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So everyone inside home network can reach the Internet thanks to NAT translation described in SOHO #004 and reach now their favorite Internet service by name thanks for SOHO #005.

Requirement

  • Basic Linux/Unix knowledge
  • Service provider networking knowledge

Overview

In the previous article during the verification we assume to have one host connected to a subnetwork cascaded behind SOHO router@sdn6. However, we did not described how the host could get connectivity.

Article objective

In this article we will pursue the SOHO network appliance installation and enable DHCP for IPv4 server. SOHO router can then answer to DHCPv4 request ingressing sdn6 interface.

  • Let's assume we have a switch connected to SOHO router@sdn6 for the future
  • sdn6 has ipv4 interface 192.168.136.1 manually configured
  • switch has IPv4 192.168.136.2
  • subnetwork behind sdn6 is 192.168.136.0/24
  • DHCPv4 will serve 192.168.136.3 → 192.168.136.254

Diagrams

[ SOHO #006 ] - "Do you need an IP ?"

First step, configure sdn6 and bind it to VRF inet. few considerations:

  • sdn6 has ipv4 192.168.136.1 manually configured
  • sdn6 will act as gateway for all connected host in subnetwork 192.168.136.0/24
  • In SOHO #004 ,  192.168.136.0/24 NAT is already taken into account


SOHO router DHCP server in VRF inet
conf t
interface sdn6
 mtu 1500
 vrf forwarding inet
 ipv4 address 192.168.136.1 255.255.255.0
 no shutdown
 no log-link-change
 exit
!

Second step, configure a DHCPv4 server@ SOHO router

SOHO router DHCP server in VRF inet
conf t
server dhcp4 dh4-16
 pool 192.168.136.3 192.168.136.254
 gateway 192.168.136.1
 netmask 255.255.255.0
 dns-server 192.168.254.1
 domain-name local
 interface sdn6
 vrf inet
 exit
!

In this case DHCPv4 will allocate IPv4 address:

  • from a pool going from 192.168.136.3 → 192.168.136.254
  • All host will set their gateway to 192.168.136.1 (which is SOHO@sdn6)
  • All connected host to sdn6 subnet will get an IP from pool within 192.168.136.0/24 subnet
  • All hosts will also consider SOHO@192.168.254.1 as DNS server as we implemented in SOHO #005
  • And we bind this DHCPv4 to interface sdn6 (without binding DHCP request coming from all interface belonging to VRF inet will be processed)
  • lastly DHCPv4 server will act into VRF inet only 

Verification

Connect a host to the switch connected to sdn6 then issue DHCP debug command

DHCP debug command
debug server dhcp?                                                     
  dhcp4      - ipv4 dynamic host config protocol
  dhcp6      - ipv6 dynamic host config protocol

