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[lwip-devel] [bug #61653] PPP Netmask Changed

From: Ashley Duncan
Subject: [lwip-devel] [bug #61653] PPP Netmask Changed
Date: Thu, 9 Dec 2021 17:37:23 -0500 (EST)
User-agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/96.0.4664.45 Safari/537.36


                 Summary: PPP Netmask Changed
                 Project: lwIP - A Lightweight TCP/IP stack
            Submitted by: ashesman
            Submitted on: Thu 09 Dec 2021 10:37:21 PM UTC
                Category: PPP
                Severity: 3 - Normal
              Item Group: Faulty Behaviour
                  Status: None
                 Privacy: Public
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any
         Planned Release: None
            lwIP version: 2.1.2




I am not sure if this is an issue, or my misunderstanding of PPP.  Please
forgive me if it is the later...

I am using a PPPoS connection to a Ublox Nina W15 WiFi module.  Has been
working great for years.

Today I tried to send UDP broadcast packets on the PPP netif.  I calculate the
broadcast IP address by ORing the compliment of the netmask with the netif IP
address.  I noticed that even though the PPP link is configured for netmask, once connected the netmask is changed to
(IPADDR_BROADCAST).  this happens in get_mask, line 1232 of ppp.c.

The above calculation results in the broadcast address being calculated as the
netifs IP address.  So any packets sent on that PPP netif are caught by
loopback and not actually sent on the PPP link.

I guess that a PPP link is always one to one so the concept of broadcast may
not exist on PPP?  Although in most PPPoS cases, the PPP link is a conduit to
another network interface so should pass all traffic in both directions?


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