diff --git a/mitmproxy/proxy/protocol/http.py b/mitmproxy/proxy/protocol/http.py
index 5f9dafab6..ac410eebd 100644
--- a/mitmproxy/proxy/protocol/http.py
+++ b/mitmproxy/proxy/protocol/http.py
@@ -268,6 +268,11 @@ class HttpLayer(base.Layer):
 
         self.log("request", "debug", [repr(request)])
 
+        # set first line format to relative in regular mode,
+        # see https://github.com/mitmproxy/mitmproxy/issues/1759
+        if self.mode is HTTPMode.regular and request.first_line_format == "absolute":
+            request.first_line_format = "relative"
+
         # update host header in reverse proxy mode
         if self.config.options.mode == "reverse":
             f.request.headers["Host"] = self.config.upstream_server.address.host
diff --git a/pathod/pathod.py b/pathod/pathod.py
index a8658361d..8d57897b7 100644
--- a/pathod/pathod.py
+++ b/pathod/pathod.py
@@ -144,6 +144,7 @@ class PathodHandler(tcp.BaseHandler):
             path = req.path
             http_version = req.http_version
             headers = req.headers
+            first_line_format = req.first_line_format
 
             clientcert = None
             if self.clientcert:
@@ -167,6 +168,7 @@ class PathodHandler(tcp.BaseHandler):
                     sni=self.sni,
                     remote_address=self.address(),
                     clientcert=clientcert,
+                    first_line_format=first_line_format
                 ),
                 cipher=None,
             )
diff --git a/test/mitmproxy/test_server.py b/test/mitmproxy/test_server.py
index 9429ab0fa..a7d8cea50 100644
--- a/test/mitmproxy/test_server.py
+++ b/test/mitmproxy/test_server.py
@@ -282,6 +282,21 @@ class TestHTTP(tservers.HTTPProxyTest, CommonMixin):
         assert d.content == b"bar"
         self.master.addons.remove(s)
 
+    def test_first_line_rewrite(self):
+        """
+        If mitmproxy is a regular HTTP proxy, it must rewrite an absolute-form request like
+            GET http://example.com/foo HTTP/1.0
+        to
+            GET /foo HTTP/1.0
+        when sending the request upstream. While any server should technically accept
+        the absolute form, this is not the case in practice.
+        """
+        req = "get:'%s/p/200'" % self.server.urlbase
+        p = self.pathoc()
+        with p.connect():
+            assert p.request(req).status_code == 200
+            assert self.server.last_log()["request"]["first_line_format"] == "relative"
+
 
 class TestHTTPAuth(tservers.HTTPProxyTest):
     def test_auth(self):