The Giants announced the moves on Monday, less than a week before their season opener against the Cowboys in Dallas.
Dahl was a member of the Giants' Super Bowl championship team in February 2008. He was released over the weekend by the San Francisco 49ers after two seasons with them.
Dahl played for defensive coordinator Steve Spagnuolo with the Rams. He also is good on special teams coverage.
Brown was re-signed last week and had an interception in the preseason finale against New England. He had previously played for the Giants for three seasons.
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