Configuring IP address aliasing
Greetings, I have 2 Redhat-9 servers that I'd like to migrate to FreeBSD in the next 3 months. So I've set-up a test server (FreeBSD-4.9 STABLE) and I'm in the midst of loading 3rd party applications (via ports) to test their operation compared to the RH-9 servers. I have a question about the configuration of IP aliases, so here is some background information. Here is the real interface (192.168.1.40): # ifconfig -a ed0: flags=8843UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST mtu 1500 inet 192.168.1.40 netmask 0xff00 broadcast 192.168.1.255 ether 00:80:ad:91:9a:bd lp0: flags=8810POINTOPOINT,SIMPLEX,MULTICAST mtu 1500 ppp0: flags=8010POINTOPOINT,MULTICAST mtu 1500 sl0: flags=c010POINTOPOINT,LINK2,MULTICAST mtu 552 faith0: flags=8002BROADCAST,MULTICAST mtu 1500 lo0: flags=8049UP,LOOPBACK,RUNNING,MULTICAST mtu 16384 inet 127.0.0.1 netmask 0xff00 Now I try to add the alias (192.168.1.41): # ifconfig ed0 alias 192.168.1.41 netmask 255.255.255.0 broadcast 192.168.1.255 ifconfig: ioctl (SIOCAIFADDR): File exists [using same netmask as real interface] However, THIS WORKED: # ifconfig ed0 alias 192.168.1.41 netmask 255.255.255.255 broadcast 192.168.1.255 [using a netmask of 255.255.255.255] And here is the ifconfig that shows the alias in place: # ifconfig -a ed0: flags=8843UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST mtu 1500 inet 192.168.1.40 netmask 0xff00 broadcast 192.168.1.255 inet 192.168.1.41 netmask 0x broadcast 192.168.1.255 ether 00:80:ad:91:9a:bd lp0: flags=8810POINTOPOINT,SIMPLEX,MULTICAST mtu 1500 ppp0: flags=8010POINTOPOINT,MULTICAST mtu 1500 sl0: flags=c010POINTOPOINT,LINK2,MULTICAST mtu 552 faith0: flags=8002BROADCAST,MULTICAST mtu 1500 lo0: flags=8049UP,LOOPBACK,RUNNING,MULTICAST mtu 16384 inet 127.0.0.1 netmask 0xff00 QUESTION: Why do I use a different netmask (255.255.255.255) for a IP alias on FreeBSD? Why isn't 255.255.255.0 used? When I set-up IP aliases using Linux, I use the same netmask (255.255.255.0) as the real IP. The IP aliasing of my BSD server works but WHY is it working? I found the reference to using the 255.255.255.255 netmask via Google, however, I have Michael Lucas's book Absolute BSD and reference on pages 103 104 (on IP aliasing) clearly show using the same netmask as the real interface when creating IP aliases. What am I missing? Thanks, Michael ___ [EMAIL PROTECTED] mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to [EMAIL PROTECTED]
Re: Configuring IP address aliasing
Mike wrote: # ifconfig -a ed0: flags=8843UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST mtu 1500 inet 192.168.1.40 netmask 0xff00 broadcast 192.168.1.255 # ifconfig ed0 alias 192.168.1.41 netmask 255.255.255.0 broadcast 192.168.1.255 ifconfig: ioctl (SIOCAIFADDR): File exists [using same netmask as real interface] However, THIS WORKED: # ifconfig ed0 alias 192.168.1.41 netmask 255.255.255.255 broadcast 192.168.1.255 [using a netmask of 255.255.255.255] QUESTION: Why do I use a different netmask (255.255.255.255) for a IP alias on FreeBSD? Why isn't 255.255.255.0 used? From the ifconfig man page on 4.9 alias Establish an additional network address for this interface. This is sometimes useful when changing network numbers, and one wishes to accept packets addressed to the old interface. If the address is on the same subnet as the first network address for this interface, a non-conflicting netmask must be given. Usually 0x is most appropriate. Since 192.168.1.41 is on the same subnet as 92.168.1.40 you must use 0x (aka 255.255.255.255) as the netmask. (I don't know *why* I'm afraid). You could always stick the alias on a different subnet at which point you can use the netmask you expect. e.g. # ifconfig ed0 alias 192.168.2.40 netmask 255.255.255.0 broadcast 192.168.2.255 --Alex ___ [EMAIL PROTECTED] mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to [EMAIL PROTECTED]
