Linux Bash: Setting iptables rules to allow both active and passive FTP

linux bash ftp iptables
user573382 picture user573382 · May 1, 2012 · Viewed 43.7k times · Source

I have a PC on which I have a FTP server installed. I want to set the iptables rules to allow both active and passive FTP. I've tried the following code that people report is working, but it seems to block all traffic for me (pages won't load anymore etc)

#!/bin/bash

IPT=/sbin/iptables
$IPT -F
$IPT -X
$IPT -t nat -F
$IPT -t nat -X
$IPT -t mangle -F
$IPT -t mangle -X
/sbin/modprobe ip_conntrack
/sbin/modprobe ip_conntrack_ftp

# Setting default filter policy
$IPT -P INPUT DROP
$IPT -P OUTPUT ACCEPT

# Allow FTP connections @ port 21
$IPT -A INPUT  -p tcp --sport 21 -m state --state ESTABLISHED -j ACCEPT
$IPT -A OUTPUT -p tcp --dport 21 -m state --state NEW,ESTABLISHED -j ACCEPT

# Allow Active FTP Connections
$IPT -A INPUT -p tcp --sport 20 -m state --state ESTABLISHED,RELATED -j ACCEPT
$IPT -A OUTPUT -p tcp --dport 20 -m state --state ESTABLISHED -j ACCEPT

# Allow Passive FTP Connections
$IPT -A INPUT -p tcp --sport 1024: --dport 1024: -m state --state ESTABLISHED,RELATED -j ACCEPT
$IPT -A OUTPUT -p tcp --sport 1024: --dport 1024:  -m state --state ESTABLISHED,RELATED -j ACCEPT

Answer

ab1965 picture ab1965 · Nov 14, 2013

From Your question I suppose you have some trivial host with common set of apps such as web-browser, mail client, may be telnet and|or ssh-client, may be ftp-client too, may be some IM etc. And having all these apps working, You want additionally allow an FTP server on this host to work both in active and passive modes for clients which will connect. Here are 3 blocks of rules applicable in this case. Block of common rules is the minimalistic set of rules applicable for most client hosts. Next is block of rules for ftp-client, if You have such on Your host. The rules for ftp-client are slightly different from rules for others clients: there are always two connections to enable data transfer: ftp-control (port 21) and ftp-data (port 20 in Active mode or random port in Passive mode). You most probably will never need client rules for Active mode because Passive mode is single choice for NATed networks.

The rules for FTP server are in the last block.

Please check You have ip_conntrack_ftp ( may be named nf_conntrack_ftp ) in the kernel:

> lsmod | grep conn

If You do not have this kernel module, the 'RELATED' rules will not work and, most probably, separate ftp-data connection will not start while primary ftp-control connection will hang somewhere after 'PORT' command. You still can enforce ftp-data connection in this case, but at the spent of degrading security provided by the tweaked rules. The tweaks are in comments preceeding the rules.

Pro

#!/bin/bash
IPT=/sbin/iptables

$IPT -F
$IPT -t nat -F
$IPT -t mangle -F

$IPT -X
$IPT -t nat -X
$IPT -t mangle -X

/sbin/modprobe ip_conntrack
/sbin/modprobe ip_conntrack_ftp

$IPT -P INPUT DROP
$IPT -P FORWARD DROP
$IPT -P OUTPUT DROP

# Block of common rules #####################################################
$IPT -A OUTPUT -o lo -j ACCEPT
$IPT -A INPUT  -i lo -j ACCEPT

$IPT -A OUTPUT -p icmp -j ACCEPT
$IPT -A INPUT  -p icmp -j ACCEPT

# allow DNS queries and replies
$IPT -A OUTPUT -p udp --dport 53 -j ACCEPT
$IPT -A INPUT  -p udp --sport 53 -j ACCEPT

# allow all Your possible client applications to work 
$IPT -A OUTPUT -p tcp -m multiport --dports ssh,telnet,http,https,xmpp-client,aol,smtp,pop3,imap2,imap3 -m state --state NEW,RELATED,ESTABLISHED -j ACCEPT
$IPT -A INPUT  -p tcp -m multiport --sports ssh,telnet,http,https,xmpp-client,aol,smtp,pop3,imap2,imap3 -m state --state RELATED,ESTABLISHED   -j ACCEPT
# End of block of common rules ##############################################


