arplookup failed host is not on local network

radnows

出错信息：     arplookup xxx_xxx_xxx_xxx failed host is not on local network

用arp命令可以双方都能显示对方的MAC地址，但却ping不到对方，dns等服务
只在本地有效。对方接收不了。

我不清楚这是什么问题？
有谁可以答复。

ihweb

看看是否开了防火墙。

radnows

并非防火墙问题：防火墙是开放这些服务的,防火墙即使关闭问题依旧！

红袖添香

[quote]原帖由 "radnows"]并非防火墙问题：防火墙是开放这些服务的,防火墙即使关闭问题依旧！[/quote 发表：
   

检查掩码和路由

radnows

这个我早已检查过，两个网络都有不同子网才会有问题；但同样的设置在linux下没问题；

ihweb

[quote]原帖由 "radnows"]这个我早已检查过，两个网络都有不同子网才会有问题；但同样的设置在linux下没问题；[/quote 发表：
     


因为Linux 变态！！   

radnows

This is a netmask problem, but not really the one that other people
have described. This is how it usually works. Your troubled machine
above, "servername," receives an ARP who-has from another machine on
the LAN called "clientname." However, the IP address that clientname
gives as a source does not match up to any local networks that
servername knows about.

For example, say servername has an address of 192.0.2.10/25. The other
machine has 192.0.2.210/24. When servername gets an ARP (which is
broadcast so servername gets it fine),

  who-has 192.0.2.10 tell 192.0.2.210

It gets confused. 192.0.2.210 is not local (as far as it is concerned)
so it logs an error.

Note that this is not a harmless error. These two machine cannot talk
to each other.

The fix, of course, is to make sure all machines on the same LAN have
the same netmask.


===============================================

我們先看看這問題出在那兒:
ARP 在正常使用時是只在同一subnet中跑而已, 而且,  IP nodes只會對Destination
Protocol Address 是自己的Address之ARP作處理.

所以當Subnet 中每一部機器都正確地Configure, 且機器正確implement ARP protocol時,
every thing is all right....呵呵~

問題會產生的架構應為 一個router interface接兩(或以上)個subnet的情況:
(我用sniffer看, 再參考if_ether.c 歸納之):

    +--------+
---| Router |----------a.b.c.0/24
    |        |          a.b.e.0/24
    +--------+

一個router interface接兩個subnet:

  正常情況底下, a.b.c.? 要與 a.b.e.? 通訊是要透過ROUTER.
  這時候, 如果有一部a.b.e.x的mask是用/16而非/24, 那它要與a.b.c.y通訊時,
會直接送ARP Request給a.b.c.y而非ROUTER, a.b.c.y會回ARP Reply及檢
查本身的ARP table(要做reflesh, update等), 一檢查就發現a.b.e.x不在同一sunet,
那
    arplookup a.b.e.x failed: host is not on local network
就出來了, 呵~

解決方法:1.給a.b.e.x正確的subnet mask(治本), 如果a.b.e.x是別人家的,就.....
         2.我建議把這個message
           完全弄掉:

        /sys/netinet/if_ether.c

       arplookup(...) 之這一後:

        if (rt->;rt_flags & RTF_GATEWAY)
                why = "host is not on local network"
        else if ((rt->;rt_flags & RTF_LLINFO) == 0)
                why = "could not allocate llinfo"
        else if (rt->;rt_gateway->;sa_family != AF_LINK)
                why = "gateway route is not ours"
        改為:
        if (rt->;rt_flags & RTF_GATEWAY){
           // why = "host is not on local network"
           return 0;
        }
        else if ((rt->;rt_flags & RTF_LLINFO) == 0)
                why = "could not allocate llinfo"
        else if (rt->;rt_gateway->;sa_family != AF_LINK)
                why = "gateway route is not ours"

        這麼一來, 人人通用囉.....