353 lines
14 KiB
Plaintext
353 lines
14 KiB
Plaintext
User PPP Packet Aliasing
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0. Contents
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1. Background
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2. Setup
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3. New commands in ppp
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4. Future Work
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5. Authors / Acknowledgments
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6. Revision History for Aliasing Code
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1. Background
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User mode ppp has embedded packet aliasing (IP masquerading) code.
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Enabling this, either by the "-alias" command line option or the
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"alias enable yes" command in a ppp.conf file, makes the ppp host
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automatically alias IP packets forwarded from a local network, making
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them appear to come from the ppp host machine. Incoming packets
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from the outside world are then appropriately de-aliased.
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The process of aliasing involves both the IP address and the TCP or UDP
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port numbers. ICMP echo and timestamp packets are aliased by their id
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numbers. ICMP error messages can be properly directed by examining the
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fragment of the offending packet which is contained in the body of the
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message.
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This software was specifically meant to support users who have
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unregistered, private address IP networks (e.g. 192.168.0.x or 10.0.0.x
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addresses). The ppp host can act as a gateway for these networks, and
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computers on the local area net will have some degree of Internet access
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without the need for a registered IP address. Additionally, there will
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be no need for an Internet service provider to maintain routing tables
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for the local area network.
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A disadvantage of packet aliasing is that machines on the local network,
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behind the ppp host, are not visible from the outside world. They can
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establish TCP connections and make UDP inquiries (such as domain name
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service requests) but the connections seem to come from the ppp host
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itself. There is, in effect, a partial firewall. Of course, if this is
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what you want, the disadvantage becomes an advantage.
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A second disadvantage is that "IP encoding" protocols, which send IP
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address or port information within the data stream, are not supported
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for the cases where exception code exists. This implementation has
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workarounds for FTP and IRC DCC, the most well known of the IP encoding
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protocols. This frees users from depending on using the ftp passive
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mode and avoiding IRC DCC sends, as is sometimes the case with other
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masquerading solutions.
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The implementation supports all standard, non-encoding TCP and UDP protocols.
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Examples of these protocols are http, gopher and telnet. The standard UDP
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mode of RealAudio is not presently supported, but the TCP mode does work
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correctly.
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The packet aliasing code also handle many ICMP messages. In particular,
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ping and traceroute are supported.
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2. Packet Aliasing Setup
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It is recommended that users first verify correct ppp operation without
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packet aliasing enabled. This will confirm that the ppp.conf file is
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properly set up and that there are no ppp problems. Then start ppp with
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the "-alias" option on the command line. The user should verify that
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the ppp host can correctly connect to the Internet in packet aliasing
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mode. Finally, check that machines on the private network can access
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the Internet.
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The masquerading software aliases all packets, whether they come from
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the host or another computer on the local area network. Thus, a correctly
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operating ppp host indicates that the software should work properly for
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other computers on the private network.
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If the ppp host can access the Internet, but other computers on the local
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network cannot, check that IP forwarding is enabled on the ppp host. Also,
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verify that the other computers use this machine as a gateway. Of course,
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you should also verify that machines within the local area network
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communicate properly. A common error is inconsistent subnet addresses
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and masks.
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3. New commands in ppp
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In order to control aliasing behavior in a simple manner (no need for
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recompilation), a new command has been added to iij-ppp: alias. This
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is in addition to the -alias command line option. System managers and
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more experienced users may prefer to use the iij-ppp command syntax
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within the ppp.conf file. The alias command also allows packet aliasing
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behavior to be more precisely specified.
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The decision to add a command instead of extending 'set' or 'option' was
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to make obvious that these options only work when aliasing is enabled.
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The syntax for 'alias' is
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ppp> alias option [yes|no]
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where option is given by one of the following templates.
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- alias enable [yes|no] (default no)
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Enable packet aliasing functionality. If disabled, no other alias
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options will have any effect. You should usually enable aliasing
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before routing any packets over the link; good points are in the
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initial script or right before adding a route. If you do not always
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want aliasing, consider using the -alias option to ppp instead of this
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command.
