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IPtables settings to access a backup FTP

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netfilter at ghz

Aug 18, 2007, 12:08 PM

Post #1 of 5 (984 views)
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IPtables settings to access a backup FTP

Hello, this is my first message so I hope I'm doing this right ! :)

I've got iptables setup and running well on my server and up to now I've
not had any problems, however I have just installed a backup system
which needs to connect by FTP to a distant server.

With my firewall disactivated all works fine, however with the firewall
activated when I use the "ls" command of the debian ftp command line
package (CWD) I get:

200 ok then a long wait then :
421 Service not available, remote server has closed connection.

I've tried this on two different FTP's and it only works if the firewall
is disactivated.

One server is a plesk server but the server I need to connect to I do
not know much about, with filezilla on my pc I've managed to connect to
it with both FTP active and FTP passive so I guess it can do both.

On my server I just do :

ftp ftp.server.com
username
password

So I'm not sure if it connects as passive or active.

These are the ports that I have got open :

-A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
-A OUTPUT -m state --state ESTABLISHED,RELATED -j ACCEPT

-A INPUT -p tcp -m tcp --dport 20 -j ACCEPT
-A INPUT -p tcp -m tcp --dport 21 -j ACCEPT

-A OUTPUT -p tcp -m tcp --dport 20 -j ACCEPT
-A OUTPUT -p tcp -m tcp --dport 21 -j ACCEPT

Which ports do you suggest I should open to connect to this FTP server ?

Thanks in advance,

Richard

m at rtij

Aug 18, 2007, 1:16 PM

Post #2 of 5 (922 views)
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Re: IPtables settings to access a backup FTP [In reply to]

Richard wrote:
> Hello, this is my first message so I hope I'm doing this right ! :)
>
> I've got iptables setup and running well on my server and up to now
> I've not had any problems, however I have just installed a backup
> system which needs to connect by FTP to a distant server.
>
> With my firewall disactivated all works fine, however with the
> firewall activated when I use the "ls" command of the debian ftp
> command line package (CWD) I get:
>
> 200 ok then a long wait then :
> 421 Service not available, remote server has closed connection.
>
> I've tried this on two different FTP's and it only works if the
> firewall is disactivated.
>
> One server is a plesk server but the server I need to connect to I do
> not know much about, with filezilla on my pc I've managed to connect
> to it with both FTP active and FTP passive so I guess it can do both.
>
> On my server I just do :
>
> ftp ftp.server.com
> username
> password
>
> So I'm not sure if it connects as passive or active.
>
> These are the ports that I have got open :
>
> -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
> -A OUTPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
>
> -A INPUT -p tcp -m tcp --dport 20 -j ACCEPT

You don't need this (see below), but it is wrong anyhow. Active ftp uses
SOURCE port 20, not destination port 20. Most guides on the inuhnet
thingie get this wrong.

> -A INPUT -p tcp -m tcp --dport 21 -j ACCEPT

Do you also want to accept FTP connections? You don't need this to make
outgoing connections.

>
> -A OUTPUT -p tcp -m tcp --dport 20 -j ACCEPT

This is nonsense.

> -A OUTPUT -p tcp -m tcp --dport 21 -j ACCEPT

This is the only rule you need to make outgoing FTP connections.

>
> Which ports do you suggest I should open to connect to this FTP server ?
>

It's not only about static ports. FTP uses dynamic ports as well that
must be tracked by a helper. Modprobe nf_contrack_ftp (ip_contrack_ftp
on older kernels) and all should be well.

HTH,
M4

netfilter at ghz

Aug 18, 2007, 1:25 PM

Post #3 of 5 (922 views)
Permalink

Re: IPtables settings to access a backup FTP [In reply to]

Hi I've got ip_conntrack running,

My server hosts FTP accounts and I also need outgoing ftp ...

No problems accessing the ftp hosted on this server, but can't do a CWD
(ls) from this server to another external ftp server unless the firewall
is disactivated.

