關於RHEL6.3 sysctl 一個錯誤問題的解決

筆者在RHEL6.3中執行sysctl -p的時候發現輸出出現以下錯誤

# sysctl -p
net.ipv4.ip_forward = 0
net.ipv4.conf.default.rp_filter = 1
net.ipv4.conf.default.accept_source_route = 0
kernel.sysrq = 0
kernel.core_uses_pid = 1
net.ipv4.tcp_syncookies = 1
error: "net.bridge.bridge-nf-call-ip6tables" is an unknown key
error: "net.bridge.bridge-nf-call-iptables" is an unknown key
error: "net.bridge.bridge-nf-call-arptables" is an unknown key
kernel.msgmnb = 65536
kernel.msgmax = 65536
kernel.shmmax = 68719476736
kernel.shmall = 4294967296

經過Google大神點擊打開鏈接後來找到的了原因所在，原來以下3個參數依賴於bridge模塊，該模塊如果沒有加載則會現上面的輸出錯誤

error: "net.bridge.bridge-nf-call-ip6tables" is an unknown key
error: "net.bridge.bridge-nf-call-iptables" is an unknown key
error: "net.bridge.bridge-nf-call-arptables" is an unknown key

加載模塊測試，發現問題解決

# modprobe bridge
# sysctl -p
net.ipv4.ip_forward = 0
net.ipv4.conf.default.rp_filter = 1
net.ipv4.conf.default.accept_source_route = 0
kernel.sysrq = 0
kernel.core_uses_pid = 1
net.ipv4.tcp_syncookies = 1
net.bridge.bridge-nf-call-ip6tables = 0
net.bridge.bridge-nf-call-iptables = 0
net.bridge.bridge-nf-call-arptables = 0
kernel.msgmnb = 65536
kernel.msgmax = 65536
kernel.shmmax = 68719476736
kernel.shmall = 4294967296

有人可能會好奇這三個值得意義所在，我在紅帽官網找到了相關說明點擊打開鏈接

以下大致的意思主要說使用以上3個選項阻止橋接流量獲得通過主機iptables規則，Netfilter是默認情況下啓用了橋樑，如果不阻止會導致嚴重的混亂

netfilter is currently enabled on bridges by default. This means, for example, that IP packets that are forwarded across the bridge are filtered by the iptables FORWARD rules.

In practice, this can lead to serious confusion where someone creates a bridge and finds that some traffic isn't being forwarded across the bridge. Because it's so unexpected that IP firewall rules apply to frames on a bridge, it can take quite some time to figure out what's going on.

The libvirt wiki has this advice:

  http://wiki.libvirt.org/page/Networking#Fedora.2FRHEL_Bridging

  The final step is to configure iptables to allow all traffic to be forwarded 
  across the bridge

  # echo "-I FORWARD -m physdev --physdev-is-bridged -j ACCEPT" > \
       /etc/sysconfig/iptables-forward-bridged
  # lokkit --custom-rules=ipv4:filter:/etc/sysconfig/iptables-forward-bridged
  # service libvirtd reload

  Alternatively, you can prevent bridged traffic getting pushed through the 
  host's iptables rules. In /etc/sysctl.conf add

  # cat >> /etc/sysctl.conf <<EOF
  net.bridge.bridge-nf-call-ip6tables = 0
  net.bridge.bridge-nf-call-iptables = 0
  net.bridge.bridge-nf-call-arptables = 0
  EOF
  # sysctl -p /etc/sysctl.conf

It sucks that people have to do this, especially since it's a very rare user who would be using iptables on a bridge for something useful.

I posted a patch to netdev which would have allowed us to disable it by default in our kernel builds:

  http://patchwork.ozlabs.org/patch/29319/

The conclusion seems to be an agreement that distros should disable this, but using sysctl.conf instead

In the thread Herbert describes a security issue with the current default:

  I still think the risk with bridging is higher, especially
  in the presence of virtualisation.  Consider the scenario where you
  have to VMs on the one host, each with a dedicated bridge with the
  intention that neither should know anything about the other's
  traffic.

  With conntrack running as part of bridging, the traffic can now
  cross over which is a serious security hole.

and goes on to say:

  FWIW I don't really care what we have as the default for bridge
  netfilter.  I just want to make sure that people who do have
  bridge netfilter (and in particular, conntrack + bridge) active
  on their machines are aware of the security implications.  Otherwise
  we'd be negligent.

  As you said distros can change the default regardless of what
  the kernel does.



In summary, I think we should add the following to sysctl.conf:

  net.bridge.bridge-nf-call-ip6tables = 0
  net.bridge.bridge-nf-call-iptables = 0
  net.bridge.bridge-nf-call-arptables = 0