Iker Casillas became the first player to feature in a 20th Champions League campaign, but it was Fahrmann who left his mark at Veltins-Arena with a string of superb saves.
Having brilliantly kept out Alex Telles' spot-kick, Fahrmann somehow denied Felipe from point-blank range early in the second half before Breel Embolo nudged Schalke ahead just after the hour.
Fahrmann could not repeat his earlier heroics in the 75th minute, though, as Otavio stepped up to convert from the spot and secure a share of the spoils after Naldo had clumsily brought down Moussa Marega.
With all eyes on Casillas, it was Fahrmann who took the limelight in the 13th minute, getting down low to tip Telles' shot wide after referee Jesus Gil Manzano deemed Naldo had deliberately handled in the area.
Naldo could have made amends at the other end soon after but, having met Daniel Caligiuri's free-kick, the veteran defender failed to keep his header down.
Fahrmann was at his best again two minutes into the second half, reacting instinctively to prevent Felipe bundling in from three yards out.
Casillas could not replicate Fahrmann's form at the other end in the 64th minute - Embolo slotting home through the 37-year-old's legs after latching onto Weston McKennie's cross.
Schalke’s lead was short-lived, however, as - following a rash challenge on Marega by Naldo - Otavio sent Fahrmann the wrong way from 12 yards.
Yevhen Konoplyanka had the chance to snatch the points in the closing stages, only for the Ukraine international to drill wide from close range as Porto held firm.
What does it mean: Schalke show their grit
It has been a dreadful start to the Bundesliga season for Domenico Tedesco's side, who have lost their opening three games, but they displayed their resilience on Tuesday against a Porto side packed with attacking talent, and would have had the points if not for Naldo's clumsiness.
Pat on the back: Fahrmann steals Casillas' thunder
Despite Casillas' landmark appearance making headlines prior to kick-off, Schalke's Fahrmann proved his quality with a string of superb stops. The German has conceded six goals in three Bundesliga fixtures so far, but today's performance will go some way to restoring his confidence.
Boot up the backside: Naldo blunders prove costly
Schalke may feel aggrieved at the referee’s penalty decisions, but Naldo only has himself to blame for a ludicrous decision to dive in on Marega, who made the most of the contact, and Fahrmann was not able to spare the defender's blushes for a second time.
It is a return to Bundesliga action for Schalke, who have the small task of hosting Bayern Munich. Porto, meanwhile, travel to Vitoria Setubal in their next outing.