I guess you are going to say most of my settings are useless but here
goes my current settings :

-------------------------

Chain INPUT (policy DROP)
target prot opt source destination
ACCEPT all -- anywhere anywhere state
RELATED,ESTAB
LISHED
ACCEPT tcp -- anywhere anywhere tcp
dpt:ftp-data
ACCEPT tcp -- anywhere anywhere tcp dpt:ftp
LOG_ACCEPT tcp -- anywhere anywhere tcp dpt:2022
LOG_ACCEPT tcp -- anywhere anywhere tcp dpt:smtp
ACCEPT tcp -- anywhere anywhere tcp dpt:whois
ACCEPT udp -- anywhere anywhere udp dpt:domain
ACCEPT tcp -- anywhere anywhere tcp dpt:www
ACCEPT tcp -- anywhere anywhere tcp dpt:pop3
ACCEPT tcp -- anywhere anywhere tcp dpt:imap2
ACCEPT tcp -- anywhere anywhere tcp dpt:https
ACCEPT tcp -- anywhere anywhere tcp dpt:ntp
ACCEPT tcp -- anywhere anywhere tcp dpt:8443
ACCEPT tcp -- anywhere anywhere tcp dpt:8880
ACCEPT tcp -- anywhere anywhere tcp dpt:42630
ACCEPT udp -- anywhere anywhere udp dpt:42630
ACCEPT tcp -- anywhere anywhere tcp dpt:imaps
ACCEPT tcp -- anywhere anywhere tcp dpt:mysql
ACCEPT all -- localhost.localdomain anywhere
icmp_packets icmp -- anywhere anywhere
LOG_DROP all -- anywhere anywhere

Chain FORWARD (policy DROP)
target prot opt source destination
ACCEPT tcp -- anywhere anywhere tcp dpt:ftp
flags:S
YN,RST,ACK/SYN

Chain OUTPUT (policy DROP)
target prot opt source destination
ACCEPT all -- anywhere anywhere state
RELATED,ESTAB
LISHED
ACCEPT tcp -- anywhere anywhere tcp
dpt:ftp-data
ACCEPT tcp -- anywhere anywhere tcp dpt:ftp
ACCEPT tcp -- anywhere anywhere tcp dpt:2022
ACCEPT tcp -- anywhere anywhere tcp dpt:telnet
ACCEPT tcp -- anywhere anywhere tcp dpt:smtp
ACCEPT tcp -- anywhere anywhere tcp dpt:whois
ACCEPT udp -- anywhere anywhere udp dpt:domain
ACCEPT tcp -- anywhere anywhere tcp dpt:www
ACCEPT tcp -- anywhere anywhere tcp dpt:pop3
ACCEPT tcp -- anywhere anywhere tcp dpt:imap2
ACCEPT tcp -- anywhere anywhere tcp dpt:https
ACCEPT udp -- anywhere anywhere udp dpt:ntp
ACCEPT tcp -- anywhere anywhere tcp dpt:imaps
ACCEPT tcp -- anywhere anywhere tcp dpt:mysql
ACCEPT all -- anywhere localhost.localdomain
icmp_packets icmp -- anywhere anywhere
LOG_DROP all -- anywhere anywhere

Chain LOG_ACCEPT (2 references)
target prot opt source destination
LOG all -- anywhere anywhere LOG level
warning t
cp-options ip-options prefix `[IPTABLES ACCEPT] : '
ACCEPT all -- anywhere anywhere

Chain LOG_DROP (2 references)
target prot opt source destination
LOG all -- anywhere anywhere LOG level
warning t
cp-options ip-options prefix `[IPTABLES DROP] : '
DROP all -- anywhere anywhere

Chain icmp_packets (2 references)
target prot opt source destination
ACCEPT icmp -- anywhere anywhere limit: avg
1/sec bu
rst 1
LOG icmp -- anywhere anywhere limit: avg
1/sec bu
rst 1 LOG level warning prefix `PING-DROP: '

--------------

Thanks for any suggestions !

