Proxy servers

On the Proxy page, pick the Proxy type and fill in Proxy hostname, Port number and, if the proxy needs a login, Username and Password. The types offered depend on the protocol:

Type	Protocols	Notes
SOCKS4, SOCKS5	All	SOCKS5 can carry a login and resolve names.
HTTP	All	Uses the CONNECT method of a web proxy.
Telnet	SFTP, SCP	Logs in to a proxy that accepts typed commands.
Local	SFTP, SCP	Runs a command on your computer that provides the connection.
SITE %host, USER %user@%host and others	FTP	Logon styles of classic FTP proxies.

Autodetect fills in your system’s HTTP proxy, as Qt reports it. On Linux that usually comes from the http_proxy environment variable.

For SSH sites, Proxy settings adds two options:

• Do DNS name lookup at proxy end lets the proxy resolve the server name. Use it when your own machine cannot resolve internal names.
• Consider proxying local host connections sends even connections to localhost through the proxy. It is off by default.

With a SOCKS or HTTP proxy, FTP sites use passive mode.

Jump hosts with a local proxy command

The Local type hands the connection to a program. Its standard input and output become the connection to the server. With OpenSSH installed, this is the simplest way to go through a jump host:

ssh -W %host:%port alice@jump.example.org

linscp replaces %host and %port with the target server’s name and port (hover over patterns for the full list) and runs the command with /bin/sh -c. The jump login must not need typing: use a key loaded in your agent, or a key without passphrase listed in ~/.ssh/config. Since ssh reads ~/.ssh/config, a command such as ssh -W %host:%port bastion can rely on the settings you already have there, including chains of ProxyJump hosts.

SSH tunnel

An SSH tunnel uses a second SSH login to forward the connection, without needing OpenSSH on your computer. It is available for SFTP and SCP sites.

1. On the Tunnel page, check Connect through SSH tunnel.
2. Under Host to setup tunnel on, enter the jump host’s Hostname, Port number, Username and, if needed, Password.
3. Under Tunnel authentication parameters, choose a Private key file if the jump host uses key login.
4. Leave Local tunnel port on Autoselect unless a firewall on your computer allows only a fixed port.

The jump host’s host key is checked like any other; see Verifying host keys. The tunnel takes one hop. For more, use a local proxy command.

Tip

Importing hosts from ~/.ssh/config with Tools › Import Sites… turns a single ProxyJump entry into the tunnel settings for you.

The update check’s proxy

The proxy choices on Options › Preferences › Updates apply only to the update check. Your sites use their own Proxy page.