On 10/02/2016 5:51 AM, Jeremy Boy wrote:
Hello list,
please CC me in replies to this mail, since I am no subscriber to this list.
For safety reasons, we enclose user input to shell commands in quotes. Until
today, the resulting command for ifconfig(8) looked like this:
ifconfig ue0 inet "192.168.2.176 netmask 255.255.255.0"
As we figured out, this means that ifconfig sometimes doesn't set the netmask
as we expect it to do:
root@csbuild:~ # ifconfig ue0
ue0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
options=80001<RXCSUM,LINKSTATE>
ether b8:27:eb:fd:58:69
inet 192.168.2.176 netmask 0xffff0000 broadcast 192.168.255.255
media: Ethernet autoselect (100baseTX <full-duplex>)
status: active
nd6 options=29<PERFORMNUD,IFDISABLED,AUTO_LINKLOCAL>
root@csbuild:~ # ifconfig ue0 inet "192.168.2.176 netmask 255.0.0.0"
root@csbuild:~ # echo $?
0
root@csbuild:~ # ifconfig ue0
ue0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
options=80001<RXCSUM,LINKSTATE>
ether b8:27:eb:fd:58:69
inet 192.168.2.176 netmask 0xffffff00 broadcast 192.168.2.255
media: Ethernet autoselect (100baseTX <full-duplex>)
status: active
nd6 options=29<PERFORMNUD,IFDISABLED,AUTO_LINKLOCAL>
root@csbuild:~ #
As you can see, ifconfig sets the netmask to 255.255.255.0, ignoring the
netmask we provided. If provided with a 10.* address, ifconfig sets the netmask
to 255.0.0.0, no matter what actual netmask we entered. Also, it doesn't exit
with a status indicating error, which made it a little harder to debug this
issue. The following works as expected:
root@csbuild:~ # ifconfig ue0
ue0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
options=80001<RXCSUM,LINKSTATE>
ether b8:27:eb:fd:58:69
inet 192.168.2.176 netmask 0xffffff00 broadcast 192.168.2.255
media: Ethernet autoselect (100baseTX <full-duplex>)
status: active
nd6 options=29<PERFORMNUD,IFDISABLED,AUTO_LINKLOCAL>
root@csbuild:~ # ifconfig ue0 inet "192.168.2.176" netmask "255.0.0.0"
root@csbuild:~ # ifconfig ue0
ue0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
options=80001<RXCSUM,LINKSTATE>
ether b8:27:eb:fd:58:69
inet 192.168.2.176 netmask 0xff000000 broadcast 192.255.255.255
media: Ethernet autoselect (100baseTX <full-duplex>)
status: active
nd6 options=29<PERFORMNUD,IFDISABLED,AUTO_LINKLOCAL>
root@csbuild:~ #
What exactly does ifconfig do? It seems to me that it reads the IP address from
the quoted string but truncates the netmask. Is this a bug in ifconfig or
intended behavior?
it's not so much as ifconfig, as the shell
you have ifconfig with 3 arguments, the last of which is
"192.168.2.176 netmask 255.255.255.0"
which ifconfig inspects to see if it has an IP address in it (followed
by a space which terminates the test successfully).
this is expected behaviour. Though admittedly a less than clear
example of it.
Best wishes
Jeremy
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