DHCP server does not include NTP server addresses in the DHCP ACK message
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I am capturing DHCP packets with tcpdump. The client sends a DHCP request asking for the DHCP configuration which contains a request for NTP servers but in the DHCP ACK message sent by the router there is no NTP server information. Shouldn't the server answer with the NTP server's IPs as it does with the domain name servers? What am I missing?
In case that the device is not getting NTP servers' addresses through DHCP because my home router does not support it, where can it get the NTP server that it is using from? It is not a manually configured parameter, is there any other service for getting NTP servers automatically?
The system uses connman as connection manager
I attatch screen shots of the request and ack messages
tcpdump command used:
tcpdump -i eth0 -n port 67 and port 68 -w capture.pcap -v
DHCP REQUEST
DHCP ACKNOWLEDGE
linux networking dhcp ntp
asked Jul 3 at 13:32
jap jap
32
add a comment |Â
up vote
0
down vote
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I am capturing DHCP packets with tcpdump. The client sends a DHCP request asking for the DHCP configuration which contains a request for NTP servers but in the DHCP ACK message sent by the router there is no NTP server information. Shouldn't the server answer with the NTP server's IPs as it does with the domain name servers? What am I missing?
In case that the device is not getting NTP servers' addresses through DHCP because my home router does not support it, where can it get the NTP server that it is using from? It is not a manually configured parameter, is there any other service for getting NTP servers automatically?
The system uses connman as connection manager
I attatch screen shots of the request and ack messages
tcpdump command used:
tcpdump -i eth0 -n port 67 and port 68 -w capture.pcap -v
DHCP REQUEST
DHCP ACKNOWLEDGE
linux networking dhcp ntp
asked Jul 3 at 13:32
jap jap
32
2
Show yourdhcpd.conf.
– Ipor Sircer
Jul 3 at 13:34
The device does not use dhcpd, it uses connman
– jap jap
Jul 3 at 13:37
1
Instead use pool.ntp.org if your device does not return NTP servers as part of its DHCP config.
– thrig
Jul 3 at 13:45
Thanks @thrig but what I am trying to figure out is where the NTP server that the device is using comes from
– jap jap
Jul 3 at 15:47
add a comment |Â
up vote
0
down vote
favorite
up vote
0
down vote
favorite
I am capturing DHCP packets with tcpdump. The client sends a DHCP request asking for the DHCP configuration which contains a request for NTP servers but in the DHCP ACK message sent by the router there is no NTP server information. Shouldn't the server answer with the NTP server's IPs as it does with the domain name servers? What am I missing?
In case that the device is not getting NTP servers' addresses through DHCP because my home router does not support it, where can it get the NTP server that it is using from? It is not a manually configured parameter, is there any other service for getting NTP servers automatically?
The system uses connman as connection manager
I attatch screen shots of the request and ack messages
tcpdump command used:
tcpdump -i eth0 -n port 67 and port 68 -w capture.pcap -v
DHCP REQUEST
DHCP ACKNOWLEDGE
linux networking dhcp ntp
asked Jul 3 at 13:32
jap jap
32
I am capturing DHCP packets with tcpdump. The client sends a DHCP request asking for the DHCP configuration which contains a request for NTP servers but in the DHCP ACK message sent by the router there is no NTP server information. Shouldn't the server answer with the NTP server's IPs as it does with the domain name servers? What am I missing?
In case that the device is not getting NTP servers' addresses through DHCP because my home router does not support it, where can it get the NTP server that it is using from? It is not a manually configured parameter, is there any other service for getting NTP servers automatically?
The system uses connman as connection manager
I attatch screen shots of the request and ack messages
tcpdump command used:
tcpdump -i eth0 -n port 67 and port 68 -w capture.pcap -v
DHCP REQUEST
DHCP ACKNOWLEDGE
linux networking dhcp ntp
asked Jul 3 at 13:32
jap jap
32
asked Jul 3 at 13:32
jap jap
32
asked Jul 3 at 13:32
jap jap
32
asked Jul 3 at 13:32
jap jap
32
32
2
Show yourdhcpd.conf.
– Ipor Sircer
Jul 3 at 13:34
The device does not use dhcpd, it uses connman
– jap jap
Jul 3 at 13:37
1
Instead use pool.ntp.org if your device does not return NTP servers as part of its DHCP config.
– thrig
Jul 3 at 13:45
Thanks @thrig but what I am trying to figure out is where the NTP server that the device is using comes from
– jap jap
Jul 3 at 15:47
add a comment |Â
2
Show yourdhcpd.conf.
– Ipor Sircer
Jul 3 at 13:34
The device does not use dhcpd, it uses connman
– jap jap
Jul 3 at 13:37
1
Instead use pool.ntp.org if your device does not return NTP servers as part of its DHCP config.
– thrig
Jul 3 at 13:45
Thanks @thrig but what I am trying to figure out is where the NTP server that the device is using comes from
– jap jap
Jul 3 at 15:47
2
2
Show your
dhcpd.conf.– Ipor Sircer
Jul 3 at 13:34
Show your
dhcpd.conf.– Ipor Sircer
Jul 3 at 13:34
The device does not use dhcpd, it uses connman
– jap jap
Jul 3 at 13:37
The device does not use dhcpd, it uses connman
– jap jap
Jul 3 at 13:37
1
1
Instead use pool.ntp.org if your device does not return NTP servers as part of its DHCP config.
– thrig
Jul 3 at 13:45
Instead use pool.ntp.org if your device does not return NTP servers as part of its DHCP config.
