Configuring Multiple IP Addresses

This material is originally from John Ioannidis (ji@polaris.ctr.columbia.edu) I have condensed it some and applied some corrections for SunOS 4.1.x courtesy of Chuck Smoko (csmoko@relay.nswc.navy.mil). Bob Baggerman (bob@bizweb.com) 12 Jan 94 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= John Ionnidis writes: This is a topic that comes up once in a while on comp.protocols.tcp-ip and other newsgroups. The question is, how to get a machine with one network interface to respond to more than one IP addresses. I have a solution than might suit you. For my doctoral work (there's a paper about it in this year's ('91) SIGCOMM, also available for anonymous FTP from cs.columbia.edu:/pub/ji/sigcomm*.ps.Z), I've developed what I call the "Virtual Interface" (VIF). To the networking code, it looks like an interface. It gets ifattach()ed when you open the /dev/vif* device, and then you can ifconfig it as you like. It does not have an if_input procedure; it only has an if_output. Packets that it receives (from higher-level protocols) which have its IP address, it simply loops back (like any well-behaved if driver). Packets that it receives that are destined for some other address, it encapsulates in an encapsulation protocol I call IPIP (IP-within-IP, protocol number IPPROTO_IPIP == 94), and sends it to another machine that groks that encapsulation protocol. This feature you won't need, but here's how to have multiple IP addresses on a machine with a single real interface: Let's say your primary interface's IP address is, and you also want it to respond to addresses and (note that these are three distinct class C addresses in three distinct class C nets). Here are the ifconfigs: ifconfig le0 up -trailers # config primary interface ifconfig vif0 up # config first virtual interface route delete net 198.4.3 # delete spurious route route add host 0 # add route for this i/f ifconfig vif1 up # config second virtual interface route delete net 198.5.4 # delete spurious route route add host 0 # add route for this i/f The route deletes are needed because the ifconfig creates a default route to the interface's network, which can cause problems; all that's needed is the (host) route to the interface's address. Now, get le0's ethernet address (say, 8:0:20:3:2:1), and add the following static ARP entries: arp -s 8:0:20:3:2:1 pub arp -s 8:0:20:3:2:1 pub This will cause any ARP requests for the VIF addresses to be replied with your machine's ethernet address. Now, make sure your default route is to your segment's gateway, through the real interface. Finally, make sure your routers and/or hosts on the same segment as yours know that and are on that cable. Here's what you've accomplished. ARP requests for any of your host's addresses will be replied to with the host's ethernet address (the real one, because that's what it is, the virtual ones because of the public static arp entries). Packets reaching your host with any of these addresses will be accepted by the ip_input routine because they match the address of one of the host's interfaces. Packets leaving your host can have any of its addresses (real and virtual). The code for vif follows. To use it, put the stuff in netinet/if_vif.c and netinet/if_vif.h, configure your kernel with the number of virtual interfaces you want using a line like: pseudo-device vif4 # Virtual IP interface in your configuration file, and the line netinet/if_vif.c optional vif device-driver in the "files" file. Also, add the appropriate entries in conf.c, so that you can access the if_attach() routine when you open the device: -------------------------- conf.c------------------------------------------ add this in the appropriate place in the headers of conf.c: -------------------- #include "vif.h" #if NVIF > 0 int vifopen(), vifclose(), vifread(), vifwrite(), vifselect(), vifioctl(); #else #define vifopen nodev #define vifclose nodev #define vifread nodev #define vifwrite nodev #define vifselect nodev #define vifioctl nodev #endif -------------------- then, way down in the definition for cdevsw[]: -------------------- vifopen, vifclose, vifread, vifwrite, /*14*/ vifioctl, nodev, nodev, 0, 0, nodev, -------------------- Make sure you remember the correct major device number, 14 in this case! --------------------------------------------------------------------------- Finally, here's the code. It has the tunneling pieces removed (you need more code to use that anyway), and it comes from a Mach 2.6 