(Previous option of the week posts.)
--socks5 was added to curl back in 7.18.0. It takes an argument and that argument is the host name (and port number) of your SOCKS5 proxy server. There is no short option version.
Proxy
A proxy, often called a forward proxy in the context of clients, is a server that the client needs to connect to in order to reach its destination. A middle man/server that we use to get us what we want. There are many kinds of proxies. SOCKS is one of the proxy protocols curl supports.
SOCKS
SOCKS is a really old proxy protocol. SOCKS4 is the predecessor protocol version to SOCKS5. curl supports both and the newer version of these two, SOCKS5, is documented in RFC 1928 dated 1996! And yes: they are typically written exactly like this, without any space between the word SOCKS and the version number 4 or 5.
One of the more known services that still use SOCKS is Tor. When you want to reach services on Tor, or the web through Tor, you run the client on your machine or local network and you connect to that over SOCKS5.
Which one resolves the host name
One peculiarity with SOCKS is that it can do the name resolving of the target server either in the client or have it done by the proxy. Both alternatives exists for both SOCKS versions. For SOCKS4, a SOCKS4a version was created that has the proxy resolve the host name and for SOCKS5, which is really the topic of today, the protocol has an option that lets the client pass on the IP address or the host name of the target server.
The --socks5 option makes curl itself resolve the name. You’d instead use --socks5-hostname if you want the proxy to resolve it.
--proxy
The --socks5 option is basically considered obsolete since curl 7.21.7. This is because starting in that release, you can now specify the proxy protocol directly in the string that you specify the proxy host name and port number with already. The server you specify with --proxy. If you use a socks5:// scheme, curl will go with SOCKS5 with local name resolve but if you instead use socks5h:// it will pick SOCKS5 with proxy-resolved host name.
SOCKS authentication
A SOCKS5 proxy can also be setup to require authentication, so you might also have to specify name and password in the --proxy string, or set separately with --proxy-user. Or with GSSAPI, so curl also supports --socks5-gssapi and friends.
Examples
Fetch HTTPS from example.com over the SOCKS5 proxy at socks5.example.org port 1080. Remember that –socks5 implies that curl resolves the host name itself and passes the address to to use to the proxy.
curl --socks5 socks5.example.org:1080 https://example.com/
Or download FTP over the SOCKS5 proxy at socks5.example port 9999:
curl --socks5 socks5.example:9999 ftp://ftp.example.com/SECRET
Useful trick!
A very useful trick that involves a SOCKS proxy is the ability OpenSSH has to create a SOCKS tunnel for us. If you sit at your friends house, you can open a SOCKS proxy to your home machine and access the network via that. Like this. First invoke ssh, login to your home machine and ask it to setup a SOCKS proxy:
ssh -D 8080 user@home.example.com
Then tell curl (or your browser, or both) to use this new SOCKS proxy when you want to access the Internet:
curl --socks5 localhost:8080 https:///www.example.net/
This will effectively hide all your Internet traffic from your friends snooping and instead pass it all through your encrypted ssh tunnel.
Related options
As already mentioned above, --proxy is typically the preferred option these days to set the proxy. But --socks5-hostname is there too and the related --socks4 and --sock4a.