– thrig
Jul 3 at 13:45
Thanks @thrig but what I am trying to figure out is where the NTP server that the device is using comes from
– jap jap
Jul 3 at 15:47
Thanks @thrig but what I am trying to figure out is where the NTP server that the device is using comes from
– jap jap
Jul 3 at 15:47
add a comment |Â
1 Answer
1
active
oldest
votes
up vote
0
down vote
accepted
1) The DHCP server will only answer with an NTP server field if it is configured to do that. From your dumps, it looks like it isn't, so this is a dead end.
2) The device will use whatever it likes as an NTP server - there's no general rule. Very likely somewhere on the device is a config file which contains the NTP server(s) it uses.
3) If you know enough about the device to say that it uses connman: Do you have access to the device as root? Or can you see the file system? If yes, look at all configuration files you can find.
4) If the device is some closed embedded device you don't have access to, another option is to monitor all traffic from the device until it contacts an NTP server, and then you'll know. Wireshark/tcpdump with a proper rule to filter for NTP traffic will do. If you can connect the device to a single custom AP or LAN port (e.g. on some Linux PC), that will make monitoring easier; but as you are already able to dump the DHCP requests + responses, it should also work that way.
answered Jul 4 at 6:15
dirkt
13.9k2930
add a comment |Â
1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
0
down vote
accepted
1) The DHCP server will only answer with an NTP server field if it is configured to do that. From your dumps, it looks like it isn't, so this is a dead end.
2) The device will use whatever it likes as an NTP server - there's no general rule. Very likely somewhere on the device is a config file which contains the NTP server(s) it uses.
3) If you know enough about the device to say that it uses connman: Do you have access to the device as root? Or can you see the file system? If yes, look at all configuration files you can find.
4) If the device is some closed embedded device you don't have access to, another option is to monitor all traffic from the device until it contacts an NTP server, and then you'll know. Wireshark/tcpdump with a proper rule to filter for NTP traffic will do. If you can connect the device to a single custom AP or LAN port (e.g. on some Linux PC), that will make monitoring easier; but as you are already able to dump the DHCP requests + responses, it should also work that way.
answered Jul 4 at 6:15
dirkt
13.9k2930
add a comment |Â
up vote
0
down vote
accepted
1) The DHCP server will only answer with an NTP server field if it is configured to do that. From your dumps, it looks like it isn't, so this is a dead end.
2) The device will use whatever it likes as an NTP server - there's no general rule. Very likely somewhere on the device is a config file which contains the NTP server(s) it uses.
3) If you know enough about the device to say that it uses connman: Do you have access to the device as root? Or can you see the file system? If yes, look at all configuration files you can find.
4) If the device is some closed embedded device you don't have access to, another option is to monitor all traffic from the device until it contacts an NTP server, and then you'll know. Wireshark/tcpdump with a proper rule to filter for NTP traffic will do. If you can connect the device to a single custom AP or LAN port (e.g. on some Linux PC), that will make monitoring easier; but as you are already able to dump the DHCP requests + responses, it should also work that way.
answered Jul 4 at 6:15
dirkt
13.9k2930
add a comment |Â
up vote
0
down vote
accepted
up vote
0
down vote
accepted
1) The DHCP server will only answer with an NTP server field if it is configured to do that. From your dumps, it looks like it isn't, so this is a dead end.
2) The device will use whatever it likes as an NTP server - there's no general rule. Very likely somewhere on the device is a config file which contains the NTP server(s) it uses.
3) If you know enough about the device to say that it uses connman: Do you have access to the device as root? Or can you see the file system? If yes, look at all configuration files you can find.
4) If the device is some closed embedded device you don't have access to, another option is to monitor all traffic from the device until it contacts an NTP server, and then you'll know. Wireshark/tcpdump with a proper rule to filter for NTP traffic will do. If you can connect the device to a single custom AP or LAN port (e.g. on some Linux PC), that will make monitoring easier; but as you are already able to dump the DHCP requests + responses, it should also work that way.
answered Jul 4 at 6:15
dirkt
13.9k2930
1) The DHCP server will only answer with an NTP server field if it is configured to do that. From your dumps, it looks like it isn't, so this is a dead end.
2) The device will use whatever it likes as an NTP server - there's no general rule. Very likely somewhere on the device is a config file which contains the NTP server(s) it uses.
3) If you know enough about the device to say that it uses connman: Do you have access to the device as root? Or can you see the file system? If yes, look at all configuration files you can find.
4) If the device is some closed embedded device you don't have access to, another option is to monitor all traffic from the device until it contacts an NTP server, and then you'll know. Wireshark/tcpdump with a proper rule to filter for NTP traffic will do. If you can connect the device to a single custom AP or LAN port (e.g. on some Linux PC), that will make monitoring easier; but as you are already able to dump the DHCP requests + responses, it should also work that way.
answered Jul 4 at 6:15
dirkt
13.9k2930
answered Jul 4 at 6:15
dirkt
13.9k2930
answered Jul 4 at 6:15
dirkt
13.9k2930
answered Jul 4 at 6:15
dirkt
13.9k2930
13.9k2930
add a comment |Â
add a comment |Â
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2
Show your
dhcpd.conf.– Ipor Sircer
Jul 3 at 13:34
The device does not use dhcpd, it uses connman
– jap jap
Jul 3 at 13:37
1
Instead use pool.ntp.org if your device does not return NTP servers as part of its DHCP config.
– thrig
Jul 3 at 13:45
Thanks @thrig but what I am trying to figure out is where the NTP server that the device is using comes from
– jap jap
Jul 3 at 15:47