Copyright © 2003, 2004, 2005, 2006 Thomas M. 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”.
2020/04/17
Table of Contents
Caution
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 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 ftp1.shorewall.net Connected to lists.shorewall.net. 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 (ftp1.shorewall.net:teastep): ftp 331-Welcome to ftp.shorewall.net 331- 331 Any password will work Password: 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 ftp>
Things to notice:
The commands that I issued are strongly emphasized.
Commands sent by the client to the server are preceded by --->
Command responses from the server over the control connection are numbered.
FTP uses a comma as a separator between the bytes of the IP address.
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.
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 -rImportant
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/.
Important
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 nda,nf_conntrack_tftp,nf_conntrack_sip,nf_conntrack_proto_sctp,nf_conntrack_pptp,nf_conntrack_proto_gre,nf_conntrack_netlink,nf_conntrack_netbios_ns,nf_conntrack_irc,nf_conntrack_ h323,nf_conntrack_ftp,xt_helper,xt_state,xt_connmark,xt_conntrack,iptable_nat,nf_nat,nf_conntrack_ipv4 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 a,nf_conntrack_netlink,iptable_nat 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.
Because of the potential for attackers to subvert Netfilter helpers like the one for FTP, the Netfilter team are in the process of eliminating the automatic association of helpers to connections. In the 3.5 kernel, it is possible to disable this automatic association, and the team have announced that automatic association will eventually be eliminated. While it is certainly more secure to add explicit rules that create these associations, for Shorewall to require users to add those rules would present a gross inconvenience during a Shorewall upgrade. To make Shorewall and kernel upgrades as smooth as possible, several new features were added to the Shorewall 4.5.7:
Shorewall automatically disables the kernel's automatic association of helpers to connections on kernel 3.5 and later.
An automatic association of helpers with connections that performs the same function as in the pre-3.5 kernels has been added. This automatic association is controlled by the AUTOHELPERS shorewall.conf option which is set to 'Yes' by default.
A HELPERS column has been added to the /etc/shorewall/rules In the NEW section: When the ACTION is ACCEPT, DNAT or REDIRECT, the specified helper is automatically associated with the connection.
HELPERS may be specified in action files, macros and in the rules file itself. In the RELATED section: The rule will only match related connections that have the named helper attached. - The standard Macros for applications requiring a helper (FTP, IRC, etc) have been modified to automatically specify the correct helper in the HELPER column.
HELPER is now a valid action in /etc/shorewall/rules. This action requires that a helper be present in the HELPER column and causes the specified helper to be associated with connections matching the rule. No destination zone should be specified in HELPER rules. HELPER rules allow specification of a helper for connections that are ACCEPTed by the applicable policy.
Example (loc->net policy is ACCEPT) - In /etc/shorewall/rules:
#ACTION SOURCE DEST FTP(HELPER) loc -
or equivalently
#ACTION SOURCE DEST PROTO DPORT HELPER loc - tcp 21 { helper=ftp }The set of enabled helpers (either by AUTOHELPERS=Yes or by the HELPERS column) can be taylored using the new HELPERS option in shorewall.conf.
By making AUTOHELPERS=Yes the default, users can upgrade their systems to a 3.5+ kernel without disrupting the operation of their firewalls. Beyond such upgrades, we suggest setting AUTOHELPERS=No and follow one of two strategies:
Use the HELPERS column in the rules file to enable helpers as needed (preferred); or
Taylor the conntrack file to enable helpers on only those connections that are required.
With either of these approaches, the list if available helpers can be trimmed using the HELPERS option and rules can be added to the RELATED section of the rules file to further restrict the effect of helpers. The implementation of these new function places conditional rules in the /etc/shorewall[6]/conntrack file. These rules are included conditionally based in the setting of AUTOHELPERS.
Example:
#ACTION SOURCE DESTINATION PROTO DPORT SPORT USER SWITCH ?if $AUTOHELPERS && __CT_TARGET ?if __FTP_HELPER CT:helper:ftp all - tcp 21 ?endif ... ?endif
__FTP_HELPER evaluates to false if the HELPERS setting is non-empty and 'ftp' is not listed in that setting. For example, if you only need FTP access from your 'loc' zone, then add this rule outside of the outer-most ?if....?endif shown above.
#ACTION SOURCE DESTINATION PROTO DPORT SPORT USER SWITCH ... CT:helper:ftp loc - tcp 21
For an overview of Netfilter Helpers and Shorewall's support for dealing with them, see https://shorewall.org/Helpers.html.
See https://home.regit.org/netfilter-en/secure-use-of-helpers/ for additional information.
If you are running kernel 3.5 or later and Shorewall 4.5.7 or later, then please read the preceding section. You can add appropriate entries into shorewall-rules(5) or shorewall-conntrack(5) to associate the FTP helpers with a nonstandard port.
Examples using port 12345:
/etc/shorewall/rules:
#ACTION SOURCE DEST PROTO DPORT
DNAT net loc:192.168.1.2:21 tcp 12345 { helper=ftp }That entry will accept ftp connections on port 12345 from the net and forward them to host 192.168.1..2 and port 21 in the loc zone.
/etc/shorewall/conntrack:
#ACTION SOURCE DESTINATION PROTO DPORT SPORT USER SWITCH ... CT:helper:ftp loc - tcp 12345
That rule automatically associates the ftp helper with TCP port 12345 from the 'loc' zone.
Otherwise, read on.
Note
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.
Caution
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
Note
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
Important
Once you have made these changes to /etc/shorewall/modules and/or /etc/modules.conf, you must either:
Unload the modules and restart shorewall:
rmmod nf_nat_ftp; rmmod nf_conntrack_ftp; shorewall restartReboot
Warning
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:
#ACTION SOURCE DESTINATION PROTO DPORT SPORT ORIGDEST ACCEPT or <source> <destination> tcp 21 - <external IP addr> if DNAT ACTION = DNAT
You need an entry in the ORIGDEST 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 DPORT column. If you post your rules on the mailing list and they show 20 in the DPORT 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 192.168.1.5 in your local zone using the standard port (21). You need this rule:
#ACTION SOURCE DESTINATION PROTO DPORT SPORT ORIGDEST FTP(DNAT) net loc:192.168.1.5
Example 4. Allow your DMZ FTP access to the Internet
#ACTION SOURCE DESTINATION PROTO DPORT SPORT ORIGDEST 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
or this one:
21:37:40 insert-master kernel: [832161.057782] nf_ct_ftp: dropping
packet IN=eth4 OUT= MAC=00:0a:cd:1a:d1:95:00:22:6b:be:3c:41:08:00
SRC=66.199.187.46 DST=192.168.41.1 LEN=102 TOS=0x00 PREC=0x00 TTL=45
ID=30239 DF PROTO=TCP SPT=21 DPT=50892 SEQ=698644583 ACK=3438176321
WINDOW=46 RES=0x00 ACK PSH URGP=0 OPT (0101080A932DFE0231935CF7) MARK=0x1I see this problem occasionally with the FTP server in my DMZ. My solution is to add the following rule:
#ACTION SOURCE DESTINATION PROTO DPORT SPORT ORIGDEST ACCEPT:info dmz net tcp - 20
The above rule accepts and logs all active mode connections from my DMZ to the net.