# If You have ftp-client too, this block of rules 
# will allow it to work with external ftp servers in both modes.
#
# First, allow ftp-control at client side:
$IPT -A OUTPUT -p tcp -m tcp --dport 21 -m state --state NEW,ESTABLISHED -j ACCEPT 
$IPT -A INPUT -p tcp -m tcp --sport 21 -m state --state ESTABLISHED -j ACCEPT 
#
# Then allow ftp-data Active Mode at client side:
# Client accepts RELATED connection from server port 20 
# to client port number negotiated in ftp-control connection.
# nf_conntrack_ftp is REQUIRED at client host 
# to pick up this client port number from payload of ftp-control packets,
# otherwise You are forced to use 'NEW' instead of 'RELATED'.
# And in the case of 'NEW' You allow connection to ANY port of Your host!
$IPT -A INPUT -p tcp -m tcp --sport 20 -m state --state RELATED,ESTABLISHED -j ACCEPT 
$IPT -A OUTPUT -p tcp -m tcp --dport 20 -m state --state ESTABLISHED -j ACCEPT 
#
# Finally, allow ftp-data Passive Mode at client side:
# Client starts RELATED connection from random own high port number 
# to server fixed high port number negotiated in ftp-control connection.
# nf_conntrack_ftp is REQUIRED again at client host 
# to pick up this client port number from payload of ftp-control packets, 
# otherwise You are forced to use 'NEW' instead of 'RELATED' !
-A OUTPUT -p tcp -m tcp --sport 1024:65535 --dport 1024:65535 -m state --state RELATED,ESTABLISHED -j ACCEPT 
-A INPUT -p tcp  -m tcp --sport 1024:65535 --dport 1024:65535 -m state --state ESTABLISHED -j ACCEPT    

#######[ Block of rules needed for Local FTP Server ]#######
# This block of rules allows clients to access Your FTP server at this host
# either in Active or Passive mode. 
# You may need to enable Passive mode in FTP server config file,
# e.g. with pasv_enable=yes in /etc/vsftpd.conf if vsftpd is Your choice.
#
# Ftp-control at server side:
# (some example rules are given below just to show 
# how You can selectively restrict access to Your FTP server):
$IPT -A INPUT -s 1.2.3.0/24 -p tcp -m tcp --dport 21 -m state --state NEW,ESTABLISHED -j ACCEPT 
$IPT -A INPUT -s 5.6.7.8/32 -p tcp -m tcp --dport 21 -m state --state NEW,ESTABLISHED -j ACCEPT 
$IPT -A OUTPUT -p tcp -m tcp --sport 21 -m state --state ESTABLISHED -j ACCEPT 
#
# Ftp-data Active Mode at server side:
# Server starts RELATED connection from server port 20 
# to client port number negotiated in ftp-control connection.
# nf_conntrack_ftp is REQUIRED to pick up this client port number
# from payload of ftp-control packets, 
# otherwise You are forced to use 'NEW' instead of 'RELATED' !
$IPT -A OUTPUT -p tcp -m tcp --sport 20 -m state --state RELATED,ESTABLISHED -j ACCEPT 
$IPT -A INPUT -p tcp -m tcp --dport 20 -m state --state ESTABLISHED -j ACCEPT 
#
# Ftp-data Passive Mode at server side:
# Server accepts RELATED client connection from random client high port number
# to own fixed high port number negotiated in ftp-control connection.
# nf_conntrack_ftp is REQUIRED to pick up this own fixed high port number
# from payload of ftp-control packets,
# otherwise You are forced to use 'NEW' instead of 'RELATED'.
# And in the case of 'NEW' You allow connection to ANY high port of Your server!
$IPT -A INPUT -p tcp -m tcp --sport 1024:65535 --dport 1024:65535 -m state --state RELATED,ESTABLISHED -j ACCEPT 
$IPT -A OUTPUT -p tcp -m tcp --sport 1024:65535 --dport 1024:65535 -m state --state ESTABLISHED -j ACCEPT 
######

Related questions

Getting exit status code from 'ftp' command in linux shell

I need to retrive the exit status code from a command line program. No worries, I used $?. But for ftp, even if it doesn't connect, it opens the ftp shell, so I'm not able to understand that the connection haven't …

Read More
Deleting a file with LFTP using variables

I'm trying to delete a file from an FTP server in my shell scrip using LFTP, but for some reason it will not use my variables, and takes them as literals. The code: USERNAME="theuser" PASSWORD="verygoodpassword" SERVER="example.com" …

Read More
Looping through the content of a file in Bash

How do I iterate through each line of a text file with Bash? With this script: echo "Start!" for p in (peptides.txt) do echo "${p}" done I get this output on the screen: Start! ./runPep.sh: line 3: syntax error …

Read More