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I am using wifi-connected computer set up near my non-wifi-capable printer as a bridge to put the printer within my subnet. The computer is set up as a vpn client with the tap port bridged with eth0. The vpn server is also bridged so that vpn clients are within the network. This works fine. The printer is accessible throughout the network. The only exception is that the printer is unable to communicate directly with the computer running the vpn server. L2 traffic works fine. The vpn server is able to send arp requests to the printer and receive responses, but the server does not receive ping replies from the printer. Watching the traffic from the vpn client, I have seen the ping reply sent from the printer.

Dump from 172.16.0.1 (vpn server)

$ sudo tcpdump -i br0 host 172.16.1.213
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on br0, link-type EN10MB (Ethernet), capture size 262144 bytes
22:14:50.419458 IP 172.16.0.1 > 172.16.1.213: ICMP echo request, id 5937, seq 1, length 64
22:14:50.424190 ARP, Request who-has 172.16.0.1 tell 172.16.1.213, length 46
22:14:50.424283 ARP, Reply 172.16.0.1 is-at 00:08:a1:01:8d:96 (oui Unknown), length 28
^C
3 packets captured
3 packets received by filter
0 packets dropped by kernel

Dump from 172.16.1.212 (bridged vpn client)

$ sudo tcpdump -i br0 host 172.16.1.213
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on br0, link-type EN10MB (Ethernet), capture size 262144 bytes
22:14:50.423435 IP 172.16.0.1 > 172.16.1.213: ICMP echo request, id 5937, seq 1, length 64
22:14:50.423719 ARP, Request who-has 172.16.0.1 tell 172.16.1.213, length 46
22:14:50.426531 ARP, Reply 172.16.0.1 is-at 00:08:a1:01:8d:96 (oui Unknown), length 28
22:14:50.426743 IP 172.16.1.213 > 172.16.0.1: ICMP echo reply, id 5937, seq 1, length 64
^C
4 packets captured
4 packets received by filter
0 packets dropped by kernel

Ping from 172.16.0.1

$ ping 172.16.1.213 -c1
PING 172.16.1.213 (172.16.1.213) 56(84) bytes of data.

--- 172.16.1.213 ping statistics ---
1 packets transmitted, 0 received, 100% packet loss, time 0ms
  • Packet sniffing on the tap and physical interfaces may be helpful. Also use the -e switch to tcpdump to show the ethernet addresses of the packets. How do they relate to the addresses shown by ifconfig? – Timothy Baldwin Jun 26 at 9:23

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