How do I set the source address on a multi-homed host?

Alin-Adrian Anton aanton at
Sun Feb 13 17:47:26 GMT 2005

Daniela wrote:
> On Saturday 12 February 2005 16:06, Volker Kindermann wrote:
>>Hi Daniela,
>>>Yes, this happens when I connect from my machine (which functions as a
>>>router with NAT to allow the other LAN machines connect to the internet)
>>>to another LAN machine. When the router establishes a connection to
>>>another point in the intranet, the source address used is my official IP,
>>>and not, which is the intranet IP of the router.
>>please post the output of the following commands:
>>ifconfig -a
> [Showing only relevant entries. My official IP is replaced with x.x.x.x]
>         inet6 fe80::202:44ff:fe66:bf4%rl0 prefixlen 64 scopeid 0x1 
>         inet x.x.x.x netmask 0xffffff00 broadcast x.x.x.255
>         ether 00:02:44:66:0b:f4
>         media: Ethernet autoselect (100baseTX <full-duplex>)
>         status: active
>         inet6 fe80::20a:cdff:fe00:c076%rl1 prefixlen 64 scopeid 0x2 
>         inet netmask 0xff000000 broadcast
>         ether 00:0a:cd:00:c0:76
>         media: Ethernet autoselect (100baseTX <full-duplex>)
>         status: active

I really don't see why you use A-class netmask. It's very probable that 
a C-class netmask would suffice:

rl1 inet netmask 0xffffff00 broadcast

Also, the routes for rl1 which is the internal interface don't look normal.

You should have only one rl1 entry, like this:          link#2             UC          0        0   rl1

And not:
> 10                 link#2             UC          2        0    rl1
>           00:0d:61:17:fc:30  UHLW        1      444    rl1    903
>     ff:ff:ff:ff:ff:ff  UHLWb       0     2453    rl1

See? Why special route for

Besides that, I hope your nat and firewall configs are not mangled too. 
Try these changes first, and see if things get normal.

PS: there is *no way* for the behaviour you explained to happen under 
normal circumstances, unless you *explicitly*, intentionally or by 
mistake have configured the gateway to do so.

Alin-Adrian Anton
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