★backend vsftpdサーバ×2

vi /etc/vsftpd/vsftpd.conf

pasv_enable=YES
pasv_min_port=60000
pasv_max_port=60001
pasv_address=lb.x.x.x

★frontend nginxサーバ

vi nginx.conf

stream {
#FTP
upstream ftp {
hash $remote_addr;
server vsftpd.nilesflow.net:21 weight=5 max_fails=3 fail_timeout=30s;
server aws-default.nilesflow.net:21 weight=5 max_fails=3 fail_timeout=30s;
# server pure-ftpd.nilesflow.net:10021 weight=5 max_fails=3 fail_timeout=30s;
}

server {
listen 20021;
proxy_connect_timeout 1s;
# proxy_timeout 3s;
proxy_pass ftp;
}

#FTP data
upstream ftpdata-v1 {
hash $remote_addr;
server vsftpd.nilesflow.net:60000 weight=5 max_fails=3 fail_timeout=30s;
server aws-default.nilesflow.net:60000 weight=5 max_fails=3 fail_timeout=30s;
}
upstream ftpdata-v2 {
server vsftpd.nilesflow.net:60001 weight=5 max_fails=3 fail_timeout=30s;
server aws-default.nilesflow.net:60001 weight=5 max_fails=3 fail_timeout=30s;
}

# vsftpd
server {
listen 60000;
proxy_connect_timeout 1s;
proxy_timeout 3s;
proxy_pass ftpdata-v1;
}
server {
listen 60001;
proxy_connect_timeout 1s;
proxy_timeout 3s;
proxy_pass ftpdata-v2;
}

★どこかのサーバから

ftp lb.nilesflow.net 20021

★補足

pasv_address= ドメイン名ではなくIPアドレス