BGP Best Practices for ISPs

Prefix List, AS PATH filters, Bogon Filters, Anycast, Mailing Lists, INOC DBA


Gaurab Raj Upadhaya
Packet Clearing House

What are Best Practices
 Established or known good ways of
doing things.
 Being a good Internet citizen.
 Trying TO restrict damage to your
network and FROM you network
 A lot of RFCs devoted on different BCP,
but here we’ll just cover a few topics

BGP Security

Protocols are vulnerable
 Routing protocols, like BGP, OSPF can
be attacked and weakness exploited
 False Information
 Man-in-middle situation
 Denial of service
 Routing Conditions

BGP Security Issues  Prefix Lists  Why. How and Where  Prefix Lists  Customers  Peers  Upstream  Protocol Authentication  MD5  BGP BCP  Bogons .

Why Prefix Filter? .

learning how to do BGP. we don't do prefix-filters AS1 AS2 We are not happy We are happy with our provider with our provider AS65535 AS65534 .. we can make mistakes AS101 We do filters AS1 We are AS1. We are new ISP.

and may be a /8 AS101 Woo!! no effect AS1 We are AS1. we'll announce our prefixes all in /24s. so we will send the garbage through to peers and customers AS1 AS2 Ouch ! something Which /8. I see wrong ?. Let's see how we can do BGP. we don't do prefix-filters. my traffic no problems not going to this /8 AS65534 5 AS65534 .

this happened with AS 7007 - which was the most visible of all. .  Garbage in – Garbage out. It's REAL.  Frequent Messages on IXes mailing list about hitting max prefix limits. does happen  In 1997.

So.Net Police  Help others and they'll help you . what should we do ?  Don't send Junk  Filter your own advertisement  Filter your customers  Don't Accept Junk  Filter your customers  Filter your peers  Help Others mitigate impact .

How .Prefix Filter .

widely used. BGP Prefix Filtering  Two ways to implement prefix filtering:  Prefix list . not highly scalable  AS-PATH . highly scalable Distribute list is now considered obsolete  Two filtering techniques:  Explicit Permit (permit then deny any)  Explicit Deny (deny then permit any) .easy to use.

0/16 le 32 ip prefix-list rfc1918 deny le 32 ip prefix-list rfc1918 permit 0.0/12 le 32 ip prefix-list rfc1918 deny le 32 ip prefix-list rfc1918 deny le 32 ip prefix-list rfc1918 deny le 32 ip prefix-list rfc1918 deny le 32 ip prefix-list rfc1918 deny le 32 ip prefix-list rfc1918 deny 10.0/0 le 32 . BGP Route Filtering Prefix-List example ip prefix-list rfc1918 deny 0.0. prefix-list rfc1918 in neighbor 198.200 remote-as 65200 neighbor prefix-list rfc1918 out no auto-summary ! .231.210 prefix-list rfc1918 in neighbor 198.210 remote-as 65210 neighbor 198.231.200 prefix-list rfc1918 out neighbor 198.231.32. BGP Filtering Applying the previous prefix-list to peers router bgp 65564 no synchronization neighbor 198.32.

Using AS-PATH filters .

0.0 mask 255.255.200 remote-as 65200 neighbor 198. Using AS-PATH filters  Filter routes based on AS path  Applied same way as prefix-list filters  AS-PATH syntax is different.200 filter-list 1 out neighbor filter-list 200 in ! ip as-path access-list 1 permit ^65564$ ip as-path access-list 200 permit ^65200$ .12. can use regular expressions  Example Configuration: router bgp 65564 network 10.0 neighbor

end. Match one character * Match any number of preceding expression + Match at least one of preceding expression ^ Beginning of line $ End of line _ Beginning. Regular Expressions • Most router OS uses Unix regular expressions . white-space. brace | Or () brackets to contain expression .

Regular Expressions examples  Examples .+
 match at least one character ^$
 match routes local to this AS _3856$

 originated by AS3856 ^3856_

 received from AS3856 _3856_

 via AS3856 _3856_42_
 via AS3856 and AS42 _(3856_)+
 multiple AS3856 in sequence
 (used to match AS-PATH prepends) .*
 match anything .


