Packet "Mixing" Between Multiple NICs on Host

From: Jonathan B. Horen (horen_at_mail.iucc.ac.il)
Date: 03/30/04 

Date: Tue, 30 Mar 2004 12:34:35 +0200
To:  fedora-list@redhat.com

Shalom!

In, perhaps, a misguided desire for elegance, I moved our DNS server from an
aged and infirm host onto our existing file/mail server (Dell PowerEdge/2450
w/dual PIII/866 CPUs and 2GB RAM, running Fedora Core 1 w/all updates). Since
the DNS server sat on a different subnet, I added a second NIC to the
file/mail server and created the appropriate files in
/etc/sysconfig/network-scripts (ifcfg-eth1 and route-eth{0,1}).

"route -n" shows:

Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
128.139.197.0 0.0.0.0 255.255.255.0 U 0 0 0 eth1
128.139.206.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
169.254.0.0 0.0.0.0 255.255.0.0 U 0 0 0 eth1
127.0.0.0 0.0.0.0 255.0.0.0 U 0 0 0 lo
0.0.0.0 128.139.197.16 0.0.0.0 UG 0 0 0 eth1
0.0.0.0 128.139.206.1 0.0.0.0 UG 1 0 0 eth0

The problem is that although packets received from the two subnets arrive
through the corresponding device, *packets sent to a host on a subnet other
than 128.139.197.0 exit through eth1.*

Running "ping 128.139.206.12" from a host on the 128.139.206.0 subnet shows
that packets exit via eth1, rather than via eth0:

root@efes network-scripts# tcpdump -i eth1 host horen.tau.ac.il
tcpdump: listening on eth1
11:54:38.192269 efes.iucc.ac.il > horen.tau.ac.il: icmp: echo reply
11:54:39.202538 efes.iucc.ac.il > horen.tau.ac.il: icmp: echo reply
11:54:40.212855 efes.iucc.ac.il > horen.tau.ac.il: icmp: echo reply

I understand that this is because the metric for eth1 is "0", while the metric
for eth1 is "1". If I understand correctly, changing the metric for eth0 to
"0" would mean that every packet would be sent to *both* interfaces, giving me
a 50% packet loss.

Is there a way to configure routing on this server so that a packet's
source-address is "honored" by the system when responding?

worst-case, I'll cobble together a separate DNS server from an unused PIII/500...

TIA! 

-- 
JONATHAN B. HOREN                            UNIX SYSTEMS ADMINISTRATOR
E: horen@mail.iucc.ac.il            Inter-University Computation Center
T: +972-(0)3-640-5203                               Tel-Aviv University
F: +972-(0)3-640-9118                           Ramat-Aviv 69978 Israel

Relevant Pages

  • Packet "Mixing" Between Multiple NICs on Host
    ... aged and infirm host onto our existing file/mail server (Dell PowerEdge/2450 ... the DNS server sat on a different subnet, I added a second NIC to the ... The problem is that although packets received from the two subnets arrive ... that packets exit via eth1, ...
    (Fedora)
  • Re: resolver latencies return in Mozilla 1.6
    ... I have watched the packets going out and I ... >> ISP's DNS server keeps rejecting. ... What's weird is that for these failing conversations my firewall doesn't seem ... Same sequential port numbers, but no ICMPs, no "ServFail" packet (whatever ...
    (comp.unix.bsd.freebsd.misc)
  • Re: problems with BT broadband connection
    ... speculation could point to DNS packets ... being lost over a dodgy wireless connection. ... improve the connection to your wireless router; ... PC's LAN interface to use 127.0.0.1 as your primary DNS server. ...
    (uk.telecom.broadband)
  • RE: Re[2]: [Full-disclosure] Personal firewalls.
    ... And setup some kind of overideing ruleset, ... If Z spoofs packets using the ip of the DNS Server (the one that is not ... Would be allowed to send those packets, SYN Packet, etc, as was stated, ad ... Checked by AVG Free Edition. ...
    (Full-Disclosure)
  • # packets for first query with personal DNS server
    ... I asked earlier about my new DNS server ... packets ). ... initial query of www.google.com. ... then it started sending packets to ...
    (comp.os.linux.networking)