ConfigServer Community Forum

Hi All,

We have seen a behaviour in all "CloudLinux 8" servers with the introduction of nftables with iptables with connections using PASV.

To this test, we have ensured that on TCP_IN ports for FTP are defined between 50000-59000 and Pure-FTPD configured to use that ports.

With CSF enabled, when we try to connect to a remote FTP and it tries to list directories, this is the result:

# ftp transfer.xxxxxxx.com
Connected to transfer.xxxxxxx.com (XX.XXXX.XXXX.XXX).
220 Welcome to FTP Server
Name (transfer.xxxxxxx.com:root): user
331 User name okay, need password for user.
Password:
230 User logged in, proceed.
Remote system type is UNIX.
Using binary mode to transfer files.
ftp> ls
227 Entering Passive Mode (XX,XXX,XXX,XXX,192,4)
ftp: connect: Connection refused
ftp> quit
221 Goodbye.
#

On dmesg, this is the result
server kernel: Firewall: *TCP_IN Blocked* IN=eno1 OUT= MAC=XXXXX SRC=(FTP_DESTINATION_IP) DST=(THIS_SERVER) LEN=52 TOS=0x02 PREC=0x00 TTL=110 ID=31020 DF PROTO=TCP SPT=20 DPT=60229 WINDOW=8192 RES=0x00 CWR ECE SYN URGP=0

If we disable the Firewall, the connection is done with success.

In TCP_OUT we are not adding the Passive Ports. The same on other CloudLinux 7 and works fine.

So, TCP_OUT without Passive Ports for FTP in CL7 or CL6 works perfectly for ftp connections but on CL8 fails. We tested this behaviour in 4 x CL8 servers and 6 x CL7 servers.

This is know or any suggestion that not result in adding the Passive Ports in TCP_OUT ?

Kind Regards,

Note:

For example, if we change on TCP_IN the port from 50000-59000 to 49152-65534 it works.

If you see the error on dmesg:
server kernel: Firewall: *TCP_IN Blocked* IN=eno1 OUT= MAC=XXXXX SRC=(FTP_DESTINATION_IP) DST=(THIS_SERVER) LEN=52 TOS=0x02 PREC=0x00 TTL=110 ID=31020 DF PROTO=TCP SPT=20 DPT=60229 WINDOW=8192 RES=0x00 CWR ECE SYN URGP=0

When the connection to the FTP is established and we do the "ls", the (FTP_DESTINATION_IP) (FTP SERVER) tries to make a TCP_IN connection to this server on Port 60229. This port is not allowed on Firewall.

Something is not establishing the entry port in the server for passive ports when the interaction is the ftp cli or php ftp_get.

After talking with CloudLinux, they mention:

v11 - https://blog.configserver.com/?p=3152
Ensure /proc/sys/net/netfilter/nf_conntrack_helper is enabled at startup to allow connection tracking to continue working on newer kernels

v12 - https://blog.configserver.com/?p=3249
Added new option USE_FTPHELPER. This enables the ftp helper via the iptables CT target on supporting kernels instead of the current method via /proc/sys/net/netfilter/nf_conntrack_helper and unrestricted use of RELATED state