243 lines
5.6 KiB
HTML
243 lines
5.6 KiB
HTML
<HTML
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><HEAD
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><TITLE
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>Routing issues on a LAN</TITLE
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NAME="GENERATOR"
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CONTENT="Modular DocBook HTML Stylesheet Version 1.52"><LINK
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TITLE="Linux PPP HOWTO"
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TITLE="Using /etc/ppp/ip-down"
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>Linux PPP HOWTO</A
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><H1
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><A
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NAME="AEN1482"
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>Chapter 27. Routing issues on a LAN</A
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></H1
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><P
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>If you are connected to a LAN but still want to use PPP on your personal
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Linux machine , you need to address some issues of the routes packets
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need to take from your machine to reach your LAN (through your Ethernet
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interface) and also to the remote PPP server and beyond.</P
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><P
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>This section does NOT attempt to teach you about routing - it deals only
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with a simple, special case of (static) routing!</P
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><P
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>I strongly urge you to read the Linux Network Administrator Guide (NAG)
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if you are NOT familiar with routing. Also the O'Reilly book "TCP/IP
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Network Administration" covers this topic in a very understandable form.</P
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><P
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>The basic rule of static routing is that the DEFAULT route should be the
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one that points to the MOST number of network addresses. For other
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networks, enter specific routes to the routing table.</P
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><P
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>The ONLY situation I am going to cover here is where your Linux box is
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on a LAN that is not connected to the Internet - and you want to dial
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out to the Internet for personal use whilst still connected to the LAN.</P
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><P
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>First of all, make sure that your Ethernet route is set up to the
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specific network addresses available across your LAN - NOT set to the
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default route!</P
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><P
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>Check this by issuing a route command, you should see something like the
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following:-</P
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><P
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> <TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><PRE
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CLASS="SCREEN"
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>[root@hwin /root]# route -n
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Kernel routing table
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Destination Gateway Genmask Flags MSS Window Use Iface
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loopback * 255.255.255.0 U 1936 0 50 lo
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10.0.0.0 * 255.255.255.0 U 1436 0 565 eth0</PRE
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></TD
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></TR
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></TABLE
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> </P
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><P
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>If your Ethernet interface (eth0) is pointing at the default route, (the
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first column will show "default" in the eth0 line) you need to change
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your Ethernet initialization scripts to make it point at the specific
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network numbers rather than the default route (consult the Net2 HOWTO
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and NAG).</P
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><P
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>This will allow pppd to set up your default route as shown below:-</P
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><P
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> <TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><PRE
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CLASS="SCREEN"
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>[root@hwin /root]# route -n
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Kernel routing table
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Destination Gateway Genmask Flags MSS Window Use Iface
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10.144.153.51 * 255.255.255.255 UH 488 0 0 ppp0
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127.0.0.0 * 255.255.255.0 U 1936 0 50 lo
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10.1.0.0 * 255.255.255.0 U 1436 0 569 eth0
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default 10.144.153.51 * UG 488 0 3 ppp0</PRE
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></TD
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></TR
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></TABLE
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> </P
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><P
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>As you can see, we have a host route to the PPP server ( 10.144.153.51)
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via ppp0 and also a default network route that uses the PPP server as
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its gateway.</P
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><P
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>If your set up needs to be more complex than this - read the routing
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documents already mentioned and consult an expert at your site!</P
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><P
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>If your LAN already has routers on it, you will already have gateways
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established to the wider networks available at your site. You should
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STILL point your default route at the PPP interface - and make the other
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routes specific to the networks they serve.</P
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><DIV
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CLASS="SECT1"
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><H1
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CLASS="SECT1"
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><A
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NAME="AEN1500"
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>27.1. Note on Security</A
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></H1
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><P
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>When you set up a Linux box on an existing LAN to link into the
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Internet, you are potentially opening your entire LAN to the Internet -
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and the hackers that reside there. Before you do this, I strongly urge
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you to consult your network administrator and site security policy. If
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your PPP connection to the Internet is used to successfully attack your
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site, you will at the very least earn the intense anger of your fellow
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users, network and system administrators. You may also find yourself in
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very much more serious trouble!</P
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><P
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>Before you connect a LAN to the Internet, you should consider the
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security implications of even a DYNAMIC connection - hence the earlier
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reference to the O'Reilly "Building Internet Firewalls"!</P
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></DIV
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HREF="c1477.html"
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>Prev</A
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>Home</A
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HREF="ppp-server.html"
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>Next</A
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><TD
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WIDTH="33%"
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ALIGN="left"
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VALIGN="top"
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>Using <TT
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CLASS="LITERAL"
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>/etc/ppp/ip-down</TT
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></TD
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><TD
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WIDTH="34%"
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ALIGN="center"
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VALIGN="top"
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> </TD
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><TD
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WIDTH="33%"
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ALIGN="right"
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VALIGN="top"
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>Setting up a PPP server</TD
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