diff --git a/mitmproxy/net/check.py b/mitmproxy/net/check.py
index f793d3971..d30c1df65 100644
--- a/mitmproxy/net/check.py
+++ b/mitmproxy/net/check.py
@@ -1,3 +1,4 @@
+import ipaddress
 import re
 
 # Allow underscore in host name
@@ -6,17 +7,26 @@ _label_valid = re.compile(b"(?!-)[A-Z\d\-_]{1,63}(?<!-)$", re.IGNORECASE)
 
 def is_valid_host(host: bytes) -> bool:
     """
-        Checks if a hostname is valid.
+    Checks if the passed bytes are a valid DNS hostname or an IPv4/IPv6 address.
     """
     try:
         host.decode("idna")
     except ValueError:
         return False
+    # RFC1035: 255 bytes or less.
     if len(host) > 255:
         return False
     if host and host[-1:] == b".":
         host = host[:-1]
-    return all(_label_valid.match(x) for x in host.split(b"."))
+    # DNS hostname
+    if all(_label_valid.match(x) for x in host.split(b".")):
+        return True
+    # IPv4/IPv6 address
+    try:
+        ipaddress.ip_address(host.decode('idna'))
+        return True
+    except ValueError:
+        return False
 
 
 def is_valid_port(port):
diff --git a/test/mitmproxy/net/test_check.py b/test/mitmproxy/net/test_check.py
index 9dbc02e02..0ffd6b2e7 100644
--- a/test/mitmproxy/net/test_check.py
+++ b/test/mitmproxy/net/test_check.py
@@ -11,3 +11,4 @@ def test_is_valid_host():
     assert check.is_valid_host(b"one.two.")
     # Allow underscore
     assert check.is_valid_host(b"one_two")
+    assert check.is_valid_host(b"::1")