Public bug reported:
[ Impact ]
* The vendored n-dhcp in NetworkManager does not set ciaddr in DISCOVER
state. This is invalid according to RFC 2131 table 1 ("Description of
fields in a DHCP message") and has previously been discussed in
https://github.com/nettools/n-dhcp4/issues/45.
* I came up with a patch to reset ciaddr in INIT state, so we do not
include the "Client-IP" in any messages, until we receive another DHCP-
ACK.
[ Test Plan ]
* execute as bash -x repro.sh
```
#!/bin/bash
trap "trap - SIGTERM && kill -- -$$" SIGINT SIGTERM EXIT
ip netns add ns1
ip link add veth0 type veth peer name veth1 netns ns1
ip link set veth0 up
ip -n ns1 link set veth1 up
ip -n ns1 address add dev veth1 172.25.1.4/24
ip netns exec ns1 dnsmasq --bind-interfaces --interface veth1 --dhcp-
range=172.25.1.100,172.25.1.150,60
tcpdump -venni veth0 udp port 67 &
sleep 5
nmcli connection add type ethernet ifname veth0 con-name veth0+ ipv6.method
disabled autoconnect no
nmcli connection up veth0+
sleep 5 # wait for DHCP ACK
ip addr flush dev veth0 # clear IP received from DHCP server
ip addr add 172.25.1.42/24 dev veth0 # add non-DHCP address
ip addr show dev veth0
killall dnsmasq
ip netns exec ns1 dnsmasq --bind-interfaces --interface veth1
--dhcp-range=172.25.1.151,172.25.1.200,60
sleep 240
nmcli connection delete veth0+
ip link delete veth0
ip netns delete ns1
```
Without the change, I can see the following DISCOVER message after the
sleep 240: => Notice the Client-IP 172.25.1.126, which is wrong.
"""
14:45:13.755789 2a:02:0c:7b:bc:81 > ff:ff:ff:ff:ff:ff, ethertype IPv4 (0x0800),
length 350: (tos 0xc0, ttl 64, id 0, offset 0, flags [DF], proto UDP (17),
length 336)
0.0.0.0.68 > 255.255.255.255.67: BOOTP/DHCP, Request from
2a:02:0c:7b:bc:81, length 308, xid 0x4acac969, secs 1, Flags [none]
Client-IP 172.25.1.126
Client-Ethernet-Address 2a:02:0c:7b:bc:81
Vendor-rfc1048 Extensions
Magic Cookie 0x63825363
DHCP-Message (53), length 1: Discover
Client-ID (61), length 7: ether 2a:02:0c:7b:bc:81
Parameter-Request (55), length 17:
Subnet-Mask (1), Time-Zone (2), Domain-Name-Server (6), Hostname
(12)
Domain-Name (15), MTU (26), BR (28), Classless-Static-Route (121)
Default-Gateway (3), Static-Route (33), YD (40), YS (41)
NTP (42), Unknown (119), Classless-Static-Route-Microsoft (249),
Unknown (252)
RP (17)
MSZ (57), length 2: 576
Hostname (12), length 30: "lp2116214-jammy-n-dhcp4-ciaddr"
"""
With the change, we should see this instead (after the sleep 240): => No
Client-IP header inside the DISCOVER message
"""
14:53:45.217406 2a:02:0c:7b:bc:81 > ff:ff:ff:ff:ff:ff, ethertype IPv4 (0x0800),
length 350: (tos 0xc0, ttl 64, id 0, offset 0, flags [DF], proto UDP (17),
length 336)
0.0.0.0.68 > 255.255.255.255.67: BOOTP/DHCP, Request from
2a:02:0c:7b:bc:81, length 308, xid 0x55d8ec3c, secs 1, Flags [none]
Client-Ethernet-Address 2a:02:0c:7b:bc:81
Vendor-rfc1048 Extensions
Magic Cookie 0x63825363
DHCP-Message (53), length 1: Discover
Client-ID (61), length 7: ether 2a:02:0c:7b:bc:81
Parameter-Request (55), length 17:
Subnet-Mask (1), Time-Zone (2), Domain-Name-Server (6), Hostname
(12)
Domain-Name (15), MTU (26), BR (28), Classless-Static-Route (121)
Default-Gateway (3), Static-Route (33), YD (40), YS (41)
NTP (42), Unknown (119), Classless-Static-Route-Microsoft (249),
Unknown (252)
RP (17)
MSZ (57), length 2: 576
Hostname (12), length 30: "lp2116214-jammy-n-dhcp4-ciaddr"
"""
[ Where problems could occur ]
* This is touching NetworkManager's internal DHCPv4 client, so if
anything goes wrong, we could see issues with acquiring dynamic IPv4
addresses.
[ Other Info ]
* Ubuntu Core20 bug in LP: #2116214
* Upstream PR: https://github.com/nettools/n-dhcp4/pull/48
** Affects: network-manager (Ubuntu)
Importance: Undecided
Status: New
** Affects: network-manager (Ubuntu Focal)
Importance: Undecided
Status: Won't Fix
** Affects: network-manager (Ubuntu Jammy)
Importance: Undecided
Status: New
** Affects: network-manager (Ubuntu Noble)
Importance: Undecided
Status: New
** Affects: network-manager (Ubuntu Plucky)
Importance: Undecided
Status: New
** Affects: network-manager (Ubuntu Questing)
Importance: Undecided
Status: New
** Also affects: network-manager (Ubuntu Noble)
Importance: Undecided
Status: New
** Also affects: network-manager (Ubuntu Jammy)
Importance: Undecided
Status: New
** Also affects: network-manager (Ubuntu Plucky)
Importance: Undecided
Status: New
** Also affects: network-manager (Ubuntu Questing)
Importance: Undecided
Status: New
** Also affects: network-manager (Ubuntu Focal)
Importance: Undecided
Status: New
** Changed in: network-manager (Ubuntu Focal)
Status: New => Won't Fix
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https://bugs.launchpad.net/bugs/2126573
Title:
n-dhcp4: Do not set ciaddr in DISCOVER state.
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