<PAGE>
Exhibit 5
---------
September 8, 2000
CacheFlow Inc.
650 Almanor Avenue
Sunnyvale, California 94539
Re: CacheFlow Inc. Registration Statement
for Offering of 1,657,429 Shares of Common Stock
Ladies and Gentlemen:
We refer to your registration on Form S-8 (the "Registration Statement")
under the Securities Act of 1933, as amended, of, (i) 77,000 shares of Common
Stock under the SpringBank Networks, Inc. 2000 Stock Incentive Plan and (ii) and
1,580,429 shares of Common Stock issued to Registered Stockholders under the
Written Compensatory Agreements (collectively, the "Plans"). We advise you that,
in our opinion, when such shares have been issued and sold pursuant to the
applicable provisions of the Plans and in accordance with the Registration
Statement, such shares will be validly issued, fully paid and nonassessable
shares of the Company's Common Stock.
We hereby consent to the filing of this opinion as an exhibit to the
Registration Statement.
Very truly yours,
/s/ Gunderson Dettmer Stough Villeneuve
Franklin & Hachigian, LLP
Gunderson Dettmer Stough Villeneuve Franklin &
Hachigian, LLP
