how to open ports with iptables

lsimon4180 · 10-02-2004, 01:29 PM

Hey,

I have a machine running fedora core2 and i am planning on installing qmail...part of the qmail installation requires to have certian ports open:

Outbound ports (tcp)

25 - SMTP
110 - POP services
143 - IMAP
783 - Spamassassin
993 - IMAPS

Inbound Ports (tcp)

25 - SMTP
80 - HTTP
110 - POP services
143 - IMAP
443 - HTTPS
783 - Spamassassin
993 - IMAPS

I ran nmap on my ip and got these results:

Starting nmap 3.50 at 2004-10-03 02:20 CDT
sendto in send_tcp_raw: sendto(3, packet, 40, 0, xxx.xxx.xxx.xxx, 16) => Operation not permitted
Interesting ports on xxx.xxx.xxx.xxx.lem-bsr1.chi-lem.il.cable.rcn.com (xxx.xxx.xxx.xxx):
(The 1654 ports scanned but not shown below are in state: closed)
PORT STATE SERVICE
20/tcp filtered ftp-data
21/tcp open ftp
22/tcp open ssh
53/tcp open domain
80/tcp open http

FIrst of all what does this mean:
sendto in send_tcp_raw: sendto(3, packet, 40, 0, xxx.xxx.xxx.xxx, 16) => Operation not permitted

its repeats about 30 times and then finally displays the open ports....

I tried to run the follow command:

iptables -A INPUT -p tcp --dport 25 -j ACCEPT

and ran service iptables save

then ran nmap again to see if port 25 would show but it doesnt..

and i missing something?

THanks,

Lenny

nukkel · 10-02-2004, 01:46 PM

Hi,

nmap only shown ports as "open" when (1) iptables allows traffic to that port and (2) some server program is actually listening on that port -- so to see if port 25 is reachable you should first run the qmail server and only then run nmap to see if it can reach the port. Then it will show either "open" or "filtered".

cheers,
nukkel

P.S. those sendto errors -- I get them too sometimes, don't know exectly what triggers them though.

Josep Lladonosa · 10-19-2004, 08:52 AM

In a working environment with iptables, it is supposed that
there are rules to accept traffic, and last rule to deny all.
If that is your case, by adding a new rule with -A,
what you do is to put the new rule AFTER the 'denyying all' rule, so it is never checked.

iptables -I INPUT -p tcp --dport 25 -j ACCEPT

Then, you can save rules as the new rule is in the correct place.

Another problem appears if the firewalling machine is also doing NAT. Then, to make ftp work, an iptables module must be loaded, as ftp (active) is an stateful protocol.
If not, make clients to use passive ftp.

pburwell · 06-16-2014, 02:43 PM

Quote:

Originally Posted by Josep Lladonosa

In a working environment with iptables, it is supposed that
there are rules to accept traffic, and last rule to deny all.
If that is your case, by adding a new rule with -A,
what you do is to put the new rule AFTER the 'denyying all' rule, so it is never checked.

iptables -I INPUT -p tcp --dport 25 -j ACCEPT

Then, you can save rules as the new rule is in the correct place.

Another problem appears if the firewalling machine is also doing NAT. Then, to make ftp work, an iptables module must be loaded, as ftp (active) is an stateful protocol.
If not, make clients to use passive ftp.

iptables -A INPUT -m state --state NEW -m tcp -p tcp --dport 25 -j ACCEPT

jacksonking112 · 06-17-2014, 03:20 AM

CentOS has an extremely powerful firewall built in, commonly referred to as iptables, but more accurately is iptables/netfilter. Iptables is the userspace module, the bit that you, the user, interact with at the command line to enter firewall rules into predefined tables. Netfilter is a kernel module, built into the kernel, that actually does the filtering. There are many GUI front ends for iptables that allow users to add or define rules based on a point and click user interface, but these often lack the flexibility of using the command line interface and limit the users understanding of what's really happening.

mcresist · 07-01-2014, 12:52 PM

Quote:

Originally Posted by lsimon4180

FIrst of all what does this mean:
sendto in send_tcp_raw: sendto(3, packet, 40, 0, xxx.xxx.xxx.xxx, 16) => Operation not permitted

its repeats about 30 times and then finally displays the open ports....

Are you running nmap as a user without elevated privileges ? I believe nmap requires elevated privileges to be able to open a raw connection.

anindyameister · 07-07-2014, 03:59 AM

You can insert a rule by mentioning position instead of appending by using -I instead of -A.

From Fedoraproject:

Code:

 Inserting Rules

Create a Rule at the top (first) position:

[root@server ~]# iptables -I INPUT 1 -p tcp --dport 80 -j ACCEPT
[root@server ~]# iptables -L
Chain INPUT (policy DROP)
target     prot opt source               destination         
ACCEPT     tcp  --  anywhere             anywhere             tcp dpt:http
ACCEPT     all  --  anywhere             anywhere             state RELATED,ESTABLISHED
ACCEPT     icmp --  anywhere             anywhere            
ACCEPT     all  --  anywhere             anywhere            
ACCEPT     tcp  --  anywhere             anywhere             state NEW tcp dpt:ssh

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination         

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination 

The number given after the chain name indicates the position before an existing Rule. So, for example, if you want to insert a Rule before the third rule you specify the number 3. Afterward, the existing Rule will then be in the fourth position in the chain.

baldur_1 · 07-07-2014, 02:06 PM

i have been using linux for many years now, 15 i think. i used to use iptables and was always very frustrated. one thing i would seriously consider it changing to shorewall. it is so, SO much easier to configure and still uses ipchains for protection. once i switched and took a few minutes to understand it, i have never had firewall issues since!!