1 Managing Multiple Internet Connections with Shorewall Tom Eastep Linuxfest Northwest April 24-25, 2010
2 Agenda Introduction Routing Refresher Introduction to Policy Routing Policy Routing and Shorewall Monitoring Link Status and Reacting to Failures Q&A
3 Introduction About Me I am the creator of Shorewall I work for Hewlett-Packard My job there has nothing to do with IP Networking Shorewall development is not supported by HP This presentation is my own and does not represent HP in any way.
4 Introduction Having two or more internet links in a SOHO environment is becoming more common. Largely a routing problem
5 Introduction Requirements Optional load balancing Control which link to use for particular traffic Failover
6 Introduction Requirements Shorewall can meet these requirements but a basic understanding of policy routing on Linux can make things go smoother.
7 Routing Refresher - Tables Routing is directed by a routing table A table entry contains (among other things): Destination Network (may be a /32, in which case it is called a host route) Interface Gateway (Optional) Usually includes a host IP address If omitted, the destination network is connected directly to the interface. Otherwise, specifies the next hop; packets are sent to the next hop using the Gateway's Layer 2 Address
8 Routing Refresher Routing Tables Example from a Desktop Linux System ip route ls /24 dev eth0 proto kernel scope global src metric /16 dev eth0 proto kernel scope link metric 1000 default via dev eth0 proto static Ordered by most-specific to least-specific via denotes the gateway default is an alias for /0 which matches any IP address First match determines routing
9 Routing Refresher Routing Tables Point-to-point routes don't need to specify an IP address for the gateway: default via dev ppp0
10 Routing Refresher Routing Vs. Rules Routing determines where packets go Firewall filtering determines if they are allowed to go there or not DNAT firewall rules can change the destination address in packets but that occurs before routing (nat PREROUTING chain on Linux). SNAT firewall rules can change the routing of response packets.
11 Routing Refresher Response Packets Response packets are not like carrier pidgons they don't instinctively know their way home Response packets can take a route other than the reverse of the corresponding request packet's route (asymmetric routing) The route taken by a response packet must be through all routers that perform DNAT on request packets. In the context of this talk, a response packet should go out the interface that the corresponding request arrived through
12 Policy Routing Linux Policy Routing Multiple Routing Tables Referred to by number and optionally by name /etc/iproute2/rt_tables gives the correspondence between name and number Routing Rules Ordered by priority If a packet matches a rule, it is routed using the related table If not routed by that table, the next rule is tested Always Enabled
13 Policy Routing - Example Example of Default /etc/iproute2/rt_tables # # reserved values # 255 local 254 main 253 default # # local # Example of Default Routing Rules ip rule ls 0: from all lookup local 32766: from all lookup main 32767: from all lookup default
14 Policy Routing Example Continued local Routing Table ip route ls table local broadcast dev lo proto kernel scope link src broadcast dev eth0 proto kernel scope link src local dev eth0 proto kernel scope host src broadcast dev eth0 proto kernel scope link src broadcast dev lo proto kernel scope link src local dev lo proto kernel scope host src local /8 dev lo proto kernel scope host src default Routing Table is normally empty ip route ls table default
15 Policy Routing Routing Rules Routing rules allow assigning a packet to a table based on its firewall mark (fwmark) value (Netfilter's Swiss Army Knife ) Other routing keys are: Input Interface Source Address (net or host) Destination Address (net or host) TOS
16 Policy Routing Balanced Routes Route with multiple next-hop gateways Round-robin assignment by connection Allows multiple default routes from a single system to be load-balanced Given that it is strictly round-robin, balancing isn't perfect.
17 Policy Routing Balanced Routes Example 1 (ppp devices): default Example 2 default nexthop via dev ppp0 weight 1 nexthop via dev ppp1 weight 2 nexthop via dev eth1 weight 1 nexthop via dev eth2 weight 2 weight > 1 causes duplication of the route in the list so it is assigned more often.
