[tor-relays] Detecting Network Attack [re: exit synflooded]

[Igor Mitrofanov]

After reading every paper and post on sysctl.conf and iptables tuning
I could find, and reading some kernel code, I have come to a
conclusion that, while there are a few settings to tune (can share
mine, but your mileage *will* vary), most of the defaults are actually
not broken in the latest kernels, while a lot of tuning
recommendations online are outdated.

I enjoyed this guide a lot:
https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/6/html-single/performance_tuning_guide/index.
It is written for Red Hat, but contains quite a few insights
applicable to all Linux distributions.

- Igor


On Sat, Nov 25, 2017 at 3:14 PM, Dhalgren Tor <dhalgren.tor at gmail.com> wrote:
>>Well, it's still going on, and is pretty much ruining Libero :( .  Running CentOS 6, here.
>>
>>When it's happening it can look like this:
>>
>># netstat -n | grep -c SYN
>>17696
>
> I run a fast exit and can offer some advice:
>
> 1) mitigate bug #18580 (also #21394); is a DNS denial-of-service and
> could be the problem.  either upgrade to 0.3.2.4+ or edit resolve.conf
> per https://trac.torproject.org/projects/tor/wiki/doc/DnsResolver#TuningeventdnscomponentofTorDaemon
>
> also check out https://arthuredelstein.net/exits/
>
> 2) if you continue to experience excessive outbound scanning SYNs,
> I've found that simply enabling connection tracking helps by
> implicitly limiting the rate a which connections can originate.  If an
> iptables "-m state --state ESTABLISHED,RELATED -j ACCEPT" rule exists
> it will turn it on or you could modprobe the module if you don't want
> to configure incoming connection rules.
>
> some useful sysctl.conf changes, run sysctl -p after or reboot
>
> # Tor Exit tuning.
> net.ipv4.ip_local_port_range = 1025 65535
> net.ipv4.tcp_synack_retries = 2
> net.ipv4.tcp_max_tw_buckets = 4194304
> net.ipv4.tcp_tw_recycle = 1
> net.ipv4.tcp_fin_timeout = 30
> net.ipv4.tcp_keepalive_time = 300
> net.ipv4.tcp_keepalive_intvl = 10
> net.ipv4.tcp_keepalive_probes = 3
> net.netfilter.nf_conntrack_tcp_timeout_established = 1000
> net.netfilter.nf_conntrack_checksum = 0
>
> you might see many messages:
>
> kernel: nf_conntrack: table full, dropping packet
>
> which indicates no connection table slots were available for an
> outbound connection and that rate limiting is effected
>
> state of connection tracking appears in /proc/net/nf_conntrack
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