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- alias deny_incoming [yes|no] (default yes)
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Set to "yes" to disable all incoming connections. This just drops
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connections to, for example, ftp, telnet or web servers. The aliasing
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mechanism prevents these connections. Technically, this option denies
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all incoming TCP and UDP requests, making the aliasing software a
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fairly efficient one-way firewall. The default is no, which will
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all incoming connections to telnetd, ftpd, etc.
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- alias log [yes|no]
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Controls logging of alias link creation to "/var/log/alias.log" - this
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is usually only useful if debugging a setup, to see if the bug is in
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the PPP aliasing. The debugging information is fairly limited, listing
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the number of aliasing links open for different prototocols.
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- alias same_ports [yes|no] (default yes)
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When a connection is being established going through the aliasing
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routines, it will normally have its port number changed to allow the
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aliasing code to track it. If same_ports is enabled, the alias
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software attempts to keep the connection's source port unchanged.
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This will allow rsh, RPC and other specialized protocols to work
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_most of the time_, at least on the host machine. Please, do not
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report this being unstable as a bug - it is a result of the way
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aliasing has to work. TCP/IP was intended to have one IP address
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per machine.
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- alias use_sockets [yes|no] (default yes)
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This is a fairly obscure option. For the most part, the packet aliasing
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software does not have to allocate system sockets when it chooses an
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aliasing port number. Under very specific circumstances, FTP data
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connections (which don't know the remote port nubmer, though it is
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usually 20) and IRC DCC send (which doesn't know either the address or
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the port from which the connection will come), there can potentially be
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some interference with an open server socket having the same port number
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on the ppp host machine. This possibility for interferience only exists
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until the TCP connection has been acknowledged on both sides. The safe
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option is yes, though fewer system resources are consumed by specifying
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no.
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- alias unregistered_only [yes|no] (default no)
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Packet aliasing normally remaps all packets coming from the local area
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network to the ppp host machine address. Set this option to only map
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addresses from the following standard ranges for private, unregistered
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addresses:
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10.0.0.0 -> 10.255.255.255
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172.16.0.0 -> 172.31.255.255
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192.168.0.0 -> 192.168.255.255 */
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In the instance that there is a subnet of public addresses and another
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subnet of private addresses being routed by the ppp host, then only the
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packets on the private subnet will be aliased.
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- alias port <proto> <local addr>:<port> <alias port>
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This command allows incoming traffic to <alias port> on the host
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machine to be redirected to a specific machine and port on the
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local area network. One example of this would be:
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alias port tcp 192.168.0.4:telnet 8066
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All traffic to port 8066 fthe ppp host would then be sent to
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the telnet port (23) of machine 192.168.0.4. Port numbers
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can either be designated numerically or by symbolic names
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listed in /etc/services. Similarly, addresses can be either
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in dotted quad notation or in /etc/hosts.
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- alias addr <local addr> <public addr>
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This command allows traffic for a public IP address to be
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redirected to a machine on the local network. This function
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is known as "static NAT". An address assignment of 0 refers
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to the default address of the ppp host. Normally static
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NAT is useful if your ISP has allocated a small block of
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IP addresses to the user, but it can even be used in the
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case of a single, dynamically allocated IP address:
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alias addr 10.0.0.8 0
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The above command would redirect all incoming traffic to
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machine 10.0.0.8.
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If several address aliases specifiy the same public addres
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as follows
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alias addr 192.168.0.2 public_addr
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alias addr 192.168.0.3 public_addr
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alias addr 192.168.0.4 public_addr
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then incoming traffice will be directed to the last
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translated local address (192.168.0.4), but outgoing
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traffic to the first two addresses will still be aliased
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to the specified public address.
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4. Future Work
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What is called packet aliasing here has been variously called masquerading,
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network address translation (NAT) and transparent proxying by others. It
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is an extremely useful function to many users, but it is also necessarily
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imperfect. The occasional IP-encoding protocols always need workarounds
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(hacks). Users who are interested in supporting new IP-encoding protocols
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can follow the examples of alias_ftp.c and alias_irc.c.
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ICMP error messages are currently handled only in the incoming direction.
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A handler needs to be added to correctly alias outgoing error messages.
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IRC and FTP exception handling make reasonable, though not strictly correct
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assumptions, about how IP encoded messages will appear in the control
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stream. Programmers may wish to consider how to make this process more
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robust.