 Match anything of origin AS42 
 and passed through AS2914 . Regular Expressions examples • Complex Examples ^[0-9]+$
 Match AS_PATH length of one ^[0-9]+_[0-9]+$

 Match AS_PATH length of two ^[0-9]*_[0-9]+$
 Match AS_PATH length of one or two ^[0-9]*_[0-9]*$
 Match AS_PATH length of one or two 
 (will also match zero) ^[0-9]+_[0-9]+_[0-9]+$
 Match AS_PATH length of three _(3856|42)_
 Match anything which has gone 
 through AS42 or AS3856 _2914(_.

Where to Prefix Filter? .

Where to Prefix Filter? "Customers "Ingress / Egress Filter to / from upstream "Upstream "Ingress / Egress AS101 "Use default if Customer s single homed Filter Peers AS1 "Peers Peers Customer s "Ingress and AS2 AS1 Egress at all points of Upstream peering Upstream Filter filter upstrea Upstream only default m AS65535 AS65534 .

What to Prefix Filter? Special Use Addresses (RFC3330) and Bogons . and multicast blocks  Certain RFC3330 addresses: 
 http://www.txt  BGP should have filters applied so that these routes are not advertised to or propagated through the Internet . Special Use Addresses  There are routes that should NOT be routed on the Internet  RFC 1918 and “Martian” networks  127.0.

0.0.0/— RFC 1918 private addresses  169.0/8—Host loopback  and 0.0. 172.0/24—TEST-NET for documentation  10. Special Use Addresses  Quick review  0.0/32—Default and broadcast  127.254.0/16—End node auto-configuration . and

0/0 ge 25 ip prefix-list deny-sua permit 0.0.0/8 le 32 ip prefix-list deny-sua deny le 32 ip prefix-list deny-sua deny le 32 ip prefix-list deny-sua deny le 32 ip prefix-list deny-sua deny 224.0/8 le 32 ip prefix-list deny-sua deny le 32 ip prefix-list deny-sua deny 192.0/0 le 32 . Special Use Addresses ip prefix-list deny-sua deny 0.16.0/12 le 32 ip prefix-list deny-sua deny 192.0/3 le 32 ip prefix-list deny-sua deny

 Filters should be applied on Ingress / Egress of your AS for all other address range  Only these blocks of IPv4 addresses should be visible and used on the Internet. Bogons  IANA publishes the list of IP Address that have been assigned to RIRs and end-users  http://www.

255.255  Not all IP Address are allocated by IANA  Few are not to be used on the public internet (RFC 1918)  Few blocks are reserved (> 223/8 )  The IP Address that are not supposed to be seen on the Internet are known as ‘Bogons’.0 to 255. Bogons  The IP Address is 32 Bits.0.0. The range is  0. sometime also referred to as “Martian” .255.

ISPs should not route them  Many DoS attacks originate from these unallocated address blocks. sanog. afnog etc. so it’s also good security  ISPs can get the bogons list from IANA and set up their prefix filters to route traffic  When new address is assigned.)  Same procedure is also applied for IPv6 address space . Bogons  To be a good Internet citizen. apops. nanog. generally the announcements are sent far and wide on many different mailing lists (eg.

0 null0  BGP prefix-list for Unallocated blocks  neighbor x.x.0.0.0 mask 255.x.x prefix-list bogons  ip prefix-list bogons seq 5 deny 10.2.0/0 le 32  ip prefix-list bogons seq 10 deny 127.0. Router Configuration  This is how you configure your Cisco routers in your networks  Static Route to Null0.255.0. good idea for RFC1918 space  ip route 192.255.0/0 le 32 .0.

new people start managing the network  New engineers may have no clue on why those configurations are in there  The network is working. so let’s leave it there triumphs over further digging . Router Configuration  The problem with the static configuration is many fold  The Bogons list keeps on changing  People move into new jobs.

then you can either  Either set the next-hop for the bogons to a static IP address and help in research work  Or set the next-hop for the bogons to a static IP address that is statically routed to Null0 . Connecting to at least two is recommended.  You can run a BGP session with the Bogon route server  You receive the bogons list through BGP. There are four Bogons Route Server. Bogon Route Server Project  Thus. the Bogon Route Server Project  Provides bogons over a eBGP Multihop session.

