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T7723: dhcp-server add DHCP Option 26 #4666

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T7723: dhcp-server add DHCP Option 26 #4666

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46b7141
Changes to support DHCP option 26 (client interface mtu)
mzpqnxow Aug 18, 2025
5fc7ec8
fix type for interface-mtu
mzpqnxow Aug 29, 2025
1bb88d8
use minimum MTU specified in RFC 790 as the lower bound
mzpqnxow Aug 29, 2025
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12 changes: 12 additions & 0 deletions interface-definitions/include/dhcp/option-v4.xml.i
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Original file line number Diff line number Diff line change
Expand Up @@ -83,6 +83,18 @@
</constraint>
</properties>
</leafNode>
<leafNode name="interface-mtu">
<properties>
<help>Interface MTU</help>
<valueHelp>
<format>u32:1-16</format>
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Why 1-16?

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Per the RFC, it's a 16bit value. But looking at it now, I think I misread how the notation in that file works and it should probably just be u16?

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As far as the values chosen for the bounds - I'm open to any ideas on the best values to use.

I admit they're somewhat arbitrary choices (though I'm sure you recognize the upper bound as Jumbo)

In terms of standards, we could use 576 for the minimum. It's unreasonably small, but mentioned in the IPv4 RFC (RFC 791 section 3.1):

All hosts must be prepared
to accept datagrams of up to 576 octets (whether they arrive whole
or in fragments). It is recommended that hosts only send datagrams
larger than 576 octets if they have assurance that the destination
is prepared to accept the larger datagrams.

The IPv6 RFC RFC 8200, section 5 requires a minimum MTU of 1280 - but this is DHCP IPv4, so I don't think we need to be troubled with that

I can't imagine 576 actually being used anywhere. At the same time, I've actually come close to needing to go below 1280 before on IPv4 interfaces (due to use of multiple layers of tunnels). So 576 seems reasonable.

<description>Client interface MTU</description>
</valueHelp>
<constraint>
<validator name="numeric" argument="--range 1200-9000"/>
</constraint>
</properties>
</leafNode>
<leafNode name="ip-forwarding">
<properties>
<help>Enable IP forwarding on client</help>
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1 change: 1 addition & 0 deletions python/vyos/kea.py
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Expand Up @@ -45,6 +45,7 @@
'ipv6_only_preferred': 'v6-only-preferred',
'captive_portal': 'v4-captive-portal',
'capwap_controller': 'capwap-ac-v4',
'interface_mtu': 'interface-mtu',
}

kea6_options = {
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7 changes: 7 additions & 0 deletions smoketest/scripts/cli/test_service_dhcp-server.py
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Expand Up @@ -225,6 +225,7 @@ def test_dhcp_single_pool_options(self):
server_identifier = bootfile_server
ipv6_only_preferred = '300'
capwap_access_controller = '192.168.2.125'
interface_mtu = '1420'

pool = base_path + ['shared-network-name', shared_net_name, 'subnet', subnet]
self.cli_set(pool + ['subnet-id', '1'])
Expand All @@ -233,6 +234,7 @@ def test_dhcp_single_pool_options(self):
self.cli_set(pool + ['option', 'name-server', dns_1])
self.cli_set(pool + ['option', 'name-server', dns_2])
self.cli_set(pool + ['option', 'domain-name', domain_name])
self.cli_set(pool + ['option', 'interface-mtu', interface_mtu])
self.cli_set(pool + ['option', 'ip-forwarding'])
self.cli_set(pool + ['option', 'smtp-server', smtp_server])
self.cli_set(pool + ['option', 'pop-server', smtp_server])
Expand Down Expand Up @@ -365,6 +367,11 @@ def test_dhcp_single_pool_options(self):
['Dhcp4', 'shared-networks', 0, 'subnet4', 0, 'option-data'],
{'name': 'ip-forwarding', 'data': 'true'},
)
self.verify_config_object(
obj,
['Dhcp4', 'shared-networks', 0, 'subnet4', 0, 'option-data'],
{'name': 'interface-mtu', 'data': interface_mtu},
)

# Time zone
self.verify_config_object(
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