Content-Length Header responses being Reset by LTM

To add an XFF HTTP header without the HTTP profile, you'd use TCP events:

when CLIENT_ACCEPTED {
    TCP::collect
}
when CLIENT_DATA {  
    if { [TCP::payload] contains "User-Agent:" } {
        set ua "\"User-Agent:\" \"X-Forwarded-For: [IP::client_addr]\r\nUser-Agent:\""
        TCP::payload replace 0 [TCP::payload length] [string map $ua [TCP::payload]]
    }   
    TCP::release
    TCP::collect
}

If the VIP is SSL-enabled, then this:

when CLIENTSSL_HANDSHAKE {
    SSL::collect
}
when CLIENTSSL_DATA {   
    if { [SSL::payload] contains "User-Agent:" } {
        set ua "\"User-Agent:\" \"X-Forwarded-For: [IP::client_addr]\r\nUser-Agent:\""
        SSL::payload replace 0 [SSL::payload length] [string map $ua [SSL::payload]]
    }   
    SSL::release
    SSL::collect
}