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I have these two computers connected through ppp.

One has ip 10.10.0.1 and the other has ip 10.10.0.2.

Both also have the interface ppp0 with netmask 255.255.255.255:

# ifconfig
ppp0: flags=4305<UP,POINTOPOINT,RUNNING,NOARP,MULTICAST>  mtu 1500
        inet 10.10.0.2  netmask 255.255.255.255  destination 10.10.0.1
        ppp  txqueuelen 3  (Point-to-Point Protocol)
        RX packets 7  bytes 105 (105.0 B)
        RX errors 3  dropped 0  overruns 0  frame 0
        TX packets 20  bytes 1522 (1.4 KiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

How can I make it possible to connect from 10.10.0.2 into 10.10.0.1 using SSH?

If I make:

# ssh -vv root@10.10.0.1

I can see that it gets frozen at:

.
.
.
password: 
debug2: input_userauth_info_req
debug2: input_userauth_info_req: num_prompts 0
debug1: Authentication succeeded (keyboard-interactive).
Authenticated to 10.10.0.1 ([10.10.0.1]:22).
debug1: channel 0: new [client-session]
debug2: channel 0: send open
debug1: Requesting no-more-sessions@openssh.com
debug1: Entering interactive session.
debug1: pledge: network

Watching everything from tcpdump I see this:

# tcpdump -i ppp0
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on ppp0, link-type LINUX_SLL (Linux cooked), capture size 262144 bytes
22:15:48.592651 IP tlcmelo > 10.100.0.1: ICMP echo request, id 17288, seq 1, length 64
22:15:48.942907 IP 10.100.0.1 > tlcmelo: ICMP echo reply, id 17288, seq 1, length 64
22:15:49.593728 IP tlcmelo > 10.100.0.1: ICMP echo request, id 17288, seq 2, length 64
22:15:49.821594 IP 10.100.0.1 > tlcmelo: ICMP echo reply, id 17288, seq 2, length 64
22:15:50.595483 IP tlcmelo > 10.100.0.1: ICMP echo request, id 17288, seq 3, length 64
22:15:50.840112 IP 10.100.0.1 > tlcmelo: ICMP echo reply, id 17288, seq 3, length 64
22:15:51.597014 IP tlcmelo > 10.100.0.1: ICMP echo request, id 17288, seq 4, length 64
22:16:00.105020 IP tlcmelo.57621 > 10.100.0.1.57621: UDP, length 44
22:16:38.104153 IP tlcmelo.57621 > 10.100.0.1.57621: UDP, length 44
22:17:04.490191 IP tlcmelo.37489 > 239.255.255.250.1900: UDP, length 171
22:17:05.490773 IP tlcmelo.37489 > 239.255.255.250.1900: UDP, length 171
22:17:06.491775 IP tlcmelo.37489 > 239.255.255.250.1900: UDP, length 171
22:17:07.492280 IP tlcmelo.37489 > 239.255.255.250.1900: UDP, length 171
22:17:16.104112 IP tlcmelo.57621 > 10.100.0.1.57621: UDP, length 44
22:17:54.103994 IP tlcmelo.57621 > 10.100.0.1.57621: UDP, length 44
22:18:32.105034 IP tlcmelo.57621 > 10.100.0.1.57621: UDP, length 44

Edit 1:

@derobert, thanks for your message: It is not possible also to ssh from 10.10.0.1 back to 10.10.0.2, same issue actually. I tried a smaller MTU, no success unfortunately.

@pizdelect, thanks for your message: I am running debian in both computers. I tried setting MTU to 1000 and 500, no success yet.

Edit 2:

In the 10.10.0.1, doing iptables -vL I see these:

Chain Block (2 references)
 pkts bytes target     prot opt in     out     source               destination         
   75  3210 LOG        all  --  any    any     anywhere            !0.0.0.255/-1         limit: avg 3/hour burst 5 LOG level warning prefix "Iptables: Block: "
    6   360 REJECT     tcp  --  any    any     anywhere             anywhere             reject-with tcp-reset
 2433  143K REJECT     udp  --  any    any     anywhere             anywhere             reject-with icmp-port-unreachable
    0     0 DROP       all  --  any    any     anywhere             anywhere 

Is that possible that it is blocking the connection?

  • 1
    It should just work. Does ssh to localhost (from 10.10.0.1) work? That'd at least demonstrate the ssh server is working. – derobert Mar 13 at 22:00
  • what are you running ppp over? try setting the mtu and mru to a lower value. also, what system is that? openbsd? – pizdelect Mar 13 at 22:17
  • 1
    @derobert a mtu = 1500 will certainly not "just work" with pppoe. icmp echos and small packets will get through though ;-) – pizdelect Mar 13 at 22:23
  • 1
    @pizdelect Well, OP's tcpdump doesn't even show any ssh traffic, but the debug output shows session established and authenticated. So something is weird. Besides, PPP should figure out the right MTU for PPPoE. – derobert Mar 13 at 22:27
  • I don't know whether ppp "should" figure out the right mtu for pppoe, but it usually doesn't -- take for instance the (vanilla) openwrt on my router, where 1492 it is passed as a parameter to pppd (and it also does mss clamping via iptables). – pizdelect Mar 13 at 22:34

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