Richard

Martijn Lievaart a �crit :
> Richard wrote:
>> Hello, this is my first message so I hope I'm doing this right ! :)
>>
>> I've got iptables setup and running well on my server and up to now
>> I've not had any problems, however I have just installed a backup
>> system which needs to connect by FTP to a distant server.
>>
>> With my firewall disactivated all works fine, however with the
>> firewall activated when I use the "ls" command of the debian ftp
>> command line package (CWD) I get:
>>
>> 200 ok then a long wait then :
>> 421 Service not available, remote server has closed connection.
>>
>> I've tried this on two different FTP's and it only works if the
>> firewall is disactivated.
>>
>> One server is a plesk server but the server I need to connect to I do
>> not know much about, with filezilla on my pc I've managed to connect
>> to it with both FTP active and FTP passive so I guess it can do both.
>>
>> On my server I just do :
>>
>> ftp ftp.server.com
>> username
>> password
>>
>> So I'm not sure if it connects as passive or active.
>>
>> These are the ports that I have got open :
>>
>> -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
>> -A OUTPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
>>
>> -A INPUT -p tcp -m tcp --dport 20 -j ACCEPT
>
> You don't need this (see below), but it is wrong anyhow. Active ftp uses
> SOURCE port 20, not destination port 20. Most guides on the inuhnet
> thingie get this wrong.
>
>> -A INPUT -p tcp -m tcp --dport 21 -j ACCEPT
>
> Do you also want to accept FTP connections? You don't need this to make
> outgoing connections.
>
>>
>> -A OUTPUT -p tcp -m tcp --dport 20 -j ACCEPT
>
> This is nonsense.
>
>> -A OUTPUT -p tcp -m tcp --dport 21 -j ACCEPT
>
> This is the only rule you need to make outgoing FTP connections.
>
>>
>> Which ports do you suggest I should open to connect to this FTP server ?
>>
>
> It's not only about static ports. FTP uses dynamic ports as well that
> must be tracked by a helper. Modprobe nf_contrack_ftp (ip_contrack_ftp
> on older kernels) and all should be well.
>
> HTH,
> M4
>

m at rtij

Aug 18, 2007, 1:29 PM

Post #4 of 5 (920 views)
Permalink

Re: IPtables settings to access a backup FTP [In reply to]

Richard wrote:

[ Please don't toppost. Thanks ]

> Hi I've got ip_conntrack running,

I was talking about ip_conntrack_ftp, not ip_conntrack. You need both.
The difference is between ftp working and not working at the first LS
command.

>
> My server hosts FTP accounts and I also need outgoing ftp ...

Clear, so you need the input rule on port 21 as well.

>
> No problems accessing the ftp hosted on this server, but can't do a
> CWD (ls) from this server to another external ftp server unless the
> firewall is disactivated.

No problems? Impossible with the rules you posted.

>
> I guess you are going to say most of my settings are useless but here
> goes my current settings :

Thanks, but that is virtually unreadable. Post the output of
iptables-save instead.

HTH,
M4

netfilter at ghz

Aug 18, 2007, 3:36 PM

Post #5 of 5 (923 views)
Permalink

Re: IPtables settings to access a backup FTP [In reply to]

Martijn Lievaart a �crit :
> Richard wrote:
>
> [ Please don't toppost. Thanks ]
>
>> Hi I've got ip_conntrack running,
>
> I was talking about ip_conntrack_ftp, not ip_conntrack. You need both.
> The difference is between ftp working and not working at the first LS
> command.
>
>>
>> My server hosts FTP accounts and I also need outgoing ftp ...
>
> Clear, so you need the input rule on port 21 as well.
>
>>
>> No problems accessing the ftp hosted on this server, but can't do a
>> CWD (ls) from this server to another external ftp server unless the
>> firewall is disactivated.
>
> No problems? Impossible with the rules you posted.
>
>>
>> I guess you are going to say most of my settings are useless but here
>> goes my current settings :
>
> Thanks, but that is virtually unreadable. Post the output of
> iptables-save instead.
>
> HTH,
> M4
>

Thankyou ! That fixed my problem ! I did a lsmod and it was not there so
I did a mprobe ip_conntrack_ftp and then added ip_conntrack_ftp to
/etc/modules

Now my backup system works well !

