No internet while downloading torrents – seems dns related

bittorrentdns

I have a very peculiar problem regarding the internet connection while downloading torrents.
Before you conclude that I should "reduce the # of half-open & user connections", let me say I have done that.(10 half open connections, 20 users, it still doesn't work , and I don't get any downloading going on anymore).

I should also say that QoS shouldn't be necessary. usually in my experience with downloading torrents (in linux/windows nad mac) the internet connection was shared among all processes. Here it seems like torrents are chewing on all the available bandwidth. (Shouldn't the kernel be divide time among processes that request to send/receive packages?)

Finally, I should say that this problem started appearing after I updated to slack 64bit v14 (from v13.37).

So, the actual problem seems to be related with dns server not responding once I start download with ktorrent or rtorrent. And no webpages load anymore. torrent will be downloading at reasonable speed, but no websites will be loading.
so "nslookup" and "dig" will tell me that the dns server (which btw is located on the same pc) was not found :

nslookup facebook.com
;; connection timed out; no servers could be reached

and 

nass@stargaze:~$ dig !$
dig facebook.com
; <<>> DiG 9.9.1-P3 <<>> facebook.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: SERVFAIL, id: 26154
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;facebook.com.                  IN      A

;; Query time: 1125 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Fri Aug  2 01:14:46 2013
;; MSG SIZE  rcvd: 41

restarting the dns server (bind) while the torrent is running will generally NOT fix things, even though sometimes I have seen this happening.
stopping the dns , deleting any *.jnl files that were generated and restarting seems to work, but again it may not be always. (I don't have a repeated pattern for this case).
I can't say I have found "a way" to get the internet back.

  • usually closing ktorrent and waiting for a few secs could even fix the internet on its own.
  • Other times closing the ktorrent client and restarting dns server would work faster than the previous case.
  • sometimes repeated restarts would NOT get the dns back working (bu waiting for a few mins would fix the prob)
  • recently I have started stopping named, deleting *.jnl files and restarting it. This has had 100% success in my (only 2) trials.

the firewall log, the /var/log/messages/ and named's logs, doesn't register anything weird.

I have not used tcpdump, wireshark, netstat so I don't know if I can use this tools to identify …something! Could anyone help with this?

Since this problem seems to be related -primarily- to the dns server, I am appending my dns file and explain my pc's configuration abit:

so ADSL internet arrives in the modem (provided by ISP, always on, even when I don't have internet). Modem is connected to this pc on eth1 where I am downloading torrents . this pc is my home network and file server (and my desktop when I am away – i connect using nx). It is running iptables, dns, & squid servers (among others). Then from eth0 of this pc, the wifi and intranet switch are fed. The squid is running on a transparent configuration but it shouldn't interfere with torrent traffic as this is done on different ports (rather than the port 80).

So initially, I am attaching my named.conf, in an attempt to get feedback on it (perhaps some logically erroneous config that is not caught from the webmin named config file checker – with which I have repeatedly verified that the named.conf file is syntactically correct)

named.conf is here

If this is fine, is there someway I could start using tcpdump (and any other tool) under your guidance to collect info as to what might be causing this?

Thank you extremely much for your help 🙂

EDIT: my /etc/resolv.conf looks like:

domain skails.home
nameserver 127.0.0.1

Best Answer

(Shouldn't the kernel be divide time among processes that request to send/receive packages?)

The typical situation with having slow or no Internet with something like Bittorrent saturating your connection is that incoming traffic on your upstream (which is usually lower than your downstream on most residential connections) is crowded out. So incoming TCP ACKs are not received timely, and connections timeout on their end, and then eventually your end.

One thing I learned from studying QoS is that there is no such thing as QoS on incoming traffic, because you can't control what's being sent to you. You can only really QoS/divide/share outgoing traffic. You can see the current Linux QoS configuration with tc - but be warned, tc is very complicated.

It's possible that a single connection could saturate your incoming bandwidth and crowd out incoming TCP ACKs, causing slowdowns, drops, etc. The number of concurrent connections doesn't really matter.

You probably need to set the total amount of bandwidth your Bittorrent program uploads to just under your maximum upstream, like 8Kbit/sec below what you know is the speed of your upstream. You also might want to look into Wondershaper if you feel like going down the rabbit hole that is QoS on Linux.

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