18 Requirements Load balancing Failover Assign link to use for particular traffic
19 Shorewall and Policy Routing Shorewall Multi-ISP feature allows you to define: Multiple Additional Routing Tables Balanced Routes (with weights) Assignment of particular connections to a specific table. Geared toward multiple internet links from a single firewall/gateway Other uses are possible
20 Shorewall Provider Each Shorewall (Internet Service) Provider: Is normally associated with an internet link Again, there are other uses described on the web site Has it's own routing table Defines a next-hop gateway Typically has an fwmark value associated with it. Defined by an entry in /etc/shorewall/providers Up to 255 providers may be defined
21 Shorewall Providers Simple Example Two internet links balanced #PROVIDER NUM MARK DUP INTERFACE GATEWAY OPTIONS COPY ISP1 1 1 main eth0 detect balance,track eth2 ISP2 2 2 main eth1 detect balance,track eth2 MARK=1 causes any packet with firewall mark value 1 to be routed using the ISP1 routing table. 'DUP=main' causes the main routing table to be copied to create the provider's table 'COPY=eth2' means that only routes out of eth2 are to be copied. 'track' insures that a response goes out the interface that the corresponding request entered through
22 Shorewall Providers Simple Example #PROVIDER NUM MARK DUP INTERFACE GATEWAY OPTIONS COPY ISP1 1 1 main eth0 detect balance,track eth2 ISP2 2 2 main eth1 detect balance,track eth2 'balance' causes a balanced default route to replace the default route in the main table. 'balance=n' causes the nexthop weight to be set to n. 'track' locks connections to providers using connection marks. Can be made the default by setting TRACK_PROVIDERS=Yes in shorewall.conf.
23 Shorewall Providers Simple Load-balancing Example #PROVIDER NUM MARK DUP INTERFACE GATEWAY OPTIONS COPY ISP1 1 1 main eth0 detect balance,track eth2 ISP2 2 2 main eth1 detect balance,track eth2 Generated Routing Rules 0: from all lookup local 10001: from all fwmark 1 lookup ISP : from all fwmark 2 lookup ISP : from addr-of-eth0 lookup ISP : from addr-of-eth1 lookup ISP : from all lookup main 32767: from all lookup default ISP1 Routing Table dev eth2 scope link src /24 dev eth2 proto kernel scope link src default via dev eth2 src
24 Shorewall Providers - Selecting Provider for a Connection Although the routes are 'balanced', you still have control over which interface is used: PREROUTING entries in /etc/shorewall/tcrules. Use provider 1 except for outgoing mail and SSH #MARK/ SOURCE DEST PROTO DEST #CLASSIFIER PORT(S) 1:P - - 2:P - - TCP 22,25 In this configuration, forwarded traffic never goes through the main routing table. May also use /etc/shorewall/route_rules which will be mentioned again later
25 Shorewall Providers VPN Issues The previous configuration works badly if the main table is being dynamically altered by VPN servers and clients because the provider tables don't get updated. Example: SSH to a host connected through a VPN. Solution 1: Add a route rule in /etc/shorewall/route_rules to route VPN traffic through the main table #SOURCE DEST PROVIDER PRIORITY # #OpenVPN clients # /24 main 1000
26 Shorewall Providers USE_DEFAULT_RT=Yes Solution 2: USE_DEFAULT_RT=Yes in shorewall.conf and use this provider configuration #PROVIDER NUM MARK DUP INTERFACE GATEWAY OPTIONS COPY ISP eth track - ISP eth track -
27 Shorewall Providers USE_DEFAULT_RT Causes a balanced default route to be added to the default table rather than to the main table (balance=1 is the default for all providers) All traffic traverses the main table; even traffic marked to be routed out of a particular provider. Only works well with static gateways because the main table has no default route in it. Dynamic IP management like DHCP wants to add a default route in the main table
28 Shorewall Providers USE_DEFAULT_RT Routing Rules when USE_DEFAULT_RT=Yes 0: from all lookup local 999: from all lookup main 10000: from all fwmark 1 lookup ISP : from all fwmark 2 lookup ISP : from addr-of-eth0 lookup ISP : from addr-of-eth1 lookup ISP : from all lookup default
29 Shorwall Providers Fallback Providers shorewall.net configuration: A fast cable link (Comcast) with a single dynamic IP address A slower DSL link (Avvanta) with 5 static IP addresses OpenVPN servers running on both Goal: Use the Comcast link except where a static IP address is required or when the Comcast link is down.
