Classical liberals and libertarians, from Frederick Douglass to Ludwig von Mises to Deirdre McCloskey, are nearly unanimous in their praise for open borders. Indeed, the free movement of people across borders seems to be implied straightforwardly by libertarian principles. However, insofar as libertarians are resistant to immigration, it's typically because of worries about the alleged incompatibility of open borders and the welfare state. Most famously, Milton Friedman said, "It's just obvious you can't have free immigration and a welfare state." Many conservatives and conservative-leaning libertarians take this point to count against immigration.
Now, I think this is a bad argument for several reasons. First, estimates of the fiscal impact of immigration "are clustered around zero." Thus, it is simply not clear that free immigration and a welfare state are incompatible for fiscal reasons.
Second, as I wrote elsewhere:
Libertarians oppose laws restricting people’s ability to do things like consume drugs and sugary beverages, gamble their savings on professional sports, ride motorcycles without helmets, etc. But note that a major argument made on behalf of supporting these laws is that they prevent an increase in the consumption of state-provided benefits. People who ruin their health and their finances are more likely to avail themselves of state assistance. Still, libertarians reject these laws—not just in principle, but here and now. I see no reason to treat free immigration differently. (Indeed, if anything, we have far stronger reason to permit free immigration because it would produce significantly greater gains in human well-being than, say, helmetless motorcycle riding.)
But I'll set these objections aside because I want to draw attention to an underappreciated part of Friedman's views on immigration. Unlike contemporary conservatives who take the alleged incompatibility of redistribution and immigration to weigh against immigration, Friedman actually took it to weigh in favor of illegal immigration. As he put it: "It’s a good thing for the illegal immigrants. It’s a good thing for the United States. It’s a good thing for the citizens of the country." Friedman is therefore far from an ally of those who would push for a border wall or tell prospective immigrants to "get in line." Insofar as there is a conflict between state-enforced redistribution and state-authorized immigration, he takes a distinctively unconservative and very much libertarian position: simply immigrate without the authorization of the state. (For a more detailed look at Friedman's views on immigration, see chapter 6 of Ilya Somin's new book Free to Move.)