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Old 09-25-2010, 12:15 PM   #1
bratch
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Registered: Nov 2004
Location: Jersey, British Isles
Distribution: Gentoo
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Interface alias does not receive broadcast traffic


When setting up an alias for eth0, the interface works as expected for normal traffic, but does not receive broadcast traffic.

Host 1's setup:
Code:
# ifconfig eth0
eth0      Link encap:Ethernet  HWaddr 00:17:31:f5:9d:63  
          inet addr:192.168.0.107  Bcast:192.168.0.255  Mask:255.255.255.0
          inet6 addr: fe80::217:31ff:fef5:9d63/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:22749 errors:0 dropped:0 overruns:0 frame:0
          TX packets:18614 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:4245622 (4.0 MiB)  TX bytes:2217688 (2.1 MiB)
          Interrupt:16 
# ifconfig eth0:0
eth0:0    Link encap:Ethernet  HWaddr 00:17:31:f5:9d:63  
          inet addr:192.168.0.150  Bcast:192.168.0.255  Mask:255.255.255.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          Interrupt:16
Pinging host 1's normal interface from host 2 works as expected:
Code:
$ ping 192.168.0.107
PING 192.168.0.107 (192.168.0.107) 56(84) bytes of data.
64 bytes from 192.168.0.107: icmp_req=1 ttl=64 time=0.274 ms
64 bytes from 192.168.0.107: icmp_req=2 ttl=64 time=0.276 ms
^C
--- 192.168.0.107 ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 999ms
rtt min/avg/max/mdev = 0.274/0.275/0.276/0.001 ms
Pinging host 1's alias interface from host 2 works as expected:
Code:
$ ping 192.168.0.150
PING 192.168.0.150 (192.168.0.150) 56(84) bytes of data.
64 bytes from 192.168.0.150: icmp_req=1 ttl=64 time=0.281 ms
64 bytes from 192.168.0.150: icmp_req=2 ttl=64 time=0.275 ms
^C
--- 192.168.0.150 ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 1000ms
rtt min/avg/max/mdev = 0.275/0.278/0.281/0.003 ms
Broadcast pinging from host 2 only gets a reply from host 1's real interface (as well as some other uninteresting devices on the network):
Code:
$ ping -b 192.168.0.255
WARNING: pinging broadcast address
PING 192.168.0.255 (192.168.0.255) 56(84) bytes of data.
64 bytes from 192.168.0.107: icmp_req=1 ttl=64 time=0.285 ms
64 bytes from 192.168.0.1: icmp_req=1 ttl=64 time=0.289 ms (DUP!)
64 bytes from 192.168.0.35: icmp_req=1 ttl=255 time=0.982 ms (DUP!)
64 bytes from 192.168.0.107: icmp_req=2 ttl=64 time=0.274 ms
64 bytes from 192.168.0.1: icmp_req=2 ttl=64 time=0.278 ms (DUP!)
64 bytes from 192.168.0.35: icmp_req=2 ttl=255 time=0.994 ms (DUP!)
^C
--- 192.168.0.255 ping statistics ---
2 packets transmitted, 2 received, +4 duplicates, 0% packet loss, time 1000ms
rtt min/avg/max/mdev = 0.274/0.517/0.994/0.333 ms
I have confirmed by listening on both interfaces using netcat, and broadcasting using netcat, and again only the real interface receives data.

Is this by design, or is it possible to get interface aliases to receive broadcast traffic?
 
Old 09-27-2010, 03:26 PM   #2
nini09
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Registered: Apr 2009
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The alias, eth0:0, thing is not an interface but only a kind of label associated to the address. The label does not exist for kernel networking functions such as routing, packet filtering and so on.
 
  


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