Re: [Networking] ERSPAN decapsulation drops DHCP unicast packets

[Bagas Sanjaya]

On Sat, Aug 26, 2023 at 09:55:30PM +0200, Volodymyr Litovka wrote:
> Hi colleagues,
> 
> I'm trying to catch and process (in 3rd party analytics app) DHCP packets
> from ERSPAN session, but cannot do this due to absence of DHCP unicast
> packets after decapsulation.
> 
> The model is pretty simple: there is PHY interface (enp2s0) which receive
> ERSPAN traffic and erspan-type interface to get decapsulated packets
> (inspan, created using command "ip link add inspan type erspan seq key 10
> local 10.171.165.65 erspan_ver 1", where 10.171.165.65 is ERSPAN target).
> Then I'm going to rewrite headers in the proper ways (nftable's netdev
> family) and forward packets to the pool of workers.
> 
> Having this, I'm expecting everything, which is encapsulated inside ERSPAN,
> on 'inspan' interface. And there is _almost_ everything except DHCP unicast
> packets - tcpdump shows about 1kps on this interface of decapsulated
> packets, but no DHCP unicast (see below traces).
> 
> To avoid any interactions, I removed and disabled everything that can catch
> DHCP in userspace - systemd-networkd, netplan, dhcp-client. There is no DHCP
> server and ifupdown - for test purposes, I'm bringing networking manually.
> Apparmor disabled as well. Kernel (Linux 5.19.0-42-generic
> #43~22.04.1-Ubuntu SMP PREEMPT_DYNAMIC) compiled without CONFIG_IP_PNP
> (according to /boot/config-5.19.0-42-generic). Nothing in userspace listens
> on UDP/68 and UDP/67:

Can you reproduce this on latest mainline?

> 
> # netstat -tunlpa
> Active Internet connections (servers and established)
> Proto Recv-Q Send-Q Local Address           Foreign Address        
> State       PID/Program name
> tcp        0      0 0.0.0.0:22 0.0.0.0:*               LISTEN      544/sshd:
> /usr/sbin
> tcp6       0      0 :::22 :::*                    LISTEN      544/sshd:
> /usr/sbin
> 
> I have no ideas, why this is happening. Decapsulation itself works, but
> particular kind of packets get lost.
> 
> I will appreciate if anyone can help me understand where is the bug - in my
> configuration or somewhere inside the kernel?
> 
> Evidence of traffic presence/absence is below.
> 
> Thank you.
> 
> Encapsulated ERSPAN session (udp and port 67/68) contains lot of different
> kinds of DHCP packets:
> 
> # tcpdump -s0 -w- -i enp2s0 'proto gre and ether[73:1]=17 and
> (ether[84:2]=67 or ether[84:2]=68)' | tshark -r- -l
>  [ ... ]
>     7   0.001942  0.0.0.0 → 255.255.255.255 DHCP 392 DHCP Discover -
> Transaction ID 0x25c096fc
>     8   0.003432  z.z.z.z → a.a.a.a         DHCP 418 DHCP ACK      -
> Transaction ID 0x5515126a
>     9   0.005170  m.m.m.m → z.z.z.z         DHCP 435 DHCP Discover -
> Transaction ID 0xa7b7
>    10   0.005171  m.m.m.m → z.z.z.z         DHCP 435 DHCP Discover -
> Transaction ID 0xa7b7
>    11   0.015399  n.n.n.n → z.z.z.z         DHCP 690 DHCP Request  -
> Transaction ID 0x54955233
>    12   0.025537  z.z.z.z → n.n.n.n         DHCP 420 DHCP ACK      -
> Transaction ID 0x54955233
>    13   0.030313  z.z.z.z → m.m.m.m         DHCP 413 DHCP Offer    -
> Transaction ID 0xa7b7
> 
> but decapsulated traffic (which I'm seeing on inspan interface) contains
> just the following:
> 
> # tcpdump -i inspan 'port 67 or port 68'
> listening on inspan, link-type EN10MB (Ethernet), snapshot length 262144
> bytes
> 17:23:36.540721 IP 0.0.0.0.bootpc > 255.255.255.255.bootps: BOOTP/DHCP,
> Request from 00:1a:64:33:8d:fa (oui Unknown), length 300
> 17:23:39.760036 IP 0.0.0.0.bootpc > 255.255.255.255.bootps: BOOTP/DHCP,
> Request from 00:1a:64:33:8d:fa (oui Unknown), length 300
> 17:23:44.135711 IP 0.0.0.0.bootpc > 255.255.255.255.bootps: BOOTP/DHCP,
> Request from 00:1a:64:33:8d:fa (oui Unknown), length 300
> 17:23:52.008504 IP 0.0.0.0.bootpc > 255.255.255.255.bootps: BOOTP/DHCP,
> Request from 00:1a:64:33:8d:fa (oui Unknown), length 300
> 

What hardware?

-- 
An old man doll... just what I always wanted! - Clara

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