Using SLIP And Proxy ARP To Connect A Local Ethernet LAN To An Internet Connected LAN Dave Kennedy November 20, 1996 --------------------------------------------------------------- $Id: slip-arp,v 1.3 1996/11/20 16:06:51 davek Exp $ --------------------------------------------------------------- --------------------------------------------------------------- 0. Introduction 1. Other Options Available 2. The Final Result 3. Setting It Up 4. Kernel And Software 5. Outstanding Issues And Questions 6. Acknowledgments 7. Further Reading --------------------------------------------------------------- 0. Introduction In late 1994 and much of 1995, I had a small Ethernet LAN at home that I connected to the Internet. At the time, I did not have my own network address and I wanted to piggyback on my employer's network address. I succeeded using SLIP and proxy ARP. This document quickly describes how I did it at the time. It also provides information on other options available now that were not there in 1993/1994. More than likely, these other options will be your best bet. But, I'm continuing to maintain this document for those who need this type of solution. --------------------------------------------------------------- 1. Other Options Available When I originally wrote this mini-HOWTO, the options to connect via a dial-up LAN were more limited than today. If you want to connect a local LAN to the Internet via a dial-up connection, you should look at: IP Masquerade http://www.hwy401.com/achau/ipmasq/ http://www.indyramp.com/masq/ Use of proxy services http://harvest.transarc.com/afs/transarc.com/public/trg/Harvest/ http://harvest.cs.colorado.edu/ All links are current as of the date of this document. I no longer use this technique; if I had the same requirements today, I would use IP Masquerade. --------------------------------------------------------------- 2. The Final Result First, I'll present the final result and then describe how to get there. The final network is all on a Class C network (a.b.c.0): Internet | o-------+---------+----------------o <- Remote Ethernet | .126 NetBlazer | .215 <- SLIP (via 14.4 modem) gw | .253 o-----------------+--------+-------o <- Local Ethernet | .254 mus --------------------------------------------------------------- 3. Setting It Up The keys to getting this working are routing and proxy ARP. Here is gw's routing table: Kernel routing table Destination Gateway Genmask Flags Metric Ref Use Iface a.b.c.254 * 255.255.255.255 UH 0 0 5628 eth0 a.b.c.253 * 255.255.255.255 UH 0 0 0 eth0 a.b.c.126 * 255.255.255.255 UH 0 0 22 sl0 127.0.0.0 * 255.0.0.0 U 0 0 32 lo default a.b.c.126 * UG 0 0 9 sl0 This table was created with the following commands: 1. The SLIP connection was established which created the default gateway entry and the a.b.c.126 entry. 2. ifconfig eth0 a.b.c.253 3. route add a.b.c.253 eth0 4. route add a.b.c.254 eth0 Gw is now set up to talk to both eth0 and sl0. (This might be a good time to note that my kernel has IP forwarding configured.) The reason for the explicit route of a.b.c.253 may not be needed, but ping .253 from .253 without the route causes the ping to take the default route out the SLIP link. Here is mus's routing table: Kernel routing table Destination Gateway Genmask Flags MSS Window Use Iface a.b.c.253 * 255.255.255.255 UH 1436 0 59 eth0 127.0.0.0 * 255.0.0.0 U 1936 0 52 lo default a.b.c.253 * UG 1436 0 21661 eth0 This table was created with the following commands: 1. ifconfig eth0 a.b.c.254 2. route add a.b.c.253 eth0 3. route add default gw 192.68.22.253 Now, at this point I attempted to ping the NetBlazer. I could see the packets being sent out the modem (SD was flashing), but there was no response. This was good since it indicated that gw was getting packets in on eth0 (Ethernet) and sending them out sl0 (SLIP). The next step is to get the other side of the SLIP connection (NetBlazer) to recognize and know how to talk to me. This was done by using its proxy arp command. Usage: proxy ether|token| proxy a.b.c.253 ether 00:c0:6d:11:66:d3 proxy a.b.c.254 ether 00:c0:6d:11:66:e4 At this point I was able to ping the Internet from mus. And all was well with the world. :) NOTE: The 'proxy' command is available only on the NetBlazer. The equivalent command under linux is 'arp -s pub'. --------------------------------------------------------------- 4. Kernel And Software The kernel used on gw was 1.1.88 with IP Forwarding configured. This is required. The kernel on mus was also 1.1.88, but IP Forwarding was not configured. This does not matter. It's also worth noting that PPP could have been used instead of SLIP just as easily. [See, I told you this was a long time ago!] --------------------------------------------------------------- 5. Outstanding Issues And Questions The main question I have came up while writing this. When I checked the ARP table on the NetBlazer, my ARP entries were no longer there, but everything was still working. I readded the entry for mus and it made no difference. If anyone has ideas on this, let me know. --------------------------------------------------------------- 6. Acknowledgments My thanks to John Garnett for his post Dec 21, 1994 "A Guide on Using SLIP with ProxyARP..." I'm not so sure that my document is anything more than a rehash of what he wrote other than I'm using a NetBlazer on the Internet side and he had a modem attached to a computer. Anyway, thanks, John! --------------------------------------------------------------- 7. Further Reading Dawson, Terry; "NET-2-HOWTO." Garnett, John; "A (client) SLIP Guide for Linux." Lots of other HOWTO's.