Re: Configuring IP address aliasing
Hello, I found the reference to using the 255.255.255.255 netmask via Google, however, I have Michael Lucas's book Absolute BSD and reference on pages 103 104 (on IP aliasing) clearly show using the same netmask as the real interface when creating IP aliases. This book is blatantly wrong then. Basically, the point is that the network route already exists and the network extension is already clear. So, you must use a non-conflicting netmask. The ifconfig manpage describes this as well, by the way. Cheers, J. ___ [EMAIL PROTECTED] mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to [EMAIL PROTECTED]
Re: Configuring IP address aliasing
On Mon, Jan 26, 2004 at 08:40:18AM -0800, Mike wrote: QUESTION: Why do I use a different netmask (255.255.255.255) for a IP alias on FreeBSD? Why isn't 255.255.255.0 used? It's all about routing of outgoing packets. Unlike linux, you can't use the route(8) command to set up a route to a directly attached network. All of the local routing is controlled through ifconfig(8) -- when you run: # ifconfig fxp0 inet 192.168.0.1 netmask 0xff00 broadcast 192.168.0.255 up amongst other things, that establishes that interface as the route to the 192.168.0.0/24 network. It also says that the source address on packets sent out of that interface will be 192.168.0.1 Now suppose you add an alias address on the same subnet: # ifconfig fxp0 add 192.168.0.2 netmask 0x broadcast 192.168.0.255 Since the netmask is a /32 it says this interface is the route to the specific single address. It also means that the 192.168.0.2 address is only used on outgoing packets from connections directly to the alias address. That's a bit of a simplification, as one of the principle uses for alias addresses is to support jail(8)ed processes, but it holds in general. Could FreeBSD use exactly the same semantics as Linux or Solaris -- yes, sure. But the FreeBSD way gives you a tad more control, and to be honest, it's the way it is mostly because the people who wrote the code thought that was the way it should be. Cheers, Matthew -- Dr Matthew J Seaman MA, D.Phil. 26 The Paddocks Savill Way PGP: http://www.infracaninophile.co.uk/pgpkey Marlow Tel: +44 1628 476614 Bucks., SL7 1TH UK pgp0.pgp Description: PGP signature
Re: Configuring IP address aliasing
Matthew Seaman wrote: On Mon, Jan 26, 2004 at 08:40:18AM -0800, Mike wrote: QUESTION: Why do I use a different netmask (255.255.255.255) for a IP alias on FreeBSD? Why isn't 255.255.255.0 used? It's all about routing of outgoing packets. Unlike linux, you can't use the route(8) command to set up a route to a directly attached network. All of the local routing is controlled through ifconfig(8) -- when you run: # ifconfig fxp0 inet 192.168.0.1 netmask 0xff00 broadcast 192.168.0.255 up amongst other things, that establishes that interface as the route to the 192.168.0.0/24 network. It also says that the source address on packets sent out of that interface will be 192.168.0.1 Now suppose you add an alias address on the same subnet: # ifconfig fxp0 add 192.168.0.2 netmask 0x broadcast 192.168.0.255 Since the netmask is a /32 it says this interface is the route to the specific single address. It also means that the 192.168.0.2 address is only used on outgoing packets from connections directly to the alias address. That's a bit of a simplification, as one of the principle uses for alias addresses is to support jail(8)ed processes, but it holds in general. Could FreeBSD use exactly the same semantics as Linux or Solaris -- yes, sure. But the FreeBSD way gives you a tad more control, and to be honest, it's the way it is mostly because the people who wrote the code thought that was the way it should be. Ah yes! What a great explanation. Thanks Matthew. Best Regards, Michael Cheers, Matthew ___ [EMAIL PROTECTED] mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to [EMAIL PROTECTED]
