Shorewall and FTP

Tom Eastep

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover, and with no Back-Cover Texts. A copy of the license is included in the section entitled “GNU Free Documentation License”.


Table of Contents

FTP Protocol
Linux FTP connection-tracking
FTP on Non-standard Ports


This article applies to Shorewall 4.0 and later. If you are running a version of Shorewall earlier than Shorewall 4.0.0 then please see the documentation for that release.

FTP Protocol

FTP transfers involve two TCP connections. The first control connection goes from the FTP client to port 21 on the FTP server. This connection is used for logon and to send commands and responses between the endpoints. Data transfers (including the output of “ls” and “dir” commands) requires a second data connection. The data connection is dependent on the mode that the client is operating in:

Passive Mode

(often the default for web browsers) -- The client issues a PASV command. Upon receipt of this command, the server listens on a dynamically-allocated port then sends a PASV reply to the client. The PASV reply gives the IP address and port number that the server is listening on. The client then opens a second connection to that IP address and port number.

Active Mode

(often the default for line-mode clients) -- The client listens on a dynamically-allocated port then sends a PORT command to the server. The PORT command gives the IP address and port number that the client is listening on. The server then opens a connection to that IP address and port number; the source port for this connection is 20 (ftp-data in /etc/services).

You can see these commands in action using your linux ftp command-line client in debugging mode. Note that my ftp client defaults to passive mode and that I can toggle between passive and active mode by issuing a “passive” command:

[teastep@wookie Shorewall]$ ftp
Connected to
220-=(<*>)=-.:. (( Welcome to PureFTPd 1.0.12 )) .:.-=(<*>)=-
220-You are user number 1 of 50 allowed.
220-Local time is now 10:21 and the load is 0.14. Server port: 21.
220 You will be disconnected after 15 minutes of inactivity.
500 Security extensions not implemented
500 Security extensions not implemented
KERBEROS_V4 rejected as an authentication type
Name ( ftp
331-Welcome to
331 Any password will work
230 Any password will work
Remote system type is UNIX.
Using binary mode to transfer files.
ftp> debug
Debugging on (debug=1).
ftp> ls
---> PASV
227 Entering Passive Mode (192,168,1,193,195,210)
---> LIST
150 Accepted data connection
drwxr-xr-x    5 0        0            4096 Nov  9  2002 archives
drwxr-xr-x    2 0        0            4096 Feb 12  2002 etc
drwxr-sr-x    6 0        50           4096 Feb 19 15:24 pub
226-Options: -l
226 3 matches total
ftp> passive
Passive mode off.
ftp> ls
---> PORT 192,168,1,3,142,58
200 PORT command successful
---> LIST
150 Connecting to port 36410
drwxr-xr-x    5 0        0            4096 Nov  9  2002 archives
drwxr-xr-x    2 0        0            4096 Feb 12  2002 etc
drwxr-sr-x    6 0        50           4096 Feb 19 15:24 pub
226-Options: -l
226 3 matches total

Things to notice:

  1. The commands that I issued are strongly emphasized.

  2. Commands sent by the client to the server are preceded by --->

  3. Command responses from the server over the control connection are numbered.

  4. FTP uses a comma as a separator between the bytes of the IP address.

  5. When sending a port number, FTP sends the MSB then the LSB and separates the two bytes by a comma. As shown in the PORT command, port 142,58 translates to 142*256+58 = 36410.

Linux FTP connection-tracking

Given the normal loc->net policy of ACCEPT, passive mode access from local clients to remote servers will always work but active mode requires the firewall to dynamically open a “hole” for the server's connection back to the client. Similarly, if you are running an FTP server in your local zone then active mode should always work but passive mode requires the firewall to dynamically open a “hole” for the client's second connection to the server. This is the role of FTP connection-tracking support in the Linux kernel.

Where any form of NAT (SNAT, DNAT, Masquerading) on your firewall is involved, the PORT commands and PASV responses may also need to be modified by the firewall. This is the job of the FTP nat support kernel function.

