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NET_ALIAS device aliasing v0.5x
===============================
  The main step taken in versions 0.40+ is the implementation of a
  device aliasing mechanism that creates *actual* devices.
  This development includes NET_ALIAS (generic aliasing) plus IP_ALIAS
  (specific IP) support.
  From version 0.50, dynamic configuration of max alias per device and
  tx/rx stats for aliases added.
  Also fixed inter-alias routing and arping problems.

Features
--------
o  ACTUAL alias devices created & inserted in dev chain
o  AF_ independent: net_alias_type objects. Generic aliasing engine.
o  AF_INET optimized
o  hashed alias address lookup
o  net_alias_type objs registration/unreg., module-ables.
o  /proc/net/aliases & /proc/net/alias_types entries
o  /proc/sys/net/core/net_alias_max entry (affects hash table size
   also)
o  tx/rx stats   


o IP alias implementation: static or runtime module.

Usage (IP aliasing)
-------------------
  A very first step to test if you are running a net_alias-ed kernel
  is to check /proc/net/aliases & /proc/net/alias_types entries:
     # cat /proc/net/alias*

  For IP aliasing you must have IP_ALIAS support included by
  static linking ('y' to CONFIG_IP_ALIAS? question), or runtime module
  insertion ('m'):
     # insmod /usr/src/linux/modules/ip_alias.o (2.0.xx) or
     # modprobe ip_alias.o
     
     Also, dynamic loading is supported (kerneld).
       You should have the following line in /etc/conf.modules (not needed
       for newer modutils):
       alias net_alias-2 ip_alias

  Module options
  --------------
  From 0.5x ip_alias module supports a new option ("no_sel" symbol).
  If no_sel is set (default is 0), alias association (device selection) with
  foreign addresses will be disabled.

    You will get:
    - Faster operation by avoiding completely routing lookups. 
      Due to the "logical nature" of aliasing, netdevice SELection can only be
      done based on info from network layer. When packet dst address isn't 
      one of my addresses, I query the routing table to see which netdevice
      would be selected for packet _source_ address. This option avoids
      doing so, and you must consider using it if you *only* have same-net
      aliases (common usage).

    You will loose:
    - Inter-alias routing
    - Proxyarp over aliases

  To activate:
    # insmod ip_alias.o no_sel=1  
      or
    # modprobe ip_alias.o no_sel=1
      or
    add the following line to /etc/conf.modules:
      options ip_alias no_sel=1


o Alias creation.
  Alias creation is done by 'magic' iface naming: eg. to create a
  200.1.1.1 alias for eth0 ...
  
    # ifconfig eth0:0 200.1.1.1  etc,etc....
                   ~~ -> request alias #0 creation (if it not exists) for eth0
    and routing stuff also ...
    # route add -host 200.1.1.1 dev eth0:0  (if same IP network as
                                            main device)
   
    # route add -net 200.1.1.0 dev eth0:0   (if completely new network wanted
                                            for eth0:0)

o Alias deletion.
  Also done by magic naming, eg:

    # ifconfig eth0:0-  0  (maybe any address)
                   ~~~ -> will delete alias (note '-' after dev name)
  alias device is closed before deletion, so all network stuff that
  points to it (routes, arp entries, ...) will be released.
                                   
o Alias (re-)configuring
  Aliases *are* devices, so you configure and refer to them as usual (ifconfig,
  route, etc).
  
o PROCfs entries
  2 entries are added to help fetching alias runtime configuration:
  a) /proc/net/alias_types
     Will show you alias_types registered (ie. address families that
     can be aliased).
     eg. for IP aliasing with 1 alias configured:

     # cat /proc/net/alias_types
     type    name            n_attach
     2       ip              1      
     
  b) /proc/net/aliases
     Will show aliased devices info, eg (same as above):

     # cat /proc/net/aliases
     device           family address                                
     eth0:0           2      200.1.1.1
     
o PROCfs dynamic configuration (from v0.50)
  You can now change the max aliases per device limit via
  /proc/sys/net/core/net_alias_max entry (default=256)
     # cat /proc/sys/net/core/net_alias_max
     256
     # echo 1000 > /proc/sys/net/core/net_alias_max
     # cat /proc/sys/net/core/net_alias_max
     1000
     # _
          
  With this funcionality you can disable net_alias creation from now on
     # echo 0 > /proc/sys/net/core/net_alias_max

  The new aliasing limit is considered (grabbed) when creating the
  FIRST alias for the main device.
  Eg:
     # echo 10 > /proc/sys/net/core/net_alias_max
     # ifconfig eth0:0 xx.xx.xx.xx  (first alias creation for eth0,
                                     eth0 will 'remember' max==10)
     # echo 1000 > /proc/sys/net/core/net_alias_max
     # ifconfig eth0:999 xx.xx.xx.xx
     SIOCIFSADDR: No such device
  Of course these semantics can be changed, please let me know.
  
  Configuration changes get logged as usual (klogd -> /var/log/messages)
  
o Alias devices rx/tx stats
  Fake rx/tx stats are accounted:
  - TX
    When the packet is ``switched'' from logical alias device to
    physical device, tx counter gets incremented.
  - RX
    When an incoming packet's address equals alias network device's addr it
    gets ``switched'' from physical to logical device, rx counter gets
    incr.
    
  Please NOTE that for ``same'' network alias devices you usually have
  one net-route through physical device (eg. eth0), so output pkts
  will NOT pass down via alias device (so, no tx++ will occur).
  
  Also NOTE that currently ifconfig does not handle the ``:'' of alias devices
  names, a little patch solves the problem:
--- ifconfig.c.dist     Tue Apr  4 17:58:32 1995
+++ ifconfig.c  Fri Oct 25 13:11:23 1996
@@ -243,7 +243,12 @@
                bp++;
        if(strncmp(bp,ifname,strlen(ifname))==0 && bp[strlen(ifname)]==':')
        {
-               bp=strchr(bp,':');
+               /* 
+                *      start bp at ifname end to prevent ':' ambiguity
+                *      with alias devices (eg. eth0:0)
+                *      
+                */
+               bp+=strlen(ifname);
                bp++;
                sscanf(bp,"%d %d %d %d %d %d %d %d %d %d %d",
                        &ife->stats.rx_packets,
  
Relationship with main device
-----------------------------
  - On main device closing, all aliases will be closed and freed.
  - Each new alias created is inserted in dev_chain just before next
    main device (aliases get 'stacked' after main_dev), eg:
      lo->eth0->eth0:0->eth0:2->eth1->0
    If eth0 is unregistered, all it aliases will also be:
      lo->eth1->0

Contact
-------
Please e-mail me:
   Juan Jose Ciarlante <irriga@impsat1.com.ar> or <jjciarla@raiz.uncu.edu.ar>
   
Acknowledments
--------------
Special thanks to Claudia for all her love an patience.
Also thanks to Antonio Trevi~o <antonio@ecord.gov.ar> great human being 
and un*x guru.

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