(previous options of the week)
--ftp-pasv has no short version of the option.
This option was added in curl 7.11.0, January 2004.
FTP uses dual connections
FTP is a really old protocol with traces back to the early 1970s, actually from before we even had TCP. It truly comes from a different Internet era.
When a client speaks FTP, it first sets up a control connection to the server, issues commands to it that instructs it what to do and then when it wants to transfer data, in either direction, a separate data connection is required; a second TCP connection in addition to the initial one.
This second connection can be setup two different ways. The client instructs the server which way to use:
- the client connects again to the server (passive)
- the server connects back to the client (active)
--ftp-pasv is the default behavior of curl and asks to do the FTP transfer in passive mode. As this is the default, you will rarely see this option used. You could for example use it in cases where you set active mode in your
.curlrc file but want to override that decision for a single specific invocation.
The opposite behavior, active connection, is called for with the
--ftp-port option. An active connection asks the server to do a TCP connection back to the client, an action that these days is rarely functional due to firewall and network setups etc.
FTP passive behavior detailed
FTP was created decades before IPv6 was even thought of, so the original FTP spec assumes and uses IPv4 addresses in several important places, including how to ask the server for setting up a passive data transfer.
EPSV is the “new” FTP command that was introduced to enable FTP over IPv6. I call it new because it wasn’t present in the original RFC 959 that otherwise dictates most of how FTP works. EPSV is an extension.
EPSV was for a long time only supported by a small fraction of FTP servers and it probably still exists such servers, so in case invoking this command is a problem, curl needs to fallback to the original command for this purpose:
curl first tries EPSV and if that fails, it sends PASV. The first of those commands that is accepted by the server will make the server wait for the client to connect to it (again) on fresh new TCP port number.
Problems with the dual connections
Your firewall needs to allow your client to connect to the new port number the server opened up for the data connection. This is particularly complicated if you enable FTPS (encrypted FTP) as then the new port number is invisible to middle-boxes such as firewalls.
Dual connections also often cause problems because the control connection will be idle during the transfer and that idle connection sometimes get killed by NATs or other middle-boxes due to that inactivity.
Enabling active mode is usually even worse than passive mode in modern network setups, as then the firewall needs to allow an outside TCP connection to come in and connect to the client on a given port number.
Ask for a file using passive FTP mode:
curl --ftp-pasv ftp://ftp.example.com/cake-recipe.txt
--ftp-skip-pasv-ip tells curl that the IP address in the server’s PASV response should be ignored and the IP address of the control connection should instead be used when the second connection is setup.