I don't know what program is generating this. Is there a way to create a IP table of blocked DNS queries?
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How do I block a specific DNS query (IP address) in a CENT/OS conputer.
This particular computer is sending out a bogus DNS query to our DNS windows 2003 sever. I would like to block the query at the source. How can this be done?
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Answers
Is the "particular" computer within your organization, or is it coming from the Internet?
If it's coming from the Internet, your choices are to firewall that IP address on TCP and UDP inbound destination port 53, or, you can lookup the technical contact information for the IP address through ARIN (http://ws.arin.net/whois/
You would put:
127.0.0.1 hostname
where hostname is the host name it is trying to find. This will not stop the queries, it will just stop them from going to your DNS server.
However, now that the program is getting back a result, it will try and connect to 127.0.0.1 to something. What you may want to do is put a invalid IP address (say 127.1.1.1) with the hostname and then issue the command:
netstat -np | grep 127.1.1.1
every now and then. the "p" will give you the pid of the task that is trying to connect to 127.1.1.1, then you can find out what program is issuing the queries and "fix" it.
Rather than redirecting the results and still taking in the traffic and trying to deal with it, why not just firewall it and forget about it?
If you have iptables installed:
iptables -A INPUT -p tcp --dport 53 -s 67.40.182.122 -j DROP
iptables -A INPUT -p udp --dport 53 -s 67.40.182.122 -j DROP
service iptables save
... and the traffic is gone. The machine can still send you email, or view websites, it just won't be able to pass you anymore DNS traffic.
Yes, you did, I am just dyslexic today, I had the two machines backwards in my head.
You can block the lookup of the host by using the /etc/hosts file, but you need to enter it as the name being looked up, not the result IP
Example
127.0.0.1 name.domain.com
Any request for name.domain.com would be resolved as 127.0.0.1 without querying DNS.
If you want to create a log of the DNS requests leaving from the CentOS machine, you can use this iptables statement to make a log:
iptables -A OUTPUT -p tcp --dport 53 -j LOG
iptables -A OUTPUT -p udp --dport 53 -j LOG
The requests will be placed in /var/log/messages
Yes it does. This IP has a block so it does not resolve and excessive traffic is created as a result. Several DNS errors. I'm going to try lanboyo's suggestion and let you all know what happens.
Thanks,
05/26/09 16:55:10 IP block 67.40.182.122
Trying 67.40.182.122 at ARIN
Trying 67.40.182 at ARIN
OrgName: Qwest Communications Corporation
OrgID: QCC-22
Address: 1801 California Street
City: Denver
StateProv: CO
PostalCode: 80202
Country: US
NetRange: 67.40.0.0 - 67.42.255.255
CIDR: 67.40.0.0/15, 67.42.0.0/16
NetName: QWEST-INET-116
NetHandle: NET-67-40-0-0-1
Parent: NET-67-0-0-0-0
NetType: Direct Allocation
I guess the problem I am having is I don't know if a way to prevent a query for a specific name/address. You can prevent queries from occurring totally, but you can't say "don't allow query for name XXXX" or "don't allow query for address a.b.c.d".
My guess is that address is attempting to connect to some service on that computer (http, ssh, ftp, smtp, or something else) that is configured to do reverse look-ups.
The only way to "get the query under control" is to find out what is doing it and stop it. That I am aware of there is no "content filtering" for dns functions.
I haven't tried it yet but maybe this would work because the query would think it got its answer..
lanboyo:If you put a host entry for the ip,
67.40.182.122 any_name_you_want
It will likely not do a reverse lookup.
A lot of commands have a -n option to not do the reverse lookup if this is a tcdump or traceroute kind of thing.
Posted via EE Mobile
Yes, this entry in /etc/hosts will stop some of the madness.
However, rhe next reverse lookup may start the problem over again.
I have had a similiar problem with a microsoft dns infrastructure, where reverse lookups for ips that are not in the local ad schema are blasted to every registered server in ad that is running dns, and every dns server asks every registered dns server.. And so on.
If your external server has cached an nxerror for this reverse lookup, then you will get used to seeing this problem. My org was disconnected from a shared dns structure for exactly this problem.
Since the originator is a linux box perhaps you can tweak the resolve.conf file to look directly to the external dns server for addresses not used internally. Apps that do a surprisingly large number of reverse lookups are tcpdump, snort and ethereal, even when they are reading old capture files.
Also traceroute and mtr.
It has been a while. Did the host entry not help? I suppose the issue is that the linux box is doing a reverse lookup on an IP address.
Some linux apps will not trust the listing in the host table.
You may need to set up a dns server on the linux box and put the servers own address in the dns search order.
I recommend dnsmasq, very lightweight, and it will serve the local addresses out of /etc/hosts .
http://dag.wieers.com/rpm/
I was able to get it working....
administrator@mail:/etc/in
; <<>> DiG 9.4.2-P2 <<>> -x 67.40.182.122
;; global options: printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 27955
;; flags: qr aa rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0
;; QUESTION SECTION:
;122.182.40.67.in-addr.arp
;; ANSWER SECTION:
122.182.40.67.in-addr.arpa
Do we know what app is doing the reverse lookup? That may be the key.
I had to go back and re-read to try and remember what the issue is.
--> "This particular computer is sending out a bogus DNS query to our DNS windows 2003 sever. I would like to block the query at the source. How can this be done?"
Basic answer, you can't.
From the posts, the request is not a bogus request, its just a request you would rather not have come to your DNS server.
The solution is to find out what is doing the reverse look-up and stop it from doing it. You can't stop a reverse lookup request (or a forward lookup) for a specific entry.
If you can't find out what is doing the look-up, then all you can do attempt to do is change what you return. Which lanboyo did give you a possible solution: add and entry in your hosts file for the IP address that is attempting to be looked up. This will not stop the look-up, but it may return a host name other than the real host name and it may return the result faster than if you forward the request to another name server.
I know that some web server report packages do reverse look-ups to do reporting, some ssh servers do reverse look-ups to validate the host name before allowing connections, and some e-mail servers also do reverse look-ups to verify that IP address is associated with the domain name that remote smtp server says it is coming from. So returning a "bad" host name could cause something else to "break."
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by: giltjrPosted on 2009-05-21 at 14:25:05ID: 24446058
To block at the source, find out what program is doing this and stop it or remove it.