dhclient with '-s ' flag












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I want to understand under which circumstance this command is valid. As i can see, using this command the discover packets have unicast destination ip same as that of the provided server ip while the destination mac is broadcast. Is this a valid packet, as I am not able to get ip using this flag.










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    I want to understand under which circumstance this command is valid. As i can see, using this command the discover packets have unicast destination ip same as that of the provided server ip while the destination mac is broadcast. Is this a valid packet, as I am not able to get ip using this flag.










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      I want to understand under which circumstance this command is valid. As i can see, using this command the discover packets have unicast destination ip same as that of the provided server ip while the destination mac is broadcast. Is this a valid packet, as I am not able to get ip using this flag.










      share|improve this question














      I want to understand under which circumstance this command is valid. As i can see, using this command the discover packets have unicast destination ip same as that of the provided server ip while the destination mac is broadcast. Is this a valid packet, as I am not able to get ip using this flag.







      dhcp ip-address ipv4 dhcp-server






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      asked Jul 4 '18 at 18:51









      user8070445user8070445

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          I think that receiving an answer with -s flag it depends on the listening dhcp server (or its configuration).



          Short answer, it works with dhcpd server but it fails with dnsmasq one.



          Long(er) answer, using "dhclient -v -s 172.16.0.1 eth1" will get an IP if the listening server is dhcpd:



          [root@c602 ~]# dhclient -v -s 172.16.0.1 eth1
          Internet Systems Consortium DHCP Client 4.1.1-P1
          Copyright 2004-2010 Internet Systems Consortium.
          All rights reserved.
          For info, please visit https://www.isc.org/software/dhcp/

          Listening on LPF/eth1/08:00:27:31:f2:d6
          Sending on LPF/eth1/08:00:27:31:f2:d6
          Sending on Socket/fallback
          DHCPDISCOVER on eth1 to 172.16.0.1 port 67 interval 7 (xid=0x6161a797)
          DHCPOFFER from 172.16.0.1
          DHCPREQUEST on eth1 to 172.16.0.1 port 67 (xid=0x6161a797)
          DHCPACK from 172.16.0.1 (xid=0x6161a797)

          bound to 172.16.0.230 -- renewal in 19087 seconds.

          [root@c602 ~]# ip -4 a show dev eth1
          3: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
          inet 172.16.0.230/24 brd 172.16.0.255 scope global eth1


          In both circumstances, when dhcpd or dnsmasq are the acting dhcp server, the DHCPDISCOVER is reaching the server but is not answered by dnsmasq; seen this with tcpdump on server machine. (tcpdump -nnnvvvi eth1 port 67 and port 68)






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            0














            I think that receiving an answer with -s flag it depends on the listening dhcp server (or its configuration).



            Short answer, it works with dhcpd server but it fails with dnsmasq one.



            Long(er) answer, using "dhclient -v -s 172.16.0.1 eth1" will get an IP if the listening server is dhcpd:



            [root@c602 ~]# dhclient -v -s 172.16.0.1 eth1
            Internet Systems Consortium DHCP Client 4.1.1-P1
            Copyright 2004-2010 Internet Systems Consortium.
            All rights reserved.
            For info, please visit https://www.isc.org/software/dhcp/

            Listening on LPF/eth1/08:00:27:31:f2:d6
            Sending on LPF/eth1/08:00:27:31:f2:d6
            Sending on Socket/fallback
            DHCPDISCOVER on eth1 to 172.16.0.1 port 67 interval 7 (xid=0x6161a797)
            DHCPOFFER from 172.16.0.1
            DHCPREQUEST on eth1 to 172.16.0.1 port 67 (xid=0x6161a797)
            DHCPACK from 172.16.0.1 (xid=0x6161a797)

            bound to 172.16.0.230 -- renewal in 19087 seconds.

            [root@c602 ~]# ip -4 a show dev eth1
            3: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
            inet 172.16.0.230/24 brd 172.16.0.255 scope global eth1


            In both circumstances, when dhcpd or dnsmasq are the acting dhcp server, the DHCPDISCOVER is reaching the server but is not answered by dnsmasq; seen this with tcpdump on server machine. (tcpdump -nnnvvvi eth1 port 67 and port 68)






            share|improve this answer




























              0














              I think that receiving an answer with -s flag it depends on the listening dhcp server (or its configuration).



              Short answer, it works with dhcpd server but it fails with dnsmasq one.



