What is Looking Glass?
A looking glass is a public web-based tool that provides read-only access to a network's BGP routing table, ping, and traceroute diagnostics from that network's perspective.
A looking glass is a web application that allows external users to query routing and connectivity information from a remote network's router. It typically provides read-only access to the router's BGP table, as well as the ability to run ping and traceroute commands. The output reflects what the network itself sees, offering a valuable external viewpoint for troubleshooting and verification.
How it works: A looking glass server accepts requests via a web interface, then connects to a router (often via SSH or telnet) and executes the requested command. The output is captured and displayed back to the user. The commands are usually restricted to show commands and diagnostic tools, preventing any configuration changes. Many large ISPs, Internet Exchange Points (IXPs), and backbone operators maintain public looking glasses for community use.
In the wider stack, a looking glass sits as a monitoring and diagnostic tool for the global routing system. It complements route collectors and route servers by giving a point-in-time snapshot of a specific network's routing state. Network engineers use it to verify BGP prefix announcements, check path selection, or test latency and reachability from a remote location. It is a lightweight but essential resource for operational debugging and research.
Key facts
- Provides read-only access to BGP routes, ping, and traceroute from a specific network's perspective.
- Typically implemented as a CGI or web application that executes commands on a router.
- Commonly offered by ISPs, IXPs, and large network operators for public use.
- Allows external users to verify BGP announcements and troubleshoot routing issues.
- Commands are restricted to show and diagnostic functions to prevent configuration changes.
How it works in practice
Related terms
References
More in Networking & Routing
Anycast
Anycast is a network addressing and routing method where a single IP address is assigned to multiple servers, and routers send traffic to the nearest server based on routing protocol metrics.
AS Path
A BGP path attribute that lists the sequence of autonomous system numbers a route has passed through, used for loop detection and path selection.
ASN
A globally unique 16 or 32 bit number assigned to an autonomous system for use in BGP routing between organizations on the Internet.
Autonomous System
An Autonomous System (AS) is a group of IP networks under a single administrative routing policy, identified by a unique ASN (Autonomous System Number) for exterior gateway routing.
BGP
BGP (Border Gateway Protocol) is the path vector routing protocol that networks use to exchange reachability information between autonomous systems on the public internet.
CIDR
CIDR (Classless Inter-Domain Routing) is a method for allocating IP addresses and routing packets using variable-length subnet masks (e.g., /24) instead of fixed classful boundaries.
Hop
A hop is one passage of a packet through a router or other layer-3 forwarding device as it travels from source to destination across an internetwork.
IPv4
IPv4 is the core Internet Protocol using 32-bit addresses, providing roughly 4.3 billion unique identifiers for network interfaces on the global internet.
IPv6
IPv6 is the most recent version of the Internet Protocol, using 128-bit addresses to provide an effectively unlimited number of unique identifiers for networked devices.
Latency
Latency (or round-trip time, RTT) is the time required for a packet to travel from a source to a destination and back, measured in milliseconds, and is a critical metric in network performance.