mjolnir#debug server dhcp4 ?                                                   
  <cr>
mjolnir#debug server dhcp4                                                     
mjolnir#terminal monitor
DHCP debug ouput when connecting laptop @ sdn6
mjolnir#..                                                          debug serv.servDhcp4worker.doer:servDhcp4.java:679 rx op=req sec=1 cia=0.0.0.0 yia=0.0.0.0 sia=0.0.0.0 gia=0.0.0.0 cha=9ceb.e8d5.2c51 srv= fil= op=discover dhcpsrv=null hstnm=MBP-de-Frederic dom=null lease=7776000 renew=0 mask=null gw=null dns1=null dns2=null req=null
debug serv.servDhcp4.sendPack:servDhcp4.java:482 tx 192.168.136.123 op=rep sec=1 cia=0.0.0.0 yia=192.168.136.123 sia=192.168.136.1 gia=0.0.0.0 cha=9ceb.e8d5.2c51 srv= fil= op=offer dhcpsrv=192.168.136.1 hstnm=null dom=lan lease=43200 renew=21600 mask=255.255.255.0 gw=192.168.136.1 dns1=192.168.254.1 dns2=null req=null
debug serv.servDhcp4worker.doer:servDhcp4.java:686 tx op=rep sec=1 cia=0.0.0.0 yia=192.168.136.123 sia=192.168.136.1 gia=0.0.0.0 cha=9ceb.e8d5.2c51 srv= fil= op=offer dhcpsrv=192.168.136.1 hstnm=null dom=lan lease=43200 renew=21600 mask=255.255.255.0 gw=192.168.136.1 dns1=192.168.254.1 dns2=null req=null
debug serv.servDhcp4worker.doer:servDhcp4.java:679 rx op=req sec=2 cia=0.0.0.0 yia=0.0.0.0 sia=0.0.0.0 gia=0.0.0.0 cha=9ceb.e8d5.2c51 srv= fil= op=request dhcpsrv=192.168.136.1 hstnm=MBP-de-Frederic dom=null lease=0 renew=0 mask=null gw=null dns1=null dns2=null req=192.168.136.123
debug serv.servDhcp4.sendPack:servDhcp4.java:482 tx 192.168.136.123 op=rep sec=2 cia=0.0.0.0 yia=192.168.136.123 sia=192.168.136.1 gia=0.0.0.0 cha=9ceb.e8d5.2c51 srv= fil= op=ack dhcpsrv=192.168.136.1 hstnm=null dom=lan lease=43200 renew=21600 mask=255.255.255.0 gw=192.168.136.1 dns1=192.168.254.1 dns2=null req=null
debug serv.servDhcp4worker.doer:servDhcp4.java:686 tx op=rep sec=2 cia=0.0.0.0 yia=192.168.136.123 sia=192.168.136.1 gia=0.0.0.0 cha=9ceb.e8d5.2c51 srv= fil= op=ack dhcpsrv=192.168.136.1 hstnm=null dom=lan lease=43200 renew=21600 mask=255.255.255.0 gw=192.168.136.1 dns1=192.168.254.1 dns2=null req=null
info ip.ipCor6.parseIPheader:ipCor6.java:95 got bad version from ::
...
mjolnir#

So based on the debug output:

  • DHCP allocated 192.168.136.123
  • Primary DNS (dns1) is 192.168.136.1
  • Network has /24 CIDR
DHCP debug command
╭─[11/2/20|4:30:09]loui@MacBook-Pro-de-Frederic.local ~  
╰─➤  ifconfig en8
en8: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
        options=6407<RXCSUM,TXCSUM,VLAN_MTU,CHANNEL_IO,PARTIAL_CSUM,ZEROINVERT_CSUM>
        ether 9c:eb:e8:d5:2c:51 
        inet6 fe80::c93:c3b1:dfb3:77c0%en8 prefixlen 64 secured scopeid 0x13 
        inet 192.168.136.123 netmask 0xffffff00 broadcast 192.168.136.255
        inet6 2a01:e0a:159:2856:832:82f5:8519:70 prefixlen 64 autoconf secured 
        inet6 2a01:e0a:159:2856:653c:d1b2:dca9:c9da prefixlen 64 autoconf temporary 
        nd6 options=201<PERFORMNUD,DAD>
        media: autoselect (1000baseT <full-duplex>)
        status: active

╭─[11/2/20|4:37:05]loui@MacBook-Pro-de-Frederic.local ~  
╰─➤  nslookup www.free.fr      
Server:         192.168.254.1
Address:        192.168.254.1#53

Non-authoritative answer:
Name:   www.free.fr
Address: 212.27.48.10

Pay attention to the DNS server that answered ! Sweet !