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The packet aliasing engine (alias.c, alias_db.c, alias_ftp.c, alias_irc.c
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and alias_util.c) runs in user space, and is intended to be both portable
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and reusable for interfaces other than ppp. To access the basic engine
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only requires four simple function calls (initialization, communication of
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host address, outgoing aliasing and incoming de-aliasing).
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5. Authors / Acknowledgments
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Charles Mott (cmott@srv.net) <versions 1.0 - 1.8, 2.0, 2.1>
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Eivind Eklund (perhaps@yes.no) <versions 1.8b - 1.9, new ppp commands>
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Listed below, in chronological order, are individuals who have provided
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valuable comments and/or debugging assistance.
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Gary Roberts
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Tom Torrance
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Reto Burkhalter
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Martin Renters
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Brian Somers
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Paul Traina
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Ari Suutari
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J. Fortes
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Andrzej Bialeki
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6. Revision History for Aliasing Code
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Version 1.0: August 11, 1996 (cjm)
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Version 1.1: August 20, 1996 (cjm)
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PPP host accepts incoming connections for ports 0 to 1023.
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Version 1.2: September 7, 1996 (cjm)
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Fragment handling error in alias_db.c corrected.
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Version 1.3: September 15, 1996 (cjm)
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- Generalized mechanism for handling incoming connections
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(no more 0 to 1023 restriction).
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- Increased ICMP support (will handle traceroute now).
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- Improved TCP close connection logic.
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Version 1.4: September 16, 1996
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Can't remember (this version only lasted a day -- cjm).
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Version 1.5: September 17, 1996 (cjm)
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Corrected error in handling incoming UDP packets
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with zero checksum.
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Version 1.6: September 18, 1996
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Simplified ICMP data storage. Will now handle
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tracert from Win95 as well as FreeBSD traceroute.
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Verstion 1.7: January 9, 1997 (cjm)
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- Reduced malloc() activity for ICMP echo and
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timestamp requests.
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- Added handling for out-of-order IP fragments.
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- Switched to differential checksum computation
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for IP headers (TCP, UDP and ICMP checksums
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were already differential).
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- Accepts FTP data connections from other than
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port 20. This allows one ftp connections
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from two hosts which are both running packet
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aliasing.
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Verstion 1.8: January 14, 1997 (cjm)
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- Fixed data type error in function StartPoint()
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in alias_db.c (this bug did not exist before v1.7)
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Version 1.8b: January 16, 1997 (Eivind Eklund <perhaps@yes.no>)
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- Upgraded base PPP version to be the sourcecode from
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FreeBSD 2.1.6, with additional security patches. This
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version should still be possible to run on 2.1.5, though -
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I've run it with a 2.1.5 kernel without problems.
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(Update done with the permission of cjm)
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Version 1.9: February 1, 1997 (Eivind Eklund <perhaps@yes.no>)
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- Added support for IRC DCC (ee)
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- Changed the aliasing routines to use ANSI style throughout -
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minor API changes for integration with other programs than PPP (ee)
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- Changed the build process, making all options switchable
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from the Makefile (ee)
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- Fixed minor security hole in alias_ftp.c for other applications
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of the aliasing software. Hole could _not_ manifest in
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PPP+pktAlias, but could potentially manifest in other
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applications of the aliasing. (ee)
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- Connections initiated from packet aliasing host machine will
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not have their port number aliased unless it conflicts with
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an aliasing port already being used. (There is an option to
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disable this for debugging) (cjm)
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- Sockets will be allocated in cases where there might be
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port interference with the host machine. This can be disabled
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in cases where the ppp host will be acting purely as a
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masquerading router and not generate any traffic of its own.
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(cjm)
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Version 2.0: March, 1997 (cjm)
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- Incoming packets which are not recognized by the packet
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aliasing engine are now completely dropped in ip.c.
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- Aliasing links are cleared when a host interface address
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changes (due to re-dial and dynamic address allocatioa).
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- PacketAliasPermanentLink() API added.
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- Option for only aliasing private, unregistered IP addresses
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added.
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- Substantial rework to the aliasing lookup engine.
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Version 2.1: May, 1997 (cjm)
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- Continuing rework to the aliasing lookup engine to support
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multiple incoming addresses and static NAT.
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- Now supports outgoing as well as incoming ICMP error messges/
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- PPP commands to support address and port redirection.
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