+ unsigned port_num;
+
+/*
+TODO: PASV command will not work for IPv6. RFC2428 describes
+IPv6-capable "extended PASV" - EPSV.
+
+"EPSV [protocol]" asks server to bind to and listen on a data port
+in specified protocol. Protocol is 1 for IPv4, 2 for IPv6.
+If not specified, defaults to "same as used for control connection".
+If server understood you, it should answer "229 <some text>(|||port|)"
+where "|" are literal pipe chars and "port" is ASCII decimal port#.
+
+There is also an IPv6-capable replacement for PORT (EPRT),
+but we don't need that.
+
+NB: PASV may still work for some servers even over IPv6.
+For example, vsftp happily answers
+"227 Entering Passive Mode (0,0,0,0,n,n)" and proceeds as usual.
+
+TODO2: need to stop ignoring IP address in PASV response.
+*/
+
+ if (ftpcmd("PASV", NULL) != 227) {
+ ftp_die("PASV");
+ }