Problem this snippet solves:
Summary: This is an advancement on the work done by Bhattman FTP proxy (https://devcentral.f5.com/codeshare/ftp-proxy-v10-and-up) - An FTP proxy that simulates FTP proxies found on Bluecoat, Cisco Ironport and Sidewinder firewall Proxies. I've added simple local user auth and code to handle different scenarios as the original Bhattman code didn't work out of the box in our user case and required some rework.
It requires virtual server to listen on port 21 for FTP and FTP profile associated.
tested on v12.
Code :
when RULE_INIT {
    
    #set authproxy to 1 to enable local authentication of FTP proxy - user DB exists in FTP-user datagroup. authproxy eq 0 proxy does not do local auth.
    set static::authproxy 1
set static::debug 1

    set static::nonauthbanner "220-220 ---------------------------------------------------------------------------\r\n220-220\
- * * * Welcome to the FTP Gateway * * *\r\n220-220\
---------------------------------------------------------------------------\r\n220-220\
-\r\n220-220\
-\r\n220-220\
-   For connecting to an external ftp server use the following syntax in the username field:\r\n220-220\
-\r\n220-220\
-   remote_user_name@remote_ftp_server\r\n220-220\
-\r\n220-220\
-   Example: user login@remoteserver\r\n220-220\
-\r\n220-220\
-\r\n220-220\
-   Notice: All operations will be logged!\r\n220-220\
-\r\n220-220\
-\r\n220-220\
--------------------------------------------------------------------------\r\n220-220\
-\r\n220 remote_user_name@remote_ftp_server\r\n"

set static::authbanner "220 Welcome to the F5 FTP Proxy - you must login first\r\n"

set static::authenticatedbanner "230-230 Proxy authentication successful\r\n230-230\
---------------------------------------------------------------------------\r\n230-230\
- * * * Welcome to the FTP Gateway * * *\r\n230-230\
---------------------------------------------------------------------------\r\n230-230\
-\r\n230-230\
-\r\n230-230\
- You must login to the remote server\r\n230-230\
-\r\n230-230\
-\r\n230-230\
- Usage: Use the user command to connect\r\n230-230\
-\r\n230-230\
-\r\n230-230\
- example: user login@remoteserver\r\n230-230\
-\r\n230-230\
-\r\n230-230\
- Notice: All operations will be logged!\r\n220-220\
-\r\n230-230\
--------------------------------------------------------------------------\r\n230-230\
-\r\n230 F5 FTP proxy\r\n"
}

when CLIENT_ACCEPTED {

if { $static::debug } { log local0. "client FTP accepted" }
    #set variable to flag whether we've sent a proxy request to remote server
 set proxyconnect 0
 
 # check if the proxy is authenticating locally first - if not set user as authenticated else unauntenticated.
    if {$static::authproxy eq 0 } {
        TCP::respond $static::nonauthbanner
        set authenticated 1
        } else {
        TCP::respond $static::authbanner
        set authenticated 0
    }    
        
  TCP::collect
}

when CLIENT_DATA {

if { $static::debug } { log local0. "client payload - [TCP::payload]" }

# if user is authenticated and USER command is seen - it's a remote server proxy connection
if { [TCP::payload] starts_with "USER" && $authenticated eq 1 } {

    
#take the USER string in form of "USER user@sitename"
        scan [TCP::payload] {%[^@]@%s} garbage sitename
        scan $garbage %s%s cmd uid
set sitename [string tolower $sitename]
if { $static::debug } { log local0. "sitename:$sitename - cmd:$cmd - uid:$uid" }
        
#is the sitename an IP or hostname - if the sitename looks like an IP save it, else try and resolve the IP address
if { $sitename matches_regex {\b(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\b} } {
set ips $sitename
} else {
set ips [RESOLV::lookup @1.1.1.1 -a $sitename]
}
        
if {$ips eq "" } {
#No IP or sitename was not resolvable - reject the connection
if { $static::debug } { log local0. "Could not determine the FTP site - sitename:$sitename"}
TCP::respond "434 Requested host unavailable - $sitename was not resolvable \r\n"
TCP::payload replace 0 [TCP::payload length] ""
reject
} else {
#clear the payload and forward the ftp connection to the destination
TCP::payload replace 0 [TCP::payload length] ""
node [lindex $ips 0] [TCP::local_port]
TCP::release
if { $static::debug } { log local0. "address [lindex $ips 0] port [TCP::local_port]" }
}


} elseif { $authenticated eq 0 }{
#user is still unauthenticated lets try and authenticate
if { [TCP::payload] starts_with "USER" } {
# get user id
scan [TCP::payload] %s%s cmd uid

if { $static::debug } { log local0. "UID: $uid\n" }
# respond to client with password request
    TCP::respond "331 Password required for $uid\r\n"
            TCP::payload replace 0 [TCP::payload length] ""
} elseif { [TCP::payload] starts_with "PASS" } {
#if password received from client lets try and authenticate
#get the password from payload
scan [TCP::payload] %s%s cmd pass

#compare cleartext password with entry in FTP-users datagroup - dataqgroup must be created.
if { [ set password [class match -value -- $uid equals FTP-users]] eq $pass } {
#user authenticated
if { $static::debug } { log local0. "user authenticated\n" }
                # if success respond with 230 and set authenticated flag to 1
TCP::respond $static::authenticatedbanner
TCP::payload replace 0 [TCP::payload length] ""
set authenticated 1
} else {
TCP::respond "430 Invalid Username or password\r\n"
TCP::payload replace 0 [TCP::payload length] ""
set authenticated 0
}

}

}

if { $proxyconnect eq 0 } {

# deal with commands sent to F5 proxy before connection establed to remote server.

if { [TCP::payload] starts_with "SYST"} {
    TCP::respond "215 F5 emulated proxy\r\n"
    TCP::payload replace 0 [TCP::payload length] ""
} elseif { [TCP::payload] starts_with "QUIT"} {
    TCP::respond "221 Goodbye\r\n"
    TCP::payload replace 0 [TCP::payload length] ""
} elseif { (!([TCP::payload] starts_with "USER") || !([TCP::payload] starts_with "PASS")) && !([TCP::payload] eq "") } { 
    if { $static::debug } { log local0. "proxyc=0 elseif TCPP=[TCP::payload]"}
    # if a command is sent to F5 proxy and there isn't a remote server connection established respond with command not recognised
TCP::respond "501-- Proxy command not recognised\r\n501 Connect to remote server first using: user login@remoteserver\r\n"
    TCP::payload replace 0 [TCP::payload length] ""
}
}


#only release the TCP payload if user has has connected to proxy and is authenticated otherwise just collect TCP payload for user authentication
 if {$proxyconnect && $authenticated} {TCP::release}
  TCP::collect
}


when SERVER_CONNECTED {
if { $static::debug } { log local0. "connected to server" }
    set proxyconnect 1
  TCP::collect
}

when SERVER_DATA {

if { $static::debug } { log local0. "server payload [TCP::payload]" }

    if { [TCP::payload] starts_with "220 " }{
    #220 connection ok - return the user name specified to destination FTP server 
if { $static::debug } { log local0. "server found 220 ok" }
    TCP::respond "USER $uid\r\n"
    TCP::payload replace 0 [TCP::payload length] ""

    } elseif { [TCP::payload] starts_with "220-" }{
    #deal with multiline banners - clear the payload ensure nothing is sent back to client
    TCP::payload replace 0 [TCP::payload length] ""
    }
    

 TCP::release
 TCP::collect

}
Tested this on version:
12.0
unauthenticated or authenticated FTP proxy