Now for my iptable settings, here is my file :

------

#The NAT portion of the ruleset. Used for Network Address Transalation.
#Usually not needed on a typical web server, but it's there if you need it.
*nat
:PREROUTING ACCEPT [127173:7033011]
:POSTROUTING ACCEPT [31583:2332178]
:OUTPUT ACCEPT [32021:2375633]
COMMIT

#The Mangle portion of the ruleset. Here is where unwanted packet types
get dropped.
#This helps in making port scans against your server a bit more time
consuming and difficult, but not impossible.
*mangle
:PREROUTING ACCEPT [444:43563]
:INPUT ACCEPT [444:43563]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [402:144198]
:POSTROUTING ACCEPT [402:144198]
-A PREROUTING -p tcp -m tcp --tcp-flags FIN,SYN,RST,PSH,ACK,URG
FIN,PSH,URG -j DROP
-A PREROUTING -p tcp -m tcp --tcp-flags FIN,SYN,RST,PSH,ACK,URG NONE -j DROP
-A PREROUTING -p tcp -m tcp --tcp-flags SYN,RST SYN,RST -j DROP
-A PREROUTING -p tcp -m tcp --tcp-flags FIN,SYN FIN,SYN -j DROP
-A PREROUTING -p tcp -m tcp --tcp-flags FIN,SYN,RST,PSH,ACK,URG
FIN,PSH,URG -j DROP
-A PREROUTING -p tcp -m tcp --tcp-flags FIN,SYN,RST,PSH,ACK,URG NONE -j DROP
-A PREROUTING -p tcp -m tcp --tcp-flags SYN,RST SYN,RST -j DROP
-A PREROUTING -p tcp -m tcp --tcp-flags FIN,SYN FIN,SYN -j DROP
-A PREROUTING -p tcp -m tcp --tcp-flags FIN,SYN,RST,PSH,ACK,URG
FIN,PSH,URG -j DROP
-A PREROUTING -p tcp -m tcp --tcp-flags FIN,SYN,RST,PSH,ACK,URG NONE -j DROP
-A PREROUTING -p tcp -m tcp --tcp-flags SYN,RST SYN,RST -j DROP
-A PREROUTING -p tcp -m tcp --tcp-flags FIN,SYN FIN,SYN -j DROP
-A PREROUTING -p tcp -m tcp --tcp-flags FIN,SYN,RST,PSH,ACK,URG
FIN,PSH,URG -j DROP
-A PREROUTING -p tcp -m tcp --tcp-flags FIN,SYN,RST,PSH,ACK,URG NONE -j DROP
-A PREROUTING -p tcp -m tcp --tcp-flags SYN,RST SYN,RST -j DROP
-A PREROUTING -p tcp -m tcp --tcp-flags FIN,SYN FIN,SYN -j DROP
COMMIT

#The FILTER section of the ruleset is where we initially drop all
packets and then selectively open certain ports.
#We will also enable logging of all dropped requests.
*filter
:INPUT DROP [1:242]
:FORWARD DROP [0:0]
:OUTPUT DROP [0:0]
:LOG_DROP - [0:0]
:LOG_ACCEPT - [0:0]
:icmp_packets - [0:0]

#test
-A FORWARD -i eth0 -o eth1 -p tcp --syn --dport 21 -j ACCEPT
#First, we cover the INPUT rules, or the rules for incoming requests.
#Note how at the end we log any incoming packets that are not accepted.
-A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
-A INPUT -p tcp -m tcp --dport 20 -j ACCEPT
-A INPUT -p tcp -m tcp --dport 21 -j ACCEPT
-A INPUT -p tcp -m tcp --dport 2022 -j LOG_ACCEPT
-A INPUT -p tcp -m tcp --dport 25 -j LOG_ACCEPT
-A INPUT -p tcp -m tcp --dport 43 -j ACCEPT
-A INPUT -p udp -m udp --dport 53 -j ACCEPT
-A INPUT -p tcp -m tcp --dport 80 -j ACCEPT
-A INPUT -p tcp -m tcp --dport 110 -j ACCEPT
-A INPUT -p tcp -m tcp --dport 143 -j ACCEPT
-A INPUT -p tcp -m tcp --dport 443 -j ACCEPT
-A INPUT -p tcp -m tcp --dport 123 -j ACCEPT
-A INPUT -p tcp -m tcp --dport 8443 -j ACCEPT
-A INPUT -p tcp -m tcp --dport 8880 -j ACCEPT
#Passive FTP connections
-A INPUT -p tcp -m tcp --dport 32876 -j ACCEPT
-A INPUT -p udp -m udp --dport 32876 -j ACCEPT

