Linux Iptables block or open DNS / bind service port 53

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The domain name service provided by BIND (named) software. It uses both UDP and TCP protocol and listen on port 53. DNS queries less than 512 bytes are transferred using UDP protocol and large queries are handled by TCP protocol such as zone transfer.

i) named/bind server – TCP/UDP port 53

ii)Client (browser, dig etc) – port > 1023

Allow outgoing DNS client request:

Following iptables rules can be added to your shell script.

SERVER_IP is your server ip address

DNS_SERVER stores the nameserver (DNS) IP address provided by ISP or your own name servers.

Following rules are useful when you run single web/smtp server or even DSL/LL/dialup Internet connections:

SERVER_IP="202.54.10.20"
DNS_SERVER="202.54.1.5 202.54.1.6"
for ip in $DNS_SERVER
do
iptables -A OUTPUT -p udp -s $SERVER_IP --sport 1024:65535 -d $ip --dport 53 -m state --state NEW,ESTABLISHED -j ACCEPT
iptables -A INPUT -p udp -s $ip --sport 53 -d $SERVER_IP --dport 1024:65535 -m state --state ESTABLISHED -j ACCEPT
iptables -A OUTPUT-p tcp -s $SERVER_IP --sport 1024:65535 -d $ip --dport 53 -m state --state NEW,ESTABLISHED -j ACCEPT
iptables -A INPUT -p tcp -s $ip --sport 53 -d $SERVER_IP --dport 1024:65535 -m state --state ESTABLISHED -j ACCEPT
done

(B) Allow incoming DNS request at port 53:

Use following rules only if you are protecting dedicated DNS server.

SERVER_IP is IP address where BIND(named) is listing on port 53 for incoming DNS queries.

Please note that here I’m not allowing TCP protocol as I don’t have secondary DNS server to do zone transfer.

SERVER_IP="202.54.10.20"
iptables -A INPUT -p udp -s 0/0 --sport 1024:65535 -d $SERVER_IP --dport 53 -m state --state NEW,ESTABLISHED -j ACCEPT
iptables -A OUTPUT -p udp -s $SERVER_IP --sport 53 -d 0/0 --dport 1024:65535 -m state --state ESTABLISHED -j ACCEPT
iptables -A INPUT -p udp -s 0/0 --sport 53 -d $SERVER_IP --dport 53 -m state --state NEW,ESTABLISHED -j ACCEPT
iptables -A OUTPUT -p udp -s $SERVER_IP --sport 53 -d 0/0 --dport 53 -m state --state ESTABLISHED -j ACCEPT

Please note if you have secondary server, add following rules to above rules so that secondary server can do zone transfer from primary DNS server:

DNS2_IP="202.54.10.2"
iptables -A INPUT -p tcp -s $DNS2_IP --sport 1024:65535 -d $SERVER_IP --dport 53 -m state --state NEW,ESTABLISHED -j ACCEPT
iptables -A OUTPUT -p tcp -s $SERVER_IP --sport 53 -d $DNS2_IP --dport 1024:65535 -m state --state ESTABLISHED -j ACCEPT

Posted by: Vivek Gite

The author is the creator of nixCraft and a seasoned sysadmin, DevOps engineer, and a trainer for the Linux operating system/Unix shell scripting. Get the latest tutorials on SysAdmin, Linux/Unix and open source topics via RSS/XML feed or weekly email newsletter.

4 comment

  2. Also say your running a name server and you need to communicate with any name servers:


    IPADDR=xxx.xxx.xxx.xxx
    UNPRIVPORTS="1024:65535"
    iptables -A OUTPUT -p udp -s $IPADDR --sport $UNPRIVPORTS -d 0/0 --dport 53 -m state --state NEW,ESTABLISHED -j ACCEPT
    iptables -A INPUT -p udp -s 0/0 --sport 53 -d $IPADDR --dport $UNPRIVPORTS -m state --state ESTABLISHED -j ACCEPT
    iptables -A OUTPUT -p tcp -s $IPADDR --sport $UNPRIVPORTS -d 0/0 --dport 53 -m state --state NEW,ESTABLISHED -j ACCEPT
    iptables -A INPUT -p tcp -s 0/0 --sport 53 -d $IPADDR --dport $UNPRIVPORTS -m state --state ESTABLISHED -j ACCEPT

  3. Hi guys, don’t know it is right place to post it but i’d like to ask does any of you know how to change that port 53 to port 25. Our profesors in scool gave us something like this to solve.

  4. For pure and simple DNS client (not hosting DNS server and so on), I scripted it as follows:
    for dnsserverip in `grep nameserver /etc/resolv.conf | sed 's/.* //'` ; do
    /usr/sbin/iptables -A INPUT -p udp --dport 53 -s $dnsserverip -m state --state NEW -j ACCEPT
    /usr/sbin/iptables -A INPUT -p tcp --dport 53 -s $dnsserverip -m state --state NEW -j ACCEPT
    done

    I also have separate lines in my generic iptables script globally allowing “ESTABLISHED,RELATED”, negating the need for adding these parameters elsewhere.

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