# [SOLVED] A host somehow forgets how to reach me.

## Zhaozhou

Now this is wierd, I have two servers, one of which just forgets how to reach the other.

My host is lunix.se, the "bad" host is linuxuser.nu.

First, ping from linuxuser.nu to lunix.se:

```

root@linuxuser:/etc/bind/zones# ping lunix.se

PING lunix.se (217.31.187.53) 56(84) bytes of data.

--- lunix.se ping statistics ---

37 packets transmitted, 0 received, 100% packet loss, time 36006ms

```

Nothing.

tracepath from linuxuser.nu to lunix.se:

```

root@linuxuser:/etc/bind/zones# tracepath lunix.se

 1:  linuxuser.se (79.99.3.34)                              0.157ms pmtu 1500

 1:  79-99-3-252.serverhotell.net (79.99.3.252)             0.082ms 

 1:  79-99-3-252.serverhotell.net (79.99.3.252)             0.054ms 

 2:  79-99-4-3.serverhotell.net (79.99.4.3)                 0.480ms asymm  3 

 3:  90-229-5-198.link.se.telia.net (90.229.5.198)          0.713ms asymm  4 

 4:  kbn-bb2-link.telia.net (80.91.249.50)                  7.694ms asymm 10 

 5:  s-bb2-pos7-0-0.telia.net (213.248.65.30)              16.139ms asymm 11 

 6:  s-b3-link.telia.net (80.91.250.152)                   15.980ms asymm 12 

 7:  bahnhof-110262-s-b3.c.telia.net (213.248.97.42)       12.228ms asymm 13 

 8:  gav-cr1.blg-cr1.bahnhof.net (85.24.151.207)           21.597ms asymm 15 

 9:  borlange-A182.stadsnat.bahnhof.net (85.24.152.77)     16.971ms asymm 16 

10:  no reply

11:  no reply

12:  no reply

.......

31:  no reply

     Too many hops: pmtu 1500

     Resume: pmtu 1500

```

No go.

Now, i ping linuxuser.nu from lunix.se:

```

~ > ping linuxuser.nu

PING linuxuser.nu (79.99.3.34) 56(84) bytes of data.

64 bytes from linuxuser.se (79.99.3.34): icmp_seq=1 ttl=55 time=17.0 ms

^C

--- linuxuser.nu ping statistics ---

1 packets transmitted, 1 received, 0% packet loss, time 0ms

rtt min/avg/max/mdev = 17.076/17.076/17.076/0.000 ms

```

That works. And now, I can suddenly connect to lunix.se from linuxuser.nu again:

```

root@linuxuser:/etc/bind/zones# ping lunix.se

PING lunix.se (217.31.187.53) 56(84) bytes of data.

64 bytes from linfast53.bitnet.nu (217.31.187.53): icmp_seq=1 ttl=47 time=15.8 ms

64 bytes from linfast53.bitnet.nu (217.31.187.53): icmp_seq=2 ttl=47 time=15.7 ms

--- lunix.se ping statistics ---

2 packets transmitted, 2 received, 0% packet loss, time 1001ms

rtt min/avg/max/mdev = 15.799/15.801/15.804/0.125 ms

```

And I'm stunned.

What is this? Is borlange-A182.stadsnat.bahnhof.net having troubles to find me?

----------

## coRpTitan

That no reply for ICMP request does not mean that problem is on borlange-A182.stadsnat.bahnhof.net . In case of problem on this node you would most likely get error: Destination host unreachable.

This No replay means that packet was sent to next server/router/whatever and it didn't return ICMP reply for that ping.

There can be multiple reasons:

 -  /proc/sys/net/ipv4/icmp_echo_ignore_all is set on that machine to 1

 - Or there is firewall which is dropping incoming or outgoing ICM packet.

Can you successfully tracepath lunix.se when you are able to ping it?

----------

## Zhaozhou

 *coRpTitan wrote:*   

> 
> 
>  -  /proc/sys/net/ipv4/icmp_echo_ignore_all is set on that machine to 1
> 
> 

 

```

net.ipv4.icmp_echo_ignore_all = 0

net.ipv4.icmp_echo_ignore_broadcasts = 1

net.ipv4.icmp_ignore_bogus_error_responses = 1

net.ipv4.icmp_errors_use_inbound_ifaddr = 0

net.ipv4.icmp_ratelimit = 1000

net.ipv4.icmp_ratemask = 6168

```

 *coRpTitan wrote:*   

> 
> 
>  - Or there is firewall which is dropping incoming or outgoing ICM packet.
> 
> 

 

```

~ > iptables -vL                         

Chain INPUT (policy ACCEPT 21M packets, 19G bytes)

 pkts bytes target     prot opt in     out     source               destination         

Chain FORWARD (policy ACCEPT 0 packets, 0 bytes)

 pkts bytes target     prot opt in     out     source               destination         

Chain OUTPUT (policy ACCEPT 11M packets, 30G bytes)

 pkts bytes target     prot opt in     out     source               destination

```

 *coRpTitan wrote:*   

> 
> 
> Can you successfully tracepath lunix.se when you are able to ping it?

 

Yes I can. :)

I'll try to disable the ignore_broadcast and see if the host forgets me again.

----------

## coRpTitan

That 2 things I listed is probably set on node from which you get response: 

10:  no reply 

But if you say you are able to successfully tracepath to destination computer after pinging in another direction then I would say there is problem with routing on another device in patch, not on lunix.se or linuxuser.nu.

----------

## Zhaozhou

The successful tracepath, after I've pinged linuxuser.se, would include "10:  linfast53.bitnet.nu (217.31.187.53)", which is me, lunix.se... which obviously does not ignore ICMP packages.

My tracepath to linuxuser.se would also include my default route (217.31.187.1) right there in between, which according to nmap is a cisco switch, however, I can ping this device, so it can't ignore _all_ ICMP packages.

EDIT:

It seems linuxuser.se can ping 217.31.187.1 as well, but not me, which would be the next hop. So my default route can't find me?

----------

## Zhaozhou

I have no idea why, but I disabled "Socket and Networking Security Hooks" in the Security options in the kernel, and now it works again.

Too much magic going on in that feature I guess.

But thanks a bunch, it was truly helpful!

----------

