Server Fault is a question and answer site for system and network administrators. It's 100% free, no registration required.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

I set my server to reject ALL incoming UDP packets, to prevent UDP floods. However, I was told that because I use my own domain and server for my nameservers, this can cause some problems. How can I get around that?

My firewall is iptables, my distro is CentOS5.5.

share|improve this question
What makes you think you need a firewall rule for that? The UDP layer should be able to handle packets to a closed port at least as fast as the firewall would. If you aren't careful with the performance of your rules the firewall approach could even end up being a lot slower. If you are concerned about the bandwidth consumed on your incoming connection, the firewall isn't going to help anyway. – kasperd Sep 30 '15 at 8:11
up vote 5 down vote accepted

If you are following the standard security practices, then your default firewall policy will be to block everything. All you should have to do is write a rule to permit tcp and udp traffic to port 53 if you want to permit incoming DNS requests.

The traffic you are talking about is UDP. UDP is stateless. This means that people interested in saturating your connection can send the packets to your address even if you just drop them. Still you may be able to do something semi-useful with the iptables recent match, to only allow a limited ammount of traffic to actually be accepted and processed by the system. Evan has a example of the usage of this for SSH here. We might have to see your entire firewall rule set to tell you what rules would have to be added.

If you have a serious DoS against your system, you would almost certainly need your ISP to help you, trying to deal with a flood with a host-based firewall on a VPS will really not be very useful.

If you don't have it already, you should consider setting up a few secondary DNS servers for your zones on a completely different network.

share|improve this answer
Well the problem with that, is then users could easily flood me on port 53, right? – Rob Jan 14 '11 at 18:38
Yes. So you're choices are: 1. Don't run your own DNS servers or 2. Live with the risk. – joeqwerty Jan 14 '11 at 18:46
I'm sorry, I'm quite terribly new at running my own server. The whole DNS server thing, is that my resolvers? I currently have those set to GoogleDNS ( and, so would it matter? – Rob Jan 14 '11 at 18:48
Yes, any port can be flooded: that's why good firewall can detect floods & can do things like packet inspection. You would still need upstream help from your ISP or their suppliers to help with serious DDOS attacks. – DutchUncle Jan 14 '11 at 18:53
@Zoredache can you show me the rule to allow UDP only on port 53, but with a connection limit? – Rob Jan 14 '11 at 19:17

In Iptables, Accept incoming UDP traffic to port 53 & reject everything in the port range for ephemeral ports.

The highest limit should not be too high otherwise, your server will be unable to resolve external domains (for instance when you do a "ping") from inside your server. On a linux OS, 32768 is the first ephemeral port (aka dynamic ports) for sockets up to 61000. Thus, 32767 is the highest port for static allocated ports. This is only true if don't use your server as DNS resolver aka DNS cache aka server with an /etc/resolv.conf pointing to nameserver or ::1

Here is a tcpdump example:

 23:10:13.315832 IP b.b.b.b.34507 > a.a.a.a.53: 23674% [1au] A? (38)
 23:10:13.377619 IP a.a.a.a.53 > b.b.b.b.34507: 23674*- 1/2/3 A
  1. b.b.b.b requests nameserver a.a.a.a from port 34507 to give A record for on port 53
  2. a.a.a.a from port 53 answers b.b.b.b to port 34507

Normally, to find your local dynamic (also called ephemeral or private) port ranges on your linux for UDP & TCP:

 cat /proc/sys/net/ipv4/ip_local_port_range

However, it only works for server that don't host the DNS resolver (for instance, when you point your /etc/resolv.conf to

Server is not a DNS resolver:

 -A INPUT -p udp -m udp --dport 53 -j ACCEPT
 -A INPUT -p udp -m udp --dport 0:32767 -j DROP

server is a DNS resolver:

-A INPUT -p udp -m udp --dport 53 -j ACCEPT
-A INPUT -p udp -m udp --dport 0:1023 -j DROP

This should be taken into account if you want to host your own DNS resolver, to resolve all domain names.

The best would be to check it yourself:

You can monitor sending ports using

tcpdump udp and port 53 and not dst host *yourserveripaddress*

then look at sending ports and try to find the lowest the number. This lowest number should not be lower than the port number xxxx in --dport 0:xxxx otherwise you block or slow down your DNS requests.

share|improve this answer

Do you actually need to allow strangers on the internet to run DNS queries against your server? I suspect you just need to make sure that your firewall allows your server to make outgoing DNS requests.

With firewalling you start by blocking everything and then being very precise/detailed about opening up specific combinations of port/service/protocol and limit it by IP address (range).

share|improve this answer

How can I reject all incoming UDP packets except for DNS lookups?

This can be stated more broad: how can I reject all traffic I didn't initiate with Linux netfilter?

Answer is simply 2 lines:

iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A INPUT -j DROP

(Taking into account you don't want to filter loopback traffic to save some CPU cycles, you can add exception for it.)

If OTOH you have DNS server on same computer and your q-n could be more precisely stated as "How can I reject all incoming UDP traffic except external queries to DNS-server?" you would use same 2 lines base and add another one which would explicitly allow DNS traffic:

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

iptables -A INPUT -p udp --dport 53 -j ACCEPT -m comment --comment "we serve DNS"
iptables -A INPUT -p tcp --dport 53 -j ACCEPT -m comment --comment "DNS uses TCP too sometimes"

iptables -A INPUT -j DROP

Notice that you typically don't need to specify any -m state things for anything but the first line there, cause the first line is a short-cut allowing any legitimate traffic to continue had its pilot packets gotten permission to enter.

Other notices

@Zoredache's advice for recent isn't any applicable at all. recent has very limited use case scope cause it doesn't use efficient data structures but list and hashes, no trees, nope. By default it can remember only 100 IPs per list which can be extended but hashing isn't very effective for searching through it anyways.

share|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.