              Long(er) answer, using "dhclient -v -s 172.16.0.1 eth1" will get an IP if the listening server is dhcpd:



              [root@c602 ~]# dhclient -v -s 172.16.0.1 eth1
              Internet Systems Consortium DHCP Client 4.1.1-P1
              Copyright 2004-2010 Internet Systems Consortium.
              All rights reserved.
              For info, please visit https://www.isc.org/software/dhcp/

              Listening on LPF/eth1/08:00:27:31:f2:d6
              Sending on LPF/eth1/08:00:27:31:f2:d6
              Sending on Socket/fallback
              DHCPDISCOVER on eth1 to 172.16.0.1 port 67 interval 7 (xid=0x6161a797)
              DHCPOFFER from 172.16.0.1
              DHCPREQUEST on eth1 to 172.16.0.1 port 67 (xid=0x6161a797)
              DHCPACK from 172.16.0.1 (xid=0x6161a797)

              bound to 172.16.0.230 -- renewal in 19087 seconds.

              [root@c602 ~]# ip -4 a show dev eth1
              3: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
              inet 172.16.0.230/24 brd 172.16.0.255 scope global eth1


              In both circumstances, when dhcpd or dnsmasq are the acting dhcp server, the DHCPDISCOVER is reaching the server but is not answered by dnsmasq; seen this with tcpdump on server machine. (tcpdump -nnnvvvi eth1 port 67 and port 68)






              share|improve this answer


























                0












                0








                0







                I think that receiving an answer with -s flag it depends on the listening dhcp server (or its configuration).



                Short answer, it works with dhcpd server but it fails with dnsmasq one.



                Long(er) answer, using "dhclient -v -s 172.16.0.1 eth1" will get an IP if the listening server is dhcpd:



                [root@c602 ~]# dhclient -v -s 172.16.0.1 eth1
                Internet Systems Consortium DHCP Client 4.1.1-P1
                Copyright 2004-2010 Internet Systems Consortium.
                All rights reserved.
                For info, please visit https://www.isc.org/software/dhcp/

                Listening on LPF/eth1/08:00:27:31:f2:d6
                Sending on LPF/eth1/08:00:27:31:f2:d6
                Sending on Socket/fallback
                DHCPDISCOVER on eth1 to 172.16.0.1 port 67 interval 7 (xid=0x6161a797)
                DHCPOFFER from 172.16.0.1
                DHCPREQUEST on eth1 to 172.16.0.1 port 67 (xid=0x6161a797)
                DHCPACK from 172.16.0.1 (xid=0x6161a797)

                bound to 172.16.0.230 -- renewal in 19087 seconds.

                [root@c602 ~]# ip -4 a show dev eth1
                3: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
                inet 172.16.0.230/24 brd 172.16.0.255 scope global eth1


                In both circumstances, when dhcpd or dnsmasq are the acting dhcp server, the DHCPDISCOVER is reaching the server but is not answered by dnsmasq; seen this with tcpdump on server machine. (tcpdump -nnnvvvi eth1 port 67 and port 68)






                share|improve this answer













                I think that receiving an answer with -s flag it depends on the listening dhcp server (or its configuration).



                Short answer, it works with dhcpd server but it fails with dnsmasq one.



                Long(er) answer, using "dhclient -v -s 172.16.0.1 eth1" will get an IP if the listening server is dhcpd:



                [root@c602 ~]# dhclient -v -s 172.16.0.1 eth1
                Internet Systems Consortium DHCP Client 4.1.1-P1
                Copyright 2004-2010 Internet Systems Consortium.
                All rights reserved.
                For info, please visit https://www.isc.org/software/dhcp/

                Listening on LPF/eth1/08:00:27:31:f2:d6
                Sending on LPF/eth1/08:00:27:31:f2:d6
                Sending on Socket/fallback
                DHCPDISCOVER on eth1 to 172.16.0.1 port 67 interval 7 (xid=0x6161a797)
                DHCPOFFER from 172.16.0.1
                DHCPREQUEST on eth1 to 172.16.0.1 port 67 (xid=0x6161a797)
                DHCPACK from 172.16.0.1 (xid=0x6161a797)

                bound to 172.16.0.230 -- renewal in 19087 seconds.

                [root@c602 ~]# ip -4 a show dev eth1
                3: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
                inet 172.16.0.230/24 brd 172.16.0.255 scope global eth1


                In both circumstances, when dhcpd or dnsmasq are the acting dhcp server, the DHCPDISCOVER is reaching the server but is not answered by dnsmasq; seen this with tcpdump on server machine. (tcpdump -nnnvvvi eth1 port 67 and port 68)







                share|improve this answer












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                answered Feb 10 at 17:24









                Cosmin MironCosmin Miron

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