Discussion:
DHCP & hostname faceplant
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Daniel Wagner
2015-09-18 02:50:08 UTC
Permalink
Hi,

The WiFi APs provided by the local ISP here in Utah does not like the
hostname in the DHCP Request packet. Without it the hostname I get the
ACK instead of the NACK as respond. Any ideas how we could handle this
problem? E.g. stop sending the hostname after the first round of retries
have failed?

cheers,
daniel
Patrik Flykt
2015-09-22 06:12:15 UTC
Permalink
Hi,
Post by Daniel Wagner
The WiFi APs provided by the local ISP here in Utah does not like the
hostname in the DHCP Request packet. Without it the hostname I get the
ACK instead of the NACK as respond. Any ideas how we could handle this
problem? E.g. stop sending the hostname after the first round of retries
have failed?
Hm, sounds like very strange way of not handing out DHCP leases. This is
definitely a first for ConnMan, congratulations! Isn't this funny, the
expected use cases are for DHCP updating DNS information but sabotaged
by this kind of interesting network setup ;-)

There of course aren't any instructions from the ISP not to send
hostnames on the network? Do you have a tcpdump of the exchange to
share?

How does a normal Windows machine cope with this? Any possibilities to
get a packet dump and check how those machines work as sending a host
name is quite a normal operation...

Cheers,

Patrik
Daniel Wagner
2015-10-08 07:31:46 UTC
Permalink
Hi Patrik,
Post by Patrik Flykt
Post by Daniel Wagner
The WiFi APs provided by the local ISP here in Utah does not like the
hostname in the DHCP Request packet. Without it the hostname I get the
ACK instead of the NACK as respond. Any ideas how we could handle this
problem? E.g. stop sending the hostname after the first round of retries
have failed?
Hm, sounds like very strange way of not handing out DHCP leases. This is
definitely a first for ConnMan, congratulations!
Yeah, I have a good hand for this :)
Post by Patrik Flykt
Isn't this funny, the
expected use cases are for DHCP updating DNS information but sabotaged
by this kind of interesting network setup ;-)
I don't know why this ISP their routers to behave like this but there
isn't much we can do.
Post by Patrik Flykt
There of course aren't any instructions from the ISP not to send
hostnames on the network? Do you have a tcpdump of the exchange to
share?
I have uploaded two traces. Both created on my Linux machine.

- This one is with an unmodified ConnMan:

http://www.monom.org/connman/dhcp-not-working.pcapng

- And this one is with a modified ConnMan which doesn't sent its
hostname:

http://www.monom.org/connman/dhcp-working.pcapng
Post by Patrik Flykt
How does a normal Windows machine cope with this?
I don't know. I have didn't have one around and didn't try it with MacOS.
Post by Patrik Flykt
Any possibilities to
get a packet dump and check how those machines work as sending a host
name is quite a normal operation...
Argh, I should have done that but forgot. And now it is too late. I am
back which means working network :)

FWIW, we could create an bug entry and add the traces and wait until we
get another user reporting problems. Without testing I'd rather not
change the current code. What do you think?

cheers,
daniel
Patrik Flykt
2015-10-21 10:19:09 UTC
Permalink
Hi,
Post by Daniel Wagner
I have uploaded two traces. Both created on my Linux machine.
http://www.monom.org/connman/dhcp-not-working.pcapng
- And this one is with a modified ConnMan which doesn't sent its
http://www.monom.org/connman/dhcp-working.pcapng
Looks like there are two ways to tame this: have a main.conf entry or
retry without sending a hostname. In the latter case people expecting
their host name to be added to DNS will be surprised if there is
something else going wrong in the network at the same time and their
hostname is therefore not sent to the server.

The first option will be clearer (will it?) to the user that no
hostnames are sent and DNS therefore cannot be expected to work
automatically. But of course this is now an option that hits all DHCP
connection, not the presumably "faulty" one. OTOH with this main.conf
one can prevent hostnames to be sent altogether.

Cheers,

Patrik
Daniel Wagner
2015-10-21 10:27:03 UTC
Permalink
Post by Patrik Flykt
Post by Daniel Wagner
I have uploaded two traces. Both created on my Linux machine.
http://www.monom.org/connman/dhcp-not-working.pcapng
- And this one is with a modified ConnMan which doesn't sent its
http://www.monom.org/connman/dhcp-working.pcapng
Looks like there are two ways to tame this: have a main.conf entry or
retry without sending a hostname. In the latter case people expecting
their host name to be added to DNS will be surprised if there is
something else going wrong in the network at the same time and their
hostname is therefore not sent to the server.
The first option will be clearer (will it?) to the user that no
hostnames are sent and DNS therefore cannot be expected to work
automatically. But of course this is now an option that hits all DHCP
connection, not the presumably "faulty" one. OTOH with this main.conf
one can prevent hostnames to be sent altogether.
My take is to gather some traces from Windows and see how they solve
this kind of problem. Though someone else has to do it. Is someone
living in Utah and accesses to a CenturyLink DSL WiFi modem?
Jussi Kukkonen
2015-10-21 10:46:36 UTC
Permalink
Post by Daniel Wagner
Post by Patrik Flykt
Looks like there are two ways to tame this: have a main.conf entry or
retry without sending a hostname. In the latter case people expecting
their host name to be added to DNS will be surprised if there is
something else going wrong in the network at the same time and their
hostname is therefore not sent to the server.
The first option will be clearer (will it?) to the user that no
hostnames are sent and DNS therefore cannot be expected to work
automatically. But of course this is now an option that hits all DHCP
connection, not the presumably "faulty" one. OTOH with this main.conf
one can prevent hostnames to be sent altogether.
My take is to gather some traces from Windows and see how they solve
this kind of problem. Though someone else has to do it. Is someone
living in Utah and accesses to a CenturyLink DSL WiFi modem?
My recollection on this is that Windows does not use option 12 at all:
clients do not request a hostname and servers won't provide it. My
experience is from a few years ago (and might have been on an older OS even
then) so testing that might still be useful.

- Jussi

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