Including FTP connection-tracking and NAT support normally means that the modules “nf_conntrack_ftp” and “nf_nat_ftp” need to be loaded. Shorewall automatically loads these “helper” modules from /lib/modules/<kernel-version>/kernel/net/netfilter/ and you can determine if they are loaded using the “lsmod” command. The <kernel-version> may be obtained by typing

uname -r


Note: If you are running kernel 2.6.19 or earlier, then the module names are ip_nat_ftp and ip_conntrack_ftp and they are normally loaded from /lib/modules/<kernel-version>/kernel/net/ipv4/netfilter/.


Because the ftp helper modules must read and modify commands being sent over the command channel, they won't work when the command channel is encrypted through use of TLS/SSL.

Example 1. Example (Kernel 3.2.20)

[root@lists etc]# lsmod
Module                  Size  Used by    Not tainted
iptable_filter          3072  1 
iptable_mangle          2816  0 
iptable_nat             7684  0 
iptable_raw             2048  0 
ip_tables              12232  4 iptable_raw,iptable_mangle,iptable_nat,iptable_filter
ipt_addrtype            1920  0 
ipt_ah                  2048  0 
ipt_CLUSTERIP           8708  0 
ipt_ecn                 2304  0 
ipt_ECN                 3072  0 
ipt_iprange             1920  0 
ipt_LOG                 6528  0 
ipt_MASQUERADE          3456  0 
ipt_NETMAP              2048  0 
ipt_owner               2048  0 
ipt_recent              9496  0 
ipt_REDIRECT            2048  0 
ipt_REJECT              4608  0 
ipt_SAME                2432  0 
ipt_TCPMSS              4096  0 
ipt_tos                 1664  0 
ipt_TOS                 2304  0 
ipt_ttl                 1920  0 
ipt_TTL                 2432  0 
ipt_ULOG                8068  0 
nf_conntrack           59864  28 ipt_MASQUERADE,ipt_CLUSTERIP,nf_nat_tftp,nf_nat_snmp_basic,nf_nat_sip,nf_nat_pptp,nf_nat_irc,nf_nat_h323,nf_nat_ftp,nf_nat_amanda,nf_conntrack_ama
nf_conntrack_amanda     5248  1 nf_nat_amanda
nf_conntrack_ftp        9728  1 nf_nat_ftp
nf_conntrack_h323      50396  1 nf_nat_h323
nf_conntrack_ipv4      17932  2 iptable_nat
nf_conntrack_irc        7064  1 nf_nat_irc
nf_conntrack_netbios_ns     3072  0 
nf_conntrack_netlink    26240  0 
nf_conntrack_pptp       6912  1 nf_nat_pptp
nf_conntrack_proto_gre     5632  1 nf_conntrack_pptp
nf_conntrack_proto_sctp     8328  0 
nf_conntrack_sip        9748  1 nf_nat_sip
nf_conntrack_tftp       5780  1 nf_nat_tftp
nf_nat                 17964  14 ipt_SAME,ipt_REDIRECT,ipt_NETMAP,ipt_MASQUERADE,nf_nat_tftp,nf_nat_sip,nf_nat_pptp,nf_nat_proto_gre,nf_nat_irc,nf_nat_h323,nf_nat_ftp,nf_nat_amand
nf_nat_amanda           2432  0 
nf_nat_ftp              3584  0 
nf_nat_h323             7808  0 
nf_nat_irc              2816  0 
nf_nat_pptp             3840  0 
nf_nat_proto_gre        3204  1 nf_nat_pptp
nf_nat_sip              4608  0 
nf_nat_snmp_basic      10372  0 
nf_nat_tftp             1920  0 
xt_CLASSIFY             1920  0 
xt_comment              1920  0 
xt_connmark             2432  0 
xt_conntrack            2944  0 
xt_dccp                 3588  0 
xt_hashlimit           10252  0 
xt_helper               2688  0 
xt_length               1920  0 
xt_limit                2688  0 
xt_mac                  1920  0 
xt_mark                 1920  0 
xt_MARK                 2304  0 
xt_multiport            3328  1 
xt_NFLOG                2176  0 
xt_NFQUEUE              2048  0 
xt_physdev              2704  2 
xt_pkttype              1920  0 
xt_policy               3840  0 
xt_state                2560  0 
xt_tcpmss               2304  0 
xt_tcpudp               3328  0 
[root@lists etc]#

If you want Shorewall to load these modules from an alternate directory, you need to set the MODULESDIR variable in /etc/shorewall/shorewall.conf to point to that directory.