cymru.x remote-as 65333 
 neighbor x.x.x.x maximum-prefix 100 threshold 90 ! ip bgp-community new-format ! ip route 192.x.x ebgp-multihop 255 
 neighbor x.x description <your description> 
 neighbor x.x.x.0. Configuring Routers for Bogon RS  Full Details are available at bogon route-servers 
 match community 10 .x route-map CYMRUBOGONS in 
 neighbor x.x password <your password> 
 neighbor x.x.x.x.html router bgp <your asn> 
 neighbor x.1 255.2.x prefix-list cymru-out out 
 neighbor x.x.x.255.255 null0 !
 ip community-list 10 permit 65333:888 ! route-map CYMRUBOGONS permit 10 
 description Filter bogons learned from cymru.255.x.

com. . if you have one (not mandatory)  The session is set up through eBGP multihop with a private ASN.  contact team-cymru@cymru.  When requesting a peering session. Your router must at least support these requirements. How do I set up the peering ?  To peer with the Bogon Route Server. It currently has 95 prefixes. you should include the following information in your email:  Your AS number  The IP address(es) you want to use for peering  If your equipment support MD5 passwords for BGP sessions?  Your PGP/GPG public key.

you can also take a look at the secure Router Configuration Template  You’ll find loads of useful information for ISPs and Network people there .com)  While you are at the Team Cymru Team Cymru  The bogon list is maintained by the Team Cymru (www.

Prefix Filters on Customers .

Prefix Filters on Customers  Apply prefix filters on all your customers! AS101 AS1 AS2 AS1 AS65535 AS65534 Peers ISP Customer .

g  If the RIR has assigned 202.  E.224.0/19 to your customer. then accept specific prefix assigned to them by the other ISP. accept only that from it  If your customer is mulihomed. if required . Customer Prefixes  Service Providers should only accept assigned or allocated prefixes from their downstream peer/customer.52.

0.0/0 le 32 .231.32.201 remote-as 201 neighbor ip prefix-list CUSTOMER-AS201 deny prefix-list CUSTOMER-AS201 in ! ip prefix-list CUSTOMER-AS201 permit 201. Receiving Customer Prefixes  Configuration example on upstream: router bgp 65534 neighbor 198.

Prefixes to Peers .

Prefixes to Peers  Apply prefix filters. AS-PATH filters to AS101 your peers! AS1 AS2 AS1 AS65535 AS65534 Peers ISP Customer .

e.  Bogons – assume garbage will leak into your iBGP. Prefixes to Peers  What do you announce to other networks?  Your prefixes. i.  Prefixes longer than /24. /25 to /32 .  Customer's Provider Independent (PI) prefixes  More specific customers prefixes (customers who are multihoming)  What do you not send to other network?  RFC3330 Prefixes – assume junk will leak into your iBGP.

Egress Filter to Peers  The egress prefix list can grow to be very large:  More specifics for customers.  Specific blocks from other ISPs  AS-PATH filters are more scalable  Peer Mailing lists generally send updates about new downstream prefixes .

Ingress Prefix Filtering from Peers .

Prefixes from Peers  Filter all routes from your peers! AS101 AS1 AS2 AS1 AS65535 AS65534 Peers ISP Customer .

full routes are not required. accept only default . Filtering Ingress Routes  Peers and Upstream provide access to the Internet routes  Ingress filters with Peers  The peering policy should have requirements so that filters can be built  Max Prefix limit are important  Ingress filters with Upstream  Unless you multihome.

Best Practices for ingress filters  Don’t accept RFC1918 etc prefixes  Don’t accept your own prefix  Don’t accept default (unless you need it)  Don’t accept prefixes longer than /24  Consider Net Police Filtering .

