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Topic: DHCP client source port? (Read 554 times)
sjm42
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Posts: 2
Karma: 0
DHCP client source port?
«
on:
April 05, 2024, 01:13:43 pm »
Well, when my OPNsense is trying to renew its WAN address with DHCP, I see its udp source port is a random upper port.
Is this behaviour normal, acceptable and per specifications? My ISP seems to only accept dhcp requests from source port 68.
I am wondering, does my firewall source NAT its own self-initiated traffic and that is why dhcp unicast gets a different port?
Slightly anonymized tcpdump follows.
The first (unicast) dhcp request never gets a response, but the second request to broadcast address does.
12:59:09.799559 IP (tos 0x10, ttl 128, id 58116, offset 0, flags [none], proto UDP (17), length 328)
x.y.169.21.6671 > x.y.168.1.67: [udp sum ok] BOOTP/DHCP, Request from 00:e2:69:65:3a:25, length 300, xid 0x11fa7b4a, secs 1508, Flags [none] (0x0000)
Client-IP x.y.169.21
Client-Ethernet-Address 00:e2:69:65:3a:25
Vendor-rfc1048 Extensions
Magic Cookie 0x63825363
DHCP-Message Option 53, length 1: Request
Client-ID Option 61, length 7: ether 00:e2:69:65:3a:25
Hostname Option 12, length 4: "opns"
Parameter-Request Option 55, length 10:
Subnet-Mask, BR, Time-Zone, Classless-Static-Route
Default-Gateway, Domain-Name, Domain-Name-Server, Hostname
Option 119, MTU
END Option 255, length 0
PAD Option 0, length 0, occurs 29
13:00:53.801032 IP (tos 0x10, ttl 128, id 0, offset 0, flags [none], proto UDP (17), length 328)
x.y.169.21.68 > 255.255.255.255.67: [udp sum ok] BOOTP/DHCP, Request from 00:e2:69:65:3a:25, length 300, xid 0x11fa7b4a, secs 1612, Flags [none] (0x0000)
Client-IP x.y.169.21
Client-Ethernet-Address 00:e2:69:65:3a:25
Vendor-rfc1048 Extensions
Magic Cookie 0x63825363
DHCP-Message Option 53, length 1: Request
Client-ID Option 61, length 7: ether 00:e2:69:65:3a:25
Hostname Option 12, length 4: "opns"
Parameter-Request Option 55, length 10:
Subnet-Mask, BR, Time-Zone, Classless-Static-Route
Default-Gateway, Domain-Name, Domain-Name-Server, Hostname
Option 119, MTU
END Option 255, length 0
PAD Option 0, length 0, occurs 29
13:00:53.806786 IP (tos 0x0, ttl 16, id 26217, offset 0, flags [none], proto UDP (17), length 306)
x.y.168.1.67 > x.y.169.21.68: [udp sum ok] BOOTP/DHCP, Reply, length 278, xid 0x11fa7b4a, Flags [none] (0x0000)
Client-IP x.y.169.21
Your-IP x.y.169.21
Server-IP x.y.168.1
Client-Ethernet-Address 00:e2:69:65:3a:25
Vendor-rfc1048 Extensions
Magic Cookie 0x63825363
DHCP-Message Option 53, length 1: ACK
Subnet-Mask Option 1, length 4: 255.255.255.128
Default-Gateway Option 3, length 4: x.y.169.1
Domain-Name-Server Option 6, length 8: x.y.170.170,x.y.171.171
Lease-Time Option 51, length 4: 4200
Server-ID Option 54, length 4: x.y.168.1
END Option 255, length 0
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Patrick M. Hausen
Hero Member
Posts: 6825
Karma: 573
Re: DHCP client source port?
«
Reply #1 on:
April 05, 2024, 01:46:31 pm »
What does your outbound NAT rule on WAN look like? If the source address is "any" and not limited to "LAN net" or similar, that might well be the case.
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Deciso DEC750
People who think they know everything are a great annoyance to those of us who do.
(Isaac Asimov)
sjm42
Newbie
Posts: 2
Karma: 0
Re: DHCP client source port?
«
Reply #2 on:
April 05, 2024, 02:07:40 pm »
Actually yes, I figured it out. The firewall was doing source NAT including source port translation to its self-initiated traffic!
I added an outbound NAT rule for destination port 67 with "static port" flag set, and it solved the problem with unicast dhcp!
It was news to me that also self-initiated traffic gets source NATed.
Phew pffffft...
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Patrick M. Hausen
Hero Member
Posts: 6825
Karma: 573
Re: DHCP client source port?
«
Reply #3 on:
April 05, 2024, 02:11:42 pm »
Quote from: sjm42 on April 05, 2024, 02:07:40 pm
It was news to me that also self-initiated traffic gets source NATed.
The firewall does what the rules specify. If the NAT rule is "source: any" then it's all traffic. That's why you should change the rule to "source: LAN net" or "source: <group of internal networks>". Then no extra rule with static port is necessary.
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Deciso DEC750
People who think they know everything are a great annoyance to those of us who do.
(Isaac Asimov)
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DHCP client source port?