[FS#3815] PPPoE interface naming creates issues for ping, traceroute, curl and scripts

[OpenWrt Bugs]

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User who did this - HikariWS (HikariWS) 

Attached to Project - OpenWrt/LEDE Project
Summary - PPPoE interface naming creates issues for ping, traceroute, curl and scripts
Task Type - Bug Report
Category - Base system
Status - Unconfirmed
Assigned To - 
Operating System - All
Severity - Low
Priority - Very Low
Reported Version - Trunk
Due in Version - Undecided
Due Date - Undecided
Details - I can confirm this happens on EdgeRouter X and AMD64. I'm currently using a Ryzen 3100 with OpenWRT 21.02.0-rc1.

When we configure WAN interfaces, we create them and point to a ifname. In my example, for a ISP that uses DHCP, I create interfaces //wan_mrd// and //wan_mrd6// both using ifname //eth2//. When we're gonna use softwares like ping, traceroute and curl that support specifying the interface, I'd use ping -I eth2.

But when a WAN uses PPPoE, apart from the interface name, OpenWRT creates a second name on the format pppoe-. In example, I have another ISP on port //eth3//, then on config I create interfaces  named //want// and //want6//. Then when I use ping I must use ping -I pppoe-want.

If I use ping -I eth3 it fails with message ping: connect: Permission denied. ping -I want fails with ping: unknown iface: want.

Oddly, //pppoe-want// works for IPv6 too. There were times that even //pppoe-want// resulted on Permission denied, I don't know what had changed and it's working at the moment.

There are some reports of this causing conflicts. In example, https://github.com/openwrt/packages/issues/15434 has people having trouble with mwan3 because of this. On https://forum.openwrt.org/t/ddns-not-working-with-pppoe-interface/96778 package ddns doesn't work because it uses ethX instead of pppoe-X.

I don't know why it was implemented this way and how to fix it or improve its compatibility. I guess there's a reason, or PPPoE connections would just use the port ifname as other protocols do.

More information can be found at the following URL:
https://bugs.openwrt.org/index.php?do=details&task_id=3815

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