30 Shorwall Providers Fallback Providers Solution make Avvanta a 'fallback' provider. #PROVIDER NUM MARK DUP IFACE GATEWAY OPTIONS COPY Comcast 1 1 main eth0 detect balance eth2,\ venet0,\ tun+ Avvanta 2 2 main eth1 detect fallback eth2,\ venet0,\ tun+ The dynamic default route for Comcast is in the main table (balance) The static default route for Avvanta is in the default table (fallback) TRACK_PROVIDERS=Yes in shorewall.conf causes 'track' to be assumed on both providers.
31 Shorwall Providers Fallback Providers /etc/shorewall/route_rules: #SOURCE DEST PROVIDER PRIORITY # #OpenVPN clients # /24 main 1000 # # Servers in OpenVZ containers routes are generated by OpenVZ # main main 1001 # # All 5 static IP addresses # /30 - Avvanta Avvanta 26000
32 Shorwall Providers Fallback Providers Generated routing rules: 0: from all lookup local <== Default 1001: from all to lookup main <== route_rules 1001: from all to lookup main <== route_rules 10000: from all fwmark 1 lookup Avvanta <== track 10001: from all fwmark 2 lookup Comcast <== track 20256: from lookup Comcast <== no 'loose' 26000: from /30 lookup Avvanta <== route_rules 26000: from lookup Avvanta <== route_rules 32766: from all lookup main <== Default 32767: from all lookup default <== Default Note: is the dynamic address of eth0
33 Dead Gateway Detection Failover Most SOHO ISP accounts don't offer routing protocol support. Linux lacks passive Dead Gateway Detection (DGD) without kernel patching. Solution Active DGD (pinging) Not a great choice but all we have Shorewall isn't a daemon so it can't do the pinging itself Solution: Link Status Monitor (LSM)-
34 Dead Gateway Detection Failover LSM keeps track of which interfaces are up and down and calls a user-provided script when an interface changes state. The isusable Shorewall extension script (user exit) can be used to help decide if the interface is up or down. The user-provider LSM script creates status files in /var/lib/shorewall/ which can be interrogated by the Shorewall isusable helper.
35 Dead Gateway Detection Failover The key is to arrange your Shorewall configuration such that if either of the interfaces fails or comes back up, a simple shorewall restart -f command will succeed On status change, LSM script re-writes status file(s) and does 'shorewall restart -f' Provider interfaces should be defined as optional in /etc/shorewall/interfaces When an optional provider is not available, its routing table and rules are deleted by restart.
36 Dead Gateway Detection LSM Log lsm: link Avvanta down event lsm: name = Avvanta, replied = 80, waiting = 20, timeout = 20, late reply = 0, cons rcvd = 0, cons wait = 2, cons miss = 2, avg_rtt = , seq = 7884 lsm: seq * lsm] used lsm: wait lsm: replied lsm: timeout lsm: error
37 Dead Gateway Detection LSM Hi, Connection Avvanta is now down. Following parameters were passed: newstate = down name = Avvanta checkip = device = eth2 warn_ = Packet counters: replied = 80 packets replied waiting = 20 packets waiting for reply timeout = 20 packets that have timed out (= packet loss) reply_late = 0 packets that received a reply after timeout cons_rcvd = 0 consecutively received replies in sequence cons_wait = 1 consecutive packets waiting for reply cons_miss = 1 consecutive packets that have timed out avg_rtt = average rtt, notice that waiting and timed out packets have rtt = 0 when calculating this Your LSM Daemon
38 More Information
39 Q & A
40 Backup Slides
41 Hairpinning Example (Why the NAT rule is Important) Router and /24 To: :80 From: :55555 To: :80 From: :55555 To: :55555 From: : / /24
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