DNS resolution from CLI (DNS query originated by router)
╭─[11/2/20|4:30:11]loui@MacBook-Pro-de-Frederic.local ~  
╰─➤  ping 8.8.8.8 -c 5
PING 8.8.8.8 (8.8.8.8): 56 data bytes
64 bytes from 8.8.8.8: icmp_seq=0 ttl=119 time=4.171 ms
64 bytes from 8.8.8.8: icmp_seq=1 ttl=119 time=4.334 ms
64 bytes from 8.8.8.8: icmp_seq=2 ttl=119 time=4.208 ms
64 bytes from 8.8.8.8: icmp_seq=3 ttl=119 time=3.856 ms
64 bytes from 8.8.8.8: icmp_seq=4 ttl=119 time=4.090 ms
Check that NAT is occuring for 192.168.136.126
mjolnir#show ipv4 nat inet translations | i 192.168.136.123                    
1      8.8.8.8 710148096           192.168.0.90 710148096   8.8.8.8 710148096        192.168.136.123 710148096   00:02:46  00:02:46  00:05:00  1       84
1      192.168.136.123 710148096   8.8.8.8 710148096        192.168.0.90 710148096   8.8.8.8 710148096           00:02:46  00:02:46  00:05:00  1       84
1      8.8.8.8 710148097           192.168.0.90 710148097   8.8.8.8 710148097        192.168.136.123 710148097   00:02:45  00:02:45  00:05:00  1       84
1      192.168.136.123 710148097   8.8.8.8 710148097        192.168.0.90 710148097   8.8.8.8 710148097           00:02:45  00:02:45  00:05:00  1       84
1      8.8.8.8 710148098           192.168.0.90 710148098   8.8.8.8 710148098        192.168.136.123 710148098   00:02:44  00:02:44  00:05:00  1       84
DNS resolution from CLI (DNS query originated by router)
╭─[11/2/20|4:36:18]loui@MacBook-Pro-de-Frederic.local ~  
╰─➤  ping www.free.fr -c 5 
PING www.free.fr (212.27.48.10): 56 data bytes
64 bytes from 212.27.48.10: icmp_seq=0 ttl=57 time=3.903 ms
64 bytes from 212.27.48.10: icmp_seq=1 ttl=57 time=5.883 ms
64 bytes from 212.27.48.10: icmp_seq=2 ttl=57 time=3.658 ms
64 bytes from 212.27.48.10: icmp_seq=3 ttl=57 time=4.872 ms
64 bytes from 212.27.48.10: icmp_seq=4 ttl=57 time=4.488 ms

--- www.free.fr ping statistics ---
5 packets transmitted, 5 packets received, 0.0% packet loss
round-trip min/avg/max/stddev = 3.658/4.561/5.883/0.787 ms
Check that NAT is occuring for 192.168.136.126 and www.free.fr IPv4 address
mjolnir#show ipv4 nat inet translations | i 212.27.48.10                       
1      192.168.136.123 1263796224  212.27.48.10 1263796224  192.168.0.90 1263796224  212.27.48.10 1263796224     00:03:47  00:03:47  00:05:00  1       84
1      212.27.48.10 1263796224     192.168.0.90 1263796224  212.27.48.10 1263796224  192.168.136.123 1263796224  00:03:47  00:03:47  00:05:00  1       84
1      192.168.136.123 1263796225  212.27.48.10 1263796225  192.168.0.90 1263796225  212.27.48.10 1263796225     00:03:46  00:03:46  00:05:00  1       84
1      212.27.48.10 1263796225     192.168.0.90 1263796225  212.27.48.10 1263796225  192.168.136.123 1263796225  00:03:46  00:03:46  00:05:00  1       84
1      192.168.136.123 1263796226  212.27.48.10 1263796226  192.168.0.90 1263796226  212.27.48.10 1263796226     00:03:45  00:03:45  00:05:00  1       84

Conclusion

In this article DHCP service has been enabled at:

  • SOHO router level 
  • All host getting an IPv4 via DHCP will get a DNS server set to SOHO@loopback0 (192.168.254.1)

RARE validated design: [ SOHO #005 ] - key take-away

In this example the key take-away are:

  • DHCP is available for IPv4 and IPv6
  • DHCP here is bind to an interface so rogue DHCP request coming from any router interface other than the speficied interface won't be honoured
  • Feel free to explore all DHCP options propose by RARE/freeRouter

DHCP for IPv6 is also available if you want to strictly reproduce IPv4 allocation scheme. In our case we will use IPv6 SLAC.


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