CDN works with DNS cache basic knowledge

Products

• VMware SD-WAN by VeloCloud
• VMware VeloCloud SD-WAN
• VMware VeloCloud SD-WAN with Premier Support

Issue/Introduction

Users rely on CDN-based content delivery and manage their domains through a CDN provider. When a request is made for Domain A, A client will reach out to its DNS server and will be directed to the "authoritative entity" for the Domain A in question. From this point on, the CDN DNS server can reply with an answer that provides an IP address binding to the "closest" where the request originated.

Environment

VeloCloud SD-WAN Edge R5234-20241206-GA-ATT-084d77ec0d

 

Resolution

The HTTP request process is as follows:

1  A user 10.0.2.161 enters the domain name of a website (for example, www.example.com) in the browser. The browser sends a DNS request to the local DNS 10.0.5.218.
2  The local DNS 10.0.5.218 checks whether its cache includes the IP address of www.example.com. If yes, the local DNS returns the cached information to the user. If no, the local DNS sends a resolution request to the authoritative DNS.
3  The authoritative DNS resolves the domain name. The domain name points to www.example.com.c.cdnveloclould.com (CNAME record of the domain name).
4  The local DNS redirects the request to the CDN service and responds to A user.
5  Check DNS cache on VCE

edge:b2-edge1:~# vctcpdump -nni br-network1 host 10.0.5.218 -c 100
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on br-network1, link-type EN10MB (Ethernet), capture size 262144 bytes
02:33:12.328079 IP 10.0.2.161.53815 > 10.0.5.218.53: 0+ A? www.example.com. (33)
02:34:07.424387 IP 10.0.5.218.53 > 10.0.2.161.53815: 0*- 2/0/0 CNAME www.example.com.c.cdnveloclould.com., A 10.10.5.254 (148)

edge:b2-edge1:~# debug.py --dns_name_cache
Total Cache Entries: 13
NAME                                    ADDRESS  TTL(s)  SOURCE
.www.example.com                    10.10.5.254     553     DNS <<<<<<<<<<<<<
changelogs.ubuntu.com              91.189.91.48   23700     DPI
changelogs.ubuntu.com              91.189.91.49   23679     DPI
.connectivity-check.ubuntu.com   185.125.190.96      -1     DNS
.connectivity-check.ubuntu.com   185.125.190.97      -1     DNS
.connectivity-check.ubuntu.com   185.125.190.98      -1     DNS

Additional Information

https://knowledge.broadcom.com/external/article?legacyId=67816

The edge cannot associate multiple domains to the same IP (more on this in the "Known limitation" section below).