kernel; it should compile on any Berkeley-derived unix with minor changes (most likely only in the includes). ---------------------netinet/if_vif.h-------------------------------------- typedef struct { struct ifnet vif_if; struct ifnet *vif_sif; /* slave interface */ int vif_flags; } vif_softc_t; #define VIFMTU (1024+512) --------------------------------------------------------------------------- and ---------------------netinet/if_vif.c-------------------------------------- /* * Virtual IP interface module. */ #include "param.h" #include "../sys/systm.h" #include "../sys/mbuf.h" #include "../sys/socket.h" #include "../sys/errno.h" #include "../sys/ioctl.h" #include "../net/if.h" #include "../net/netisr.h" #include "../net/route.h" #ifdef INET #include "../netinet/in.h" #include "../netinet/in_systm.h" #include "../netinet/in_var.h" #include "../netinet/ip.h" #endif #include "in_pcb.h" #include "vif.h" typedef struct { struct ifnet vif_if; struct ifnet *vif_sif; /* slave interface */ int vif_flags; } vif_softc_t; #define VIFMTU (1024+512) vif_softc_t vif_softc[NVIF]; int vifs_inited = 0; vifattach() { register int i; register struct ifnet *ifp; int vifoutput(), vififioctl(); for (i=0; i<NVIF; i++) { ifp = &vif_softc[i].vif_if; ifp->if_name = "vif"; ifp->if_unit = i; ifp->if_mtu = VIFMTU; ifp->if_flags = IFF_LOOPBACK | IFF_NOARP; ifp->if_ioctl = vififioctl; ifp->if_output = vifoutput; if_attach(ifp); } } vifopen(dev, flag) int dev, flag; { int unit; if (!vifs_inited) { vifattach(); vifs_inited = 1; printf("vif initialized\n"); } unit = minor(dev); if ((unit < 0) || (unit >= NVIF)) { return ENXIO; } return 0; } vifclose(dev, flag) int dev, flag; { return 0; } vifread() { return ENXIO; } vifwrite() { return ENXIO; } vifselect() { return ENXIO; } vifoutput(ifp, m0, dst) struct ifnet *ifp; register struct mbuf *m0; struct sockaddr *dst; { int s; register struct ifqueue *ifq; struct mbuf *m; struct sockaddr_in *din; if (dst->sa_family != AF_INET) { printf("%s%d: can't handle af%d\n", ifp->if_name, ifp->if_unit, dst->sa_family); m_freem(m0); return (EAFNOSUPPORT); } din = (struct sockaddr_in *)dst; if (din->sin_addr.s_addr == IA_SIN(ifp->if_addrlist)->sin_addr.s_addr) { /* printf("%s%d: looping\n", ifp->if_name, ifp->if_unit); */ /* * Place interface pointer before the data * for the receiving protocol. */ if (m0->m_off <= MMAXOFF && m0->m_off >= MMINOFF + sizeof(struct ifnet *)) { m0->m_off -= sizeof(struct ifnet *); m0->m_len += sizeof(struct ifnet *); } else { MGET(m, M_DONTWAIT, MT_HEADER); if (m == (struct mbuf *)0) return (ENOBUFS); m->m_off = MMINOFF; m->m_len = sizeof(struct ifnet *); m->m_next = m0; m0 = m; } *(mtod(m0, struct ifnet **)) = ifp; s = splimp(); ifp->if_opackets++; ifq = &ipintrq; if (IF_QFULL(ifq)) { IF_DROP(ifq); m_freem(m0); splx(s); return (ENOBUFS); } IF_ENQUEUE(ifq, m0); schednetisr(NETISR_IP); ifp->if_ipackets++; splx(s); return (0); } return EHOSTUNREACH; } /* * Process an ioctl request. */ /* ARGSUSED */ vififioctl(ifp, cmd, data) register struct ifnet *ifp; int cmd; caddr_t data; { int error = 0; switch (cmd) { case SIOCSIFADDR: ifp->if_flags |= IFF_UP; /* * Everything else is done at a higher level. */ break; default: error = EINVAL; } return (error); } vifioctl(dev, cmd, arg, mode) dev_t dev; int cmd; caddr_t arg; int mode; { int unit; unit = minor(dev); if ((unit < 0) || (unit >= NVIF)) return ENXIO; return EINVAL; } ---------------------------------------------------------------------------- To use it, compile your kernel, and reboot. Then create the vif device: # mknod /dev/vif c 14 0 (or whatever major number it ended up being), and echo something into it: # echo > /dev/vif This will cause the device to be opened, which will if_attach the interfaces. If you feel like playing with the code, you may want to kmem_alloc() the vif_softc structrure at open time, and use the minor number of the device to tell it how many interfaces to create. Now you can go ahead and ifconfig etc. I'll be happy to answer minor questions, and hear about success and failure stories, but I cannot help you if you don't already know how to hack kernels. Good luck! /ji In-Real-Life: John "Heldenprogrammer" Ioannidis E-Mail-To: ji@cs.columbia.edu V-Mail-To: +1 212 854 8120 P-Mail-To: 450 Computer Science \n Columbia University \n New York, NY 10027

Note: there is also a commercial-product-turned-freeware called "Col. Patch" which does this as a loadable kernel module for SunOS 4.1.3_U1.

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