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Phuong Nguyen
Asked: 2011-09-27 09:16:12 +0800 CST

Cannot modify iptables on CentOS

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My question: How to add a custom iptables rule to accept connection on a certain port?

I'm trying to open port 3500 on my server but failed. I started by using this command: (From http://wiki.centos.org/HowTos/Network/IPTables)

iptables -A INPUT -p tcp --dport 3500 -j ACCEPT

But then I run iptables -L I still do not see the new rules being listed: (My assumption it should include 3500 in the output)

Chain INPUT (policy ACCEPT)
target     prot opt source               destination         
RH-Firewall-1-INPUT  all  --  anywhere             anywhere            
ACCEPT     tcp  --  anywhere             anywhere            tcp dpt:ssh 
ACCEPT     tcp  --  anywhere             anywhere            tcp dpt:rtmp-port 

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination         
RH-Firewall-1-INPUT  all  --  anywhere             anywhere            

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination         

Chain RH-Firewall-1-INPUT (2 references)
target     prot opt source               destination         
ACCEPT     all  --  anywhere             anywhere            
ACCEPT     icmp --  anywhere             anywhere            icmp any 
ACCEPT     esp  --  anywhere             anywhere            
ACCEPT     ah   --  anywhere             anywhere            
ACCEPT     udp  --  anywhere             224.0.0.251         udp dpt:mdns 
ACCEPT     udp  --  anywhere             anywhere            udp dpt:ipp 
ACCEPT     tcp  --  anywhere             anywhere            tcp dpt:ipp 
ACCEPT     all  --  anywhere             anywhere            state RELATED,ESTABLISHED 
ACCEPT     udp  --  anywhere             anywhere            state NEW udp dpt:snmp 
ACCEPT     udp  --  anywhere             anywhere            state NEW udp dpt:snmptrap 
ACCEPT     tcp  --  anywhere             anywhere            state NEW tcp dpt:ssh 
ACCEPT     tcp  --  anywhere             anywhere            state NEW tcp dpt:http 
ACCEPT     tcp  --  anywhere             anywhere            state NEW tcp dpt:ftp 
REJECT     all  --  anywhere             anywhere            reject-with icmp-host-prohibited

Edit So I try to insert ACCEPT rule to the INPUT chain and my iptables now look like this:

Chain INPUT (policy ACCEPT)
target     prot opt source               destination
ACCEPT     tcp  --  0.0.0.0/0            0.0.0.0/0           tcp dpt:3500 
RH-Firewall-1-INPUT  all  --  0.0.0.0/0            0.0.0.0/0

But it does not allow me to connect to 3500 from outside. (I can still telnet from inside). When I'm trying to telnet my_host 3500, I get this: telnet: Unable to connect to remote host: Connection refused

Edit 2: My netstat -an | grep "LISTEN " output:

tcp 0 0 127.0.0.1:3500 0.0.0.0:* LISTEN
tcp 0 0 0.0.0.0:973 0.0.0.0:* LISTEN
tcp 0 0 0.0.0.0:111 0.0.0.0:* LISTEN
tcp 0 0 127.0.0.1:631 0.0.0.0:* LISTEN
tcp 0 0 127.0.0.1:25 0.0.0.0:* LISTEN
tcp 0 0 :::22 :::* LISTEN

Edit 3: I followed lain advise, also have my service bind to 0.0.0.0:3500 instead of 127.0.0.1:3500 and it works.

firewall centos iptables

1 Answers

  1. Best Answer

    Your rules is listed, rtmp-port is port 3500 according to IANA port/service names. To get a listing of port numbers rather than their service names use the -n switch

    iptables -L -n

    You have also use the -A switch to add your rule to the INPUT chain. This has added after the packets have been sent to the RH-Firewall-1-INPUT chain, the last rule of which is a blanket REJECT so packets destined for port 3500 will be REJECTed before they get tested in the INPUT Chain.

    You have a couple of possible solutions - use the -I switch to insert your rule to the INPUT chain or the RH-Firewall-1-INPUT chain

    iptables -I INPUT -p tcp --dport 3500 -j ACCEPT

    or

    iptables -I RH-Firewall-1-INPUT -p tcp --dport 3500 -j ACCEPT

    You should probably clean up your rules too, you can use 

    iptables -D INPUT -p tcp --dport 3500 -j ACCEPT

    (several times) to delete the existing rules for port 3500 before adding the new rules.

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