一.iptables简介
1.iptables数据包处理流程
以本机为目的的包，由上至下，走左边的路
本机产生的包，从local process开始走左边的路
本机转发的包，由上至下走右边的路
简化流程如下：
2.iptables表结构
在neutron中主要用到filter表和nat表
filter表:
Chain INPUT
Chain FORWARD
Chain OUTPUT
filter表用于信息包过滤，它包含INPUT、OUTPUT和FORWARD 链。
nat表:
Chain PREROUTING
Chain OUTPUT
Chain POSTROUTING
nat表用于网络地址转换，PREROUTING链由指定信息包一到达防火墙就改变它们的规则所组成，而 POSTROUTING 链由指定正当信息包打算离开防火墙时改变它们的规则所组成。
More：
Traversing of tables and chains
Linux Firewalls Using iptables
二.l3 agent消息处理
_rpc_loop --- _process_router --- _router_added
--- process_router
--- _router_removed
--- _process_router_delete --- _router_removed
在上面几个方法中，会涉及到iptables的处理。
三.iptables_manager初始化
iptables_manager的初始化是在class IptablesManager中完成的，它对iptables的链进行了包装。
源码目录：neutron/neutron/agent/linux/iptables_manager.py
主要操作：
新建一个neutron-filter-top链，这个是没有包装的，加在原生的FORWARD和OUTPUT链上。
对filter表的INPUT，OUTPUT，FORWARD链进行包装，将到达原链的数据包转发到包装链，还增加一个包装的local链。
对于nat表，PREROUTING，OUTPUT，POSTROUTING链进行包装，另外在POSTROUTING链之后加了snat链。
代码分析：
对于l3 agent，binary_name是neturon-l3-agent。
filter表的操作：
增加一个链neutron-filter-top，增加规则：
-A FORWARD -j neutron-filter-top
-A OUTPUT -j neutron-filter-top
增加一个包装链neutron-l3-agent-local，增加规则：
-A neutron-filter-top -j neutron-l3-agent-local
# Add a neutron-filter-top chain. It's intended to be shared
# among the various nova components. It sits at the very top
# of FORWARD and OUTPUT.
for tables in [self.ipv4, self.ipv6]:
tables['filter'].add_chain('neutron-filter-top', wrap=False)
tables['filter'].add_rule('FORWARD', '-j neutron-filter-top',
wrap=False, top=True)
tables['filter'].add_rule('OUTPUT', '-j neutron-filter-top',
wrap=False, top=True)
tables['filter'].add_chain('local')
tables['filter'].add_rule('neutron-filter-top', '-j $local',
wrap=False)
包装IPv4和IPv6 filter表的INPUT，OUTPUT，FORWARD链，以及IPv4 nat表的PREROUTING，OUTPUT，POSTROUTING链。
将到达原链的数据包转发到包装链：
# Wrap the built-in chains
builtin_chains = {4: {'filter': ['INPUT', 'OUTPUT', 'FORWARD']},
6: {'filter': ['INPUT', 'OUTPUT', 'FORWARD']}}
if not state_less:
self.ipv4.update(
{'nat': IptablesTable(binary_name=self.wrap_name)})
builtin_chains[4].update({'nat': ['PREROUTING',
'OUTPUT', 'POSTROUTING']})
for ip_version in builtin_chains:
if ip_version == 4:
tables = self.ipv4
elif ip_version == 6:
tables = self.ipv6
for table, chains in builtin_chains[ip_version].iteritems():
for chain in chains:
tables[table].add_chain(chain)
tables[table].add_rule(chain, '-j $%s' %
(chain), wrap=False)
包装链neutron-l3-agent-INPUT，neutron-l3-agent-OUTPUT，neutron-l3-agent-FORWARD，添加规则：
-A INPUT -j neutron-l3-agent-INPUT
-A OUTPUT -j neutron-l3-agent-OUTPUT
-A FORWARD -j neutron-l3-agent-FORWARD
nat表的操作：
（承上面的代码）
包装链neutron-l3-agent-PREROUTING，neutron-l3-agent-OUTPUT，neutron-l3-agent-POSTROUTING，添加规则：
-A PREROUTING -j neutron-l3-agent-PREROUTING
-A OUTPUT -j neutron-l3-agent-OUTPUT
-A POSTROUTING -j neutron-l3-agent-POSTROUTING
nat表中添加neutron-postrouting-bottom链，增加规则：
-A POSTROUTING -j neutron-postrouting-bottom
nat表中添加包装链neutron-l3-agent-snat，增加规则：
-A neutron-postrouting-bottom -j neutron-l3-agent-snat
nat表中添加包装链neutron-l3-agent-float-snat，增加规则：
-A neutron-l3-agent-snat -j neutron-l3-agent-float-snat
代码如下：
if not state_less:
# Add a neutron-postrouting-bottom chain. It's intended to be
# shared among the various nova components. We set it as the last
# chain of POSTROUTING chain.
self.ipv4['nat'].add_chain('neutron-postrouting-bottom',
wrap=False)
self.ipv4['nat'].add_rule('POSTROUTING',
'-j neutron-postrouting-bottom',
wrap=False)
# We add a snat chain to the shared neutron-postrouting-bottom
# chain so that it's applied last.
self.ipv4['nat'].add_chain('snat')
self.ipv4['nat'].add_rule('neutron-postrouting-bottom',
'-j $snat', wrap=False)
# And then we add a float-snat chain and jump to first thing in
# the snat chain.
self.ipv4['nat'].add_chain('float-snat')
self.ipv4['nat'].add_rule('snat', '-j $float-snat')
四.l3 agent代码中关于iptables的处理
1._router_added
_router_added方法，创建和metadata相关的iptables规则：
def _router_added(self, router_id, router):
ri = RouterInfo(router_id, self.root_helper,
self.conf.use_namespaces, router)
self.router_info[router_id] = ri
if self.conf.use_namespaces:
self._create_router_namespace(ri)
for c, r in self.metadata_filter_rules():
ri.iptables_manager.ipv4['filter'].add_rule(c, r)
for c, r in self.metadata_nat_rules():
ri.iptables_manager.ipv4['nat'].add_rule(c, r)
ri.iptables_manager.apply()
super(L3NATAgent, self).process_router_add(ri)
if self.conf.enable_metadata_proxy:
self._spawn_metadata_proxy(ri.router_id, ri.ns_name)
1.metadata_filter_rules方法中，如果enable_metadata_proxy为True，增加规则
def metadata_filter_rules(self):
rules = []
if self.conf.enable_metadata_proxy:
rules.append(('INPUT', '-s 0.0.0.0/0 -d 127.0.0.1 '
'-p tcp -m tcp --dport %s '
'-j ACCEPT' % self.conf.metadata_port))
return rules
然后在filter表中增加这条规则，接受所有从外面进来到达metadata_port端口的数据包：
-A neutron-l3-agent-INPUT -s 0.0.0.0/0 -d 127.0.0.1 -p tcp -m tcp –dport 9697 -j ACCEPT
2.metadata_nat_rules方法，如果enable_metadata_proxy为True，增加规则
def metadata_nat_rules(self):
rules = []
if self.conf.enable_metadata_proxy:
rules.append(('PREROUTING', '-s 0.0.0.0/0 -d 169.254.169.254/32 '
'-p tcp -m tcp --dport 80 -j REDIRECT '
'--to-port %s' % self.conf.metadata_port))
return rules
然后在nat表中增加这条规则做DNAT转换，在route之前，将虚拟机访问169.254.169.254端口80的数据包重定向到metadat_port端口：
-A neutron-l3-agent-PREROUTING -s 0.0.0.0/0 -d 169.254.169.254/32 -p tcp -m tcp –dport 80 -j REDIRECT –to-port 9697
再调用iptables_manager.apply()方法，应用规则：
iptables-save -c ，获取当前所有iptables信息；
iptables-restore -c ，应用最新的iptables配置；
2.process_router
process_router方法：
1.perform_snat_action，为external gateway处理SNAT规则
def _handle_router_snat_rules(self, ri, ex_gw_port, internal_cidrs,
interface_name, action):
# Remove all the rules
# This is safe because if use_namespaces is set as False
# then the agent can only configure one router, otherwise
# each router's SNAT rules will be in their own namespace
ri.iptables_manager.ipv4['nat'].empty_chain('POSTROUTING')
ri.iptables_manager.ipv4['nat'].empty_chain('snat')
# Add back the jump to float-snat
ri.iptables_manager.ipv4['nat'].add_rule('snat', '-j $float-snat')
# And add them back if the action if add_rules
if action == 'add_rules' and ex_gw_port:
# ex_gw_port should not be None in this case
# NAT rules are added only if ex_gw_port has an IPv4 address
for ip_addr in ex_gw_port['fixed_ips']:
ex_gw_ip = ip_addr['ip_address']
if netaddr.IPAddress(ex_gw_ip).version == 4:
