Dahl was an undrafted free agent out of North Dakota State in 2007 and signed initially with the New York Giants. He played nine games, starting two, before he tore his ACL and missed the rest of the season. He was made an unrestricted free agent after he wasn't tendered as an exclusive-rights free agent and signed with the Rams.
Over the next four years in the Rams, Dahl only missed three games, though he only started the majority of the team's games in 2010 and all of them in 2012. He was a backup in 2009 and 2011. Over his career, Dahl has made 321 tackles, three sacks and picked off four passes.
The 49ers lost free safety Dashon Goldson to the Tampa Bay Buccaneers in free agency once the new contract year began. They could be signing Dahl as his replacement, or they could be signing him primarily for depth in the secondary. The Niners had the fourth-ranked pass defense in 2012, only giving up an average of 200.2 yards per game. It was one of the biggest reasons the team made it to Super Bowl XLVII.