Re: DNS ACL ?
From: Richard C Lewis (mrlew1_at_earthlink.net)
Date: 11/14/05
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To: John Hally <JHally@epnet.com> Date: Sun, 13 Nov 2005 20:19:09 -0500
Whenever a DNS server returns a response of over 512 bytes it will set
the Truncation bit to tell the requesting server to reissue the same
query over a Virtual Circuit (TCP connection). This is normally seen
with requests for web server information for large server farms. If you
block TCP/53 to your DNS server you *MAY* not experience any problems,
but the problems will likely occur on the requesting side of someone
seeking your information. If you do not have a lot of systems with the
same name on multiple IP addresses or multiple CNAMES or a large mail
server farm you *MAY* come out okay. Just keep in mind that your DNS
system will not be functioning the way it *should* be... but then we
wouldn't have a need for security professionals if everything did what
it *should*...
You can limit your exposure by reducing who can perform zone transfers
via the allow-transfer option and use the query-source option to control
the port used for your outgoing queries. Coupled with router/firewall
ACLs you can serious limit the TCP connections to your DNS server.
attached mail follows:
To: "'pen-test@securityfocus.com'" <pen-test@securityfocus.com> Date: Fri, 11 Nov 2005 08:35:06 -0500
Hello All,
I need a sanity check regarding DNS ACLs. For external facing DNS servers
you need to allow only udp/53 inbound, correct? I know tcp/53 is used for
zone transfers and requests/replies greater than a certain size, but they
shouldn't typically happen for general dns queries correct?
Thanks in advance!
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Audit your website security with Acunetix Web Vulnerability Scanner:
Hackers are concentrating their efforts on attacking applications on your
website. Up to 75% of cyber attacks are launched on shopping carts, forms,
login pages, dynamic content etc. Firewalls, SSL and locked-down servers are
futile against web application hacking. Check your website for vulnerabilities
to SQL injection, Cross site scripting and other web attacks before hackers do!
Download Trial at:
http://www.securityfocus.com/sponsor/pen-test_050831
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