#uncomment the next line if you are running Spamassassin on your server
#-A INPUT -p tcp -m tcp --dport 783 -j ACCEPT
-A INPUT -p tcp -m tcp --dport 993 -j ACCEPT
-A INPUT -p tcp -m tcp --dport 3306 -j ACCEPT
-A INPUT -s 127.0.0.1 -j ACCEPT
-A INPUT -p icmp -j icmp_packets
-A INPUT -j LOG_DROP

#Next, we cover the OUTPUT rules, or the rules for all outgoing traffic.
#Note how at the end we log any outbound packets that are not accepted.
-A OUTPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
-A OUTPUT -p tcp -m tcp --dport 20 -j ACCEPT
-A OUTPUT -p tcp -m tcp --dport 21 -j ACCEPT
-A OUTPUT -p tcp -m tcp --dport 2022 -j ACCEPT
-A OUTPUT -p tcp -m tcp --dport 23 -j ACCEPT
-A OUTPUT -p tcp -m tcp --dport 25 -j ACCEPT
-A OUTPUT -p tcp -m tcp --dport 43 -j ACCEPT
-A OUTPUT -p udp -m udp --dport 53 -j ACCEPT
-A OUTPUT -p tcp -m tcp --dport 80 -j ACCEPT
-A OUTPUT -p tcp -m tcp --dport 110 -j ACCEPT
-A OUTPUT -p tcp -m tcp --dport 143 -j ACCEPT
-A OUTPUT -p tcp -m tcp --dport 443 -j ACCEPT
-A OUTPUT -p udp -m udp --dport 123 -j ACCEPT
#uncomment the next line if you are running Spamassassin on your server
#-A OUTPUT -p tcp -m tcp --dport 783 -j ACCEPT
-A OUTPUT -p tcp -m tcp --dport 993 -j ACCEPT
-A OUTPUT -p tcp -m tcp --dport 3306 -j ACCEPT
-A OUTPUT -d 127.0.0.1 -j ACCEPT
-A OUTPUT -p icmp -j icmp_packets
-A OUTPUT -j LOG_DROP

#Here we have 2 sets of logging rules. One for dropped packets to log
all dropped requests and one for accepted packets, should we wish to log
any accepted requesets.
-A LOG_DROP -j LOG --log-prefix "[IPTABLES DROP] : " --log-tcp-options
--log-ip-options
-A LOG_DROP -j DROP

-A LOG_ACCEPT -j LOG --log-prefix "[IPTABLES ACCEPT] : "
--log-tcp-options --log-ip-options
-A LOG_ACCEPT -j ACCEPT

#And finally, a rule to deal with ICMP requests. We drop all ping
requests except from our own server.
# Make sure you replace 1.2.3.4 with the IP address of your server.
#-A icmp_packets -p icmp -m icmp --icmp-type 0 -j ACCEPT
#-A icmp_packets -p icmp -m icmp --icmp-type 8 -j ACCEPT
#-A icmp_packets -p icmp -m icmp --icmp-type 3 -j ACCEPT
#-A icmp_packets -p icmp -m icmp --icmp-type 11 -j ACCEPT
-A icmp_packets -p icmp -m limit --limit 1/s --limit-burst 1 -j ACCEPT
-A icmp_packets -p icmp -m limit --limit 1/s --limit-burst 1 -j LOG
--log-prefix "PING-DROP: "

COMMIT

-----

So as far as I understand, I must remove the two following lines :

-A INPUT -p tcp -m tcp --dport 20 -j ACCEPT
-A OUTPUT -p tcp -m tcp --dport 20 -j ACCEPT

Thanks again !

Richard

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