hy ABHISHEK , the syllabus of indian foreign services is almost the same as that of IAS.

The major difference between them is the difference of the job profile and the number of vacancy available. The number of seats available in IFS is quite less that is why it is a bit more difficult than IAS. Otherwise there is no difference between the two in terms of syllabus aa both are conducted by the same examination body that is UPSC .

Hope it helps !!

Good luck!!