Ingress example - Cisco IOS
router bgp 65534
network mask
neighbor remote-as 201
neighbor prefix-list in-filter in
ip prefix-list in-filter deny ! Block default
ip prefix-list in-filter deny le 32
ip prefix-list in-filter deny le 32
ip prefix-list in-filter deny le 32
ip prefix-list in-filter deny le 32
ip prefix-list in-filter deny le 32
ip prefix-list in-filter deny le 32
ip prefix-list in-filter deny le 32
ip prefix-list in-filter deny le 32 ! Block local prefix
ip prefix-list in-filter deny le 32
ip prefix-list in-filter deny ge 25 ! Block prefixes >/24
ip prefix-list in-filter permit le 32

Packet Filtering Principles

 Filter as close to the edge as possible
 Filter as precisely as possible
 Filter both source and destination
where possible

Routing Protocol Updates MD5

Route Authentication
Hashed Routing

Router1 Network
Signs Route

Certifies Authenticity of Neighbor
and Integrity of Route Updates

2.1 password 7 cisco  Router2 Example router bgp 201 neighbor description Link to AS-200-Peer neighbor remote-as 200 neighbor password 7 cisco . BGP Route Authentication  Router1 Example router bgp 200 neighbor 2.1 description Link to AS-201-Peer neighbor 2.2.1 remote-as 201 neighbor

Additional BGP Knobs .

BGP Maximum Prefix Tracking  Allow configuration of the maximum number of prefixes a BGP router will receive from a peer  Two level control  Warning threshold: Log warning message  Maximum: Tear down the BGP peering. can be setup renew the session after a time period  Vendor Implementation varies a little bit .

x. BGP Maximum-prefix .x> maximum-prefix <max> [<threshold>] [warning-only]  Threshold is an optional parameter between 1 to 100 percent  Specify the percentage of <max> that a warning message will be generated.Cisco neighbor <x.x. Default is 75%  Warning-only is an optional keyword which allows log messages to be generated but peering session will not be torn down .

then this traffic will go to this one router and get space for the latest macro allocations.  Dropping backscatter traffic might overload the router.  DOS/DDOS attack use spoofed addresses from the un-allocated IPV4 space. .iana.  See http://www. Avoid Default Routes  ISPs with full BGP feeds should avoid default routes.  Backscatter traffic from DOS/DDOS targets need to go somewhere. If there is a default.

RFC 2827/BCP 38 RFC 2827/BCP 38 Network Ingress Filtering: Defeating Denial of Service Attacks which employ IP Source Address Spoofing "Thou shalt only sendth and receiveth IP packets you have rights for" .

consistent with the topology and space allocation . allocated address space. RFC 2827/BCP 38 Ingress Packet Filtering  Packets should be sourced from valid.

Guidelines for BCP38  Networks connecting to the Internet Must use inbound and outbound packet filters to protect network  Configuration example Outbound—only allow my network source addresses out Inbound—only allow specific ports to specific destinations in .

NO BCP38 may mean :  Devices can (wittingly or unwittingly) send traffic with spoofed and/or randomly changing source addresses out to the network  Complicates traceback immensely  Sending bogus traffic is not free .

Techniques for BCP 38 Filtering  Static ACLs on the edge of the network  Dynamic ACLs with AAA profiles  Unicast RPF strict mode  IP source guard  Cable source verify (DHCP) .

Using ACLs to Enforce BCP38  Static ACLs are the traditional method of ensuring that source addresses are not spoofed: Permit all traffic whose source address equals the allocation block Deny any other packet  Principles: Filter as close to the edge as possible Filter as precisely as possible Filter both source and destination where possible .

0.X/24 96.0.255 any 96.0.3 description T1 Link to XYZ.0.0.0/19 BCP 38 Filter = Allow Only Source Addresses from the Customer’s 96.0/24 96.19.0/24 access-list 121 deny ip any any log ! interface serial 1/1/1.20.X.18. ip access-group 121 in ! BCP 38 Filter Applied on Leased Line Aggregation Router .0/24 access-list 121 permit ip Internet Core Static Allocation ACL example .0 0.Cisco Block: 96.

BCP ACL Guidelines  ISPs Make sure your customers install filters on their routers—give them a template they can use  Customer end-sites Make sure you install strong filters on routers you use to connect to the Internet First line of defense—never assume your ISP will do it .

IPv4 Anycast Routing .

or network.  Doesn’t require any special capabilities in the servers. clients. .  Doesn’t break or confuse existing infrastructure. nobody’s proprietary technology. What isn’t Anycast?  Not a protocol. not a different version of IP.

