TCP Wrappers
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Video Transcription
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>> Hey, cybrarians. Welcome back to
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the Linus Plus Course here at Cybrary.
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I'm your instructor Robby Gill and in today's lesson,
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we're going to be covering TCP Wrappers.
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Upon completion of today's lesson,
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you're going to be able to understand
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the purpose of TCP Wrappers and then
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locate the files that are used to control
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access when we're using TCP Wrappers.
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TCP Wrappers were used to
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control access to network-enabled services.
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This is a legacy control process,
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and it was used to control things such as SSH or NFS.
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Now, this functionality has been
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largely replaced by firewalls,
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but there are services that still
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support using TCP Wrappers.
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The way we can find them is they will have
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the libwrap library compiled
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with them. How do we find that?
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Well, we can use or
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friendly ldd command to list
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the libraries that are used by each service.
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Recall back to module 7 where we covered LDD.
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In order to do this, it's a four-step process
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or a three-step process.
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The first thing we're going to want to do is
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find the full path to the daemon.
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We can do that with the which command,
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we can do which sshd,
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and it tells us the full path to that is
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>> /usr/sbin/sshd.
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>> Then we can use LDD to list all
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>> of the shared libraries.
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>> Your LDD user sbin SSHD.
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Then from there, we can just grep for libwrap.
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Now, if we see libwrap,
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that means that this service still
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supports using TCP Wrappers.
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For example, when we do that for SSHD,
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we do see that it comes back and returns the result.
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But there is a newer services
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out there that don't support this at all.
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If we did the same thing for OpenVPN,
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which OpenVPN, we find out its /usr/sbin/openvpn.
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We do ldd /usr/sbin/openvpn,
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we got for libwrap, we get nothing back.
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That tells us that OpenVPN has no use
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for TCP Wrappers and to work with that,
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we would probably need to configure a
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firewall to secure it.
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When we are working with TCP Wrappers,
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there are two files that we use to control
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access to this service and this control via IP address.
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There's the /etc/host.allow file,
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and that allows access to IPs that are on lists and
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the /etc/host.deny file blocks
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access to IPs that are on list.
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There's generally order of
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operations that we have to
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follow when we're using these files,
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and it's very specific and it may be on the exam,
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so definitely go over this a few times if
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it is not making sense
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The first step is we look at the host.allow
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file and it's checked for IP address of the requester.
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If the IP address is found, great,
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we get access to that service,
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nothing further is required.
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But if the IP address is not found,
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then we go into the host.deny file
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and we check to see if this IP Is denied.
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Now, if the IP address is found,
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access is denied for that system,
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the requester can't get in, all good.
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But if the IP is not found in either file,
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the axis is granted by default.
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Access can be allowed if
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the IP is not found in either file.
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Therefore, a lot of times when
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people are using these things,
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they will add,/ etc/hosts.deny.
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They'll add a line to the very end of it.
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This has all colon all.
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This is really only necessary when we're using
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host.allow to make sure that
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somebody can't drop through and get
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access because they're not included in either file.
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But with that we've reached the end of this lesson.
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In this lesson we covered the purpose of
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TCP wrappers and then we learned about
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controlling access to network services
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that allow us to use TCP wrappers.
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We do that using host.allow and host.deny files.
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Thanks so much for being here and I look
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forward to seeing you in the next lesson.
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