FTP on Non-standard Ports


If you are running kernel 2.6.19 or earlier, replace nf_conntrack_ftp with ip_conntrack_ftp in the following instructions. Similarly, replace nf_nat_ftp with ip_nat_ftp.

The above discussion about commands and responses makes it clear that the FTP connection-tracking and NAT helpers must scan the traffic on the control connection looking for PASV and PORT commands as well as PASV responses. If you run an FTP server on a nonstandard port or you need to access such a server, you must therefore let the helpers know by specifying the port in /etc/shorewall/modules entries for the helpers. You should create /etc/shorewall/modules by copying /usr/share/shorewall/modules.


You must have modularized FTP connection tracking support in order to use FTP on a non-standard port.

Example 2. if you run an FTP server that listens on port 49 or you need to access a server on the Internet that listens on that port then you would have:

loadmodule nf_conntrack_ftp ports=21,49
loadmodule nf_nat_ftp                   # NOTE: With kernels prior to 2.6.11, you must specify the ports on this line also


you MUST include port 21 in the ports list or you may have problems accessing regular FTP servers.

If there is a possibility that these modules might be loaded before Shorewall starts, then you should include the port list in /etc/modules.conf:

options nf_conntrack_ftp ports=21,49
options nf_nat_ftp


Once you have made these changes to /etc/shorewall/modules and/or /etc/modules.conf, you must either:

  1. Unload the modules and restart shorewall:

    rmmod nf_nat_ftp; rmmod nf_conntrack_ftp; shorewall restart
  2. Reboot



If you run an FTP server behind your firewall and your server offers a method of specifying the external IP address of your firewall, DON'T USE THAT FEATURE OF YOUR SERVER. Using that option will defeat the purpose of the ftp helper modules and can result in a server that doesn't work.

If the policy from the source zone to the destination zone is ACCEPT and you don't need DNAT (see FAQ 30) then you need no rule.

Otherwise, for FTP you need exactly one rule:

#                                                PORT(S)    PORT(S)     DESTINATION
ACCEPT or    <source>   <destination>  tcp       21         -           <external IP addr> if
DNAT                                                                    ACTION = DNAT

You need an entry in the ORIGINAL DESTINATION column only if the ACTION is DNAT, you have multiple external IP addresses and you want a specific IP address to be forwarded to your server.

Note that you do NOT need a rule with 20 (ftp-data) in the DEST PORT(S) column. If you post your rules on the mailing list and they show 20 in the DEST PORT(S) column, we will know that you haven't read this article and will either ignore your post or tell you to RTFM.

Shorewall includes an FTP macro that simplifies creation of FTP rules. The macro source is in /usr/share/shorewall/macro.FTP. Using the macro is the preferred way to generate the rules described above. Here are a couple of examples.

Example 3. Server running behind a Masquerading Gateway

Suppose that you run an FTP server on in your local zone using the standard port (21). You need this rule:

#                                                 PORT(S)    PORT(S)     DESTINATION
FTP(DNAT)    net       loc:

Example 4. Allow your DMZ FTP access to the Internet

#                                                 PORT(S)    PORT(S)     DESTINATION
FTP(ACCEPT)   dmz        net

Note that the FTP connection tracking in the kernel cannot handle cases where a PORT command (or PASV reply) is broken across two packets or is missing the ending <cr>/<lf>. When such cases occur, you will see a console message similar to this one:

Apr 28 23:55:09 gateway kernel: conntrack_ftp: partial PORT 715014972+1

I see this problem occasionally with the FTP server in my DMZ. My solution is to add the following rule:

#                                                 PORT(S)    PORT(S)     DESTINATION
ACCEPT:info  dmz        net             tcp       -          20

The above rule accepts and logs all active mode connections from my DMZ to the net.