rules = self.external_gateway_nat_rules(ex_gw_ip,
internal_cidrs,
interface_name)
for rule in rules:
ri.iptables_manager.ipv4['nat'].add_rule(*rule)
break
ri.iptables_manager.apply()
先清空nat表的neutron-l3-agent-POSTROUTING链和neutron-l3-agent-snat链；
再在nat表的neutron-l3-agent-snat链添加规则：
-A neutron-l3-agent-snat -j neutron-l3-agent-float-snat
然后对应add_rules操作，则处理external_gateway_nat_rules，处理完后在nat表中添加规则：
def external_gateway_nat_rules(self, ex_gw_ip, internal_cidrs,
interface_name):
rules = [('POSTROUTING', '! -i %(interface_name)s '
'! -o %(interface_name)s -m conntrack ! '
'--ctstate DNAT -j ACCEPT' %
{'interface_name': interface_name})]
for cidr in internal_cidrs:
rules.extend(self.internal_network_nat_rules(ex_gw_ip, cidr))
return rules
规则命令如下：
-A neutron-l3-agent-POSTROUTING ! -i qg-XXX ! -o qg-XXX -m conntrack ! –ctstate DNAT -j ACCEPT
这条命令的意思是除了出口和入口都为qg-XXX，（qg即是router上的外部网关接口）匹配除了DNAT之外的其他状态。
然后处理internal_network_nat_rules：
def internal_network_nat_rules(self, ex_gw_ip, internal_cidr):
rules = [('snat', '-s %s -j SNAT --to-source %s' %
(internal_cidr, ex_gw_ip))]
return rules
规则命令如下：
-A neutron-l3-agent-snat -s internal_cidr -j SNAT –to-source ex_gw_ip
2.process_router_floating_ip_nat_rules方法，处理floating ip，作SNAT/DNAT转换。
def process_router_floating_ip_nat_rules(self, ri):
"""Configure NAT rules for the router's floating IPs.
Configures iptables rules for the floating ips of the given router
"""
# Clear out all iptables rules for floating ips
ri.iptables_manager.ipv4['nat'].clear_rules_by_tag('floating_ip')
# Loop once to ensure that floating ips are configured.
for fip in ri.router.get(l3_constants.FLOATINGIP_KEY, []):
# Rebuild iptables rules for the floating ip.
fixed = fip['fixed_ip_address']
fip_ip = fip['floating_ip_address']
for chain, rule in self.floating_forward_rules(fip_ip, fixed):
ri.iptables_manager.ipv4['nat'].add_rule(chain, rule,
tag='floating_ip')
ri.iptables_manager.apply()
def floating_forward_rules(self, floating_ip, fixed_ip):
return [('PREROUTING', '-d %s -j DNAT --to %s' %
(floating_ip, fixed_ip)),
('OUTPUT', '-d %s -j DNAT --to %s' %
(floating_ip, fixed_ip)),
('float-snat', '-s %s -j SNAT --to %s' %
(fixed_ip, floating_ip))]
先清理nat表所有的floationg ip规则；然后floating_forward_rules方法,在nat表中处理floating ip和fixed ip的NAT转换：
具体规则如下：
-A neutron-l3-agent-PREROUTING -d floating_ip -j DNAT –to fixed_ip
-A neutron-l3-agent-OUTPUT -d floating_ip -j DNAT –to fixed_ip
-A neutron-l3-agent-float-snat -s fixed_ip -j SNAT –to floating_ip
3._router_removed
_router_removed方法，删除和metadata相关的规则：
def _router_removed(self, router_id):
ri = self.router_info.get(router_id)
if ri is None:
LOG.warn(_("Info for router %s were not found. "
"Skipping router removal"), router_id)
return
ri.router['gw_port'] = None
ri.router[l3_constants.INTERFACE_KEY] = []
ri.router[l3_constants.FLOATINGIP_KEY] = []
self.process_router(ri)
for c, r in self.metadata_filter_rules():
ri.iptables_manager.ipv4['filter'].remove_rule(c, r)
for c, r in self.metadata_nat_rules():
ri.iptables_manager.ipv4['nat'].remove_rule(c, r)
ri.iptables_manager.apply()
if self.conf.enable_metadata_proxy:
self._destroy_metadata_proxy(ri.router_id, ri.ns_name)
del self.router_info[router_id]
self._destroy_router_namespace(ri.ns_name)
五.总结
l3 agent初始化完成后，iptables处理流程如下：