Re: [coLinux-users] change in slirp behavior from 0.7.1 to 0.7.3 with udp forwarded ports

[Henry N. <hen...@ar...>]

Mike Frysinger wrote:
> On Mon, Jun 23, 2008 at 4:27 PM, Henry Nestler wrote:
>> With rev 479/480 you pointed to a bigger set of changes.
>>
>> By the while I found the place where it goes wrong. Please see the
>> source snip or:
>> http://colinux.svn.sourceforge.net/viewvc/colinux/branches/devel/src/colinux/user/slirp/udp.c?revision=677&view=markup
>>
>> file udp.c, line 323, function udp_output()
>>
>> In the line 323 the source address saddr.sin_addr (192.168.x.x) is
>> overwritten with 10.0.2.2
>> One line before was checked the listen address (so->so_faddr).
>> I'm afraid, there should check the source address instead? But why there
>> is a check for broadcast? Is that a hack for a hack? It sems me, that it
>> would better work without the lines from 322 to 326 there?
>>
>> Background:
>> special_addr = 10.0.2.2 (const)
>>
>> All UDP sockets are listen on address 0.0.0.0, that address is set as
>> faked address 10.0.2.2 in line 383:
>> "so->so_faddr = alias_addr;"
>>
>> -----------------------------------
>> devel/src/colinux/user/slirp/udp.c
>>  315 int udp_output(struct socket *so, struct mbuf *m,
>>  316                struct sockaddr_in *addr)
>>  317
>>  318 {
>>  319     struct sockaddr_in saddr, daddr;
>>  320
>>  321     saddr = *addr;
>>  322     if ((so->so_faddr.s_addr & htonl(0xffffff00)) ==
>> special_addr.s_addr) {
>>  323         saddr.sin_addr.s_addr = so->so_faddr.s_addr;
>>  324         if ((so->so_faddr.s_addr & htonl(0x000000ff)) == htonl(0xff))
>>  325             saddr.sin_addr.s_addr = alias_addr.s_addr;
>>  326     }
>>  327     daddr.sin_addr = so->so_laddr;
>>  328     daddr.sin_port = so->so_lport;
>>  329
>>  330     return udp_output2(so, m, &saddr, &daddr, so->so_iptos);
>>  331 }
> 
> i'd like to assist in analysis, but i'm not familiar with any of the
> colinux source base, so i cant really even make an educated guess; i'd
> be shooting blind.  i'll gladly test any proposed changes though.
> 
> a comment block here in the source explaining what it's supposed to be
> doing would have been useful :/.

Yes, I know. But is not from me. It was copied from some others.
As I understand, after deep debugging with you helpful simple netcat and
tcp example I would say, that the if condition wand to check if the
source is from the host (localhost or the host's self ipaddress) and
then it replaced the source from sender with faked 10.0.2.2

But after the change SVN rev 479/480 the address is fix as localhost or
bind all times to all (0.0.0.0). In that case, this can not be use for
that test any more.

Have fixed. Here is the new executable. Your Netcat + tcpcopy example
was working:
http://www.henrynestler.com/colinux/testing/stable-0.7.3/20080623-slirp/colinux-slirp-net-daemon-0.7.3-2.zip

I don't know, what the broadcast address (10.0.2.255) should do there.
Have removed. This is the new code snip:

int udp_output(struct socket *so, struct mbuf *m,
               struct sockaddr_in *addr)

{
    struct sockaddr_in saddr, daddr;

    saddr = *addr;

    /* Translate connections from localhost to the alias hostname */
    if(is_localhost(saddr.sin_addr)) {
        saddr.sin_addr.s_addr = so->so_faddr.s_addr;
    }

    daddr.sin_addr = so->so_laddr;
    daddr.sin_port = so->so_lport;

    return udp_output2(so, m, &saddr, &daddr, so->so_iptos);
}

-- 
Henry N.