DHCP server

For cases where a host requires IPAM (IP Address Management) without an external DHCP server, or where DHCP relay to underlay is not possible, IPAM information can be stored locally on the SR Linux device, which can assign an IP address and other DHCP options to the host using a local DHCP server.

SR Linux supports static IP allocations on both DHCPv4 and DHCPv6 servers. The SR Linux DHCP server can be enabled under regular Layer 3 or IRB subinterfaces. On Layer 3 or IRB subinterfaces, the DHCP server can only be enabled under subinterfaces where DHCP relay is disabled.

When an incoming DHCP Discover or Solicit message is received from a host (DHCP client), and its MAC address matches an entry in the SR Linux DHCP server configuration, the SR Linux DHCP server starts the process of IP address assignment and sends other DHCP options if configured to do so.

For DHCPv4, in addition to IP address allocation, SR Linux can send the following DHCP options to a host:

• router (option 3) – IPv4 address of the gateway for the DHCP client
• dns-server (option 6) – List of up to 4 DNS servers for the DHCP client to use
• hostname (option 12) – The hostname for the DHCP client
• domain-name (option 15) – The domain name the client can use when resolving hostnames via DNS
• interface-mtu (option 26) – The MTU to use on this interface. The MTU is specified as a 16-bit unsigned integer. The minimum legal value for the MTU is 68.
• ntp-server (option 42) – List of up to 4 NTP servers for the DHCP client to use
• server-id (option 54) – IP address the DHCP server must match within the network-instance, such as the subinterface primary address or loopback address
• tftp-server-name (option 66) – The FQDN of the TFTP server the client uses to download the bootfile/configuration script
• bootfile-name (option 67) – The name of the configuration file the client uses during booting
• domain-search-list (option 119) – The domain search list the client uses when resolving hostnames with DNS
• static-route (option 121) – Classless Static Route option, which contains one or more static routes, each consisting of a destination descriptor and the IP address of the router to be used to reach that destination
• tftp-server-address (option 150) – List of IP addresses of the TFTP servers the client uses to download the bootfile/configuration script

For DHCPv6, in addition to IP address allocation, SR Linux can send the following DHCP options to a host:

• dns-server (option 23) - List of up to 4 DNS servers for the DHCP client to use
• domain-search-list (option 24) - The domain search list the client uses when resolving hostnames with DNS

Notes:

• The DHCP server can be enabled under regular Layer 3 or IRB subinterfaces.
• The SR Linux DHCP server supports static IP address allocation only. Dynamic allocation is not supported.
• On Layer 3 subinterfaces, you can enable either DHCP relay or the DHCP server, but not both. DHCP servers can be enabled only on subinterfaces where DHCP relay is disabled.
• It is assumed there is no DHCP relay agent between the DHCP client and the SR Linux DHCP server. Relayed frames are not supported.
• For IPv6, DHCP configuration uses MAC-to-IPv6 address binding. The IPv6 address is assigned to the client based on the client's MAC address, not IAID. The client's MAC address is derived from the client identifier. The recommended client identifier type is DUID type DUID-LLT or DUID-LL.

--{ * candidate shared default }--[  ]--
# info interface ethernet-1/1 subinterface 1
    interface ethernet-1/1 {
        subinterface 1 {
            admin-state enable
            ipv4 {
                admin-state enable
                address 192.14.1.4/27 {
                }
                dhcp-server {
                    admin-state enable
                }
            }
            ipv6 {
                admin-state enable
                address 2001:192:14:1::4/120 {
                }
                dhcpv6-server {
                    admin-state enable
                }
            }
        }
    }

--{ * candidate shared default }--[  ]--
# info system dhcp-server
    system {
        dhcp-server {
            admin-state enable
            network-instance default {
                dhcpv4 {
                    options {
                        domain-name lan
                        router 192.168.1.1
                        dns-server [
                            192.168.1.53
                            192.168.1.54
                        ]
                        ntp-server [
                            192.168.1.50
                        ]
                    }
                }
                dhcpv6 {
                    options {
                        dns-server [
                            2001:192:14:1::4
                            2001:192:14:1::5
                        ]
                    }
                }
            }
        }
    }

--{ * candidate shared default }--[  ]--
# info detail system dhcp-server network-instance default dhcpv4
    system {
        dhcp-server {
            admin-state enable
            network-instance default {
                dhcpv4 {
                    admin-state enable
                    static-allocation {
                        host 00:1D:FE:E0:E9:7C {
                            ip-address 192.168.1.1/24
                            options {
                                router 192.168.1.1
                                dns-server [
                                    192.168.1.53
                                ]
                            }
                        }
                    }
                }
            }
        }
    }

--{ * candidate shared default }--[  ]--
# info detail system dhcp-server network-instance default dhcpv6
    system {
        dhcp-server {
            admin-state enable
            network-instance default {
                dhcpv6 {
                    admin-state enable
                    static-allocation {
                        host 92:93:47:30:32:CA {
                            ip-address 2001:1::192:168:12:1/126
                            options {
                                dns-server [
                                    2001:192:14:1::4
                                ]
                            }
                        }
                    }
                }
            }
        }
    }