. What is Anycast?  Just a configuration methodology.  It’s been the basis for large-scale content- distribution networks since at least 1995. although not described in detail. in numerous RFCs since time immemorial.  Mentioned.  It’s gradually taking over the core of the DNS infrastructure. as well as much of the periphery of the world wide web.

. How Does Anycast Work?  The basic idea is extremely simple:  Multiple instances of a service share the same IP address.  The routing infrastructure directs any packet to the topologically nearest instance of the service.  What little complexity exists is in the optional details.

Example Router 2 Server Instance A Client Router 1 Router 3 Router 4 Server Instance B .

168.1 10.0.2 10.0.0. Example 192.0.1 Router 2 Server Instance A Client Router 1 Router 3 Router 4 Server Instance B .

0.server.1 Router 2 Server Instance A Client Router 1 Router 3 Router 4 Server Instance B DNS lookup for http://www.168.server.1 .1 produces a single answer: www. Example IN A 10.2

 0 10.2
 2 .1 Router 2 Server Instance A Client Router 1 Router 3 Router 4 Server Instance B Routing Table from Router 1: Destination
 192. Example 10.0.1
 1 10. Routing Table from Router 1: Destination
 Distance 192.2
 2 .
 1 10.1 10.1
 192.168.0. Example 192.1 Router 2 Server Instance A Client Router 1 Router 3 Router 4 Server Instance B 192.1
 0 10.0.0
 /32 Router 2 Server Instance A Client Router 1 Router 3 Router 4 Server Instance B Routing Table from Router 1: Destination
 2 .0.2 10.1 Example 192.1
 1 10.1
 0 10.0.

 127.0.1 Router 2 10.1 Client Router 1 Server Router 3 Router 4 192.0.2 Routing Table from Router 1: Destination
 Distance 192.0.1
 0 10.2
 2 .0.0. Example What the routers think the topology looks like:

. A Security Ramification  Anycast server clouds have the useful property of sinking DOS attacks at the instance nearest to the source of the attack.  This is still of some utility even when DOS sources are widely distributed. leaving all other instances unaffected.

More things to know .

Update your whois Information 
 “Please expect that advanced parts of the community are building filters straight .” 
 Ruediger Volk. Deutsche Telecom 
 (VIX Mailing List. August 17. 05) .and strictly .from routing registry data (at least for the RIPE community this is fairly solid data .a benefit which is painfully missing in most other RIR's service areas).

What else should I know ?  Good ISPs know about their networks. and never block ICMP as a result of some virus activity  ISPs should not be involved in blocking ports over their network  But they can always create a best practice document for their customers not to send internal information on the public internet  Keep their abuse desk functional and have valid e- mails address for abuse  Anycast can provide critical service redundancy .

list is long  Security updates from vendors are sent to most of the list above plus  NSP-Sec Mailing List  Cisco-NSP.. Juniper-NSP and other vendor specific lists  CERT mailing list . EOF is for Europe. NANOG is North America. AfNOG is Africa. Mailing Lists  Most countries have regional if not local mailing lists for operational / bogons updates  SANOG is South Asia. APOPS is Asia Pacific. NZNog is Kiwis…. Swinog is Swiss.

the IX mailing list can be the ideal list for such technical discussion  Remember. a rouge user anywhere on the Internet can effect you. . so you should participate in these forums. and chances of attacks originating near your own network is always higher. Participate  Networks are only useful when people can use it  The only way you can make your network work for everyone is by talking to others.  If you have an IX.

. INOC DBA  INOC-DBA: Inter-NOC Dial-by-ASN  Global Voice-over-IP hotline phone system. exchange points. ISPs. and vendors. directly interconnecting NOCs and SIRTs within carriers.

it’ll ring the phones belonging to that person. . (example: 42 )  If you dial an ASN and an extension number. routing. abuse. How does it work?  If you just dial an Autonomous System Number. it’ll ring a predefined group of phones within that AS. well-known extensions for NOC. SIRT. et cetera. (example: 42*WEW )  Also.

Questions ? .

Thank You Gaurab Raj Upadhaya Peering and Network Group Packet Clearing House With acknowledgements to Philip Fredrick Smith. Rob Thomas.pch. Merike Kaeo and Bill Woodcock The Best Practices for ISPs tutorial can be found at http:// / resources / tutorials / ispbcp .