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Add --dhcp-split-relay option.

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This makes a DHCPv4 relay which is functional when
client and server networks aren't mutually route-able.
This commit is contained in:
Simon Kelley
2025-07-25 18:47:33 +01:00
parent ea5b0e64e2
commit fc9f6985ab
8 changed files with 247 additions and 127 deletions
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23
man/dnsmasq.8
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@@ -1487,7 +1487,28 @@ DHCPv4 to a DHCPv6 server or vice-versa.
The DHCP relay function for IPv6 includes the ability to snoop
prefix-delegation from relayed DHCP transactions. See
.B --dhcp-script
for details.
for details.
.TP
.B --dhcp-split-relay=<local address>,[<server address>[#<server port>]],<server-facing-interface>
A usefully enchanced version of DHCPv4 relay. IPv4 DHCP normally uses a single address
for two functions; it is used by the DHCP server to determine which network to allocate
an address on, and it is used as the address of the relay to which the server sends packets.
This version of DHCP relay splits these functions. It uses the address of the server-facing relay
interface as the address that the server talks to. The address of the client-facing interface
(the first item in the config) is used as to determine the client's subnet. The
local address is also used as server-ID override so that the client always sends requests
via the relay. The effect of this is that server doesn't require
a route to the client network and the clients don't require a route to the server.
The interface parameter is mandatory and a cannot be a wildcard.
If setting up a network where the client networks have limited routing, be careful
about configuring the DHCP server. Dnsmasq, as DHCP server, will set the default route to the
client-facing relay interface unless explicitly configured: that is a sensible default.
The normal default DNS server (the same address as the DHCP server)
will not be appropriate when there is no route bewteen the
two, so this will have to be explicitly configured.
.TP
.B \-U, --dhcp-vendorclass=set:<tag>,[enterprise:<IANA-enterprise number>,]<vendor-class>
Map from a vendor-class string to a tag. Most DHCP clients provide a