Re: [PATCH 13/13] TProxy: use the interface primary IP address as a default value for --on-ip

[Balazs Scheidler <bazsi@balabit.hu>]

On Tue, 2009-09-22 at 10:17 -0400, Brian Haley wrote:
> Balazs Scheidler wrote:
> > On Mon, 2009-09-21 at 14:00 -0400, Brian Haley wrote:
> >> Balazs Scheidler wrote: 
> >>>  #if defined(CONFIG_IPV6) || defined(CONFIG_IPV6_MODULE)
> >>> +
> >>> +static inline const struct in6_addr *
> >>> +tproxy_laddr6(struct sk_buff *skb, const struct in6_addr *user_laddr, const struct in6_addr *daddr)
> >>> +{
> >>> +	struct inet6_dev *indev;
> >>> +	struct inet6_ifaddr *ifa;
> >>> +	struct in6_addr *laddr;
> >>> +	
> >>> +        if (!ipv6_addr_any(user_laddr))
> >>> +                return user_laddr;
> >>> +	
> >>> +        laddr = NULL;
> >>> +        rcu_read_lock();
> >>> +        indev = __in6_dev_get(skb->dev);
> >>> +        if (indev && (ifa = indev->addr_list)) {
> >>> +		laddr = &ifa->addr;
> >>> +	}
> >>> +        rcu_read_unlock();
> >>> +        
> >>> +        return laddr ? laddr : daddr;
> >>> +}
> >> You should call ipv6_dev_get_saddr() to get a source address based on the target
> >> destination address.
> > 
> > Thanks for this hint, however this is not selecting a source address for
> > a given destination, rather it selects the address where tproxy is
> > redirecting the connection in case the user specified no --on-ip
> > parameter.
> > 
> > e.g. 
> > 
> > ip6tables -A PREROUTING -p tcp --dport 80 -j TPROXY --on-port 50080
> > 
> > This should redirect the connection to the primary IP address of the
> > incoming interface. In fact I spent 2 hours to figure out how to find
> > the proper address, and at the end I used the first IP address
> > configured to the interface, seeing that those addresses are sorted in
> > 'scope' order, e.g. link-local and site-local addresses are at the end
> > of the list, thus the front should be ok.
> 
> Yes, the addresses are sorted by scope, but just because they're in the
> list doesn't mean they can be used, for example that address might have
> failed DAD or be Deprecated.  ipv6_dev_get_saddr() will follow the rules
> from RFC 3484 in picking the best address to use, or none if there isn't
> anything appropriate.
> 
> > Since I'm not that much into IPv6, I'd appreciate some help, is
> > ipv6_dev_get_saddr(client_ip_address) indeed the best solution here?
> 
> Probably.  An alternative might be to use ip6_dst_lookup() (see tcp_v6_connect()),
> but a lot more code for you.

One question, how fast is ipv6_dev_get_saddr()? Since the TPROXY target
can execute on a per-packet path (although the socket match should take
care of packets that belong to already-established connections), this
may be performance critical. Is ipv6_dev_get_saddr() something that I
can call so often?

-- 
Bazsi