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Re: How to determine source ip address

To: Padraig Brady <padraig@xxxxxxxxxxxxx>
Subject: Re: How to determine source ip address
From: Gigi Sullivan <sullivan@xxxxxxxxxxxxx>
Date: Tue, 11 Sep 2001 21:50:36 +0200
Cc: Julian Anastasov <ja@xxxxxx>, Christopher Friesen <cfriesen@xxxxxxxxxxxxxxxxxx>, netdev@xxxxxxxxxxx
In-reply-to: <>; from on Tue, Sep 11, 2001 at 06:47:07PM +0100
References: <Pine.LNX.4.33.0109102359080.1850-100000@u.domain.uli> <>
Sender: owner-netdev@xxxxxxxxxxx
User-agent: Mutt/1.2.5i
Aiee :)


On Tue, Sep 11, 2001 at 06:47:07PM +0100, Padraig Brady wrote:
> >>Padraig Brady wrote:
> >>
> >>>I'm writing a network app (currently on linux 2.2.18,
> >>>but soon moving to 2.4) that needs to build up ip &
> >>>tcp packets manually. So my question is how do I determine
> >>>the source ip address to use for a given destination
> >>>ip address. I.E. take account of multiple interfaces/routes/
> >>>aliases/...
> >>>
> Chris, I'm only filling in the TCP header, and I realise the IP header 
> will be
> filled appropriately when sent. However I need to calculate the checksum for
> the TCP header which is actually calculated for the pseudo header:
> struct pseudo_header {
>     unsigned int src_address; /* <==-- */
>     unsigned int dest_address; >     unsigned char placeholder;
>     unsigned char protocol;
>     unsigned short tcp_length;
>     struct tcphdr tcp;
> }  pseudo_header;
> So I need to determine the source address before sending...

   When sending IP datagram, source IP address is choosen by the
   kernel just picking up the one bound to the outgoing interface
   (actually I never thought about this could work when multiple 
   addresses are assigned to one interface).

   So, what you have to do is just this;
   look at destination IP address, use the kernel routing table to
   get the outgoing interface, retrieve the IP address associated with

   You can achieve all this just using AF_NETLINK socket(2) or by playing 
   with /proc/net/route and doing a few ioctl(2) calls (SIOCGIFADDR))

   If I misenderstood something, I apologize; point me out to the 
   beginning thread then, thanks! :)

> Thanks,
> Padraig.

bye bye

                        -- gg sullivan

Lorenzo Cavallaro       `Gigi Sullivan' <sullivan@xxxxxxxxxxxxx>

Until I loved, life had no beauty;
I did not know I lived until I had loved. (Theodor Korner)

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